# Variable Speed Induction Motor Drives Biology Essay

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This report describes the experiment in detail that was performed in the Power Electronics Drives laboratory to understand the basic principle and working of variable speed induction machine. It explains the performance of such drive at different load torques with the help of graphs and recorded measurements. Soft start and regenerative breaking are also discussed later in this report. At the end conclusion is made on the basis of all the measurement results.

Induction machines are widely used in the industry for performing various tasks. These machines are usually variable speed drives. The main reason is that, most of the tasks in the industry are so critical that the entire setup cannot afford shut down of any critical process due to the failure of the drives. These drives are when started draws up to six times of their rated current at full rated voltage which reduces to the rated value as the machine reaches maximum speed. This huge current at the start could damage the drive and hence in order to protect it a variable frequency drive operation (VFD) is implemented. The other reason for the use of variable speed induction drive is because it can run at any desirable speed up to the rated speed. The method used to control the speed of induction motor is discussed briefly as below

## Figure (1) Variable Frequency Drive Controller

Left hand side is the three phase rectifier where the three phase input is applied. This three phase input is converted to the dc which is applied to the three phase inverter. Here the PWM signal is applied at gates of the switches in the inverter. By controlling the frequency of the modulating signal of the PWM the output frequency of the inverter can be adjusted similarly in order to adjust the output amplitude of the inverter the modulating index is adjusted accordingly. The machine whose variable speed operation is desired is connected at the output of the inverter as shown in the figure above. In order to change the starting and maximum torque at low speed the frequency and voltage ratio is kept constant. However in order to get the optimum performance out of your drive at low frequency operation the voltage and frequency is no longer kept constant instead it is adjusted accordingly.

## EXPERIMENT PROCEDURE

The main objective of the experiment was to understand the operation of induction machine drive at variable speed and its performance at different load torques. To achieve it we used MOD SIM used for adjustment of Torque, MOD MECA used for the measurement of Torque, Speed Power, MOD ELEC used for the measurement of a single phase current voltage and finally DIGIDRIVE which was variable speed controller that accepted single phase input and provided three phase output. This equipment was connected to an induction machine drive with the ratings as three phase, four pole and power of 1.5kW. In order to apply the load at the machine, a balanced-mounted magnetic powder brake of 1.5 kW was used. For the measurement of line to line voltage, phase voltage and current 20MHz digital oscilloscopes was used. After setup of the equipments three tests were performed. These tests are at zero mechanical braking torque and non-zero mechanical torque. For non-zero mechanical torque further two tests constant and variable load were performed. Each one is discussed in detail below.

## ZERO MECHANICAL BRAKING TORQUE

After the setup of the equipment as discussed earlier the speed controller on DIGIDRIVE was set to 50%, 60%, 70%, and 90% and then phase to neutral, line to line voltages and phase current waveform were monitored. Following are the recorded measurements

## RESULTS AND DISCUSSION

% Speed Setting

Torque (Nm)

Speed (rev/min)

Vrms (V)

Irms (A)

fm (HZ)

Slip

Three Phase Pin(W)

Pout (W)

Power Factor

% Efficiency

50

0.7

749

122

0.9

25

0.0013

90

52

0.68

57.5

60

0.7

899

131

0.9

30

0.001

102

62

0.58

60.7

70

0.6

1045

139

0.9

35

0.004

114

70

0.4927

61.4

90

0.7

1348

155

0.9

45

0.001

144

91

0.32

63.19

## Figure (2) Measurements at Zero Mechanical Breaking Torque

If we observe the power factor calculated for various measurements in the above table, it appears that it is very low. Since there was no load attached to the motor for these sets of measurements hence the reactive power had much higher influence than the real power. This is the reason for low power factor. The efficiency was noted to be very low. This was mainly due to the power losses that were due to frictional losses.

The frequency modulation index is calculated as follows

Number of pulses - fm = fs

27-2 -50 = 3 KHz

Therefore fs = 3 KHz

Modulation Index (mf) = fs/fm

The following figure shows calculated modulation index against the different modulating frequencies.

fm (Hz)

fs (KHz)

Modulation Index ( mf)

25

3

120

30

3

100

35

3

85.71

45

3

66.66

## Figure (3) Frequency Modulation Index at Zero Mechanical Breaking Torque

The following figure shows calculated values of voltage and frequency ratio tabulated below

Voltage (Vrms)

Frequency (fm)

V/f

122

25

4.88

131

30

4.3

139

35

3.97

155

45

3.4

## Figure (4) Voltage to Frequency Ratio at Zero Mechanical Breaking Torque

The recorded waveforms for the line to line voltages, phase voltages and currents are displayed below along with their corresponding values at different speed settings.

