Special Issue of Motor Drive Applications［ New Products ］

Low Starting Current Motor
Developed for Top Runner Model
Keywords

Top runner, Low starting current

In April 2015, the Top Runner Standard (equivalent of IE3) for 3-phase induction motors started in Japan. The applicable category objective items of thissystem are 3-phase low-voltage induction motors with capacities of 0.75kW to375kW (referred to as “the Top Runner motors”). These motors are stipulated toimprove the energy consumption efﬁciency from IE1 to IE3.
A Top Runner motor generally tends to have an increased starting current toraise its efﬁciency. Such a feature can affect our customers who want to have a
low starting currents speciﬁcation motor.
As such, we developed low starting current type Top Runner motors.

Abstract

1 Preface
It is estimated that about one hundred million
motors are currently used in Japan and 97% of
these motors belong to the IE1 efﬁciency class(“IE1” hereafter). If all 3-phase induction motors areimproved to the IE3 efﬁciency class (“IE3” hereafter), it is estimated that about 1.5% of total powerconsumption in Japan (i.e. 15.5 billion kWh/year) could
be reduced (1). Fig. 1 shows efﬁciencies of IE1 andIE3 for 4-pole induction motors.
In 2011, AC motors were the applicable machines for the Top Runner Standard. In April 2015,
the Top Runner Standard for 3-phase induction
motors began. As a result of this expansion of the100
99989796
9594(％)
9392919089
88870Fig. 1Kazuhiro Tomizawa,
Kiyoshi Nagasawa,Yasuo Hayashi,
Tadahiro Matsumoto

IE3 (50Hz)
IE3 (60Hz)

IE1 (50Hz)
IE1 (60Hz)
20406080100 120 140 160 180(kW)200

Efficiencies of IE1 and IE3

Efﬁciencies of IE1 and IE3 are shown. These efﬁciencies are thevalues deﬁned in JIS C 4213-2014 and JIS C 4034-30-2011.

applicable range, the efﬁciency class of 3-phase induction motors had to be improved to IE3. When theefﬁciency class is improved, some energy saving
can be expected. On the other hand, however, thestarting current tends to be increased as a result. Itis, therefore, necessary to take some appropriate
measures for the customers who call for low starting
current speciﬁcations. Given the aforementioned,
we developed Top Runner motors that feature low
starting currents. This paper introduces our low
starting currents technology for Top Runner motors.

2 Changes in Characteristics Due toMigration to IE3
When an electrical input is converted into amechanical output in the motor, part of the energy isconsumed in the form of losses. Motor efﬁciency isdeﬁned as a value that is the input minus the losses(＝ output) divided by the input. To improve the efﬁciency class from IE1 to IE3, a required amount oflosses may be deducted. Fig. 2 shows the deﬁnitionof motor losses and Table 1 shows the measures toreduce motor losses.
When the efﬁciency class is improved to IE3,
there will be characteristic changes in the IE1 motor
as described below.
( 1) Compared with motor size of a totally enclosed
motor of IE1, the total length becomes longer.
Compared with drop-proof motors, the frame number becomes larger.
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17

Primary copper loss (Stator coils)
Loss caused by resistance in coils

Stray load losses
Surface loss, interbar current loss,
pulsation loss, high frequency loss,
Leakage magnetic flux loss, etc.

Secondary copper loss (Rotor coils)
Loss caused by resistance in coils

Iron loss (Stator core)
Loss caused by characteristics
singular to iron core
Iron loss (Rotor core)
Loss caused by characteristics
singular to iron core

Mechanical loss (Cooling fan)
Loss caused by a cooling fan
Mechanical losses (Bearings)
Losses caused by friction, etc.

Fig. 2

Definition of Motor Losses

Types of motor losses are shown.

Table 1 Measures to Reduce Motor Loss Reduction
Measures to reduce motor losses reduction are shown.
LossMeasures to reduce losses

Primary・Increase in conductor cross section
copper loss ・Improvement of space factor for coils
・Reduction of coil end length
Secondary ・Increase in conductor cross section
copper loss
Iron loss

・Adoption of low loss electrical steel laminations
・Adoption of thin electrical steel laminations
・Optimization of magnetic flux density

Mechanical ・Reduction of cooling fan diameterloss
・Adoption of low loss grease
Stray loadlosses

・Optimization of gap length
・Optimization of the number of rotor slots
・Optimization of rotor skew
・Insulation of rotor slots
・Iron core annealing( 2 ) The motorʼs starting current is greater.
( 3 ) The rated speed of the motor becomes higher.
( 4 ) Generated torque of the motor becomes greater.
For IE1 motors, the starting current value is 6
to 8 times the rated current. For IE3 motors, thisvalue becomes 7 to 10 times. This is because motor
impedance (consisting of ohmic component andinductance component, and the former is relatedto loss generation) is reduced in order to curtail
losses, and this treatment results in an increase instarting current.
When the starting current is reduced, motor
impedance is increased and motor losses are also
increased. To realize a low starting current, it is necessary to reduce losses inclusive of the increasedcomponent. As a result, the frame number becomes
greater compared with Top Runner motors.

