37

CHAPTER 3

EFFICIENCY IMPROVEMENT IN CAGE INDUCTION

MOTORS BY USING DCR TECHNOLOGY

3.1 INTRODUCTION

This chapter describes, a comparison of the performance

characteristics of a 2.2 kW induction motor with conventional Die-cast

Aluminium Rotor to that of a 2.2 kW induction motor with proposed Die- cast

Copper Rotor. Both the motors are fabricated and its steady state

characteristics are compared using MATLAB 7 software, whose inputs are

No-load and Blocked rotor tests, obtained from the above motors. These test

results are used to determine the equivalent circuit parameters of each motor.

The actual load tests were also performed in the motors in accordance with IS

12615: 2004 standard. The possibility of the efficiency improvement for the

three phase, low voltage, squirrel-cage induction motor is also experimentally

verified between the ratings from 0.37 kW to 11 kW.

3.2 INTERNATIONAL RECOMMENDATIONS ON ENERGY

EFFICIENCY

In today's power scenario, we are facing a major power crunch.

Day by day, the gap between demand and supply of electrical energy is

widening at the rate of 3%. Bridging this gap from supply side is very

difficult and expensive proposition. Due to the pressure of energy shortages,

energy cost and environmental considerations, various Government/Non-

38

Government agencies through out the world started looking at the ways of

improving the efficiency and creating awareness amongst the industry by

enacting mandatory laws/modifying standards for voluntary adoption.

Comparative picture of laws and standards as established today in

USA, EU and India is given in Table 3.1.

Table 3.1 International Recommendations on Energy Efficiency

Factors USA (EPAct)EU (CEMEP Agreement)

INDIA

(Energy Conservation Act)

Legislation Mandatory Voluntary Voluntary

Type of EE motor 3 Phase Induction

3 Phase Induction

3 Phase Induction

Ratings 0.75 kW – 150 kW

1.1 kW–90 kW 0.37kW – 160 kW

Polarities 2,4 & 6 Pole 2 & 4 Pole 2,4,6 & 8 Pole

Efficiency Standard Minimum Specified

Minimum EFF category to be

labelled

Minimum Specified as per IS : 12615

Testing Specification

IEEE – 112 B IEC 600 34.2 IS 12615

Minimum Efficiency Specified for (11kW, 4 Pole, TEFC)

91.0 %

(60 Hz)

88.4 %

(50 Hz)

88.4 %

(50 Hz)

39

3.3 FOCUS ON ENERGY EFFICIENCY OF ELECTRICAL

MOTORS

In simplest terms, energy-efficient electric motors are high-quality

versions of standard motor products. They pack more of 'active' electric

materials (steel laminations and copper) into essentially the same physical

package. By introducing this motor which have different characteristic from a

standard motor, less energy is required to produce the same output torque.

The usage of energy efficient motor, can reduce financial cost of industrial

sector such as the cost of motor maintenance and cost of buying a new motor

because it has long life span.

The importance of energy saving in induction motor was

emphasized about 20 years ago, in the academic area, but the motor

manufacturer’s interest is focused only on the maximum benefit. As a

costumer, it is better to take into account not only the motor price, but also the

cost of the used energy during the whole lifetime of the motor. The new

requirement to improve the motor efficiency is a serious research subject,

which must be about the possibility of loss minimization in the induction

motor.

For the three phase, low voltage, induction motor, the used material

for the squirrel cage is aluminum because of the lower price when compared

to copper, which is convenient for the existing technological solutions. As it is

known, the copper’s resistivity is lower than that of aluminium, and therefore

the copper squirrel cage losses decrease with the ratio of resistivity of copper

to resistivity of aluminium. Till now, the actual technology has no solutions

for low voltage motors and hence new solutions are necessary in the

technological area. This chapter analyzes a series of industrial induction

machines with squirrel cage, considering the rotor losses decreasing, the

material cost increase and the saved energy for the whole machine life.

40

3.4 ENERGY EFFICIENT MOTOR

An ‘Energy Efficient’ motor produces the same shaft output (HP), but absorbs less input power (kW) than a standard motor.

