Variable Speed Driven Pumps

Best Practice Guide

AUTOMATION

INSTRUMENTATION & CONTROL

LABORATORY TECHNOLOGY

H

0

0 Q

P1 P1+2 P1+2+3
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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PREFACE AND ACKNOWLEDGEMENTS ........................................................................................................................3

SECTION 1 INTRODUCTION..................................................................................................................................................4

SECTION 2 PUMPING SYSTEM HYDRAULIC CHARACTERISTICS....................................................................5

2.1 SYSTEM CHARACTERISTICS............................................................................................................................................5

2.3 PUMP CURVES ........................................................................................................................................................................6

2.3 PUMP OPERATING POINT ..................................................................................................................................................7

SECTION 3 ROTODYNAMIC PUMPS..................................................................................................................................8

3.1 PUMP PRINCIPLES & PERFORMANCE CHARACTERISTICS.................................................................................8

3.1.1 EFFECT OF SPEED VARIATION....................................................................................................................................8

3.1.2 EFFECTS OF IMPELLER DIAMETER CHANGE......................................................................................................10

3.1.3 PUMP SUCTION PERFORMANCE (NPSH)...............................................................................................................11

3.2 METHODS OF VARYING PUMP PERFORMANCE....................................................................................................12

3.2.1 THE NEED FOR PERFORMANCE VARIATION.......................................................................................................12

3.2.2 PUMP CONTROL BY VARYING SPEED ....................................................................................................................12

3.2.3 PUMPS IN PARALLEL SWITCHED TO MEET DEMAND ....................................................................................14

3.2.4 STOP/START CONTROL.................................................................................................................................................15

3.2.5 FLOW CONTROL VALVE...............................................................................................................................................16

3.2.6 BY-PASS CONTROL.........................................................................................................................................................16

SECTION 4 POSITIVE DISPLACEMENT PUMPS ...................................................................................................... 17

4.1 PUMP PRINCIPLES, TYPES AND PERFORMANCE CHARACTERISTICS.........................................................17

4.2 METHODS OF VARYING PUMP PERFORMANCE....................................................................................................18

4.2.1 PUMP CONTROL BY VARYING SPEED ....................................................................................................................18

4.2.2 FLOW CONTROL USING PUMPS IN PARALLEL...................................................................................................21

4.2.3 PUMPS IN SERIES (ROTARY) .......................................................................................................................................21

4.2.4 FLOW CONTROL VALVE...............................................................................................................................................21

4.2.5 BY-PASS CONTROL.........................................................................................................................................................21

4.2.6 LOAD/UNLOAD CONTROL...........................................................................................................................................21

SECTION 5 MOTORS .............................................................................................................................................................. 22

5.1 MOTOR PRINCIPLES ...........................................................................................................................................................22

5.2 MULTI-SPEED MOTORS ....................................................................................................................................................22

5.3 ENERGY EFFICIENCY........................................................................................................................................................22

5.4 EFFICIENCY LABELLING.................................................................................................................................................23

5.5 OTHER ENERGY SAVING OPPORTUNITIES..............................................................................................................24

5.5.1 MOTOR SIZING..................................................................................................................................................................24

5.5.2SWITCH IT OFF!.................................................................................................................................................................25

5.5.3 A MOTOR MANAGEMENT POLICY...........................................................................................................................25

5.5.4 SHAFT ALIGNMENT........................................................................................................................................................26

5.5.5 PULLEY SIZING.................................................................................................................................................................26

SECTION 6 VARIABLE SPEED DRIVES ......................................................................................................................... 27

6.1 VARIABLE FREQUENCY DRIVE PRINCIPLES 27

6.2 THE FREQUENCY CONVERTER.....................................................................................................................................27

6.2.1 RECTIFIER...........................................................................................................................................................................28

6.2.2 INTERMEDIATE CIRCUIT..............................................................................................................................................28

6.2.3 INVERTER............................................................................................................................................................................28

6.2.4 CONTROL UNIT.................................................................................................................................................................28

6.3 PULSE WIDTH MODULATION ........................................................................................................................................29

6.4 INTEGRATED VARIABLE SPEED MOTORS ...............................................................................................................29

SECTION 7 SELECTING A VARIABLE SPEED DRIVE FOR A NEW INSTALLATION............................... 30

7.1 SIZING & SELECTION.............................................................................. ...................30

7.2 CONTROL...................................................... ............................................................33

7.2.1 CONTROL BY FIXING PRESSURE BUT VARYING FLOW .................................................................................33

7.2.2 HEATING SYSTEM CONTROL.....................................................................................................................................33

7.2.3 CONTROL BY FIXING FLOW BUT VARYING PRESSURE:................................................................................34

7.2.4 IMPLEMENTATION ..........................................................................................................................................................34

7.2.5 SOFT STARTING AND STOPPING...............................................................................................................................34

SECTION 8 RETROFITTING A VARIABLE SPEED DRIVE TO EXISTING EQUIPMENT........................ 35

8.1 JUSTIFICATION.......................................................................................... ................35

8.2 MOTOR DERATING.................................................................................... .................35
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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8.3 SIZING AND SELECTION OF A VSD ON EXISTING EQUIPMENT ...... ..............................36

SECTION 9 EFFECTS OF NOISE & VIBRATION WHEN VARYING SPEED ................................................... 39

9.1 TYPICAL NOISE LEVELS............................................................... ..............................39

PUMPS................................ 40

MOTORS 41

APPENDIX ADDITIONAL INFORMATION 41

A1 MAGNETIC DRIVE PUMPS ........................................................................................................................................... 41

A1.1 EFFECTS OF PUMP SPEED 41

A1.2 EFFECTS OF DIAMETER CHANGE .............................................................................................................................41

A2 MOTOR CONSIDERATIONS ......................................................................................................................................... 41

PROTECTION................................................................................................................................................................................41

SPEED ..............................................................................................................................................................................................42

INSULATION DESIGN ...............................................................................................................................................................42

MOTOR BEARINGS ....................................................................................................................................................................42

A3 LEGISLATIVE REQUIREMENTS................................................................................................................................ 43

A3.1 THE MACHINERY DIRECTIVE.....................................................................................................................................43

A3.2 THE EMC DIRECTIVE......................................................................................................................................................43

A3.2.1 RADIATED EMISSIONS...............................................................................................................................................44

A3.2.2 CONDUCTED EMISSIONS ..........................................................................................................................................44

A3.2.3 HARMONICS....................................................................................................................................................................44

A3.3 THE LOW VOLTAGE DIRECTIVE ...............................................................................................................................44

A3.4 THE ATEX DIRECTIVE ....................................................................................................................................................45

A3.5 THE CE MARKING DIRECTIVE ...................................................................................................................................45

A4 ABBREVIATIONS............................................................................................................................................................... 45

A5 REFERENCES AND FURTHER READING............................................................................................................... 46
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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PREFACE AND ACKNOWLEDGEMENTS 

This guide is the result of co-operation between three different industries whose goal was to 

produce a document that would clearly define; in simple terms the information required

when planning to use an electronic Variable Speed Driven Pumping System. The guide 

focuses mainly on applications within the Industrial Sector, however the principles used will 

be applicable to most pumping applications.

Members from the British Pump Manufacturers’ Association (BPMA), Gambica’s Variable 

Speed Drive group and experts from the Electric Motor industry assisted with this guide. 

The chairman of the working group is indebted to all the team members for their

contributions. 

In particular a special mention should be given to: -

Geoff Brown – ABB Ltd., Drives & Motors Darren Henighan – Brook Crompton Motors 

Ian James – HMD Sealless Pumps John Bower – Flowserve

John Drane – Mono Pumps John Thorne – ITT Goulds Pumps

Mike Moss – Weir Pumps Peter Thompson – David Brown Union Pumps

Robin Piper- GAMBICA George Fraser - GAMBICA

Justin Brock – Action Energy.

Sponsorship has been provided by Action Energy (formerly the Energy Efficiency Best 

Practice Programme). This continued support for energy saving equipment ensures that 

documents like this can be published.

To ensure the guide achieved its goal, copies were circulated to Technical Experts outside the 

working group who offered comment and direction. The entire working group are grateful 

for this extra guidance.

A special mention should be given to: -

Steve Moore – Ineos Chlor Bob Went – Thames Water Utilities

Morris Yates – AEMS Derek Burgoyne- Pump Consultant

Dr Hugh Falkner – FES Brian Nesbitt- Pump Consultant

Steve Schofield 

Director Technical Services BPMA

(Chairman of the VSD working group)
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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SECTION 1 INTRODUCTION 

Pump systems are often operated inefficiently. The reasons will vary from process to process 

and application to application, but the constant outcome is the cost to industry through 

wasted energy, which runs into millions of pounds per year, and the cost to the environment 

through the generation of this wasted energy.

It is estimated that in the United Kingdom, pumps use a total of 20TWh/annum, responsible 

for the emission of 2.7MtC/annum (2.7 million tons of carbon). Pumps therefore represent 

the largest single use of motive power in industry and commerce as shown in the breakdown 

of energy usage by motor driven equipment:

Pumps-31%

Fans- 23%

Air Compressors- 8%

Other Compressors – 14%

Conveyors – 8%

Others 16%

A pump installation is often sized to cope with a maximum predicted flow, which, may never 

happen. This principle of over sizing is frequently used in Industry, which subsequently 

leads to wasted energy and damage to parts of the pump installation.

Procurement costs of the pump equipment in general amount to less than 1% of the total 

investment of a plant, yet the operational quality of a pump may be the decisive factor in the 

overall functionality of the plant and its associated running costs.

Flow control by speed regulation of pumps, is one of today’s best methods of varying the 

output on both Rotodynamic and Positive Displacement pumps and this guide describes its 

many advantages and potential system drawbacks.

