8.8-i

APPENDIX 8.8. POWER CONSUMPTION OF BC/ECM+ BLOWER MOTORS

TABLE OF CONTENTS

8.8.1 INTRODUCTION ................................................... 8.8-1

8.8.2 SHAFT POWER AND STATIC PRESSURE AT CONSTANT SPEED ......... 8.8-2

8.8.3 BLOWER AT OTHER SPEEDS AND AIRFLOWS ........................ 8.8-3

8.8.4 OTHER SIZE BLOWERS ............................................. 8.8-4

8.8.5 SYSTEM EFFECT FACTOR (SEF) OF BLOWERS IN FURNACES .......... 8.8-5

8.8.6 BLOWER SPEED AND SHAFT POWER AS FUNCTION OF FURNACE 

STATIC PRESSURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8-6

8.8.7 ECM 4.0 MOTOR EFFICIENCY ....................................... 8.8-9

8.8.8 BC/ECM+ BLOWER MOTOR ELECTRICITY CONSUMPTION ........... 8.8-10

LIST OF TABLES

Table 8.8.2.1 Coefficients for Shaft Power and Static Pressure at Constant Speed ...... 8.8-3

Table 8.8.4.1 Scalars for Airflow and Shaft Power for the Backward-Inclined Blowers . . 8.8-4

Table 8.8.5.1 SEF for BC/ECM+ Blowers in Virtual Model Furnaces ................ 8.8-6

Table 8.8.6.1 Coefficients for Blower Speed Equation ............................ 8.8-8

Table 8.8.6.2 Coefficients for Blower Shaft Power Equation ....................... 8.8-9

LIST OF FIGURES

Figure 8.8.2.1 Pressure and Shaft Power versus Airflow at Constant Speed ............ 8.8-2

Figure 8.8.5.1 SEF Versus Nominal Airflow for Virtual Model Furnaces ............. 8.8-6

Figure 8.8.7.1 Efficiencies and Operating Range of Prototype Motor ................ 8.8-10
8.8-1

APPENDIX 8.8. POWER CONSUMPTION OF BC/ECM+ BLOWER MOTORS

8.8.1 INTRODUCTION

General Electric Industrial Systems (GE) has developed a prototype backward-inclined

blower impellor for use in residential furnace and air handlers.1 Backward-inclined blowers

have significantly wider blades and must rotate at higher speeds to provide the same static

pressure rise as forward-curved blowers of the same size. As part of this project, GE also

developed a new, smaller diameter, motor, ECM 4.0, which is able to drive the new blower at

higher speeds.

The Department developed the equations to predict blower motor electrical power

consumption for the BC/ECM+ design option based on this prototype.

From a plot of efficiencies and pressures versus airflow of the GE prototype, DOE

developed equations for shaft power and static pressure as functions of airflow. These equations,

combined with the fan laws, allowed the Department to determine shaft power and static

pressure for any speed and airflow for this prototype blower. The Department developed

modifications of these equations to estimate the performance of blowers of other sizes to cover

virtual model furnaces of other nominal maximum airflow capacities. 

The GE prototype is designed to provide airflow for a furnace built to accommodate a 3-

ton air conditioner. The Department assumed it would be matched to a furnace designed to have

a system effect factor (SEF) giving a static pressure rise across the furnace of 0.5 in.w.g. at an

airflow of 1200 cfm. The ratio of the SEF for other size furnaces to the SEF of this furnace was

assumed to follow the pattern seen in furnaces with forward-curved blowers. 

Furnaces with these backward-inclined blowers are intended to operate at constant

airflow, regardless of the static pressure. Once the equations based on airflow were developed,

the Department applied them to determine the blower speeds required to provide nominal airflow

across a range of static pressures. Shaft power was determined at these same operating points to

fit equations of blower speed and shaft power as polynomial functions of furnace static pressure.

GE reported the efficiency of the ECM 4.0 at two operating conditions. The two points

had almost identical efficiencies. The points roughly span the intended operating range of the

motor in a furnace with a nominal airflow capacity of 1200 cfm. Based on this, the Department

assumed the new motor would operate at constant efficiency. The shaft power of the BC/ECM+

blower in generic model furnaces divided by this efficiency gives the blower motor electrical

power consumption.