Efficiency = Output / Input= (Input – Losses) / Input= 1 – (Losses* / Input)

* Lesser the loss, higher the efficiency.

The efficiency can be improved by reducing the various losses in motor. The various losses encountered in an induction motor are given in Table 3.2.

Table 3.2 Losses Encountered in Induction Motor

S.No. Motor Component Loss Share on Total Loss

1 Stator copper loss 37% 2 Rotor copper (Conductor) loss 18% 3 Iron Loss 20% 4 Friction and Windage loss 9% 5 Stray loss 16%

Various attempts are made to reduce the above category of losses in

induction motors, primarily to reduce rotor copper loss using DCR

technology.

Energy - efficient motors, also called premium or high efficiency

motors. Motors qualify as ‘energy efficient’ if they met or exceed the

efficiency level listed in the National Electric Manufactures Association’s

(NEMA’s) MG1-1993 Publication (NEMA Standards Publication MG1

1993 and 1996).

41

100

99989796959493929190898887

Historical Motors - 1975

Today’s Standard Motor

Today’s Premium Efficiency Motor

Die Cast Copper Rotor Motor

Amorphous Steel Laminations

Super Conducting

Nirvana

20 HP Induction

motor Efficiency

Comparison with the

Past and Future

3.4.1 Die-cast Copper Rotor Motor - The Economical way of Energy Conservation

Figure 3.1 Technology Vs Efficiency of Induction Motors

In conventional design, the cost of motor increases while attempting to reduce the losses.

The task of efficiency improvement by various methods is illustrated in Figure 3.1 for a 20 HP rating of motor. DCR has helped to achieve efficiencies to meet EFF1 standards in the previous experiences. It also gave confidence that with optimisation of design. It may be possible to achieve efficiencies above EFF1 level also. The DCR Technology increases the efficiency of motor with a nominal increase in cost. As shown in the Figure 3.2, the DCR technology is the best way in the 21st century to increase the motor efficiency above the premium level.

It is an interesting fact that, Two-third (2/3) of electricity generated globally is used to run motors, which is almost equal to 2 Trillion (2×1012) kWhr/Year. Out of this, about 8.5% of all electricity is consumed to meet the loss in Electrical motors. Of these, motors up to 20HP constitute approximately two third of losses (i.e.) 5.4% of all electricity is wasted as loss for these motors.

42

MOTOR LOSSES MOTOR COST

PAST

DCR Motor

21ST

CENTURY FUTURE

Figure 3.2 Induction Motor Efficiency Technology Vs Cost

Hence, more than in any other areas, improving efficiency of motors caught the attention of government bodies, producers and consumers of electricity, designers and manufacturers of motors. Improvement in efficiency of motors assumes more significance in India where motors consume over 75% of the electrical energy with very wide coverage of industrial, agriculture and rural sectors.

The DCR motor which improves efficiency by 1.5% to 3% would save 30 Billion (30× 109) kWhr/year, at Rs.4.50 / kWhr, this amounts to Rs. 135 Billion and is equivalent to 78 Million Barrels of oil (Deivasahayam2005).

3.5 EFFICIENCY IMPROVEMENT IN 2.2 KW (3 HP) INDUCTION MOTOR

A lot of research articles have been published in the past 10 years

where the results are related with high rating, 4 pole motors and very few low

rating (up to 5 HP) 2-pole motors only (Brush et al 2004 and Dale.T.Peters

et al., 2007). Therefore this work investigates the performance comparison of

DCR and DAR motors of 2.2 kW (3 HP), 2 pole induction motor both

43

theoretical calculation using Matlab 7 software and by experimentation in

accordance with IS 12615 standard.

3.5.1 IS 12615: 2004 Standard

Several international standards exist for testing the efficiency of

induction motors. The standards include:

IEEE 112, IEC 60034-2, CSA 390 and JEC 37. The standards differ

mainly in their treatment of the stray losses in an induction machine (Renier

et al 1999). A detailed comparison of the different efficiency results between

the standards was presented. The Indian standard IS 12615: 2004 was used in

the research related to this chapter. The IS 12615: 2004 standard uses the

segregation of losses method to determine the efficiency of an induction

motor as in the same way of reference international standard IEC 60034-2-1.

