ADJUSTABLE SPEED INDUCTION MOTOR DRIVE

INSTRUCTED BY: Dr. D. P. Chandima

NAME:A.G.N.Bandara

INDEX NO: 090041G

FIELD: EE

GROUP: G2

DATE OF PER: 3rd June 2013

DATE OF SUB: 17th June 2013

Motor Input Waveform:

Measurement of speed control characteristics

1. The variation of Nr (Rotor Speed) with fs (Inverter Frequency).

Speed-Input potentiometersettingInverter Frequencyfs (Hz)Rotor SpeedNr (rpm)

11.170.00

22.5070.00

34.33128.20

46.17180.70

58.00231.40

610.00286.50

713.17375.10

825.50741.20

942.831242.70

1050.001463.80

2. The variation of inverter output current (Is) with inverter frequency (fs)

Speed-Input potentiometerInverter FrequencyInverter Output Current

settingfs (Hz)Is (A)

11.170.80

22.501.10

34.331.10

46.171.00

58.000.80

610.000.70

713.170.65

825.500.80

942.830.90

1050.000.90

3. The variation of inverter output power (Ws) with inverter frequency (fs)

Speed-Input potentiometerInverter FrequencyInverter Output Power

settingfs (Hz)Ws (W)

11.1720

22.5020

34.3320

46.1720

58.0020

610.0020

713.1720

825.5060

942.83120

1050.00140

4. The variation of the ratio between inverter output voltage (Vs/fs) with inverter frequency (fs)

Speed-Input potentiometersettingInverter Frequencyfs (Hz)Ratio between Inverter OutputVoltage and Inverter FrequencyVs/fs (V/Hz)

11.170.00

22.508.00

34.336.92

46.175.84

58.004.75

610.003.80

713.173.19

825.503.29

942.833.22

1050.003.16

Measurement of speed control characteristics with voltage control

1. The variation of rotor speed (Nr) with the inverter output voltage (Vs)

Voltage feedback potentiometer settingInverter Output VoltageVs (V)Rotor SpeedNr (rpm)

180728.60

280729.30

3102736.80

4118740.00

5132742.10

6146743.80

7160745.00

8178745.90

9190746.40

2. The variation of inverter output current (Is) with the inverter output voltage (Vs)

Voltage feedback potentiometer settingInverter Output VoltageVs (V)Inverter Output CurrentIs (A)

1800.70

2800.70

31020.80

41180.90

51321.00

61461.15

71601.30

81781.50

91901.65

3. The variation of inverter output power (Ws) with the inverter output voltage (Vs)

Voltage feedback potentiometer settingInverter Output VoltageInverter Output Power

Vs (V)Ws (W)

18060

28060

310240

411860

513260

614680

716080

8178100

9190100

01. What is the motive of keeping v/f constant with adjustable speed drivesBy controlling the supply frequency of the induction motor smoothly, the synchronous speed can becontrolled over a wide range. This gives smooth speed control of an induction motor. But the expression for the air gap flux is given by,

It can be seen from this expression that if the supply frequency f is changed, the value of air gap flux also gets affected. This may result into saturation of stator and rotor cores. Such a saturation leads to the sharp increase in the (magnetisation) no load current of the motor. Hence it is necessary to maintain air gap flux constant when supply frequency f is changed.To achieve this, it can be seen from the above expression that along with f, V also must be changed so as to keep (V/f) ratio constant. This ensures constant air gap flux giving speed control without affecting the performance of the motor.

02. Why not V is increased with f for speeds above the rated speed and how this affects the motor torqueWith supply voltage equal to its rated value depending on the motor design, beyond a speed somewhere in a range of 1.5-2 times the rated speed is reduced so much that the motorapproaches its pull-out torque. At still higher speeds, the motor can deliver only a fixed percentage of the pull-out torques power becomes constant. Both the torque and the motor current decline with speed.Keeping V, constant, the motor torque in this region is not limited by the current handlingcapability of the motor, since the current at the limit is less than its rated value anddeclines with speed rather, it is limited by the maximumtorque produced by the motor.

Torque Speed Characteristics for higher speeds

03. Compare the degree of speed controllability with frequency control and voltage control.By using variable frequency control, it is possible to adjust the speed of the motor either above or below base speed. A properly designed variable frequency induction motor can control the speed over a range from as little as 5% of base speed up to about twice base speed.

Variable Speed by Frequency control

The torque developed by an induction motor is proportional to the square of the applied voltage. Therefore the speed of the motor may be controlled over a limited range by varying the line voltage. This method is sometimes used on small motor driving fans.

Variable Speed by Voltage Control04. Comment on the motor voltage and current waveforms

Both current and voltage waveforms obtained has a frequency of 40 Hz. Voltage wave form has taken considerably the shape of a square waveform. Which is the theoretical output of the six step voltage source inverter. Peaks at the rising edges and dips at the falling edges due to inductance are clearly visible. Noise can be observed in the waveform. Current output is also obey to the theory excluding small deviation. According to the theory the curvy shape of the waveform is generated by inductances. Those inductances are not perfect therefore the waveform is somewhat deviated from the expected. And the waveform is also not perfect due to harmonics.

05. Outline the salient features of control circuit giving reasons for use of different potentiometersIn the slow-up/slow-down circuit two potentiometers VR1 and VR2 are used to vary the rate of change of speed input signal.VR1 potentiometer is used to adjust the ratio of the voltage feedback.VR2 potentiometer adjusts the control signal transient. That control signal compared with the current feedback from the output of the converter.VR3 is used to adjust the current feedback ratio of N-1 amplifier circuit. This is for stability reasons and protection purposes.Before feeding to the comparator VR5 is used to adjust the final signal range. By adjusting these potentiometers the dc output of the converter can be distinguished without varying the frequency.