The benefits covered include:

• Energy cost savings

• Reliability improvements 

• Simplified pipe systems (elimination of control valves & by-pass lines)

• Soft start & stop

• Reduced maintenance

All amounting to lower life cycle costs.

Whilst other methods of control are available, this guide concentrates on Pulse Width 

Modulated Variable Speed Drive because it has the greatest benefits of control, energy 

efficiency, and ease of retrofitting.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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SECTION 2 PUMPING SYSTEM HYDRAULIC CHARACTERISTICS

2.1 SYSTEM CHARACTERISTICS

In a pumping system, the objective, in most cases, is either to transfer a liquid from a source to a 

required destination, e.g. filling a high level reservoir, or to circulate liquid around a system, e.g.

as a means of heat transfer.

A pressure is needed to make the liquid flow at the required rate and this must overcome head 

‘losses’ in the system. Losses are of two types: static and friction head.

Static head is simply the difference in height of the supply and destination reservoirs, as in Figure 

2.1. In this illustration, flow velocity in the pipe is assumed to be very small. Another example of 

a system with only static head is pumping into a pressurised vessel with short pipe runs. Static 

head is independent of flow and graphically would be shown as in Figure 2.2. 

Figure 2.1 Figure 2 .2

Static head Static head vs. flow

Friction head (sometimes called dynamic head loss) is the friction loss, on the liquid being moved, 

in pipes, valves and equipment in the system. The losses through these are proportional to the 

square of the flow rate. A closed loop circulating system without a surface open to atmospheric 

pressure, would exhibit only friction losses and would have a system head loss vs. flow

characteristic curve as Figure 2.3

Figure 2.3 

Friction head vs. flow

STATIC

HEAD

FLOW

STATIC HEAD

FLOW

FRICTION HEAD
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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Most systems have a combination of static and friction head and the system curves for two cases 

are shown in Figures 2.4 and 2.5. The ratio of static to friction head over the operating range

influences the benefits achievable from variable speed drives (see section 3.2.2)

Figure 2.4

System with high static head

Figure 2.5

System with low static head

Static head is a characteristic of the specific installation and reducing this head where this is

possible, generally helps both the cost of the installation and the cost of pumping the liquid. 

Friction head losses must be minimised to reduce pumping cost, but after eliminating unnecessary

pipefittings and length, further reduction in friction head will require larger diameter pipe, which

adds to installation cost.

2.3 PUMP CURVES

The performance of a pump can also be expressed graphically as head against flow rate. See Fig 

2.6 for rotodynamic pumps and Fig 2.7 for positive displacement (PD) pumps.

Figure 2.6 Figure 2.7

Rotodynamic Pump Positive displacement Pump

The Rotodynamic pump, (usually a centrifugal pump) has a curve where the head falls gradually 

with increasing flow, but for a PD pump, the flow is almost constant whatever the head. It is 

customary to draw the curve for PD pumps with the axes reversed (see Section 4), but to 

understand the interaction with the system, a common presentation is used here for the two pump 

types.

FLOW

SYSTEM HEAD

FRICTION HEAD

STATIC HEAD

SYSTEM CURVE

FLOW

SYSTEM HEAD

STATIC HEAD

SYSTEM CURVE

FRICTION HEAD

FLOW

HEAD

FLOW

HEAD
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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2.3 PUMP OPERATING POINT

When a pump is installed in a system the effect can be illustrated graphically by superimposing 

pump and system curves. The operating point will always be where the two curves intersect. (Fig 

2.8 and Fig 2.9).

Figure 2.8 Figure 2.9

Rotodynamic pump with system curve P.D. pump with system curve

If the actual system curve is different in reality to that calculated, the pump will operate at a flow 

and head different to that expected.

For a PD pump, if the system resistance increases, the pump will increase its discharge pressure 

and maintain a fairly constant flow rate, dependant on viscosity and pump type. Unsafe pressure 

levels can occur without relief valves.

For a rotodynamic pump, an increasing system resistance will reduce the flow, eventually to zero, 

but the maximum head is limited as shown. Even so, this condition is only acceptable for a short 

period without causing problems. An error in the system curve calculation is also likely to lead to 

a rotodynamic pump selection, which is less than optimum for the actual system head losses. 

Adding comfort margins to the calculated system curve to ensure that a sufficiently large 

pump is selected will generally result in installing an oversized pump, which will operate at 

an excessive flow rate or in a throttled condition, which increases energy usage and reduces 

pump life. 

FLOW

HEAD

HEAD FLOW CURVE

OPERATING POINT

SYSTEM CURVE

FLOW

HEAD

SYSTEM CURVE

HEAD FLOW CURVE

OPERATING POINT
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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SECTION 3 ROTODYNAMIC PUMPS 

3.1 PUMP PRINCIPLES & PERFORMANCE CHARACTERISTICS

A rotodynamic or centrifugal pump is a dynamic device for increasing the pressure of liquid. In 

passing through the pump, the liquid receives energy from the rotating impeller. The liquid is 

accelerated circumferentially in the impeller, discharging into the casing at high velocity which is 

converted into pressure as effectively as possible.

Since the pump is a dynamic device, it is convenient to consider the head generated rather than the 

pressure. The pump generates the same head of liquid whatever the density of the liquid being 

pumped. The actual shapes of the hydraulic passages of the impeller and the casing are extremely 

important, in order to attain the highest efficiency possible.

The standard convention for rotodynamic pump is to draw the pump performance curves showing 

Flow on the horizontal axis and Head generated on the vertical axis. Efficiency, Power & NPSH 

Required (see Section 3.1.3 for explanation of NPSH), are also all conventionally shown on the 

vertical axis, plotted against Flow, as illustrated in Fig 3.1.

Figure 3.1: 

Example of Pump performance curves

3.1.1 EFFECT OF SPEED VARIATION

As stated above, a centrifugal pump is a dynamic device with the head generated from a rotating 

impeller. There is therefore a relationship between impeller peripheral velocity and generated 

head. Peripheral velocity is directly related to shaft rotational speed, for a fixed impeller diameter 

and so varying the rotational speed has a direct effect on the performance of the pump. All the 

parameters shown in fig 3.1 will change if the speed is varied and it is important to have an 

appreciation of how these parameters vary in order to safely control a pump at different speeds.

The equations relating rotodynamic pump performance parameters of flow, head and power 

absorbed, to speed are known as the Affinity Laws:

NPSH Required

 POWER

 EFFICIENCY

HEAD

 FLOW RATE
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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3

2

P N

H N

Q N

∝

∝

∝

Where:

Q = Flow rate

H = Head

P = Power absorbed

N = Rotating speed

Efficiency is essentially independent of speed

The implication of the squared and cubic relationships of head and power absorbed, is that 

relatively small changes in speed give very significant changes in these parameters as shown in an 

example of a centrifugal pump in fig 3.2.

Figure 3.2

Example of speed variation effecting rotodynamic pump performance.

Points of equal efficiency on the curves for the 3 different speeds are joined to make the isoefficiency lines, showing that efficiency remains constant over small changes of speed providing 

the pump continues to operate at the same position related to its best efficiency point (BEP).

The affinity laws give a good approximation of how pump performance curves change with speed 

but in order to obtain the actual performance of the pump in a system, the system curve also has to 

be taken into account, as will be explained later.

Magnetically driven pumps, with metallic containment shell, as well as the hydraulic power, which 

obeys the affinity laws, have a magnetic power absorbed, which follows a square law with speed.

0

10

20

30

40

50

60

70

80

0 200 400 600 800 1000 1200

Flow Rate m3/h

Total Head m

0

50

100

150

200

250

300

350

400

Power kW

71%

83%

86%

83%

1480rpm

1350rpm

1184rpm

1480rpm

1350rpm

1184rpm
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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The two types of power must therefore be calculated separately for a change of speed. 

In Appendix A1-1 this is explained further.

3.1.2 EFFECTS OF IMPELLER DIAMETER CHANGE

Changing the impeller diameter gives a proportional change in peripheral velocity, so it follows 

that there are equations, similar to the affinity laws, for the variation of performance with impeller 

diameter D:

Efficiency varies when the diameter is changed within a particular casing. Note the difference in 

iso-efficiency lines in Figure 3.3 compared with Figure 3.2. The relationships shown here apply to 

the case for changing only the diameter of an impeller within a fixed casing geometry, which is a 

common practise for making small permanent adjustments to the performance of a centrifugal 

pump. Diameter changes are generally limited to reducing the diameter to about 75% of the 

maximum, i.e. a head reduction to about 50%. Beyond this, efficiency and NPSH are badly 

affected. However speed change can be used over a wider range without seriously reducing 

efficiency. For example reducing the speed by 50% typically results in a reduction of efficiency by 

1 or 2 percentage points. The reason for the small loss of efficiency with the lower speed is that 

mechanical losses in seals and bearings, which generally represent <5% of total power, are 

proportional to speed, rather than speed cubed.

It should be noted that if the change in diameter is more than about 5%, the accuracy of the 

squared and cubic relationships can fall off and for precise calculations, the pump manufacturer’s 

performance curves should be referred to.

Figure 3.3

Example of impeller diameter reduction on rotodynamic pump performance.

3

2

P D

H D

Q D

∝

∝

∝

0

10

20

30

40

50

60

70

80

0 200 400 600 800 1000 1200

Flow Rate m3/h

Total Head m

0

50

100

150

200

250

300

350

400

Power kW

400mm

373mm

326mm

400mm

373mm

326mm

70%

79%

83% 85%

86%

81%
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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The illustrated curves are typical of most rotodynamic pump types. Certain high flow, low head 

pumps have performance curve shapes somewhat different and have a reduced operating region of 

flows. This requires additional care in matching the pump to the system, when changing speed 

and diameter.

Magnetically driven pumps, may also need to be treated differently because a change of impeller 

diameter affects only the hydraulic power. Mechanical power loss in the drive is independent of 

diameter and so if the speed is unchanged the magnetic losses will not change.