(Rotating Electrical Machines Testing Standard IEC60034-2 and IEC60034-

30). The efficiency can be expressed in terms of output power (Pout) and the

sum of losses ( losses), as:

out out

in out losses

P PP P P

(3.1)

Three tests are performed in order to determine the losses in an

induction machine accurately. The tests and results associated with each are

as follows:

3.5.2 Temperature Test

The motor is loaded and allowed to run until its temperature

stabilizes. The temperature and winding resistances are recorded.

44

3.5.3 Load Test

The motor is loaded at six different loading points ranging from

25%-150% of rated load. The stator and rotor copper losses are calculated

from this.

3.5.4 No-load Test

The motor is run at no-load with a varying supply voltage between

125% to 20% of rated voltage. The friction and windage and core losses are

calculated from this.

Temperature correction (to 25C ) is done on the Stator and Rotor

losses using the winding temperature and resistance from the temperature test.

The stray load losses (SLL) are then found by subtracting all the calculated

losses from the measured loss.

The loss segregation method is regarded as the most accurate

method for calculating efficiency. This of course depends on the accuracy of

the equipment. It also has the advantage of very high repeatability, due to the

temperature correction of the losses.

3.6 PERFORMANCE CHARACTERISTICS

In order to analyze the performance of an induction motor, first, the

conventional aluminium die cast material is placed in its rotor. After that,

DCR is substituted in place of former one. A locked rotor test and a no-load

test can be used to determine the equivalent circuit parameters of an induction

machine. (Nasar et al 1979; Rajput 1993 and Engelmann et al 1995). So, these

tests were performed on each of the motor and the test results are shown in

Table 3.3 and 3.4 respectively. The equivalent circuit parameters for the DCR

motor and the comparison with the DAR motor are shown in Table 3.5.

45

Subsequently, the calculated equivalent circuit parameters are fed in to

Matlab 7 software to determine the steady-state characteristic of the induction

motor. From Table 3.5 it is observed that the value of no load resistance (R0)

in the DCR motor is more than that of DAR motor. This is due to slot

geometry which is designed for DAR motor. i.e., the slot design had not been

optimized for copper.

Table 3.3 No-Load test results for 2.2 kW (3 HP), 415 V, 50 Hz, 2-pole,

3-phase induction motor

Parameters DAR

MotorDCR

Motor

No load voltage/ phase 240V 240V

No load current / phase 2A 1.90A

No load power 220W 180W

Table 3.4 Locked rotor test results for 2.2 kW (3 HP), 415 V, 50 Hz, 2-

pole, 3-phase induction motor

Parameters DAR

Motor DCR Motor

L.R. voltage / phase 48.5V 47.3V

L.R. current / phase 4.6A 4.47A

L.R. power 410W 320W

Stator resistance per phase

46

Table 3.5 Comparison of equivalent circuit parameters of 2.2 kW

(3 HP) DCR motor with DAR motor

Parameters DAR Motor DCR Motor

Iw (A) 0.30556 0.25

Iu (A) 1.9765 1.8835

I0 (A) 0.30556-1.9765i 0.25-1.8835i

R0 ( ) 785.45 960

X0 ( ) 121.43 127.42

Z01 ( ) 10.543 10.592

R01 ( ) 6.4587 5.3384

X01 ( ) 8.3333 9.1478

R21 ( ) 3.7287 2.6084

R1 ( ) 2.73 2.73

Figure 3.3 Equivalent circuit of an induction motor

47

(a) (b)

(c) (d)

(e)

Figure 3.4 Performance characteristics of a 2.2 kW (3 HP) induction

motor

48

The equivalent circuit model of an induction machine can be used

to predict its performance characteristics (Sen 1999 and Wildi 2000). One

such circuit is shown in Figure 3.3. The variation (with output power) of its

torque, current, input power factor, efficiency, and slip are determined using

Matlab 7 which is shown in Figure 3.4 (a-e) (Ong 1998 and Okoro 2004).

