Power electronics Viva voce or interview questions part-9

 Q-201 On what basis choppers are classified in quadrant configurations?

Ans The choppers are classified depending upon the directions of current and voltage

flows. These choppers operate in different quadrants of V-I plane. There are

broadly following types of choppers: class a chopper (first quadrant); class B

(second quadrant) Class C and class D (two quadrant choppers), class C in II

quadrant and I whereas class D in IV quadrants, and I class E is four quadrant

operator.

Q-202 What are different control strategies found in choppers?

Ans The different control strategies are pulse width modulation, frequency

modulation and current limit control, variable pulse width and frequency.

Q-203 Explain the principle of operation of a chopper?

Ans A chopper acts as a switch, which connects and disconnects the load, hence

producing variable voltage.

Q-204 What are the advantages of DC choppers?

Ans * High ripple frequency, so small filters are required.

*Power factor is better.

*Efficiency is better.

*Small and compact.

*The dynamic response of choppers is fast due to switching nature of the

device.

Q-205 Define duty cycle.

Ans The duty cycle of chopper controls its output voltage. The value of duty cycle

lies between 0 and 1 and is given by Ton/(Ton+Toff).

Q-206 How can ripple current be controlled?

Ans Ripple current is inversely proportional to the frequency and hence can be

controlled by having higher frequency.

Q-207 What is step up chopper?

Ans If the output average voltage is greater than the supply voltage, then the chopper

is called step up chopper.

Q-208 On what does the commutating capacitor value depend on?

Ans It depends on the load current.

Q-209 What are the disadvantages of choppers?

Ans *They can operate only at low frequencies.

*The commutation time depends on the load current.

*The output voltage is limited to a minimum and maximum value beyond which

we cannot get the output voltage.

Q-210 How do they have high efficiency?

Ans DC choppers uses switching principle, hence they have high efficiency.

Q-211 What are the applications of dc choppers?

Ans Battery operated vehicles, switched mode power supplies, traction devices,

lighting and lamp controls, trolley cars, marine hoists, and forklift trucks. Mine

haulers etc.

Q-212 What is principle of dc motor?

Ans An electric motor is a machine, which converts electrical energy into mechanical

energy. Its action is based on the principle that when a current carrying

conductor is placed in a magnetic field it experiences a mechanical force whose

direction is given by flemings left hand rule and whose magnitude is given by

F=BIL When the field magnets of a multipolar dc motor are excited and its

armature conductors are supplied with current from supply mains they

experience a force tending to rotate the armature .By Fleming’s left hand rule it

is noted that each conductor experiences a force which tends to rotates the

armature in anticlockwise direction. These forces collectively produce a driving

torque (or twisting moment), which sets the armature rotating.

Q-213 How can the speed of the series motor controlled?

Ans *flux control method

-field divertors

-Armature divertor

*variable resistance in series with the motor.

Q-214 What are the advantages of field method?

Ans *economical,more efficient

*It gives speeds more /above the normal speed.

Q-215 What arethe disadvantages of field method?

Ans Commutation becomes unsatisfactory.

Q-216 What are the factors controlling speed?

Ans Speed can be controlled by controlling flux, resistance, voltage.

Q-217 What is the significance of back emf?

Ans When the motor armature rotates the conductors also rotates and hence cut flux.

Therefore emf is induced and direction is in opposition with the applied voltage

(Fleming’s right hand rule). Because of its opposing direction it is referred to as

back emf Eb. V has to drive Ia against the opposition of Eb.The power required

to overcome this opposition is EbIa.

Q-218 What is torque?

Ans Torque is twisting or turning moment of a force about an axis. The torque

developed by the armature of a motor is armature torque. The torque available

for useful work is known as shaft torque (available at the shaft).

Q-219 How can dc motors be classified?

Ans separately excited

*self-excited.

Q-220 What are the main losses in motors?

Ans *stator losses

*rotor losses

 *mechanical losses

Q-221 Why are starter used in dc motor?

Ans Initially Eb =0 and R is usuallly very small,therefore the armature current is very

high which could damage the motor.Hence starters which is basically a

resistance connected in series with the motor.

Q-222 What is the parameter that is being varied by varying the firing angle?

Ans The armature voltage is varied which inturn varies the speed of the motor by

varying the firing angle.

Q-223 What are the operating modes of dc motor?

Ans Motoring, regenerative braking, dynamic braking, plugging.

Q-224 Why is it called TRIAC and what are its applications?

Ans The word TRIAC is abbreviated from TRI from Triode and AC. The TRIAC is

the best device for AC phase control. The input and load is AC. The power is to

be controlled in positive as well as negative half cycles. The TRIAC is triggered

in every half cycle. Thus, the necessity of anti-parallel SCR’s is eliminated by

TRIAC. Thus, it is used mainly in AC power controllers, heater, fan, triggering

device for SCR’s.

Q-225 How many terminals does a TRIAC have?

Ans A TRIAC has three terminals, main terminal (MT 1), and main terminal (MT 2),

gate (G).