1. Attempt the following objective type questions:

In a single phase full converter, for continuous conduction, each pair of SCRs conduct for

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The minimum current required by a thyristor for turn on is known as

latching current

holding current

continuous current

None of the above

The frequency of the ripple in the output voltage of a 3-phase semi-converter depends on

firing angle and load resistance

firing angle and load reactance

the supply frequency

firing angle and supply frequency

A converter which can operate in both 3-pulse and 6-pulse mode is a

I-phase full converter

3-phase half converter

3-phase semi-converter

3-phase full converter

In dc choppers, the waveforms for input and output voltages are respectively,

discontinuous, continuous

both continuous

both discontinuous

continuous, discontinuous

A chopper can be used on

PWM only

FM only

AM only

both PWM and FM

In V/f control method, an induction motor drive operates in

constant power mode

constant torque mode

None of the above

both and

2. A 200 875 rpm, 150 A separately excited dc motor has an amature resistance of 0·06 ohm. It is fed from a single phase fully controlled rectifier with an ac source voltage of 220 50 Hz. Assuming continuous conduction, calculate:

Firing angle for rated motor torque and 750 rpm.

Motor speed for a 160° and rated torque.

3. Explain the operation of a single phase fully controlled converter fed dc separately excited motor. Draw the wave shapes corresponding to continuous conduction. Also derive the expression for output voltage.

4. Explain the operation of 3-phase fully controlled converter connected to dc separately excited motor. Draw the wave shapes for a 30° and derive the expression for speed.

A 230 960 rpm and 200 A separately excited dc motor has an armature resistance of 0·02 ohm. The motor is fed from a chopper. Calculate the duty ratio of the chopper, if the motor is running at 350 rpm.

Describe the operation of a chopper fed dc series motor. Draw the speed -torque characteristics for increasing duty ratio

6. How is variable frequency control of induction motor obtained

Voltage source inverter?

Current source inverter?

7. Write short notes on any two of the following:

Closed loop operation of multi-motor drive.

PWM control of induction motor.

Four quadrant chopper fed separately excited dc motor.