MCQs on Electrical Machines

Page 27 of 114. Go to page
01․ Which type of motors are used for lathes?
DC shunt motors.
Cage induction motor.
Synchronous motor.
Either A or B.

Lathes machines requires uniform torque. Hence squirrel cage induction motor or DC shunt motors are preferred.

02․ Which of the following alternators contain large number of poles?
Turbo alternators.
Hydro alternators.
Aame in both alternators.
None of the above.

The alternators used in hydro power plants which have water turbine runs at below 500 rpm. It is designed by salient pole rotor alternator. For projection it has small synchronous speed. It has large diameter and small axial length. Therefore they contain large number of poles and run at slow synchronous speed.

03․ What is the reason for stationary armature essential in alternators above 5 KVA?
Insulation requirement is less.
Operating cost is less.
Collecting of large current is simple.
All of the above.

As the collection of large current, slip rings become impractical when armature rotates at high speed. At high rating armature requires heavy insulation and there are difficulties in rotation. So, rotating armature requires high operating cost, mechanically unstable, less operating speed, complicate design of conductors as well as insulation requirement is high. Rotating field system is lighter and can be rotated at very high speed. Therefore the alternators above 5 KVA are essentially designed at stationary armature structure.

04․ What is the highest possible speed of turbo alternators?
3000 rpm
1500 rpm
1000 rpm
4000 rpm

Steam turbines have good efficiency at higher speeds only. Therefore maximum speed of turbo alternators used in those power plants are 3000 rpm with two poles and next possible speed is 1500 rpm with 4 poles.

05․ For a standard frequency the machine operates at a particular speed known as
slip speed.
normal speed.
synchronous speed.
any of the above.

For a standard frequency the magnetic field rotates at a particular speed known as synchronous speed. If the rotor runs as the same speed of the rotating magnetic field then it is known as synchronous machine. The speed depends on number of poles and frequency.

06․ Which of the following is the largest conversion device?
DC machine
Induction machine
Synchronous machine
All of the above

As the power generation is through a grid (parallel operation) all these generators have to generate voltage at standard frequency. The conventional power generation is synchronous generators only. All the power generating plants universally adapt this generation of 3-phase ac power generation. Synchronous machine is the largest power energy conversion device as it has revolving field system. So it can be designed for up to 500 MVA.

07․ For the speed control of polyphase squirrel cage induction motor, if frequency of operation is decreased with
constant supply voltage V, starting torque Tst decreases
constant V, Tst increases
either of these
none of the above

Flux is proportional to voltage / frequency. So, if frequency is decreased and the supply voltage is constant, then flux will increase. Due to this starting torque will increase.

08․ Rotor resistance speed control is used in
squirrel cage induction motor.
slip ring induction motor.
dc shunt motor.
synchronous motor.

Rotor resistance control can be only done in slip ring motors. The external resistance to the rotor can be added only through the slip rings. When the motor is running at the constant load torque condition, as the rotor resistance increases the motor will react to the change and maintain the torque by increasing its slip which means decrease in its speed. In Squirrel cage I.M. there are rotor bars, so external resistance can't be added on the bars.

09․ For satisfactory performance of 3-phase 480V, 60 Hz induction motor, the supply voltage at 50 Hz should be equal to
480 V.
400 V.
420 V.
350 V.

For satisfactory performance, the V / f ratio must be constant.

10․ In a 3-phase induction motor, if the supply voltage and frequency are reduced by the same ratio, then slip at which maximum torque occurs is
more and maximum torque remains constant
more and maximum torque decreases
less and maximum torque decreases
less and maximum torque increases

The speed depends on the supply frequency i.e. If the frequency is reduced, speed also will reduce. This will increase the slip. Torque ᝠV/f As the reduction in voltage and frequency is in the same ratio, the torque remains constant but it will occurs at more slip and less speed.