See Appendix A1-2.

3.1.3 PUMP SUCTION PERFORMANCE (NPSH) 

Liquid entering the impeller eye turns and is split into separate streams by the leading edges of the 

impeller vanes, an action which locally drops the pressure below that in the inlet pipe to the pump. 

If the incoming liquid is at a pressure with insufficient margin above its vapour pressure, then 

vapour cavities or bubbles appear along the impeller vanes just behind the inlet edges. This 

phenomenon is known as cavitation and has three undesirable effects:

1) The collapsing cavitation bubbles can erode the vane surface, especially when pumping waterbased liquids.

2) Noise and vibration are increased, with possible shortened seal and bearing life.

3) The cavity areas will initially partially choke the impeller passages and reduce the pump

performance. In extreme cases, total loss of pump developed head occurs.

The value, by which the pressure in the pump suction exceeds the liquid vapour pressure, is 

expressed as a head of liquid and referred to as Net Positive Suction Head Available – (NPSHA). 

This is a characteristic of the system design. The value of NPSH needed at the pump suction to 

prevent the pump from cavitating is known as NPSH Required – (NPSHR). This is a characteristic 

of the pump design.

The three undesirable effects of cavitation described above begin at different values of NPSHA 

and generally there will be cavitation erosion before there is a noticeable loss of pump head. 

However for a consistent approach, manufacturers and industry standards, usually define the onset 

of cavitation as the value of NPSHR when there is a head drop of 3% compared with the head with 

cavitation free performance. At this point cavitation is present and prolonged operation at this 

point will usually lead to damage. It is usual therefore to apply a margin by which NPSHA should 

exceed NPSHR.

As would be expected, the NPSHR increases as the flow through the pump increases, see fig 3.1. 

In addition, as flow increases in the suction pipework, friction losses also increase, giving a lower 

NPSHA at the pump suction, both of which give a greater chance that cavitation will occur.

NPSHR also varies approximately with the square of speed in the same way as pump head and 

conversion of NPSHR from one speed to another can be made using the following equations.

It should be noted however that at very low speeds there is a minimum NPSHR plateau, NPSHR 

does not tend to zero at zero speed 

It is therefore essential to carefully consider NPSH in variable speed pumping.

2 NPSHR N

Q N

∝

∝
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3.2 METHODS OF VARYING PUMP PERFORMANCE

3.2.1 THE NEED FOR PERFORM ANCE VARIATION

Many pumping systems require a variation of flow or pressure. To do so, either the system curve 

or the pump curve must be changed to get a different operating point. Where a single pump has 

been installed for a range of duties, it will have been sized to meet the greatest output demand, it 

will therefore usually be oversized, and will be operating inefficiently for other duties. There is

therefore an opportunity to achieve an energy cost saving by using control methods which reduce 

the power to drive the pump during the periods of reduced demand. Not all control methods

achieve this goal as explained in this section.

Varying pump performance by changing speed is explained first, it is the main focus of this guide,

and in many cases is a cost effective approach with good pay back and even though the capital

expenditure is relatively high, there can be savings on other equipment e.g. control valves. Other 

methods of control are then explained so that the most appropriate approach, to minimise life cycle 

cost, can be chosen. To make an effective evaluation of which control method to use, all of the 

operating duty points and their associated run time and energy consumption have to be identified, 

so that the total costs can be calculated and alternative methods compared.

Changing pump impeller diameter also effectively changes the duty point in a given system, (see

Section 3.1.2), and at low cost, but this can be used only for permanent adjustment to the pump 

curve and is not discussed further as a control method.

3.2.2 PUMP CONTROL BY VARYING SPEED

To understand how speed variation changes the duty point, the pump and system curves are overlaid. Two systems are considered, one with only friction loss and another where static head is high 

in relation to friction head. It will be seen that the benefits are different. In Figure 3.4, reducing 

speed in the friction loss system moves the intersection point on the system curve along a line of

constant efficiency. The operating point of the pump, relative to its best efficiency point, remains 

constant and the pump continues to operate in its ideal region. The affinity laws are obeyed which 

means that there is a substantial reduction in power absorbed accompanying the reduction in flow 

and head, making variable speed the ideal control method for systems with friction loss.

Figure 3.4 

Example of the effect of pump speed change in a system with only friction loss

0

10

20

30

40

50

60

70

80

0 200 400 600 800 1000 1200

Flow Rate m3/h

Total Head m

0

50

100

150

200

250

300

350

400

Power kW

71%

83%

86%

83%

1480rpm

1350rpm

1184rpm

1480rpm

1350rpm

1184rpm

System

Curve

Iso-efficiency

Lines

Operating points
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In a system where static head is high, as illustrated in Figure 3.5, the operating point for the pump 

moves relative to the lines of constant pump efficiency when the speed is changed. The reduction 

in flow is no longer proportional to speed. A small turn down in speed could give a big reduction 

in flow rate and pump efficiency, which could result in the pump operating in a region where it 

could be damaged if it ran for an extended period of time even at the lower speed. At the lowest 

speed illustrated, (1184 rpm), the pump does not generate sufficient head to pump any liquid into 

the system, i.e. pump efficiency and flow rate are zero and with energy still being input to the 

liquid, the pump becomes a water heater and damaging temperatures can quickly be reached.

Figure 3.5

Example of the effect of pump speed change with a system with high static head.

The drop in pump efficiency during speed reduction in a system with static head, reduces the 

economic benefits of variable speed control. There may still be overall benefits but economics 

should be examined on a case -by -case basis.

Usually it is advantageous to select the pump such that the system curve intersects the full speed 

pump curve to the right of best efficiency, in order that the efficiency will first increase as the 

speed is reduced and then decrease. This can extend the useful range of variable speed operation 

in a system with static head. The pump manufacturer should be consulted on the safe operating 

range of the pump.

It is relevant to note that flow control by speed regulation is always more efficient than by control 

valve.

In addition to energy savings there could be other benefits of lower speed. The hydraulic forces on 

the impeller, created by the pressure profile inside the pump casing, reduce approximately with the 

square of speed. These forces, are carried by the pump bearings and so reducing speed increases 

bearing life. It can be shown that for a rotodynamic pump, bearing life is inversely proportional to 

the 7

th power of speed. In addition, vibration and noise are reduced and seal life is increased

providing the duty point remains within the allowable operating range.

0

10

20

30

40

50

60

70

80

0 200 400 600 800 1000 1200

Flow Rate m3/h

Total Head m

0

50

100

150

200

250

300

350

400

Power kW

71%

83%

86%

83%

1480rpm

1350rpm

1184rpm

1480rpm

1350rpm

1184rpm

System

Curve

Iso-efficiency

Lines

Operating points
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The corollary to this is that small increases in the speed of a pump significantly increase power 

absorbed, shaft stress and bearing loads. It should be remembered that the pump and motor must 

be sized for the maximum speed at which the pump set will operate. At higher speed the noise and 

vibration from both pump and motor will increase, although for small increases the change will be 

small. If the liquid contains abrasive particles, increasing speed will give a corresponding increase 

in surface wear in the pump and pipework.

The affect on the mechanical seal of the change in seal chamber pressure, should be reviewed with 

the pump or seal manufacturer, if the speed increase is large. Conventional mechanical seals 

operate satisfactorily at very low speeds and generally there is no requirement for a minimum 

speed to be specified, however due to their method of operation, gas seals require a minimum 

peripheral speed of 5 m/s.

3.2.3 PUMPS IN PARALLEL SWITCHED TO MEET DEMAND

Another energy efficient method of flow control, particularly for systems where static head is a 

high proportion of the total, is to install two or more pumps to operate in parallel. Variation of 

flow rate is achieved by switching on and off additional pumps to meet demand. The combined 

pump curve is obtained by adding the flow rates at a specific head. The head/flow rate curves for 

two and three pumps are shown in Figure 3.6

Figure 3.6

Typical head-flow curves for pumps in parallel

The system curve is usually not affected by the number of pumps that are running. For a system

with a combination of static and friction head loss, it can be seen, in Fig 3.7, that the operating

point of the pumps on their performance curves moves to a higher head and hence lower flow rate 

per pump, as more pumps are started. It is also apparent that the flow rate with two pumps running 

is not double that of a single pump. If the system head were only static, then flow rate would be 

proportional to the number of pumps operating.

It is possible to run pumps of different sizes in parallel providing their closed valve heads are

similar. By arranging different combinations of pumps running together, a larger number of

different flow rates can be provided into the system.

30

35

40

45

50

55

60

65

70

0 50 100 150 200 250 300 350 400 450 500

FLOW RATE

HEAD

Two Pumps

In Parallel

Single Pump Three Pumps

In Parallel
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Care must be taken when running pumps in parallel to ensure that the operating point of the pump 

is controlled within the region deemed as acceptable by the manufacturer. It can be seen from fig 

3.7 that if 1 or 2 pumps are stopped then the remaining pump(s) would operate well out along the 

curve where NPSHR is higher and vibration level increased, giving an increased risk of operating

problems.

Figure 3.7

Typical Head-flow curves for pumps in parallel, with system curve illustrated.

3.2.4 STOP/START CONTROL

In this method the flow is controlled by switching pumps on or off. It is necessary to have a

storage capacity in the system e.g. a wet well, an elevated tank or an accumulator type pressure 

vessel. The storage can provide a steady flow to the system with an intermittent operating pump. 

When the pump runs, it does so at the chosen (presumably optimum) duty point and when it is off, 

there is no energy consumption. If intermittent flow, stop/start operation and the storage facility

are acceptable, this is an effective approach to minimise energy consumption.

The stop/start operation causes additional loads on the power transmission components and 

increased heating in the motor. The frequency of the stop/start cycle should be within the motor

design criteria and checked with the pump manufacturer.