The following sections in this appendix provide the details of these calculations and

assumptions.
8.8-2

8.8.2 SHAFT POWER AND STATIC PRESSURE AT CONSTANT SPEED

The GE topical report on the development of the prototype blower and motor includes a

plot of efficiencies and pressures versus airflow at a constant speed.1 The Department measured

the values from one and a half dozen points on the curves on this plot. It then converted the

values of these points to units of cfm for airflow and in.w.g. for pressure using a guide for SI

units from ASTM.2

 See Figure 8.8.2.1.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 500 1000 1500 2000

Q, Airflow (CFM)

P, Static Pressure (in.w.c)

0

50

100

150

200

250

H, Shaft Power (W)

P

H

N = 1290 RPM

Figure 8.8.2.1 Pressure and Shaft Power versus Airflow at Constant Speed

The Department fit these points to equations for shaft power and static pressure at

constant blower speed as polynomial functions of airflow. The equations are:

PQ C C ( ) Q C Q C Q C Q = +×





 



 + ×





 



 + ×





 



 + ×





 

 0 1  1000

2

1000

2

3

1000

3

4

1000

4

HQ C C ( ) Q C Q C Q = +× 



 



 + × 



 



 + × 



 

 0 1  1000

2

1000

2

3

1000

3

where:

P = static pressure rise across blower (in.w.g.),

Q = airflow (cfm),

H = blower shaft power (W), and
8.8-3

C0 ,C1 ,C2 ,C3 ,C4 = empirical coefficients from curve fitting.

The coefficients for these equations are shown in Table 8.8.2.1.

Table 8.8.2.1 Coefficients for Shaft Power and Static Pressure at Constant Speed

Static Pressure (P) Shaft Power (H)

C0 0.91982 46.32071

C1 0.34136 127.57380

C2 -0.11357 -13.27624

C3 -0.32487 -7.27523

C4 0.08524

8.8.3 BLOWER AT OTHER SPEEDS AND AIRFLOWS

These equations, combined with the fan laws, allowed the Department to determine shaft

power and static pressure for any speed and airflow for this prototype blower. To do this

requires finding the equivalent airflow at the test speed, and then scaling the pressure to new

speed.3

 The equations for this are:

Qequiv Q

Ntest

N

= ×

PNQ P ( ) test( ) Qequiv

N

Ntest , = ×





 





2

HNQ H ( ) test( ) Qequiv

N

Ntest , = ×





 





3

where:

P = static pressure rise across blower (in.w.g.),

N = blower speed (rpm),

Ntest = blower speed during test (rpm),

Qequiv = equivalent airflow at test speed (cfm), 

Ptest = static pressure rise across blower at test speed (in.w.g.),

H = shaft power of blower (W), and

Htest = shaft power of blower at test speed (W).
8.8-4

8.8.4 OTHER SIZE BLOWERS

The prototype blower developed by GE was thought to be designed for only one size

furnace, with a nominal airflow of 1200 cfm. Wider blowers provide more airflow and require

more shaft power to develop the same static pressure. The Department examined the airflow for

four forward-curved blowers of the same diameter with different widths (Lau Industries blowers

DD10-7A, DD10-8A, DD10-9A, and DD10-10A) when operating at 1075 rpm, the nominal

speed for PSC motors they are usually driven with.4 The ratio of average airflows for these

blowers across a representative range of pressures, roughly scaled as the ratio of widths raised to

a power of 0.795. The Department assumed that the shaft power and airflow wider backwardinclined blowers would scale with width at this same rate. The assumed width ratios and

resulting scalars for airflow and shaft power for the backward-inclined blowers for the four

airflow capacities of the generic model furnaces are shown in Table 8.8.4.1.

Table 8.8.4.1 Scalars for Airflow and Shaft Power for the Backward-Inclined Blowers

nominal airflow (cfm) 800 1200 1600 2000

generic model forward-curved blower

size

9×8 10×8 10×10 11×10

width ratio 0.875 1 1.25 1.5

scalar 0.89932 1 1.19402 1.38018

Using these assumptions, finding the pressure for a given airflow for a different width

blower means finding the equivalent airflow to calculate the pressure for the prototype blower at

the test speed. This is a modification of the techniques used in section 8.8.3, Blower at Other

Speeds and Airflows. The equations to find the equivalent airflow and scaled shaft power are:

Qequiv Q

Ntest

N W

=× × W

1

HW N Q H ( ) test( ) Qequiv

N

Ntest W , = ×





 



 ×

3 1

where:

N = blower speed (rpm),

Ntest = blower speed during test (rpm),

QW = airflow of wider blower (cfm),

Qequiv = equivalent airflow at test speed (cfm), 

HW = shaft power of wider blower (W), and

Htest = shaft power of blower at test speed(W).
8.8-5

The equation to calculate pressure is the then same as in section 7.8.3 with the new

Qequiv.