The experimental set up for conducting the full load test is shown

in Figure 3.5.

Figure 3.5 Experimental setup of 3-HP DCR Motor

The results from the tests obtained on one of the 2-pole motors of

3-phase, 2.2 kW (3 HP), 415V, 50 Hz are shown in Table 3.6. The important

parameters responsible for the induction motor characteristics like Efficiency,

Locked rotor Torque, Slip etc., are calculated from Matlab 7 and compared to

the tested results as shown in Table 3.7. Both the results are found equivalent

with each other.

49

Table 3.6 Comparison of test results of 2.2 kW (3 HP) DAR and DCR

motors

Parameters DAR DCR

75% 100% 110% 75% 100% 110%Voltage (Volts) 415 415 415 415 415 415 Current (Amps) 3.8 4.6 4.93 3.7 4.47 4.8 Input Power (Watts) 2040 2700 2990 1980 2620 2890 Speed (RPM) 2892 2874 2840 2931 2918 2902 Output Power (Watts) 1645 2203 2422 1656 2203 2425 Efficiency (%) 80.6 81.58 80.9 83.65 84.18 83.9 Power factor 0.74 0.82 0.84 0.76 0.82 0.84 Pull out torque (Break down) % of rated value 352 365

LR Torque (% of full load Torque) 268 246

Table 3.7 Comparison of tested and calculated values of 3 HP DAR

and DCR motors under rated conditions

Parameters Results DAR motor DCR motor

Efficiency(%)

Calculated 81.63 84.14

Tested 81.58 84.18

Speed (RPM)

Calculated 2875 2892

Tested 2874 2918

Power factor Calculated 0.84 0.84

Tested 0.82 0.82

Input Power(Watts)

Calculated 2654 2632

Tested 2644 2620

Output power(Watts)

Calculated 2199 2210

Tested 2203 2203

50

3.7 EFFICIENCY IMPROVEMENTS IN SMALL/MEDIUM

RATED MOTORS

3.7.1 Experimental Results

During the first test process, seven rotors were cast for 15-HP

(11 kW), 4 Pole, 50 Hz motor and were 150 mm in diameter with 165 mm

core length containing 7 kg of copper in the conductor bars and end rings. It

is important to note that, the laminations used here were designed for

aluminium, i.e., the slot design had not been optimized for copper. Rotor watt

loss averaged 135 watts with a range of 131 to 145 W losses. Table 3.8 shows

the CSA390 test results for the seven rotors tested. Similarly, the CSA390

test results for the 10-HP (7.5 kW), 4 Pole, 50 Hz motor and the segregation

of losses is given in Table 3.9. Relative costs are summed up for each rating

as per present research.

Table 3.8 Test readings of 7.5 kW (10 HP), 4 Pole, 50 Hz motor

Types of Lossesand performance

parameters

Std. Eff. motor(DAR)

Energy Eff. motor(DAR)

Energy Eff. motor(DCR)

Constant loss

(Iron loss + Friction and Windage loss)

505 W 342 W 202 W

Stator copper loss 452 W 424 W 422 W

Rotor copper loss 357 W 243 W 166 W

Stray loss 37 W 37 W 38 W

Speed (Rpm) 1432 1453 1468

Efficiency (%) 84.70 87.80 90.4

Cost (%) 100 107 115

51

Table 3.9 Test readings of 11 kW (15 HP), 4 Pole, 50Hz motor

Types of Losses

and performance parameters

Std. Eff. motor

(DAR)

Energy Eff. motor

(DAR)

Energy Eff. motor

(DCR)

Constant loss

(Iron loss + Friction and Windage loss)

555 W 422 W 222 W

Stator copper loss 761 W 714 W 666 W

Rotor copper loss 287 W 266 W 135 W

Stray loss 56 W 56 W 55 W

Speed (Rpm) 1462 1465 1482

Efficiency (%) 87.0 88.4 91.0

Cost (%) 100.0 107.0 117.5

Table 3.10 Performance comparison of motors with DAR and DCR

Motor Rating

Efficiency (Al) (%) / RPM

Efficiency (Cu)