It may also be used to benefit from “off peak” energy tariffs by arranging the run times during the

low tariff periods.

To minimise energy consumption with stop start control it is better to pump at as low flow rate as 

the process permits. This minimises friction losses in the pipe and an appropriately small pump

can be installed. For example, pumping at half the flow rate for twice as long can reduce energy

consumption to a quarter.

30

35

40

45

50

55

60

65

70

0 50 100 150 200 250 300 350 400 450 500

FLOW RATE

HEAD

SYSTEM 

CURVE

Single Pump Two Pumps

In Parallel

Three Pumps

In Parallel

Flow-1 Flow-2 Flow-3
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16

3.2.5 FLOW CONTROL VALVE

With this control method, the pump runs continuously and a valve in the pump discharge line is

opened or closed to adjust the flow to the required value.

25

30

35

40

45

50

55

60

65

70

0 50 100 150 200 250 300 350 400 450 500

FLOW RATE

Flow 2 Flow 1

System Curve with Half Open Valve

System Curve 

with Fully Open Valve

HEAD

Head Drop Across 

Half Open valve

Figure 3.8

Control of pump flow by changing system resistance using a valve.

To understand how the flow rate is controlled see Figure 3.8. With the valve fully open, the pump 

operates at “Flow 1”. When the valve is partially closed it introduces an additional friction loss in 

the system, which is proportional to flow squared. The new system curve cuts the pump curve at 

“Flow 2”, which is the new operating point. The head difference between the two curves is the 

pressure drop across the valve.

It is usual practice with valve control to have the valve 10% shut even at maximum flow. Energy is 

therefore wasted overcoming the resistance through the valve at all flow conditions.

There is some reduction in pump power absorbed at the lower flow rate (see Figure 3.1), but the 

flow multiplied by the head drop across the valve, is wasted energy. It should also be noted that, 

whilst the pump will accommodate changes in its operating point as far as it is able within its

performance range, it can be forced to operate high on the curve where its efficiency is low, and 

where its reliability is impaired.

Maintenance cost of control valves can be high, particularly on corrosive and solids-containing 

liquids. So the lifetime cost could be unnecessarily high.

3.2.6 BY-PASS CONTROL

In this approach, the pump runs continuously at the maximum process demand duty, with a

permanent by-pass line attached to the outlet. When a lower flow is required the surplus liquid is 

bypassed and returned to the supply source. An alternative configuration may have a tank

supplying a varying process demand, which is kept full by a fixed duty pump running at the peak 

flow rate. Most of the time the tank overflows and recycles back to the pump suction. This is even 

less energy efficient than a control valve because there is no reduction in power consumption with

reduced process demand.

The small by-pass line sometimes installed to prevent a pump running at zero flow is not a means 

of flow control, but required for the safe operation of the pump.
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SECTION 4 POSITIVE DISPLACEMENT PUMPS

4.1 PUMP PRINCIPLES, TYPES AND PERFORMANCE CHARACTERISTICS.

Positive Displacement Pumps can be classified into two main groups: Rotary and Reciprocating.

Rotary pumps (typical pressures up to 25 bar), transfer liquid from suction to discharge through

the action of rotating screws, lobes, gears, valves, rollers etc, which operate inside a rigid casing. 

Rotary pumps do not require non-return valves on the inlet and outlet sides of the pump.

Reciprocating pumps (typical pressures up to 500 bar) discharge liquid by changing the internal

volume of the pump. Reciprocating pumps incorporate both inlet and outlet non-return valves. 

These are generally integral with the pump body.

Flow rate and Pressure

The relationship between flow rate and pressure of the two types is shown in Figures 4.1 and 4.2.

MAX

MAX

q = Slip flow

High Speed

0

Low Speed

Pressure

q

Theoretical

MIN

q

Flow rate

 

MIN

Pressure

MAX

Volumetric

Efficiency

100 %

MAX

0

Theoretical

Flow rate

Figure 4.1 Figure 4.2

Rotary Reciprocating

There is only a small fall off in flow rate with increasing pressure. This flow rate discrepancy is

referred to as ‘slip flow’ for rotary pumps. Different types of PD pumps have different magnitudes 

of ‘slip flow’

Discharge pressure will match the system’s demand. Very high and dangerous pressures can be

created by ‘Dead Heading’ i.e. operating the pump at zero flow. This condition is usually avoided, 

to comply with Statuary requirements, by fitting a pressure relief device before any isolating valve 

or potential blockage.
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18

EFFICIENCY

Typical pump efficiency curves are shown in figures 4.3 and 4.4 below.

MAX

Pressure

MIN

10

20

30

40

50

60

70

80

90

100

Efficiency %

 

Pressure

MIN

10

20

30

40

50

60

70

80

90

100

Efficiency %

MAX

Figure 4.3 Figure 4.4

Rotary Pump Efficiency Reciprocating Pump Efficiency

SUCTION PERFORMANCE ( NPIP) 

In a similar way to that described in 3.1.3, a PD pump needs the incoming liquid to have a pressure 

margin above liquid vapour pressure to prevent cavitation in low-pressure areas in the suction

passages of the pump. For a PD pump this required pressure is known as Net Positive Inlet

Pressure (NPIP), sometimes referred to as Net Positive Suction Pressure. (NPSP)

4.2 METHODS OF VARYING PUMP PERFORMANCE

Unlike a rotodynamic pump, a PD pump cannot be controlled in flow by changing system

resistance e.g. by closing a valve and so this simple but inefficient method is ineffective and

potentially unsafe for PD pumps. Therefore, apart from changing stroke length on a limited

number of reciprocating pumps, variable speed is the method generally used.

4.2.1 PUMP CONTROL BY VARYING SPEED

SPEED AND FLOW RATE

The flow rates of both types of PD pumps can be varied by changing the operating speed.

In general flow rates are directly proportional to speed and volumetric flow rate does not change 

significantly with pressure, below 200 bar. See figure 4.5 below.
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19

0 20 40 60 80 100

SPEED (% max)

HIGH

PRESSURE

LOW

HIGH

LOWFlow rate

Figure 4.5 

Speed/Flow Rate

Speed and Torque

The torque required to drive a PD pump, in general, is directly proportional to the differential

pressure across the pump and independent of speed (see Figure 4.6). Note that this is very

different to the characteristic for a centrifugal pump where torque is proportional to speed squared.

Speed

MIN MAX

MAXTorque

INCREASING

PRESSURE

Figure 4.6 

Generalised PD Pump Speed Torque relation

Some types of PD pump have a high starting torque; this can be a significant factor in sizing a

drive. Some types of PD pumps (disc/diaphragm) have an increasing torque at the highest speeds.

PD pumps, which depend on the pumped fluid for lubrication of their moving mechanical

components, can show a torque characteristic which increases rapidly at low speed, see figure 4.7.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

20

0

Torque

Speed

INCREASING

PRESSURE

MAX

Figure 4.7 

Progressive Cavity Pump Speed Torque Relation

Speed and Power Absorbed

Typical power absorbed curves vary linearly with speed and pressure; examples are shown as 

figures 4.8 and 4.9. Flow control by varying speed is therefore the most efficient method, as it does 

not waste any of the input energy.

The liquid’s viscosity can have an effect on absorbed power and the liquid density effects the

system, the pump manufacturer’s data should be consulted.

Speed (% Max.)

20 40 60 80 100

MIN

MAX Absorbed Power (kW)

Pressure

MIN MAX

0

MAX

Pressure

20

40

60

80

100

Absorbed Power (kW)

Speed

Figure 4.8 

Rotary Pump Power Curve

Figure 4.9 

Reciprocating Pump Power Curve

Other Considerations of Variable Speed with PD Pumps

The pressure can be maintained with reduced speed and flow, so large speed turndowns can be 

useful. It is important therefore to avoid a speed which is too low for stable and reproducible

operation and to consider, at the system design stage, the constant torque characteristic and

possible low speed torque effects, because of its demand on electronic variable speed drives. In

particular low speed running at constant torque requires that motor cooling is carefully studied.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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Running some types of pump too slowly can have detrimental effects on wear rates when handling 

slurries containing settling solids. PD pumps with directly driven lubrication systems may also

have limitations on minimum speeds.

PD pumps are not inherently high in noise and vibration and any perceived noise from a pump unit 

can usually be attributed to either the drive, or to hydraulic noise in the associated pipes and

valves. Some PD pumps do contain out of balance moving components, but increasing speed

within practical limits does not normally produce abnormal vibration.

4.2.2 FLOW CONTROL USING PUMPS IN PARALLEL

PD pumps can generally be run in parallel without problems. This gives increased flow rates at 

the pressure rating of a single pump. The principle considerations are, the correct design of inlet 

and outlet pipe work to avoid problems of NPIP in the inlet, over pressure in the discharge pipe 

work, and back-flow through a stationary pump.

Different PD pumps can be operated in parallel; they do not have to be ‘matched’.

With reciprocating pumps, synchronising the strokes in order to minimise pressure pulsation is a 

consideration.

4.2.3 PUMPS IN SERIES (ROTARY)

Rotary PD pumps can be run in series, this gives increased pressure capability at the flow rate of a 

single pump. Careful design of the control logic and overpressure prevention/relief are important.

Matching the speed of the two pumps is important in order to avoid over or under feeding of the 

secondary pump. Over feeding the secondary pump will cause overpressure of the primary pump. 

Under feeding the secondary pump will cause cavitation and NPIP problems.

The use of one motor to drive both pumps ensures synchronised starting of the two pumps and

speed variations, due to the electrical supply or motor loading, are automatically compensated. 

Some types of PD pumps (e.g. progressive cavity) tend to balance out any small mismatches in

flow rates and this minimises operational problems.

4.2.4 FLOW CONTROL VALVE

This is not an acceptable technique. Throttling a PD pump will change pump pressure but will not 

change the flow rate and can lead to excessive pressures.