8.8.5 SYSTEM EFFECT FACTOR (SEF) OF BLOWERS IN FURNACES

The SEF is the reduction in pressure from the blower operating by itself compared to

when the blower is operating in a furnace. The SEF accounts for the effect of obstructions to the

airflow and the inlet and outlet geometry of the blower enclosure of the furnace. The

Department made the assumption that the fractional change in SEF for every 400 cfm change of

nominal airflow in the virtual model furnaces would also apply for the BC-ECM+ blowers in the

virtual model furnaces. 

The Department fit the SEF of the virtual model furnaces to an exponential function of

nominal airflow. Figure 8.8.5.1, shows values and the fit equation for forward-curved blowers

and PSC motors. The equation for SEF as a function of nominal airflow is:

SEF b m = × Qnom

where

SEF = system effect factor,

= 1.227249 x 106, b

m = 0.999039, and

Q = nominal airflow of blower(cfm). nom

The SEF was calculated independently for the BC-ECM+ at a nominal airflow of 1200

cfm. The equation is:

( ) SEF

Pblower Pfurn

Qnom

=

−

2

where

SEF = system effect factor,

Pblower = static pressure of blower, 0.781 (in.w.g.), calculated at 1290,

rpm and 1200 cfm,

Pfurn = static pressure of furnace, 0.5 (in.w.g.), and

Q = nominal airflow of blower, 1200 (cfm). nom
8.8-6

0.E+00

1.E-07

2.E-07

3.E-07

4.E-07

5.E-07

6.E-07

7.E-07

800 1200 1600 2000

Nominal Airflow (CFM)

SEF (in.w.g./CFM2

)

Exponential Fit

Generic Model Furnaces

Figure 8.8.5.1 SEF Versus Nominal Airflow for Virtual Model Furnaces

To get the SEF for the other virtual model furnaces with BC/ECM+ blowers, DOE

calculated the ratio of SEF for the other nominal maximum airflow virtual furnaces to the SEF

for this furnace. This was done using the exponential function of SEF versus nominal maximum

airflow of the virtual model furnaces. The resulting ratios are shown in Table 8.8.5.1.

Table 8.8.5.1 SEF for BC/ECM+ Blowers in Virtual Model Furnaces

Nominal Airflow (CFM) 800 1200 1600 2000

SEF ratio 1.468938 1 0.680764 0.463439

SEF (in.w.g./CFM2

) 2.86931E-07 1.953E-07 1.32975E-07 9.05245E-08

8.8.6 BLOWER SPEED AND SHAFT POWER AS FUNCTION OF FURNACE STATIC

PRESSURE

For furnaces with improved ECM motors and backward-inclined impellors in the blower,

the Department determined the airflow and static pressure at the operating conditions from the

intersection of the fan curve of the furnace and the system curve of the ducts. This is done in the

same way as for standard PSC motors. However, furnaces with these motors are programmed to

provide a constant airflow regardless of static pressure. Thus, the fan curves for these furnaces

are a vertical line of constant airflow.
8.8-7

The Department evaluated the furnace static pressure at the nominal air flow for each of

the virtual model furnaces with BC/ECM+ blowers for a range of blower speeds.

P ( ) furn Pblower = −× Q N SEF Q , 2

where

P = furnace static pressure (in.w.g.), furn

P = static pressure across blower alone (in.w.g.), blower

Q = airflow (cfm),

N = motor speed (rpm), and

SEF = system effect factor as explained in Appendix 7.4, Furnace Fan

Curves, section 7.4.1, General Air Speed and Airflow

Relationship.

From these static pressures, the Department developed an equation for blower speed as a

function of static pressure at the nominal airflows of the virtual model furnaces.

N P( ) furn = +× + × CCP 01 2 furn C Pfurn2

where

N = motor speed (rpm),

P = furnace static pressure (in.w.g.), and furn

C0 ,C1 ,C2 = empirical coefficients from curve fitting.