(%) / RPM

Incremental DifferenceEff. / RPM

0.37kW – 2 Pole 72.8 / 2856 76.1 / 2911 3.3 / 55

1.5 kW – 2 Pole 82.39 / 2880 84.56 / 2910 2.4 / 23

3.7 kW – 2 Pole 84.0 / 2945 86.8 / 2947 2.8 / 22

0.75 kW – 4 Pole 73.1 / 1442 74.5 / 1470 1.4 / 28

1.1 kW – 4 Pole 82.0 / 1424 84.5 / 1457 2.5 / 33

2.2 kW – 4 Pole 83.5 / 1411 85.7 / 1451 2.2 / 40

3.7 kW– 4 Pole 84.0 / 1429 86.9 / 1469 2.9 / 40

Average Efficiency improvement 2.5%

52

Improved efficiencies obtained by using DCR in the existing

designs are given in Table 3.10 for certain popular ratings. This goes to prove

that the DCR motor is the only economical way of obtaining energy

efficiency above levels acceptable.

Table 3.11 Test readings of 10 HP (7.5 kW) 4 pole Induction motor

under Full load condition

Parameter DAR DCR

Voltage (Volts) 415 415

Current (Amps) 14.4 14.73

Input Power (Watts) 8750 8500

Speed (RPM) 1442 1458

Output Power (Watts) 7487 7508

Efficiency (%) 85.57 87.78

Power factor 0.825 0.808

Slip (%) 3.87 2.8

Pull out torque (Break down) % of rated value

298 355

LR Torque (in kg-m) 14.12 14.06

LR Current (Amps) 85.0 86.6

As expected, in the higher conductivity (copper) rotor material, the

speed of DCR motor is increased slightly, the slip and input currents are

reduced and the efficiency is increased. For the same output power, by merely

replacing the DAR with DCR (without changing the other parameters), the

efficiency of the motor is increased by nearly 2.6% points in the 2.2 kW, 2

pole motor. The efficiencies of small/medium rated motors between the

53

ratings (0.37 kW- 3.7 kW) were increased by about 1.5% and nearly 3%

points, a rather remarkable improvement. The test methods for the above

motors are loss separation method of CSA 390 and the test conditions are 3

Phase, 415 V, 50 Hz. Results from tests on one of the 4 pole, 7.5 kW motor

and one of the 2 pole, 1.5 kW motor is shown in Tables 3.11 and 3.12

respectively.

It may be noted that the breakdown torque values of the DCR

motor has 30% points over those for similar motors with aluminium rotor.

Table 3.12 Test readings of 2 HP (1.5 kW) 2 pole 50 Hz induction

motor under Full load condition

Parameter DAR DCR

Voltage (Volts) 415 415

Current (Amps) 2.79 2.81

Input Power (Watts) 1840 1780

Speed (RPM) 2880 2910

Output Power (Watts) 1516 1505

Efficiency (%) 82.39 84.56

Power factor 0.92 0.891

Slip (%) 4 3

Pull out torque (Break down) % of rated value

390 448

LR Torque (in kg-m) 1.46 1.33

LR Current (Amps) 18.47 20.39

54

It is observed that this increment in efficiency in DCR is due to the

better conductivity of DCR. Starting (locked rotor) Torque is reduced

somewhat for aluminium laminations with slots designed for aluminium. In

applications where the lower starting torque is a problem, a modified design

of the rotor slots offers the solution.

The pull out (Breakdown) torque values are also reported in Table

3.6 for a 2.2 kW motor. It is observed that the pull out torque in DCR is high.

i.e., 365 % of the rated torque compared to 352 % of the rated torque for the

same motor with DAR. From the mechanical characteristics shown in Figure

3.4 (c), the DCR motor has the advantage of a higher torque at running speed.

This characteristic of a DCR motor often finds very useful in applications

such as centrifugal pumps, which need high torque at high speed.