4.2.5 BY-PASS CONTROL

Control is by either ‘modulating’ or on/off control of the bypass flow, but is not commonly used 

because of the energy wasted and wear on control valves in the bypass circuit.

4.2.6 LOAD/UNLOAD CONTROL

Unlike a rotodynamic pump this can be an effective way of controlling a PD pump discharge rate.

Load/unload control is similar to bypass control and energy wastage is relatively low, however,

wear of the load/unload valve is a problem.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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SECTION 5 MOTORS 

Whilst there are many types of prime movers available (such as diesel engines, steam turbines, dc 

motors, permanent magnet motors, synchronous reluctance motors and wound rotor motors) the

majority of pumps are driven by an ac induction motor. Although this document is principally

about pumps and Variable Speed Drives it should be mentioned that, on a typical industrial site, 

motor driven equipment accounts for approximately two thirds of electricity costs and

improvements in motor efficiency can offer major energy savings. The principles outlined will

apply to all motors on a given site, not just those used as pump drivers.

5.1 MOTOR PRINCIPLES

The speed of an induction motor is normally fixed because the supply frequency is fixed, as is the 

number of poles in the motor. The speed ( ignoring slip ) is calculated from the formula :

SPEED (R/MIN) = 120 X SUPPLY FREQUENCY (HZ) 

number of poles

i.e.: - a 2 pole motor on 50 Hz supply has a speed of 120 x 50 = 3000 rpm

2 

Equally a 2 pole motor on a 60 Hz supply has a speed of 3600 r/min

Therefore, by varying the frequency the speed can also be varied.

5.2 MULTI-SPEED MOTORS

Varying the frequency can give a step-less change of speed, but if a small number of

predetermined speeds are acceptable, a multi-speed motor is an effective solution. 

Two, three or four fixed speeds can be achieved by special windings within the same stator or

frame and a dedicated controller. 

5.3 ENERGY EFFICIENCY

The average electric motor will consume its capital cost in energy in less than 2 months, typically

a motor, costing £500 will consume over £50000 in its lifetime. Therefore a single percentage

point increase in efficiency will save lifetime energy cost generally equivalent to the purchase

price of the motor. This illustrates the importance of giving close attention to efficiency criteria.

The calculation for the energy cost per annum of any electric motor is:

Hours used per year x kWh tariff x operating point kW

Efficiency at operating point

Typically for a pumping system: -

Design duty point 80 kW Installed motor rating 90 kW

Operating point 67.5 kW Operating 6000 hrs/yr

Motor efficiency 95.0% Tariff 4.5 p/kWh

Energy cost = 6000 x £0.045 x 67.5

0.95

= £ 19, 184 per year
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Using this formula, comparisons can be made between different types of motor. Based on a typical 

fourteen-year life of an electric motor, lifetime cost savings for high efficiency motors are in the 

order of 3-4 times the purchase cost.

Efficiency depends not only on motor design, but also on the types and quantity of active materials 

used. The efficiency can therefore vary considerably from manufacturer to manufacturer. 

Manufacturers have focused on the following key factors to improve the efficiency of a motor:

• Electromagnetic design – Making the best use of copper by winding techniques and lamination

design.

• Magnetic steel – Utilising a low loss, high permeability steel.

• Thermal design – Ensuring optimum fit between stator, frame and laminations.

• Aerodynamics – Using a more efficient cooling system by change of fan and/or fan cover

design.

• Manufacturing quality – Improving assembly techniques.

By adopting these techniques, manufacturers have made efficiency improvements in the range of

3%, on motors up to 400kW. The percentage gains on the lower kW output motors could be

greater than 3%, the gains on the higher kW output motors will not be as great.

There are several international standards for measuring the efficiency of a motor. European (IEC 

34) and North American (IEEE 112) standards vary and will inevitably produce differing results.

In comparing any manufacturers’ data, the supply input and test method utilised must be common

to each set of data.

5.4 EFFICIENCY LABELLING

With the backing of the European Commission, manufacturers representing 80% of the European

production of standard motors have agreed to establish three efficiency bands or classes for their 3-

phase TEFV (totally enclosed fan ventilated), 2 and 4 pole, cage induction motors in the range 1.1-

90 kW. Most industrial equipment manufacturers and users are therefore affected by this scheme 

which covers the vast majority of 3 phase motors purchased.

The agreement also commits manufacturers, who sign up to the scheme, to a 50% reduction in the 

supply of motors in the lowest efficiency class,‘Eff 3’, by the end of 2003.

The efficiency class of each motor, within the scope of the scheme, will be identified in catalogues 

and on nameplates as a minimum. Some manufacturers will go further and include stickers on the 

motors for greater visibility. This will assist equipment manufacturers to demonstrate their more

energy efficient equipment to their end-user customers. These labels may, in some cases, be

incorporated into other stickers or branding used by the manufacturer to identify his product range. 

Figure 5.11 below illustrates the minimum basic labels.

Figure 5.1

Basic efficiency classification labels

 

1

Trade Marks of Gemelec
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Figure 5.2 

Graph of Efficiency Classification Bands

Motor efficiency classification bands are illustrated in Figure 5.2. It is apparent that the spread of

Eff 2 motor efficiencies is very wide, (up to 7.6%) in smaller sizes and becomes much narrower 

(1.1%) in larger sizes. The convergence in larger sizes is realistic since most motors in larger sizes 

have similar numerical efficiencies, but small differences are very significant in terms of lifetime

energy consumption and cost for larger motors.

5.5 OTHER ENERGY SAVING OPPORTUNITIES

5.5.1 MOTOR SIZING

Electric motors are designed to operate at full rated output, at rated voltage, twenty-four hours per 

day, three hundred and sixty five days per year. However, it is estimated that only 20% of

machines in operation are running at their full rated output. The practice of utilising a 10% or

perhaps 15% margin can often lead to the selection of a higher power rating and, in some cases an 

increase in the physical size, and therefore cost of the machine.

The loading of the motor affects its efficiency; Fig 5.3 shows a typical comparison of efficiency

between a standard and a high efficiency motor. In most cases the motor will be operating below 

its rated output. As can be seen in the diagram, the difference in efficiency at full load between the 

standard and high efficiency motor may be as small as 2 – 3%, but at half load the difference is 

considerably greater.

Eff 1

Eff 2

Eff 3

2 pole

4 pole

% Efficiency

kW

1.1 90
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25

Figure 5.3

Relative Efficiency Levels of Motors

5.5.2 SWITCH IT OFF!

The first rule of energy savings is ‘If it isn’t being used, switch it off’. This is a low cost maxim, 

which has great effect, but is not frequently enough applied. Put simply, the operator does not feel 

the pain of the energy expenditure. 

5.5.3 A MOTOR MANAGEMENT POLICY

When a motor is rewound, its efficiency is reduced, and the modest cost saving of the rewind

compared with a new machine, is very soon lost against the additional energy losses. Institute a 

motor management policy to provide a structured approach to your replace/repair decisions.

Rewinds should be in accordance with the best practices detailed by the Association of Electrical

and Mechanical Trades (AEMT2). Replacements should be high efficiency, designated Eff1, and

correctly sized for the application. 

 

2

 AEMT Best Practice Guide 

82

84

86

88

90

92

94

96

1/4 load 1/2 load 3/4 load full load 11/4 load

Efficiency %

EFF 3

EFF 1
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5.5.4 SHAFT ALIGNMENT

Misalignment of motor couplings is also surprisingly wasteful. An 0.6 mm angular offset in a pin

coupling can result in as much as 8% power loss and eventual coupling failure with attendant

production downtime. Check and realign motor drive couplings, starting with the largest motors.

5.5.5 PULLEY SIZING

Significant energy savings can be often be made simply by changing pulley sizes, to ensure a fan

or pump runs at a more appropriate duty point. This doesn’t provide the flexibility of variable

speed control but costs very little and can probably be done within the maintenance budget and 

doesn’t require capital approval.
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SECTION 6 VARIABLE SPEED DRIVES

6.1 VARIABLE FREQUENCY DRIVE PRINCIPLES 

As seen earlier in section 5.1, a motor is capable of operating over a range of speeds if correctly 

fed at a varying frequency. 

In section 3.1.1 we have seen that a rotodynamic pump performance curves show a power demand 

that follows the affinity laws, and therefore torque is proportional to (speed) 2 See also Fig 6.1.This 

means that in principal a rotodynamic pump (without influence from the system curve), when

slowed by 10% will demand only around 70% of the energy at full speed.

For a great majority of Positive displacement pumps, torque remains constant over the operating

speed range. This is significant in the selection of the drive system, and in determining motor derating

0

20

40

60

80

100

120

0 200 400 600 800 1000 1200 1400 1600

Speed (r/min)

Power/Torque %

Rotodynamic Pump

Torque a Speed2

PD Pump

Torque = Constant

PD Pump

Power a Speed

Rotodynamic Pump

Power a Speed3

Figure 6.1 

Power and torque vs. speed

6.2 THE FREQUENCY CONVERTER

The most commonly used type of electronic variable speed drive is a frequency converter used in 

conjunction with an induction motor.

The frequency converter may be referred to by several terms and abbreviations, including an

inverter (which is only part of the converter system), or as a VVVF (variable voltage, variable

frequency drive) also VFD (variable frequency drive).

Irrespective of type, a frequency converter will consist of four basic parts, and the combination of

these parts will affect the final performance of the system. Parts described below in 6.2.1 to 6.2.4 

and Figure 6.2
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In addition to the electronics described here the drive system will require conventional switching

components in the supply and safety circuitry.

6.2.1 RECTIFIER

A frequency converter will operate by rectifying the incoming AC supply to a DC level. The type 

of rectifier can vary depending on the type of performance required from the drive.

