Motor Test Results: Copper Compared to Aluminum

Die Cast Copper Motor Rotors: Motor Test Results, Copper Compared to Aluminum**

BBF Associates, 68 Gun Club Lane, Weston, MA 02193 USA
Vice President, Copper Development Association Inc., 260 Madison Ave., New York, NY 10016 USA
Senior Advisor, CDA Inc. 27 Raintree Lane, Hilton Head Island, SC 29926 USA
Van Son Consultants, 4500 Mount Harmony Road, Greenwood, AR 72936 USA

Introduction

Copper rotors were cast for four motor companies for evaluation in their facilities. The rotors were pressure die-cast using a 660-tonne real-time shot-controlled Buhler horizontal machine with H-13 die inserts. These ordinary tool steel dies were used as only a few rotors were needed for testing. The copper was inductively melted just in time to avoid large holding furnaces and control oxygen and hydrogen content.

The copper was heated to 1230°C, providing about 150°C of superheat, maintained using a heated shot sleeve. The real-time shot control capability of the die-casting machine allowed the study of various die-casting variables affecting cast copper quality and rotor performance. Concerns about potential welding of copper to iron laminations and property compromise due to heat treatment were addressed by water quenching half the rotors upon ejection from the machine, while the other half air-cooled.

Experimental Method

Motor Performance Tests

Approximately 140 rotors were cast for evaluation by four motor manufacturers using different testing methods. Three companies used dynamometer efficiency tests per IEEE Specification 112, test method B, while the fourth used IEC 34-2 test method. The IEEE test segregates energy losses into iron core losses, stator resistance, rotor resistance, windage and friction, and stray load losses. Copper rotors are expected to reduce stray load losses, making their accurate measurement important.

15 Hp (11.2 kW) Motor

The first copper rotors tested were for a 15 Hp (11.2 kW) motor. Seven rotors covering different process variables were compared to a database of aluminum rotor motors. Results showed consistent efficiency at 90.7%, with variations of only ±0.1 percentage points. Rotor resistance losses averaged 157 watts, with a maximum variation of 14 watts. The process appeared robust, as process variations did not affect final performance significantly.

Test Results

The test results were remarkably consistent across all process variables. The key measure of efficiency yielded virtually no difference with 90.7% as average and variation of only plus or minus 0.1 percentage points. Rotor watts loss aver-aged 157 watts with a maximum variation from 153 to 167 watts. With only seven tests, no pattern could be discerned relative to any of the process variables. The conclusion is that the process is very robust and process variations within the range tested have no predictable effect on final performance results. Although the post-casting cooling method seemed to have no effect on the results, water quenching reduced handling time to one minute versus a 20-minute air-cooling time. This would allow much faster production in a manufacturing plant.

From the remarkable consistency of the test results, we conclude that the cast-ing process is most viable. Results variations were all within test measurement accuracy and no pattern emerged reflecting die casting variables. When compared to historical variation in aluminum rotor motors, these copper rotors were so con-sistent as to deem the data variation insignificant.

Below table shows the IEEE test results as averages for seven rotors tested Rotor re-sistance losses are the key item in rotor material substitution and yielded a 40%reduction in measured losses. This represents 80% of the theoretical maxi-mum value possible in the conductivity difference between rotor materials. This is a very good start for a first attempt at real motors and may be improved further with detail lamination slot design.

-

Al (W)

Cu (W)

