Mumbai University > Electronics Engineering > Sem7 > Power Electronics 2

Marks: 10M

Year: May15, Dec12, May11

1) In this type of control, stator voltage is varied to vary the speed of induction motor. The stator frequency is maintained constant.

2) Equivalent circuit per phase of Induction Motor

• $R_1$ and $jX_1$ are the phase values of stator resistance and stator leakage reactance.$E_1$ is the per phase stator voltage and N_1 the number of stator turns per phase.
• $Ro$= Core loss
• Xo = magnetizing component of no load stator current

• $R_1^2 \ \ and \ \ X_1^2$ are the values of rotor resistance and reactance transferred to the stator side

  $I_2^1=\frac{E_1}{\big[(R_1+R_2^1/S)^2+(X_1+X_2^1)^2\big]^ \frac{1}2}$

  The Rotor copper per phase= $\big(I_2^1\big)R_2^1$

  Total Rotor copper loss= $\big(I_2^1\big)R_2^1$

  $P_c=(w_s-w)T \\ P_c=sW_{sT} \\ \therefore P_c=(I_2)^1R_2^1 \times 3 \\ sW_{sT}=\frac{E_1^2 \times R_2^1 \times 3}{\big[(R_1+R_2^1/S)^2+(X_1+X_2^1)^2\big]}$

  Above is the relation between motor torque and slip ‘S’

  It indicates that torque is proportional to the square of the stator voltage.

  Therefore as $E_1$ decreases then $T_1$ also decreases.

3) $E_1 = bE_1 ( b \lt 1)$

$T=\frac{3E_1^2R_2^1}{sW_s\big[(R_1+R_2^1/S)^2+(X_1+X_2^1)\big]^2}$

Torque speed Char. With variable STATOR VOLTAGE

• The point of intersection with load torque line define the stable operating point i.e. the point at which the Motor torque = Load torque

• N1 > N2 (speed)

• In this way by varying the stator voltage it is possible to vary the motor speed. With increase in the stator voltage , the motor speed will increase.

4) Implementation of stator voltage control

5) operation

• In an A.C. regulator circuit, SCR are connected in antiparallel manner in 3phase of the incoming line.

• The firing angle of these thyristors is changed to vary the stator voltage applied to the motor.

• Stator voltage can be varied in stepless manner from zero value to full value of supply

6) Disadvantages

• Due to change in firing angle ‘α’, the output voltage waveform is distorted and it will be rich in harmonics.

• Motor heating will take place

• Produces very slow starting torque

• Not suitable for constant torque load.