The rectifier design will essentially control the harmonic content of the rectifier current, as the

rectifier may not draw current for the full cycle of the incoming supply. It will also control the 

direction of power flow.

6.2.2 INTERMEDIATE CIRCUIT

Having rectified the incoming AC supply, the resultant will be an uneven rectified DC. This is

smoothed in the intermediate circuit, normally by a combination of inductors and capacitors.

Over 98% of drives currently in the marketplace use a fixed voltage DC link.

6.2.3 INVERTER

The inverter stage converts the rectified and smoothed DC back into a variable AC voltage and 

frequency.

This is normally done with a semiconductor switch. The most common switches in low voltage 

systems are currently IGBTs – Insulated Gate Bipolar Transistors. To complete the circuit when

one semiconductor is switched on, each switch is bridged in the reverse polarity by a “flywheel 

diode”.

6.2.4 CONTROL UNIT

The control unit gives and receives signals to the rectifier, the intermediate circuit and the inverter 

to achieve the correct operation of the equipment.

Intermediate

Rectifier circuit Inverter

MMI

Control Unit

Power

I/O

Figure 6.2

 Basic Elements of frequency converter
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6.3 PULSE WIDTH MODULATION

For commercial purposes the inverter control may be described differently, however, there is one 

fundamental technique that is used – Pulse Width Modulation (PWM)

The principle of PWM is to generate a pulse train by switching between the positive and the 

negative legs of the DC link, to simulate the required voltage and frequency as shown in fig 6.3 

below.

Pulse Width Modulation (PWM) Principle

+

-

M

3~

+

-

+

-

+

-

u0

SPEED 100%

100 % Volts

SPEED 50%

50 % Volts

Figure 6.3

PWM Waveform Generation

When a frequency converter is connected to a motor, it provides a voltage and frequency; the

current is controlled solely by the motor internal impedances.

One major benefit of this is that there is no starting inrush current from the supply, compared to a 

direct on line system, allowing soft start both electrically and mechanically.

6.4 INTEGRATED VARIABLE SPEED MOTORS

A recent trend amongst many of the motor manufacturers has been to develop a motor with an

integral inverter. These packages are currently available to approximately 7.5kW. The major

advantages to the pump designer are include the reduced control panel space, the simplified

installation, the straightforward electromagnetic compatibility compliance, ensured matching of

motor and drive leading to greater life cycle cost savings. 
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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SECTION 7 SELECTING A VARIABLE SPEED DRIVE FOR A NEW 

INSTALLATION

7.1 SIZING & SELECTION

When the pump maximum duty is known, the peak power and speed for the drive will become 

clear. It is essential to commence the sizing exercise with the hydraulic system, and to work

systematically to select the pump, motor and drive.

It would be normal to add a tolerance equal to the potential fall off in efficiency over the

maintenance life of the system. It is recommended when selecting a Rotodynamic pump that the

maximum flowrate is to right hand side of the best efficiency point. 

Some operating profiles will be best satisfied by sharing the duty between multiple Variable Speed 

Driven Pumps.

The following selection flow chart (A) is a step-by-step approach to see if a Variable Speed Driven 

pump will benefit the system and explain the decisions required.

Throughout this process we recommend consultation with the suppliers whose expertise should

assist your selection.
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Start

Define Duty Required

Head, Flow, Liquid

Details

Does the Pump

Duty Vary ? No

Select Fixed

Speed Pump

Variable speed

potentially useful

Yes

Define all operating

conditions & run time at

each operating point

Consider reducing the system losses

as far as economically possible.

Produce system curve.

Is duty fairly equally shared

between all flow rates ?

Low or high capacity

predominates No

2 separate pumps may

be better than VSD

Yes

System curve mostly friction ? No Mostly static ?

Check overall benefitseconomics need careful

checking

Consider switched

pumps in parallel

VSD most likely

beneficial

Yes

Does the pump run for most of

the time ?

VSD may still give

Overall benefit verall No

VSD will almost certainly

give overall benefit

Yes

Flowchart to assess the suitability of a VSD for a pump system

Continue on next page

Flow Chart

A

7.1.1
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Consider control parameters

- Flow ? Head ? Temperature ? Level ?

Can a control valve or bypass be eliminated ?

What condition monitoring is required

- Low Flow ? Low power ? Vibration ?

Temperature ?

Is pump/motor in

Hazardous area ?

No Consider integral

motor/inverter.

Provide details to pump

vendor to supply full

package and quantify the

savings

Continuation of Flowchart (A) to assess the suitability of a VSD

for a pump system

Yes

Determine kW rating of drive

Determine supply voltage

Check compatibility of

VSD with motor

Examine cable runs from

VSD to motor

Consider EMC issues ,

G5/4 (harmonics on

Supply)

Receive pump performance

curves for all duties/speeds

Calculate operating cost at

each duty, taking account

ofOf drive/motor efficiencies

at different speeds.

Calculate total annual operating

costs and compare with

alternatives e.g. flow control

valve, multiple pumps.

Receive quotes for equipment

and installation – Consider Life

Cycle Cost benefit.

Consider other VSD benefitsNo control valve, reduced

maintenance, improved pump &

motor condition monitoring

benefits

Make pay-back calculations on

VSD vs Fixed Speed vs multiple

pumps.

Review further economic benefits of improved

pump efficiency e.g. coatings, low loss seals, and

the use of high efficiency motors.

Or quantify savings and

purchase VSD

separately.
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7.2 CONTROL

A pump with variable speed capability will need to be controlled to unlock all the benefits

available from VS operation.

The effect of varying speed with a centrifugal pump is to vary both head and flow. Variation of

speed with a positive displacement pump will vary only the flow rate.

Process Controlled 

Parameter 

External influence

Heating system Temperature Ambient

Tank filling Level Outflow

Pipeline Flow Level

Distribution Pressure Draw-off

Table 7.1

Examples of control parameter

7.2.1 CONTROL BY FIXING PRESSURE BUT VARYING FLOW

The most common form of control is by use of a discharge pressure sensor which sends a signal to 

the VFD which in turn varies the speed allowing the pump to increase or decrease the flow

required by the system. 

This form of control is common in water supply schemes where a constant pressure is required but 

water is required at different flows dependant on the number of users at any given time. Capacity

changes at constant pressure are also common on centralised cooling and distribution systems and 

in irrigation where a varying number of spray heads or irrigation sections are involved.

7.2.2 HEATING SYSTEM CONTROL

In heating and cooling systems there is a requirement for flow to vary based on temperature.

In this instance the VFD is controlled by a temperature sensor, which allows the flow of hot, or 

cold liquid in the system to increase or decrease based on the actual temperature required by the 

process. 

This is similar in operation to pressure control, where the flow is also the variable entity, but a 

constant temperature requirement from a temperature sensor replaces that from a pressure sensor.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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7.2.3 CONTROL BY FIXING FLOW BUT VARYING PRESSURE:

In irrigation and water supply systems constant flow is often required, even though the water 

levels both upstream and downstream of the pumping station vary.

Also many cooling, chiller, spraying and washing applications require a specific volume of water

to be supplied even if the suction and delivery conditions vary. Typically suction conditions vary

when the height of a suction reservoir or tank drops and delivery pressure can change if filters

blind or if system resistance increases occur through blockages etc.

The VSD system is usually the optimum choice to keep constant the flow rate in the system using 

a control signal from a flowmeter, which can be, installed in the suction, but more commonly the 

discharge line.

7.2.4 IMPLEMENTATION

In many cases there will be an external control system, such as a PLC or PC, which will provide 

the start/stop control and an analogue speed reference to the drive or will pass this information to 

the drive by a serial communications link. In other cases the drive may have adequate on board 

intelligence. 

All modern drive systems rely on microprocessor control, and this allows the manufacturer to

integrate the basic signal processing functions into the drive. In some instances manufacturers

have installed bespoke control software to the drive to allow specific requirements to be met.

As every case has its own specific requirements, it is important that the control requirements are 

understood in order to achieve the optimum system performance.

7.2.5 SOFT STARTING AND STOPPING

When a induction motor is started direct on line, it will generate a high level of torque, which will 

cause a very fast breakaway, and it will then accelerate up to speed in an uncontrolled fashion.

In this case the network has to supply a large inrush current, a very high initial level to establish

flux in the motor, followed by a high level which decreases as the drive accelerates.

The effect on the pump is to place mechanical stresses on the rotating components, followed by

stresses in the hydraulic system, which may include a high initial flow rate causing a vacuum to be 

drawn on the suction side, or surge on the discharge, and possible NPSH problems.

Equally when stopping the rate of deceleration is totally uncontrolled, which can lead to further

mechanical stresses, and surges in the hydraulic circuit. This can lead to requirements for

additional inertia added to a pump, generally in the form of a flywheel, or to surge control vessels 

in the hydraulic system.

The use of electronic starting systems allows smooth acceleration and deceleration of a drive

system.

Electronic soft starters will reduce the voltage at the motor terminals in a controlled manner, but 

are generally short time rated devices, while a frequency converter is usually continuously rated

and so can be used to give very controlled rates of change.

The only drawback with either electronic scheme is that generally the equipment must be

connected to the network, and therefore problems of uncontrolled deceleration could arise in a

power failure.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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SECTION 8 RETROFITTING A VARIABLE SPEED DRIVE TO EXISTING 

EQUIPMENT

8.1 JUSTIFICATION 

There are approximately 20 times more pumps in service in existing installations than are supplied 

new every year.

It is common for fixed speed pumps to be oversized; most system designers allow a contingency

on the system head required. It follows therefore that retrofitting variable speed drives could match 

pumps to actual system requirements more accurately and save considerable amounts of energy.

Also many existing systems use control valves and bypasses, all of which absorb energy not

required to satisfy system demands.

