# Objective Questions on Electrical Drives | 1

1. Which braking is not possible in series motor?

In case of regenerative braking back emf is greater than the supply voltage. In series motor back emf cannot exceed the supply voltage. So regenerative braking is not possible.

2. Polarity of supply voltage is reversed in which type of braking?

The polarity of supply voltage is reversed while motor is running, the type of braking is called counter electric current braking /plugging. In this case direction of rotation is reversed and direction of motor torque is also reversed.

3. In which braking back emf exceeds supply voltage?

When back emf > suplly voltage machine acts as a generator. This phenomenon is called regeneration and this form of braking is called regenerative braking.

4. In industries which electrical braking is preferred?

Usually braking means converting K.E to electrical energy and then to heat energy but in case of regenerative braking the K.E converts to E.E and then feeding back to supply but it is not possible to feeding back to supply because of power electronic devises so that E.E will be charge into capacitors. Hence it is advantage to industries.

5. Full form of VVVF control

VVVF stands for variable voltage variable frequency control. The effect of VVVF speed control is to retain the shape of the torque-speed curve, but shift it along the speed axis.For VVVF control, because the shape of the torque-speed curve is the same at all frequencies, it follows that the torque of an induction motor is the same whenever the slip speed (rpm) is the same. With VVVF control, the speed range possible is from about 10 % to 150 % of rated speed

6. The heating time constant of an electrical machine gives an induction of its

The heating time constant of an electrical machine gives an induction of its cooling.

7. What type electric drive is used in cranes?

In multimotor drive system there are several drives, each of which serves to actuate one of the working parts of the driven mechanism.

8. An elevator drive is required to operate in

Elevator drive should have four quadrant operation.
Elevator drives operate in four quadrants -
Forward motoring ( 1st quadrant )
Forward braking ( 2nd quadrant )
Reverse motoring ( 3rd quadrant )
Reverse braking ( 4th quadrant )

9. The slip of an induction motor during DC rheostatic braking is

The rheostatic braking with a ployphase induction motor can be obtained by disconnecting the stator winding from the ac supply and exciting it from a dc source to produce a stationary dc field. In rheostatic braking, the stator winding is employed as a dc field winding & rotor winding as an armature winding. While the machine is operating normally as a motor, its stator magnetic field is rotating at a synchronous speed in the same direction as that of the rotor, but slightly faster than the rotor conductors. When the stator windings are disconnected from the ac supply and excited with dc, the magnetic field produced will be stationary in space, thus making the rotor conductors move past the field with a speed (1-s)Ns or SNs. Here slip is ( Ns - Nr ) / Ns = Ns - Ns / Ns ⇒ slip = s

10. Electric drive is becoming more and more popular because

Electric drive is having following advantages.
1. It is simple, clean, compact, and reliable.
2. It provides easy and smooth control, flexibility in layout, easy starting and facility for remote control.
3. It is cheaper in initial as well as in maintenance cost.

11. The basic elements of a electric drive are

The system or motor which employed for motion control is called electric drive. Hence basic elements of an electric drive is electric motor and control system.

12. Speed control by variation of field flux results in

Variation of field flux results in variable torque and constant power drive operation.

13. A four quadrant operation requires

Full converter is operated in two quadrants. Hence in order to obtain four quadrants the drive requires another two quadrants, this will obtained by reversal of current. Reversal of electric current can be done by connecting two full converters in back to back.

14. A motor has a thermal heating time constant of 50 mm. When the motor runs continuous of full scale, its final temperature rise is 80° C, what would be the temperature rise after 1 hour, if the motor runs continuously on full load?

t / tH = 60 / 50 = 1.2, τ = 80 ( 1 - e-1.2) = 55.9°.

16. For an IM to operate in braking region slip should be always

For an IM to operate in braking region the rotor is rotated by means of a prime mover in the direction opposite to the rotation of air gap flux.
∴ Rotor speed is given as - Nr.
So slip = [ Ns - ( - Nr ) ] / Ns = ( Ns + Nr) / Ns. Now since ( Ns + Nr) is greater than Ns slip is always greater than 1.

17. High braking torque produced in

In plugging, in order to limit the armature electric current to a safe value an external electric current limiting resistor is connected thus armature electric current is reversed and high braking torque is produced.

18. In case of kiln drives, starting torque is.

In kiln drives starting torque is double than that of running torque. Hence for kiln drives ward Leonard controlled dc shunt motors, slip ring IM, cascade controlled ac motors is employed.

19. A 3 - Φ induction motor draws an electric current of 50 A from mains when starts by direct switching. If an auto transformer with 60 % tapping is used for starting the electric current drawn from the mains will be

K2 × Ist = ( 60 / 100 )2 × 50 = 18 A.

20. The power input to a 3 - Φ induction motor is 60 kW and stator loss is 1 kW the rotor cu loss per phase is

Stator input = 60 kW
Stator loss = 1 kW
Rotor input = 60 - 1 = 59kW
Rotor loss = slip X 59
rotor loss / phase = slip × 59 / 3.