## Figure (5) Speed Setting=50 1. Line To Line Voltage, 2. Phase Voltage, 3. Phase Current

The measured values from the above graphs are

RMS Current 0.9A

RMS Phase Voltage 122

Peak AC Voltage -122 = 172.53V

Line to Line Voltage -172.53 = 298.8V (STAR)

## Figure (6) Speed Setting=60 1. Line To Line Voltage, 2. Phase Voltage, 3. Phase Current

The measured values from the above graphs are

RMS Current 0.9A

RMS Phase Voltage 131

Peak AC Voltage -131 = 185.26V

Line to Line Voltage -185.26 = 320V (STAR)

## Figure (7) Speed Setting=70 1. Line To Line Voltage, 2. Phase Voltage, 3. Phase Current

The measured values from the above graphs are

RMS Current 0.9A

RMS Phase Voltage 139

Peak AC Voltage -139 = 196.57V

Line to Line Voltage -196.57 = 340.47V (STAR)

## Figure (8) Speed Setting=90 1. Line To Line Voltage, 2. Phase Voltage, 3. Phase Current

The measured values from the above graphs are

RMS Current 0.9A

RMS Phase Voltage 155

Peak AC Voltage -155 = 219.20V

Line to Line Voltage -219.20 = 379.37V (STAR)

The speed control of induction machine can be obtained by varying the stator frequency and voltage by the same ratio if maximum torque is to be kept constant. This was achieved in the laboratory with the help of adjustment of the modulation index of the inverter which is tabulated above in the figure 4.

## NON ZERO MECHANICAL BRAKING TORQUE

This section is further classified in to the following two types of tests. Each one is discussed in detail below.

In this test the POT B was adjusted to get the mechanical braking torque of 1Nm, 2Nm and 5Nm against the speed settings of 50, 60, 70, 80, 90 and 100. For every single value of torque the speed, modulating frequency, rms supply voltage, current, power and torque values were recorded which are tabulated later in the report.

% Speed Setting

Torque (Nm)

Speed (rev/min)

Vrms (V)

Irms (A)

fm (HZ)

Slip

3 Phase Pin (W)

Pout (W)

% Efficiency

50

0.6

749

122

0.9

25

0.0013

78

44

56.41

50

1.5

737

122

1.1

25

0.017

150

114

76

50

5

664

122

2.7

25

0.11

519

346

66.6

60

0.6

899

130

0.9

30

0.001

93

56

60.21

60

1.5

885

130

1.1

30

0.016

177

139

78.53

60

5

815

130

2.7

30

0.09

585

421

71.96

70

0.6

1045

139

0.9

35

0.004

108

62

57.4

70

1.5

1040

139

1.1

35

0.009

204

158

77.457

70

5

970

139

2.7

35

0.07

675

503

74.51

90

0.6

1348

155

0.9

45

0.001

135

86

63.7

90

1.5

1343

155

1.2

45

0.005

258

211

81.78

90

5

1278

155

2.7

45

0.05

802

661

82.41

100

0.6

1480

157

0.9

49.5

0.003

144

87

60.41

100

1.5

1475

157

1.2

49.5

0.006

285

226

79.29

100

5

1412

157

2.7

49.5

0.04

876

735

83.9

## Figure (9) Measurements at Non-Zero Mechanical Breaking Torque (Constant Load)

For the above table it can be seen that the values of current for different speed setting are same. The reason for this is that since the torque is constant for the different values of speed setting hence the current remains the same as well. If the table is further analyzed it can be seen that power calculated was different and not the same this was because the speed of the motor was different for different speed settings.

The torque speed characteristics of the motor at different speed setting are plotted below against the above tabulated data.

Following graph represents the torque speed characteristics for the zero breaking torque. This graphs clearly indicates that higher the torque, the lesser will be the speed. Five plotted results in the following figure were plotted at different values of frequency and voltage ratios.

## Figure (10) Measurements at Non-Zero Mechanical Breaking Torque (Constant Load)

The torque current characteristic of the tabulated data for different speed settings was same. It is displayed below

## Figure (11) Measurements at Non-Zero Mechanical Breaking Torque (Constant Load)

In this section the MOD' SIM was set to T = A-n+B. On the other hand POT A was set to 0% and POT B to 10%. Following are the recorded values.

## RESULTS AND DISCUSSION

For this test the recorded values for torque, speed, rms voltage, rms current, output power and efficiency are tabulated as below.

% Speed Setting

Torque (Nm)

Speed (rev/min)

Vrms (V)

Irms (A)

fm (HZ)

Slip

3 Phase Pin (W)

Pout (W)

% Efficiency

20

0.1

294

94

0.9

10

0.02

39

20

51.28

30

0.9

442

105

0.9

15

0.01

63

43

68.25

40

1.2

592

114

1

20.1

0.01

102

73

71.56

50

1.6

735

123

1.2

25

0.02

159

123

77.35

60

2.1

876

131

1.3

29.9

0.02

234

187

79.91

70

2.5

1023

139

1.5

35

0.02

327

266

81.34

80

3.2

1161

146

1.7

39.9

0.03

450

380

84.44

90

3.7

1307

152

2

45.1

0.03

603

508

84.24

100

4.4

1439

153

2.4

50

0.04

735

653

88.84

## Figure (12) Measurements at Non-Zero Mechanical Breaking Torque (Variable Load)

Following are the torque speed characteristics for the tabulated data above for the variable load test.

## CONCLUSION

It is concluded briefly in the end that in order to get the same slip speed but different starting torques the voltage and frequency ratio applied at the input must remain same. For a constant load test the results for torque speed characteristics remains fairly linear. It was also observed in the laboratory that for the variable load the torque speed characteristics do not remain linear instead the behavior changes as displayed in figure 13. In non-zero mechanical braking torque tests with constant load the torque was observed to be constant due to constant current similarly power was no constant because of speed difference. In the first test the efficiency was observed to be very low and that too was due to friction and other internal losses.

## REFRENCES

Electric Motors and Drives, 1990 by Austin Hughes

Electric Machines, Drives and Power Systems, fifth edition by Theodore Wildi