18MEIDEN REVIEW Series No.172 2018 No.1

3 Techniques for Reducing StartingCurrents
To reduce starting currents, the following measures are generally taken:( 1) Utilizing the skin effect (a phenomenon where
the current distribution is concentrated at the upper
part of slots due to a high secondary frequency at
the time of starting), the starting secondary resistance is raised. For this purpose, the slot cross section on the gap side is decreased in the case of adouble cage rotor construction.
( 2 ) In the case of full-voltage starting (a method ofstarting by applying the rated voltage directly to
motor terminals), the effect of magnetic saturation isminimized in the vicinity of rotor core gaps. For thispurpose, semi-closed slots are adopted.
( 3 ) The number of coil turns is increased in order
to raise motor impedance.
As a result of above measure (1), however,secondary copper loss is increased. As a result ofabove measure (2), the harmonic component ofmagnetic ﬂux in gaps is increased. Inﬂuenced bythese results, stray load losses (eddy current losses
caused by harmonic magnetic ﬂux) are increased.
Still more, by virtue of above measure (3), primary andsecondary copper losses are greatly increased. It isdifﬁcult for the Top Runner motors to carry out motor
design where the starting current is suppressed.
As a technique to reduce the starting current
while securing high efﬁciencies, we would like tointroduce a rotor with a double cage rotor type construction as below.

This type of rotor has a slot structure as shownin Fig. 3 by applying a technique of the above measures (1) and (2). The starting current can bedecreased by minimizing the cross section of eachround slot part near the gap surface. At that time,
however, a proper distance between round slot partand trapezoidal part is adequately taken into consideration in order to reinforce the effect of the skineffect at the time of starting. If the cross section ofeach trapezoidal part, distant from the gap surface,is increased, secondary copper loss can be suppressed during motor operation. Regarding roundand trapezoidal slot sizes together with length andwidth of slit grooves, we are working on the optimum design by numerical analysis method.
In regard to the above measure (3), the conductor cross section is increased (making the ironcore size larger) so that the resistance value can bedecreased. By such a treatment, the increased
amount of copper losses is cancelled. For the stator, measures speciﬁed in Table 1 were taken.
As an approach to reduce losses, insulation may
be applied to the rotor slots and/or no treatment may
be applied to the outer periphery of the rotor as follows:( 1) Rotor slot insulation
The inner surface of slots in the rotor iron core
is insulated by a heat-resisting coating. By this treatment, short-circuit currents among rotor bars can besuppressed. It is, therefore, possible to suppress
the interbar current loss that is one of the stray load
losses. Since the surface is exposed to a high heatof molten aluminum at about 700℃ during aluminum die cast treatment, it is necessary to use aheat-resisting coating material. Immersion treatment is carried out in an inorganic coating material
that is superb in terms of heat resistant properties.

Round-shaped slot
Trapezoid-shaped slot

Gap surface( 2 ) No treatment to the outer periphery of the rotor
Since the outer periphery of the rotor is processed after aluminum die cast treatment, this outer
periphery is inevitably short-circuited. This eventually gives rise to surface losses consisting mainly ofeddy current loss, generated by the effect of highfrequency ﬂux around air gaps. If no treatment isapplied to the outer periphery of the rotor, it is possible to reduce the surface loss that is one of thestray load losses. To realize this no surface treatment, a high level of manufacturing expertise isrequired to produce high-level dimensional accuracy without the need for additional processing of therotor external surface. This is required in the process of aluminum die cast treatment and shaft pressin (or shrink ﬁtting).

4 Prototype Evaluation of a Low
Starting Current Type Top Runner Motor
Using approaches introduced in Sections 2and 3, we manufactured and evaluated a prototype
40kW 6-pole low starting current motor with the IE3class efﬁciency. Since the inrush current at the timeof starting the motor tends to be reduced as thestarting power factor is raised higher, conﬁgurationof slots has been determined in consideration of thisfact at the time of designing the prototype.
Table 2 shows characteristics of the demonstration unit. In regard to efﬁciencies, the machine
attained almost the same level of energy consumption efﬁciency of 94.5% that is stipulated for the Top
Runner motor. As a result of adopting the semiclosed slots, the effect of magnetic saturation upon
the starting current has been found to be almost 1/3
compared with the totally closed slots. It was suppressed to be about 5 times of the rated current.
The starting power factor was raised approximately
by 20% compared with regular machines of thecompany with the same capacity. In addition, theTable 2 Characteristics of the Demonstration Unit
Efﬁciency and starting current values of the low current starting
Top Runner motor are shown.
40kW-6P 440V-60Hz
Result of prototype modelRotor

Fig. 3Gap Structure of a Low Starting Current Motor

The gap structure of the rotor is shown.

Frame No.

225SX

Rated current

62.9A

Starting current

464％

Efficiency

94.4％

MEIDEN REVIEW Series No.172 2018 No.1

19inrush current at the time of starting could be
reduced by about 10%. The overall length of thistype of motor is also longer than that of standard
Top Runner motors.

high-efﬁciency motors to meet applications not limited to top runner motors. In so doing, we will meetthe various requirements of our customers.
・All product and company names mentioned in this paper are

5 Postscript
This paper introduced our technologies for thedevelopment of low starting current Top Runnermotors. When such technologies are applied, we canexpect to realize high efﬁciencies for other range
motors which are not covered by the Top RunnerStandard. In the future, we will work on developing

20MEIDEN REVIEW Series No.172 2018 No.1

the trademarks and/or service marks of their respective owners.
《Reference》( 1 ) Agency for Natural Resources and Energy (2009 Report ofInvestigation into Actual Conditions of Energy Consuming Equipment),
JAE-091907, Introduction of Top Runner Standards (draft) for JEMA
3-Phase Induction Motors