The coefficients for the equations of blower speed as a function of furnace static pressure

for virtual furnaces with improved ECM motors and forward-curved impellors is shown in

Table 8.8.6.1.
8.8-8

Table 8.8.6.1 Coefficients for Blower Speed Equation

Q C0 C1 C2

Cooling

2 ton 800 771.297930225 779.26756805 -139.008728615

3 ton 1200 1004.4257405 612.718105072 -82.3858179298

4 ton 1600 1115.11438648 556.374528078 -66.4367804917

5 ton 2000 1186.32793032 524.106011654 -58.6030844866

High Fire

2 ton 640.0 620.504604012 933.280077841 -205.025304436

3 ton 960.0 805.339732049 744.862082929 -126.832574748

4 ton 1280.0 893.475264237 679.413794851 -103.848338196

5 ton 1600.0 950.264980926 641.197789459 -91.9907933216

Low Fire

2 ton 533.3 521.413603686 1068.28226839 -271.834021254

3 ton 1200 673.447604033 866.36860443 -175.654364949

4 ton 1066.7 746.403441995 793.875638502 -145.692110892

5 ton 1333.3 793.449632938 751.002693796 -129.8241

For the same air flow and speeds that were used to evaluate furnace static pressure, DOE

evaluated the blower shaft power using the equations explained earlier in this appendix. From

these results, the Department developed an equation for blower shaft power as a function of

static pressure at the nominal airflows of the virtual model furnaces.

H Pfurn CCPfurn C Pfurn C Pfurn ( ) = +× + × + × 01 2 2 3 3

where

H = blower shaft power (which is also the motor output power) 

(W),

P = furnace static pressure (in.w.g.), and furn

C0 ,C1 ,C2 ,C3 = empirical coefficients from curve fitting.

The coefficients for the equations of blower shaft power as a function of furnace static

pressure for virtual furnaces with ECM motors and forward-curved impellors is shown in Table

8.8.6.2.
8.8-9

Table 8.8.6.2 Coefficients for Blower Shaft Power Equation

Q C 0 C1 C2 C3

Cooling

2 ton 800 34.7637865629 101.257496416 30.819237206 -4.393198813

3 ton 1200 86.8964114725 149.648658529 24.686091998 -1.351144007

4 ton 1600 142.379224744 199.258007379 25.830716411 -0.75624128

5 ton 2000 199.168681228 247.410917528 26.886578305 -0.163068387

High Fire

2 ton 640 17.7814395418 81.4599546929 36.760753941 -6.820695201

3 ton 960 44.4918524165 119.54001273 31.765403864 -3.666136066

4 ton 1280 72.9202558294 159.014401208 33.753367048 -2.927933696

5 ton 1600 101.991203084 197.482373726 35.406395106 -2.130855013

Low Fire

2 ton 533.3 10.2554469313 68.6525919364 41.182715094 -8.986586445

3 ton 800 25.7529563782 99.6988982718 37.766224552 -6.017676309

4 ton 1066.7 42.197172414 132.467356467 40.800818335 -5.419757563

5 ton 1333.3 59.0319912122 164.29823146 43.598086888 -4.793958254

8.8.7 ECM 4.0 MOTOR EFFICIENCY

The GE topical report listed the efficiency of the prototype ECM 4.0 motor at two

operating conditions.1

 The two points, which roughly span the intended operating range of the

prototype motor, are at the target operating condition and at the maximum operating conditions. 

The motor efficiencies at these two operating conditions were 86.0% and 84.7%, respectively. 

Figure 8.8.7.1, shows these data points against the targeted operating range of the motor. Based

on this, the Department assumed the new motor would operate at constant efficiency of 85%.
8.8-10

84.7%

86.0%

1200

1300

1400

1500

1600

1700

1800

100 150 200 250 300 350 400 450 500

H, Shaft Power (W)

N, Speed (RPM)

ECM+ Max

ECM+ SEER

Operating Range

Figure 8.8.7.1 Efficiencies and Operating Range of Prototype Motor

8.8.8 BC/ECM+ BLOWER MOTOR ELECTRICITY CONSUMPTION

The shaft power of the BC/ECM+ blower in virtual model furnaces divided by efficiency

gives the blower motor electrical power consumption.

BE Hw N QW

motor = (, )

η

where

N = blower speed at operating static pressure of furnace (rpm),

QW = airflow of wider blower at operating static pressure of furnace

(cfm), 

HW = shaft power of wider blower (W), and

η = motor efficiency. motor
8.8-11

REFERENCES

1. Wang, S. and H. Wiegman, Topical Progress Report for the Variable Speed Integrated

Intelligent HVAC Blower, November 14, 2001, GE Corporate Research and

Development. Niskayuna, NY. 

2. American Society for Testing and Materials, Standard Practice for Use of the

International System of Units (SI) (the Modernized Metric System), 1992. Philadelphia,

PA. PCN 03-543-092-34. Report No. E 380-92. 

3. Air Movement and Control Association International Inc., Fans and Systems, in AMCA

Publication 201-90. 1990: Arlington Heights, IL. 

4. Lau Industries, Whirl Wind IV, Version 4.0, Fan Application Software. 2000.

http://www.lau-ind.com/whirlwind.htm