^ W

%

Stator Resistance

507

0 Iron Core Loss

286

0 Rotor Resistance

261

157 -104

-40

Windage & Friction

115

72 -43

-37

Stray Load Losses

137

105 -32

-23

Totals

1306

1127

-179

-14

Stator Resistance

AI (W) : 507

CU (W) : 507

^ W : 0

% : 0

Iron Core Loss

AI (W) : 286

CU (W) : 286

^ W : 0

% : 0

Rotor Resistance

AI (W) : 261

CU (W) : 157

^ W : -104

% : -40

Windage & Friction

AI (W) : 115

CU (W) : 72

^ W : -43

% : -37

Stray Load Losses

AI (W) : 137

CU (W) : 105

^ W : -32

% : -23

Totals

AI (W) : 1306

CU (W) : 1127

^ W : -179

% : -14

Windage and friction losses are mechanical losses retarding rotation. Al-though these seem to have no relevance to rotor material, they do in this case. The copper rotors cast were had smooth end rings except for projections for balancing weights. They did not include cooling fins on the end rings. With a lower resis-tance rotor, less heat is generated to be dissipated. These rotors, lacking fins, were adjoined on the shaft with an internal circulating fan for stator cooling. These fans are more efficient as they can be sized for their circulating job without having to dissipate rotor heat. As a result, when compared to aluminum rotors with fins, to-tal windage loses were down 37% from 115 watts to 72 watts. Friction in the bearings is assumed to be the same. The cooler running copper rotors allow re-duced windage losses via a more efficient internal fan and reduce the amount of copper required by eliminating the rotor end ring fins.

Stray load losses are the cumulative effect of magnetic transfer efficiency be-tween the stationary stator and the rotating rotor as experienced in the air gap be-tween the two. Consistent air gap and rotor balance also affect stray load losses and there is an electrical component to the magnetic transfer efficiency. Consis-tency in conductivity of rotor conduction bars is critical to proper induction mag-netic transfer. Porosity or nonmetallic inclusions in cast rotor bars can result in variation in effective rotor bar cross sectional area, and therefore resistance, result-ing in variation in the magnetic field in the air gap. This increases stray load losses via inconsistent magnetic flux density between stator and rotor reducing overall efficiency. The seven copper rotors exhibited such rotor bar consistency so as to reduce stray load losses by 23%, from 137 watts to 105 watts. A more ac-curate and consistent casting process might possibly produce similar stay load im-provements in aluminum rotors. It is clear that the die-cast copper rotors contrib-uted to the overall motor efficiency via a consistency not normally achieved in typical motor production.

The substitution of copper as rotor material directly achieved 58% of the total savings and was materially involved in saving the other 24% in windage losses and 18% in casting accuracy stray load losses. The combination resulted in 179 watts of savings or a total of 14% reduction in total losses. These results support the efficacy of both the material and the process. The rotors did not require bal-ancing weights usually used to compensate for rotor bar inconsistencies.

Other Performance Measures

In addition to the loss measurements, the test method itemizes performance issues such as temperature rise above ambient, full load speed and power factor (Table 2). These data reveal a motor having different characteristics than a typical alu-minum rotor motor. Overall efficiency resulted in a solid addition of 1.2 percent-age points added directly to the motor nameplate efficiency. This is significant in that 20 years of motor efficiency improvements have already utilized all of the easy things that reduce losses. Copper rotors represent one of the largest possible reductions in losses without using amorphous steels or superconducting, still ex-otic and very expensive alternatives.

-

Al

Cu

Difference

% Change

Efficiency

89.5

90.7 +1.2

+1.4

Temperature Rise, °C

64.0

59.5 -4.5

-7.0

Full Load RPM

1760

1775

+15

+0.85

Slip, %

2.22

1.37 -0.85

-38

Power Factor, %

81.5

79.0 -2.5

-3

Efficiency

AI : 89.5

CU : 90.7

Difference : +1.2

% Change : +1.4

Temperature Rise, °C

AI : 64.0

CU : 59.5

Difference : -4.5

% Change : -7.0

Full Load RPM

AI : 1760

CU : 1775

Difference : +15

% Change : +0.85

Slip, %

AI : 2.22

CU : 1.37

Difference : -0.85

% Change : -38

Power Factor, %

AI : 81.5

CU : 79.0

Difference : -2.5

% Change : -3

25 Hp (18.5 kW) Motor

Tests for a 25 Hp motor showed even more dramatic results, with rotor losses 40% lower in the copper rotors and overall losses reduced by 23%. Lower running temperatures were observed, and smaller internal cooling fans were used, reducing parasitic friction and windage losses. Copper rotors are expected to enhance motor longevity and reliability.

4 Hp (3 kW) and 5 Hp (3.7 kW) Motors

Copper rotors for 4 Hp and 5 Hp motors also showed significant loss reductions. The 4 Hp motor achieved a 21% overall loss reduction, while the 5 Hp motor saw a 38% reduction in rotor resistance losses.