It is therefore apparent that a major opportunity exists for modifying installed systems to make

them more energy efficient. The fitting of variable speed drives and the removal of control valves 

and bypasses will save energy and often the payback for the modifications is short. Just consider a 

speed reduction of 10% with a rotodynamic pump will save approximately 30% of the electrical

energy absorbed and 75% of pump systems are oversized, many by between 10 and 20%

Pump Manufacturers should be contacted to ensure pumps can be run at slower speeds with no 

detrimental effects and the motor manufacturers to ensure motors are suitable for use with variable 

speed drives. There may also be other alternatives; for example if a pump is oversized but operates 

at a single duty, the impeller diameter can be permanently reduced, which achieves the same

energy reduction as a VSD but at a small cost.

8.2 MOTOR DERATING

When considering adding a variable speed drive to an existing motor, care should be taken to

match the electrical characteristics of the motor and frequency converter; otherwise the risk of

premature failure is introduced into the system.

Early frequency converters produced outputs that had a very high harmonic content in the

waveform. This resulted in substantial additional heating of motor windings, and therefore motors 

were de-rated for inverter use. Modern inverter outputs, cause relatively small levels of harmonic 

current distortion in the motor windings, and therefore little de-rating is normally required. Whilst 

this de-rating will be minimal it will vary from one motor design to another. It is also dependent on 

the type of inverter used. 

The following parameters vary from one converter design to another and will affect the drive

system performance:

• Total harmonic distortion

• Peak voltage

• Maximum rate of change of voltage

• Switching frequency

• Cable length between the motor and inverter

Motor de-rating may also be required to compensate for the reduced cooling at lower speeds when 

the motor shaft mounted fan is not generating the airflow achieved at normal synchronous speed.

The motor manufacturer’s expertise must be used to determine how much de-rating is required and 

their recommendations applied. There are no definitive rules that can be applied to all motors. 
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

36

In general, little or no de-rating is required with most rotodynamic pump drives, however PD

pumps with a constant torque characteristic are more likely to require de-rating of the motor for 

reduced speed operation.

Typically motors with a fixed speed-cooling fan can deliver a constant torque continuously over a 

wider speed range.

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0 10 20 30 40 50 60 70 80

Frequency (Hz)

Torque (%)

Constant voltage

(Field weakening) area

Motor delivers constant power

Motor with fixed speed fan

Constant flux

(Volts α frequency) area

Motor delivers constant torque (Subject to cooling)

Motor with shaft mounted fan

Typical Centrifugal pump

Speed/Torque relation

Figure 8.1

Typical Isothermal Loadability Curves

Figure 8.1 shows the effect of varying the frequency (speed) of a typical motor, with a power

rating based on Class B (800 K temperature rise).

8.3 SIZING AND SELECTION OF A VSD ON EXISTING EQUIPMENT

To assist with decision on whether or not to retrofit on existing equipment, please refer to flow

chart (B) below.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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IS THIS NEW EQUIPMENT OR

RETROFIT ?

Define Duty Required

Head, Flow , Liquid

Details

Does the Pump

Duty Vary ? No

Retain Fixed

Speed Pump

Yes

Define all operating conditions &

run time at each operating point

as far as economically possible.

Produce system curve.

Is duty fairly equally shared

between all flow rates ?

Low or high capacity

predominates No 2 separate pumps maybe better than VSD

Yes

System curve mostly friction ? No Mostly static ?

Check overall benefitseconomics need careful

checking

Consider switched

pumps in parallel

VSD most likely

beneficial

Yes

Does the pump run for most of

the time ?

VSD may give overall

benefit No

VSD will almost certainly

give overall benefit

Yes

Flowchart to assess the suitability of retrofitting a VSD to an

existing pump system

Continue on next page

Flow Chart

B

8.3.1

Retrofit

Consider reducing the system losses

Variable speed

potentially useful

New Equipment

Please refer to Flowchart

7.1.1.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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Consider control parameters

- Flow ? Head ? Temperature ? Level ?

Can a control valve or bypass be eliminated ?

What condition monitoring is required

- Low Flow ? Low power ? Vibration ?

Temperature ?

Is pump/motor in

Hazardous area ?

No Consider integral

motor/inverter.

Obtain pump curves

Continuation of Flowchart (B) to assess the suitability of

retrofitting a VSD to an existing pump system

Yes

Determine kW rating of drive

Determine supply voltage

Check compatibility of

VSD with motor

Examine cable runs from

VSD to motor

Consider EMC issues ,

G5/4 (harmonics on

Supply)

Calculate operating cost at

each duty point, taking into

account the drive/motor

efficiencies at different

speeds

Calculate total annual operating

costs and compare with current

costs.

Receive quotes for equipment

and installation – Consider Life

Cycle Cost benefit.

Consider other VSD benefitsNo control valve , reduced

maintenance , improved pump &

motor condition monitoring

benefits

Make pay-back calculations for

installing VSD
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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SECTION 9 EFFECTS OF NOISE & VIBRATION WHEN VARYING 

SPEED 

Vibration occurs in all rotating machinery when in operation. Vibrations are normally caused by

imbalance of the rotating components, but can also be caused by hydraulic flow through a pump or 

valve. It is obviously desirable to maintain sufficiently low vibration levels in order to prevent

mechanical damage. Noise is also created and is undesirable from an environmental point of view.

In centrifugal pumps, energy is transmitted via the impeller vanes to the pumping medium. 

Because the number of vanes is finite, pressure pulsations will arise. Flow around the vanes and

flow separation can occur if the pump is operating away from its design duty, leading to an

unsteady state of flow. The pressure pulsations and unsteady flow may excite vibrations in the

pump casing and associated piping, with the result that noise may be transmitted to the

surrounding environment.

The trend towards higher rotational speeds has an unfavourable effect on the noise developed by

centrifugal pumps, since the overall dimensions of the machine are reduced and the energy

conversion consequently takes place in a much smaller volume (increased power density). 

It follows, therefore that a reduction in operating speed leads to lower noise and vibration levels. 

Since the operation of a motor driven pump in combination with a variable speed drive system, 

often results in lower average speeds (the unit will only operate at a speed to achieve the desired 

duty condition), a general reduction in noise levels is to be expected. This is, of course, a benefit 

where noise level is a consideration. Typical noise level restrictions in current specifications call

for sound pressure levels to be below 85 dBA.

The adoption of a variable speed drive system can also lead to a lowering of hydraulic noise levels 

in the piping system. Additionally, the elimination of items such as control valves, will also lead 

to a reduction in hydraulic noise levels.

As most pumps are driven by electric motors, consideration should also be given to the noise

emitted by the driver. Combined noise levels for pump/motor combinations are typically 2dBA

more than the item with the higher noise level.

It should also be noted that an increase of between 3 and 6 dBA in motor noise is generally

apparent when motors are used in combination with inverter drives. The overall reduction in

speed level, however, normally leads to an overall decrease in noise levels.

A change in the rotating speed will vary the frequencies of the vibration exciting forces, this can

create a structural resonance not found at the fixed running speed this can occur equally with

reduction or increase in the speed, and is mostly associated with pumps mounted on light

structures, or vertical machines which are inevitably less rigidly mounted. The manufacturer

should always be consulted in these areas.

9.1 TYPICAL NOISE LEVELS

PUMPS

The table below indicates the noise levels that can be expected from typical single stage

centrifugal pumps operating at different speeds:
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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Power requirement at 

maximum speed (kW)

Speed (rpm) Sound pressure level 

(dBA @ 1m)

3.0 2900 72

1450 63

980 58

37 2900 80

1450 72

980 67

Table 9.1

MOTORS

Small motors are relatively quiet in operation; as motor speed and size increases the noise level

also increases. 

It should be noted that energy efficient motors (EFF1) are quieter in operation than older designs 

which generally fall into the EFF3 rating – a comparison between these is shown below which is 

based on two sizes of typical squirrel cage motors with IP 55 enclosure.

TYPICAL MOTOR NOISE LEVELS

Motor Power (kW) Motor Speed (rpm) EFF3 EFF1

No load Sound pressure level (dB(A) @ 1m)

3.0 2900 78 64

1450 58 53

980 54 51

37 2900 85 72

1450 76 63

980 72 61

Table 9.2
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APPENDIX - ADDITIONAL INFORMATION 

A1 MAGNETIC DRIVE PUMPS 

A1.1 EFFECTS OF PUMP SPEED 

The affinity laws described in section 3.1.1 apply to pumps where the impeller is driven directly, 

with the shaft sealed by gland packing or mechanical seal where it enters the pump casing.

Pumps which require a higher leak free integrity are hermetically sealed with a stationary device 

called a Containment Shell or Sheath. With this design, the pump shaft is driven by Magnetism 

across the stationary containment shell, either by the use of permanent magnets (Synchronous 

Drive or Canned Magnet Drive Pumps) or by the use of an induced magnetic field (Canned Motor 

Pumps). These react differently to speed change.

With magnetically driven pumps, there are usually magnetic power losses to be taken into account, 

as well as hydraulic power. These are proportional to speed squared. (The exception to this rule is 

that there are generally no magnetic losses incurred for non-metallic magnetically driven pumps).

Hydraulic Power follows the Affinity Laws, i.e. P2 = P1 x (RPM2/RPM1)

3 

Magnetic Power(M) follows the physical law:- M2 = M1 x (RPM2/RPM1)

2 

Then, Total Power = Hydraulic Power (P) + Magnetic Losses (M)

The magnetic power is typically 10% of the total power so the deviation caused by this departure 

from the Affinity Laws is only small.

NOTE:-

1. For any significant increase in speed, always verify with the pump manufacturer that the magnetic coupling is 

adequately rated for this new power & speed.

2. Non-metallic magnetic drive pumps usually have no magnetic losses and so power can be calculated by the 

affinity laws, similar to mechanically sealed pumps. However, the pump manufacturer must still always be 

consulted to check that the magnetic coupling is adequately rated for the new power & speed.

