Adjustable Speed Induction Motor Drive

Name : S.P.M Sudasinghe

Index No. : 100523G

Group : G-17

Date of Performance : 2014/06/16

Date of Submission : 2014/07/07

EE4092

Laboratory Practice

VII

Instructed By: Dr. D.P Chandima

OBSERVATIONS

Name : S.P.M Sudasinghe

Index No. : 100523G

Group : G-17

Date : 2014/06/16

Instructed by : Dr. D.P Chandima

b) Measurement of speed control characteristics

Speed-input potentiometer setting

VDC (V) VS (V) IS (A) WS (W) Fs (Hz) Nr (rpm)

VS/fS (V/Hz)

0 12 0 0 20 2.0 0 0 1 16 0 0.2 40 0.02 75 0 2 24 0 0.5 40 2.5 78 0 3 36 0 0.5 60 4.7 139 0 4 44 40 0.5 60 6.2 184 6.45 5 49 40 0.5 60 8.3 234 4.82 6 56 44 0.5 60 10.1 301 4.35 7 72 52 0.5 60 13.7 387 3.79 8 132 104 1 80 25.2 611 4.12

c) Measurement of speed control characteristics with voltage control

Voltage feedback potentiometer setting

VS (V) VDC (V) Nr

(rpm) IS (A) WS (W)

1 108 132 731.2 1 80 2 128 136 735.2 1 80 3 144 180 737.4 1.1 100 4 160 200 738.6 1.2 100 5 176 220 739.5 1.5 120 6 184 228 739.9 1.7 140

d) Measurement of motor input waveforms

b) Measurement of speed control characteristics

Speed Vs Inverter frequency

fs (Hz) Nr (rpm)

2.0 0

0.02 75

2.5 78

4.7 139

6.2 184

8.3 234

10.1 301

13.7 387

25.2 611

Graph 1: Motor speed Vs Frequency (frequency control)

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Nr

(rp

m)

Fs (Hz)

Motor Speed Vs Inverter Frequency

VS/fS Vs Inverter Frequency

fS (Hz) VS/fS (V/Hz)

2.0 0

0.02 0

2.5 0

4.7 0

6.2 6.45

8.3 4.82

10.1 4.35

13.7 3.79

25.2 4.12

Graph 2: Vs/fs Vs Inverter Frequency (frequency control)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

0 2 4 6 8 10 12 14 16 18 20 22 24 26

VS/

f S(V

/Hz)

fS (Hz)

VS/fS Vs Inverter frequency

IS Vs Inverter Frequency

fS (Hz) IS (A)

2.0 0

0.02 0.2

2.5 0.5

4.7 0.5

6.2 0.5

8.3 0.5

10.1 0.5

13.7 0.5

25.2 1.0

Graph 3: Motor current Vs Inverter frequency (frequency control)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 2 4 6 8 10 12 14 16 18 20 22 24 26

I S(A

)

fS (Hz)

Motor Current Vs Inverter Frequency

Motor Power Vs Inverter Frequency

fS (Hz) WS (W)

2.0 20

0.02 40

2.5 40

4.7 60

6.2 60

8.3 60

10.1 60

13.7 60

25.2 80

Graph 4: Motor power Vs Frequency (frequency control)

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

0 2 4 6 8 10 12 14 16 18 20 22 24 26

WS

(W)

fS (Hz)

Motor power Vs Inverter frequency

c) Measurement of speed control characteristics with voltage control

Motor speed Vs Inverter output voltage

VS (V) Nr (rpm)

108 731.2

128 735.2

144 737.4

160 738.6

176 739.5

184 739.9

Graph 5: Motor speed Vs Inverter output voltage (Voltage control)

731

732

733

734

735

736

737

738

739

740

100 110 120 130 140 150 160 170 180 190

Nr

(rp

m)

VS (V)

Motor Speed Vs Inverter output voltage

Motor current Vs Inverter output voltage

VS (V) Nr (rpm)

108 1.0

128 1.0

144 1.1

160 1.2

176 1.5

184 1.7

Graph 6: Motor current Vs inverter output voltage (voltage control)

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

100 110 120 130 140 150 160 170 180 190

I S(A

)

VS (V)

Motor current Vs Inverter output voltage

Motor power Vs Output voltage

VS (V) WS (W)

108 80

128 80

144 100

160 100

176 120

184 140

Graph 7: Motor power Vs Inverter output voltage (voltage control)

60

70

80

90

100

110

120

130

140

150

160

100 110 120 130 140 150 160 170 180 190

WS

(W)

VS (V)

Motor power Vs Inverter output voltage

DICUSSION

1. What is the motive for keeping V/f constant with adjustable speed drives

Induced emf (E) of an induction motor is proportional to frequency and the air gap flux. As the

stator resistance is negligible induced emf is almost equal to the supply voltage (VS).

If the frequency id reduced while keeping the supply voltage constant then air gap flux increases

and core gets saturated and magnetization current increases. Also torque of the motor depends

on air gap flux. If the frequency increases keeping the voltage constant then torque will be reduced.

Hence air gap flux should be kept constant of an induction motor. To do that V/f is kept constant.

2. Why not V is increased with f for speeds above the rated speed, and how this affects the

motor torque.

At rated speed supply voltage is at its rated value. All the parameters of the motor are designed

for this rated voltage level such as insulation level. If we exceed this voltage limit motor may be

get damaged due to insulation failures or similar problems. Hence we keep the v/f constant up to

the rated speed (rated voltage) only and beyond this voltage only the frequency is increased. If

only the frequency is increased then air gap flux gets reduced because

Due to this reduction in air gap flux, motor torque is reduced while the speed (frequency) is

increasing. Hence beyond the rated voltage power is kept constant.

Figure 1: Frequency control of induction motor

3. Compare the degree of speed controllability with frequency control and voltage control

With frequency control speed of the induction motor can be adjusted either above or below the

rated speed. Frequency controlling method can change the motor speed from about 5% of rated

speed up to as twice the rated speed. It can achieve the speeds beyond the rated speed but with

less torque at constant power.

Figure 2: Frequncy control characteristics of an induction motor

With voltage control of induction motor only limited range of speeds are achievable. As the torque

of the induction motor is proportional to the square of the voltage, torque decreases rapidly for

lower speeds. This method cannot used for go beyond the rated speed.

Figure 3: Voltage control characteristics of an induction motor

4. Comment on the motor voltage and current waveforms.

Both current and voltage waveforms have the frequency of 40Hz. Voltage waveform is nearly a

square waveform. There is a phase shift between current and voltage waveforms. Current wave

from is not a pure sine waveform because it includes some harmonics.

5. Outline the salient features of control circuit giving reasons for the use of different

potentiometers

Slow-up/slow-down circuit

VR1 & VR2 -: adjust the rate of change of speed input signal

N-1 error amplifier

VR1 -: adjust the voltage feedback ratio

VR2 -: adjust the control signal transient of the error signal of voltage feedback and the reference

VR3 -: adjust the current feedback ratio

VR4 -: adjust the control signal transient of the error between the current feedback and output of

the previous voltage feedback error signal.

VR5 -: adjust the final signal range and feed the error signal to comparator

graphs.pdfpractical.pdf