The p.f of the motor under various load conditions of both the DAR

and DCR motors are nearly equal. The pictorial views of 11 kW and 1.5 kW

DCR’s are shown in Figure 3.6. A photomicrograph of a 2.2 kW DCR is

shown in Figure 3.7.

3.7.2 Efficiency Comparison

The graphical representations of efficiency comparison of two 4

pole motors are shown in Figure 3.8 and 3.9. Both these motors are 415V, 50

Hz, 3 phase. Figure 3.9 shows the DCR of 11kW and 1.5 kW respectively. A

core plate of a 5 HP DCR is shown in Figure 3.10.

55

Figure 3.6 DCR of 11 kW and 1.5 kW ratings

Figure 3.7 A photomicrograph of a 2.2 kW (3 HP) DCR

56

2 HP (1.5 kW)

73.9

79.3

81.1

75.9

81.3

83.5

68

70

72

74

76

78

80

82

84

86

50% LOAD 75 % LOAD 100 % LOAD

LOAD (%)

ALUMINIUM COPPER

Figure 3.8 Efficiency Comparison of 2 HP Motor

0.5 HP (0.375 kW)

55.3

66.5

72.8

58.2

69.7

76.1

0

10

20

30

40

50

60

70

80

50% LOAD 75 % LOAD 100 % LOAD

LOAD (%)

ALUMINIUM COPPER

Figure 3.9 Efficiency Comparison of 0.5 HP Motor

57

Figure 3.10 Core plate of 5HP DCR

3.8 SAMPLE WORKING OF ENERGY SAVINGS AND

PAYBACK

The slightly higher (15%) initial cost of DCR motors is often

misunderstood as a demerit. It is not at all true. The increase in initial cost is

offset by the energy saved and the following equation outlines a method for

calculating the cost savings from improved motor efficiency.

The difference in losses for the two motors is:

FLSM EM

1 1L kW x F xE E

(3.2)

where,

kWFL = Full load rating (kW) (taken from motor nameplate)

ESM = Full load efficiency (%) of Standard Motor

EEM = Full load efficiency (%) of EE Motor

F = Running load as a % of full load

58

Capital Cost DifferentialPay Back Period Months Savings per Month

(3.3)

In the above equation for SM at part loads

falls steeply to a lower value unlike EEM which remains almost constant.

Hence the payback period reduces more, than what is calculated using the

above equation.

Consider an example, assume we have a standard-efficiency 11 kW

motor. Assume the motor operates 600 hours, and that power costs

Rs.4.50/kWh. Such motors have an average efficiency rating of 87% at full

load and the capital cost is $350. This motor may be replaced with a DCR

motor having an efficiency of 91% and the capital cost is $380.

The pay back period is calculated as follows:

Increased cost of motor with 91% efficiency is $30

Let,

X - Input power per hour for 87% efficiency motor

= 11/0.87 = 12.6 units

Y - Input power per hour for 91% efficiency motor

= 11/0.91 = 12.08 units

Reduction in electricity cost for 91% efficiency motor per

month = (X-Y)*600*0.1= $31.2

Hence payback of incremental cost is less than 2 months.

59

3.9 CONCLUSION

This chapter is a comparative analysis of performance

characteristics of 2.2 kW, 3-phase 2 pole TEFC induction motor and practical

verification using conventional Die-cast Aluminium Rotor with proposed Die-

cast Copper Rotor. The equivalent circuit parameters of an induction motor is

obtained from No load and Blocked rotor tests are fed to Matlab 7 software to

draw the performance characteristics of both DAR and DCR motors. The

experimental results are compared with the calculated results. Both the results

are almost tallying with each other. It was found that by using DCR technique

the efficiency of 2.2 kW, 3-phase 2 pole TEFC induction motor is improved

by 2.6%. Similarly, performance comparison with DAR and DCR were

carried out between the ranges from 0.37 kW to 11 kW. By merely replacing

the DAR with DCR (without change in the other parameters), the efficiency

of the motor were proven to improve by 1.5% to 3% and the average

efficiency improvement of 2.5% is achieved.