A 1 2 EFFECTS OF IMPELLER DIAMETER CHANGE

The equations in section 3.1.2 define the variation of pump hydraulic performance with impeller 

diameter for conventionally driven pumps. For Magnetically Driven pumps, the effect of diameter 

change on power, only applies to the Hydraulic Power. If the speed is unchanged the Magnetic 

losses will not change.

Using the usual hydraulic power/diameter relationship, P2 = P1 x (D2/D1)

3

Then total Power at the new diameter = Hydraulic Power (P2) + Constant Magnetic Losses

A2 MOTOR CONSIDERATIONS

PROTECTION

As the inverter control monitors the motor current, it will also provide motor protection, based on 

the load and speed, and compensate for reduced loadability at reduced speed.

Some more sophisticated drives offer both over and under load protection, which allows

monitoring of imminent seizure, coupling failure, low flow and dry run.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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SPEED

A motor is not limited to its synchronous speed; it can be both reduced and increased. Figure 8.1 

shows that beyond base speed the torque available will drop as the inverter cannot increase its

output voltage, and the motor becomes progressively under fluxed. This is known as field

weakening

There is a specific maximum output speed for any motor, and the advice of the manufacturer

should be sought before running a motor beyond its base speed.

INSULATION DESIGN

Modern motor insulation systems are suitable for inverter use; however, the manufacturer should

be asked to confirm. Further information is available from the Gambica/REMA Technical Report

No 13

MOTOR BEARINGS

Motor bearings may run hotter with an inverter fed motor, as some additional losses will occur in 

the rotor, and are dissipated down the shaft, potentially requiring high temperature lubricants, or

more frequent lubrication. In larger motors (greater than 100kW) electric discharge through

bearings may require insulated bearings to be fitted. Further information is available from the

Gambica/REMA Technical Report No 24

 

3

 See Appendix A6

4

 See Appendix A6
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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A3 LEGISLATIVE REQUIREMENTS

Within Europe the inter-relation of a system with its environment is laid down by a number of EU 

Directives, and by local regulation, these are direct obligations on users, and their agents to be

applied when implementing a system.

A number of EU Directives are particularly important when considering variable speed pumping

applications. EU Directives are implemented into UK law by parliamentary Statutory Instruments.

A3.1 THE MACHINERY DIRECTIVE5

The Directive is implemented by listing a number of harmonised standards, which must be

applied.

From the point of view of an electrically driven pump the most important of these standards is

likely to be EN 60204-1

A3.2 THE EMC DIRECTIVE6

EMC is the ability of a device or system to function without error in its intended electromagnetic 

environment without disturbing other devices in that environment.

Supply Network

Distribution Network

Converter

Motor

Power Drive System

Radiated 

electromagnetic

noise

Immunity

Emissions

Emissions Immunity

Conducted

electromagnetic

noise

Figure A3.1 –

EMC aspects of a Power Drive System

The EMC Directive details the levels of electrical disturbances, both radiated and conducted that 

the drive system is permitted to emit and also the levels to which it is immune. In general the

immunity limits also appear within the scope of the Machinery Directive, and the Low Voltage

Directive.

 

5

 Directive 98/37/EC published by the EU Commission, Rue de la Loi 200, B-1049, Brussels.

6

 Directive 89/336/EEC, modified by 91/263/EEC and 93/68/EEC published by the EU Commission, Rue de la Loi 

200, B-1049, Brussels.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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A3.2.1 RADIATED EMISSIONS 

The drive is a source of high frequencies due to the controlling microprocessors, and the rapidly

rising wave fronts of the output voltage pulses. 

Unsuitable cabling and incorrect grounding can allow these high frequencies to radiate and disturb 

the environment. 

A3.2.2 CONDUCTED EMISSIONS 

Lower frequency disturbances can be passed back through the supply network to cause problems 

for other users of that network.

To meet the requirements of the Directive a “Power Drive System” must comply with the

harmonised Product Specific Standard EN61800-3.

To avoid problems, it is essential that the manufacturers installation recommendations be strictly

followed. 

A3.2.3 HARMONICS

Another aspect of EMC is low frequency emissions, or harmonics. The level of harmonics

reflected back to the supply network is usually regulated by the electricity supply utility.

Harmonics are voltages and currents in the electrical system at frequencies that are multiples of the 

fundamental frequency (50 Hz in UK power systems). 

Harmonics are associated with any load that uses a rectifier based power supply such as radio and 

TV, computers, lighting ballasts, other domestic equipment such as washing machines and 

microwave ovens etc. 

Harmonics also come from generators, and transmission equipment. They can affect the equipment 

performance and are both caused by and can interfere with the function of VSDs. 78

A3.3 THE LOW VOLTAGE DIRECTIVE9

This Directive concerns all electrical equipment with nominal voltages from 50 V to 1000 V AC 

(and 75 V to 1500 V DC).

The aim is to provide protection against electrical, mechanical fire and radiation hazards.

The Low Voltage Directive does not apply to products for use in potentially explosive

atmospheres.

 

7

 Harmonics in the UK are governed by the Electricity Association Engineering Recommendation G5/4, published by 

The Electricity Association, Millbank, London.

8

 Information on compliance is contained in Gambica Technical Guide No 1.

9

 Directive 72/23/EEC, modified by 93/68/EEC published by the EU Commission, Rue de la Loi 200, B-1049, 

Brussels.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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A3.4 THE ATEX DIRECTIVE10

This Directive provides a framework for determining the essential health and safety requirements

for use of any product in a potentially explosive atmosphere. 

The Directive applies to the entire equipment INCLUDING the pump. It also includes operation

within dust hazards, as well as flammable gases.

Under the terms of the directive it is unlikely that any electric motor manufactured under previous 

standards, or meeting earlier versions of this directive will be permitted to be retrofitted with a

variable speed drive.

When considering an installation in a hazardous area the advice of the motor and converter

manufacturer must be sought to enable a properly matched and certified system to be achieved.

A3.5 THE CE MARKING DIRECTIVE11

Under the terms of the EU Directives it is a statutory requirement that all electrical and electronic 

systems installed in Europe meet the requirements of all appropriate directives, and carry the

appropriate “CE” compliance indication. 

For all equipment covered by EU Directives the principal document showing conformity is the

Manufacturers’ Declaration of Conformity. The pumping system should not be commissioned

without the appropriate declarations to the EMC, Low Voltage and Machinery Directives, plus the 

ATEX Directive if applicable, being available. 

A4 ABBREVIATIONS

PUMP

NPIP Net positive inlet pressure

NPSP Net positive suction pressure

NPSH Net positive suction head

NPSHA Net positive suction head available

NPSHR Net positive suction head required

H Head

Q Flow

Rpm Revolutions per minute

PD Positive displacement pump

BEP Best efficiency point

 

10 Directive 94/9/EC (to be fully implemented from 1st. July 2003) published by the EU Commission, Rue de la Loi 

200, B-1049, Brussels.

11 Directive 93/68/EEC published by the EU Commission, Rue de la Loi 200, B-1049, Brussels.
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

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MOTOR AND DRIVE

HEM High efficiency motor

PWM Pulse width modulated

AC Alternating current (voltage)

DC Direct current (voltage)

EMC Electromagnetic Compatibility

PLC Programmable logic controller

VSD Variable speed drive

VFD Variable frequency drive

VVVF Variable voltage variable frequency (drive)

TEFV Totally enclosed fan ventilated

A5 REFERENCES AND FURTHER READING

GAMBICA/REMA Technical Report No 1. Motor insulation voltage stresses under PWM 

inverter operation

GAMBICA/REMA Technical Report No 2 Motor shaft voltages and bearing currents

GAMBICA Technical Guide Managing Harmonics, A guide to EA Engineering Recommendation 

G5/4

Both may be freely downloaded from the appropriate organisation website (See below)

NEMA Standards Publication - Application Guide For AC Adjustable Speed Drive Systems May 

be freely downloaded from the NEMA website (See below)

Pump Life Cycle Costs - by Europump & Hydraulic Institute, available from BPMA website 

( See below)

Energy Efficiency report, 1987, Greenwood PB

Energy Efficiency of Electric Motors by Design, 1994, Walters DG & Williams IJ

Good Practice Guide 2, Energy Savings with Electric Motors and Drives, 1998, Active Energy

Good Practice Guide 49, Energy Savings in Pumping Systems, 1998, Active Energy

General Information Leaflet 56, Implementing a motor management policy, Active Energy

Gambica /REMA guide ‘Making The Most of The Climate Change Levy Package’

European motor manufacturers catalogues.

European Pumps and Pumping by McGraw Hill

Pumping Handbook by McGraw Hill

AEMT Good Practice Guide, The repair of Induction Motors.

Good Practice Guide 249, Energy Savings in Industrial Water Pumping Systems - Action Energy

Best practice Case Studies, Hundreds of actual energy saving examples are detailed in these case 

studies - Action Energy.

Action Energy makes available a huge volume of detailed guidance including Best Pratice guides 

and Case Studies. These are available on the web (www.action energy.org.uk) or as hard copies 

from the helpline (0800 585794)

Websites:-

www.gambica.org.uk

www.rema.uk.com

www.nema.org

www.bpma.org.uk

www.actionenergy.org.uk
VARIABLE SPEED DRIVEN PUMPS-BEST PRACTICE GUIDE.

47

Contact details :-

British Pump Manufacturers Association 

The McLaren Building ,

35Dale End ,

Birmingham 

B47LN

Tel : 0121 2001299

Fax : 0121 200 1306

e-mail : techdir@bpma.org.uk

GAMBICA Association Ltd.

St George's House,

195-203 Waterloo Road

London SE1 8WB

Tel: +44 (0)20 7642 8080

Fax: +44 (0)20 7642 8096