Deck 1: Synchronous Motors: Part A

Question 1

Synchronous motors are generally not self-starting because

A)the direction of rotation is not fixed
B)the direction of instantaneous torque reverses after half cycle
C)startes cannot be used on these machines
D)starting winding is not provided on the machines

Answer 1

the direction of instantaneous torque reverses after half cycle

Question 2

In case one phase of a three-phase synchronous motor is short-circuited the motor will

A)not start
B)run at 2/3 of synchronous speed
C)run with excessive vibrations
D)take less than the rated load

Answer 2

not start

Question 3

A pony motor is basically a

A)small induction motor
B)D.C. series motor
C)D.C. shunt motor
D)double winding A.C./D.C. motor

Answer 3

small induction motor

Question 4

A synchronous motor can develop synchronous torque

A)when under loaded
B)while over-excited
C)only at synchronous speed
D)below or above synchronous speed

Question 5

A synchronous motor can be started by

A)pony motor
B)D.C. compound motor
C)providing damper winding
D)any of the above

Question 6

A three-phase synchronous motor will have

A)no slip-rings
B)one slip-ring
C)two slip-rings
D)three slip-rings

Question 7

Under which of the following conditions hunting of synchronous motor is likely to occur ?

A)Periodic variation of load
B)Over-excitation
C)Over-loading for long periods
D)Small and constant load

Question 8

When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected

A)the motor stops
B)it runs as a reluctance motor at the same speed
C)it runs as a reluctance motor at a lower speed
D)none of the above

Question 9

When V is the applied voltage, then the breakdown torque of a synchronous motor varies as

A)V
B)V^3/2
C)V²
D)1/V

Question 10

The power developed by a synchronous motor will be maximum when the load angle is

A)zero
B)45°
C)90°
D)120°

Question 11

A synchronous motor can be used as a synchronous capacitor when it is

A)under-loaded
B)over-loaded
C)under-excited
D)over-excited

Question 12

A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased

A)power factor as well as armature current will decrease
B)power factor as well as armature current will increase
C)power factor will increase but armature current will decrease
D)power factor will decrease and armature current will increase

Question 13

Mostly, synchronous motors are of

A)alternator type machines
B)induction type machines
C)salient pole type machines
D)smooth cylindrical type machines

Question 14

The synchronous motor is not inherently self-starting because

A)the force required to accelerate the rotor to the synchronous speed in an instant is absent
B)the starting device to accelerate the rotor to near synchronous speed is absent
C)a rotating magnetic field does not have enough poles
D)the rotating magnetic field is produced by only 50 Hz frequency currents

Question 15

As the load is applied to a synchronous motor, the motor takes more armature current because

A)the increased load has to take more current
B)the rotor by shifting its phase backward causes motor to take more current
C)the back e.m.f. decreases causing an increase in motor current
D)the rotor strengthens the rotating field casuing more motor current

Question 16

Synchronous motor always runs at

A)the synchronous speed
B)less than synchronous speed
C)more than synchronous speed
D)none of the above

Question 17

An over-excited synchronous motor takes

A)leading current
B)lagging current
C)both (a) and (b)
D)none of the above

Question 18

The working of a synchronous motor is similar to

A)gear train arrangement
B)transmission of mechancial power by shaft
C)distribution transformer
D)turbine

Question 19

The minimum armature current of the synchronous motor corresponds to operation at

A)zero power factor leading
B)unity power factor
C)0.707 power factor lagging
D)0.707 power factor leading

Question 20

In a synchronous motor, the magnitude of stator back e.m.f. £& depends on

A)d.c. excitation only
B)speed of the motor
C)load on the motor
D)both the speed and rotor flux

Question 21

If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is

A)2
B)3
C)4
D)6

Question 22

For V-curves for a synchronous motor the graph is drawn between

A)field current and armature current
B)terminal voltage and load factor
C)power factor and field current
D)armature current and power factor

Question 23

The back e.m.f. of a synchronous motor depends on

A)speed
B)load
C)load angle
D)all of the above

Question 24

A synchronous motor can operate at

A)lagging power factor only
B)leading power factor only
C)unity power factor only
D)lagging, leading and unity power factors

Question 25

In a synchronous motor which loss varies with load ?

A)Windage loss
B)Bearing friction loss
C)Copper loss
D)Core loss