The following table summarizes rotor I²R losses for motors with copper versus aluminum rotors:

Motor Size (HP)

KW

Poles

Al (W)

Cu (W)

Difference (W)

Change (%)

4

3

4

221

92 -129

-58

5

3.7

4 *

-38

15

11

4

262

157 -104

-40

25

19

4

410

292 -118

-40

*Actual loss values not reported.

Copper Rotor Literature Data

The table below summarizes overall motor efficiencies and loss reductions from this study and literature:

HP

KW

Poles

Eff. Al

Eff. Cu

Difference

Loss Reduction

% Reference

4

3

4

83.2

86.4

3.2

19.0 This study

7.5

5.5

4

74.0

79.4

5.0

19.2

3

10

7.5

4

85.0

86.5

1.5

10.0

4

15

11.2

4

89.5

90.7

1.2

11.4 This study

25

18.8

4

90.5

92.5

1.6

17.6 This study

40

30

4

88.8

90.1

1.3

11.6

5

120

90

2

91.4

92.8

1.4

16.3

5

270

200

4

92.0

90.7

1.0

12.5

5

HP - 4

KW : 3

Poles : 4

Eff AI: 83.2

Eff. Cu : 86.4

Diiference : 3.2

Loss Reduction : 19.0

% Reference : This Study

HP - 7.5

KW : 5.5

Poles : 4

Eff AI: 74.0

Eff. Cu : 79.4

Diiference : 5.0

Loss Reduction : 19.2

% Reference : 3

HP - 10

KW : 7.5

Poles : 4

Eff AI: 85.0

Eff. Cu : 86.5

Diiference : 1.5

Loss Reduction : 10.0

% Reference : 4

HP - 15

KW : 11.2

Poles : 4

Eff AI: 89.5

Eff. Cu : 90.7

Diiference : 1.2

Loss Reduction : 11.4

% Reference : This Study

HP - 25

KW : 18.8

Poles : 4

Eff AI: 90.5

Eff. Cu : 92.5

Diiference : 1.6

Loss Reduction : 17.6

% Reference : This Study

HP - 40

KW : 30

Poles : 4

Eff AI: 88.8

Eff. Cu : 90.1

Diiference : 1.3

Loss Reduction : 11.6

% Reference : 5

HP - 120

KW : 90

Poles : 2

Eff AI: 91.4

Eff. Cu : 92.8

Diiference : 1.4

Loss Reduction : 16.3

% Reference : 5

HP - 270

KW : 200

Poles : 4

Eff AI: 92.0

Eff. Cu : 90.7

Diiference : 1.0

Loss Reduction : 12.5

% Reference : 5

Conclusions

The motor performance tests have verified the calculations of motor manufacturers about the benefits of incorporating copper in squirrel cage structures. The results conclusively show that overall motor energy losses are reduced by an average of 14%, and nameplate efficiency is increased by at least a full percentage point.

Acknowledgements

This project was sponsored by the world copper industry, the International Copper Association, Ltd., and managed by the Copper Development Association Inc. Additional funding was provided by the U. S. Department of Energy Office of Indus-trial Technologies and the Air Conditioning and Refrigeration Technical Institute. Several major motor manufacturers underwrote the costs of rotor lamination mate-rial, die inserts and in-house dynamometer testing of motors equipped with die-cast rotors. Formcast, Inc., Denver, Colorado, under Dr. Stephen P. Midson’s di-rection, provided the die casting equipment and the casting expertise. Mr. Ruedi Beck of DieTec, GmbH, Gossau, Switzerland, served to provide the die casting tooling design and innovative approaches to the heated nickel alloy die insert technology that is to be used in commercial production of copper rotors.

References

DOE/CS-0147 – U. S. Department of Energy (1980) Classification and evaluation of electric motors and pumps.
Peters DT, Van Son DJ, Cowie JG, Brush EF Jr.(2002) Improved energy efficiency
And performance through the die-cast copper rotor. International Conference on Electric Machines, Brugge, Belgium
Lie S, Di Pietro C (1995) Copper die-cast rotor efficiency improvement and economic consideration. IEEE Trans. Energy Convers. 10 No. 3: 419-424
Poloujadoff M, Mipo JC, Nurdin M (1995) Some economical comparisons between aluminum and copper squirrel cages. IEEE Trans Energy Convers. 10 No. 3: 415-418
Private communication with manufacturer.