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Power Electronics and Drives

PX 142 SOLID STATE AC DRIVES

Time : Three
hours Maximum : 100 marks

Answer ALL questions.

PART A (10 × 2 = 20 marks)

Draw the speed-torque characteristics of an induction
motor with variable stator voltage obtained by back to back thyristors
triggered at different triggering angle .
Why stator voltage control is suitable for speed
control of induction motors in fan and pump drives?
What are the two disadvantages of ac drives with
PWM inverters?
What are the advantages of ac drives with current
source inverter?
If is the resistance and 0.75 is the duty cycle
for the induction motor speed control using chopper, what is the effective
value of resistance ?
Draw the power transistor scheme of static rotor
resistance control. Assume it works as ON-OFF chopper.
What are the applications of slip power recovery
scheme induction motor drives?
State at least two modifications to improve the
power factor of the slip energy recovery schemes.
Give two advantages of margin angle control of
synchronous motor on variable frequency supply.
Compare cycloconverter fed synchronous motor with
dc link converter/inverter synchronous motor.

PART B (5 × 16 = 80 marks)

A 3 phase, star connected, 50 Hz, 4 pole induction
motor has the following parameters in ohms per phase referred to the
stator :



and .



The motor is controlled by the variable frequency control with a constant
Determine the following for an operating frequency of 15 Hz.



(i) The breakdown torque as a ratio of its value at the rated frequency
for motoring and braking.


(ii) The starting torque and rotor current in terms of their values
at the rated frequency.
(a) (i) Discuss the induction motor speed control
by variation of the stator voltage using solid state devices.
 (ii) Explain with necessary diagrams how their speed torque curves
are influenced by varying triggering angle.
 (iii) Draw and explain the operation of four quadrant ac voltage controller.

Or

(b) Explain the operation of induction motor fed by current source
inverter. Derive an expression for optimum slip which maximizes the
efficiency of an induction motor fed by a current source of variable
magnitude and fixed frequency.
(a) (i) Derive an expression for the torque T for
the rotor resistance control using the chopper scheme.
 (ii) A three phase, 400 V, 50 Hz, 960 rpm, six pole, star connected,
wound rotor induction motor has the following parameters per phase referred
to the stator :
   stator to rotor turns ratio is 2. The motor speed is controlled
by the static rotor resistance. The filter resistance is 0.01 The value
of the external resistance is chosen such that the brake-down torque
is obtained at standstill. Determine the value of external resistance.

Or

(b) (i) State the merits and demerits of rotor resistance control of
wound rotor induction motor. What are its applications?
 (ii) A three phase, four pole 50 Hz induction motor has a chopper controlled
resistance in the rotor circuit for speed control. Load torque is square
of the angular velocity. When the thyristor is ON, the torque is 30
N-m at a slip of 0.03. If compute the average torque and speed. The
motor develops a torque of 80% of ON torque when the thyristor is OFF.
The speed variation ranges down to 1200 rpm from synchronous speed.
Determine the ratio to give an average torque of 25 N-m.
(a) Explain with schematic diagram two methods
of super synchronous speed control of slip ring induction motor under
slip power recovery scheme. What are the advantages and disadvantages
of slip power recovery scheme?

Or

(b) With block diagram explain the closed loop operation of slip power
recovery scheme of induction motor. Discuss on the power factor and
locus diagram of supply current for constant torque operation of slip
power recovery scheme.
(a) Explain the working of a self controlled synchronous
mode fed from a three phase inverter. Why a self controlled synchronous
motor is free from hunting oscillations?

Or