# SSC JE synchronous motor & synchronous generator question

Ques 41. A 10 pole 25 Hz alternator is directly coupled to and is driven by 60 Hz synchronous motor then the number of poles in a synchronous motor is? (SSC-2016)

1. 24 poles
2. 48 poles
3. 12 Poles
4. None of the above

Explanation

Number of poles of alternator Pa = 10

F = 25 Hz (alternator)

F = 60 Hz (motor)

Then the number of poles of motor Pm =?

Since synchronous motor is directly coupled hence

Synchronous speed of an alternator = Synchronous speed of the motor

(120 x 25)/10 = (120 x 60)/ Pm

Pm = 24

Ques.42. A silent pole synchronous motor is operating at one-fourth full load if the field current is suddenly switched off, it would? (SSC-2016)

1. Run at super-synchronous Speed
2. Stop Running
3. Run at sub-synchronous Speed
4. Continue to run at synchronous speed

Answer.4. Continue to run at synchronous speed

Explanation

Silent pole synchronous motor runs either at synchronous speed or not at all. That is while running,  it maintains a constant speed. The speed is independent of load.

Ques.43. An alternator is supplying a load of 300 kW at a power factor of 0.6 lagging. If the power factor is raised to unity, How many more kW can the alternator supply? (SSC-2016)

1. 300 kW
2. 100 kW
3. 150 kW
4. 200 kW

Explanation

PF = active power/apparent power. So from question

0.6=300/apparent power.

Apparent power=500 Kva.

At unity power factor, active power=apparent power. So active power=500kw. So you will get an additional 200kw at unity power factor

Ques.44. The reactive power generated by a synchronous alternator can be controlled by? (SSC-2016)

1. Changing the alternator speed
2. Changing the field Excitation
3. Changing the terminal Voltage
4. Changing the prime mover input

Explanation

If alternator is Overexcited, it will deliver reactive power with the lagging current while in Underexcited, it absorbs reactive power with a leading current.

Ques.45.  Which of the following motor is not self-starting? (SSC-2016)

1. DC series motor
2. Slip ring Induction motor
3. Synchronous motor
4. Squirrel cage induction motor

Explanation

• At first synchronous motor has Stator supplied with 3 phase supply and rotor with dc supply.
• When dc supply is given to rotor, alternate poles form on the rotor.
• Because of the three-phase supply, the rotating Magnetic field will generate rotating torque at synchronous speed.
• Consider during the positive half cycle (positive torque), the N pole of the stator and S pole of the rotor is in front of each other then they will experience the force of attraction, and the tendency of the rotor will be to rotate in an anti-clockwise direction.
• Now consider, during the negative half cycle (Negative torque), the Pole of the stator will change to S. Then S pole of the stator and rotor will experience the force of repulsion and the tendency of the rotor will be to rotate in the clockwise direction.
• So combine effect of the whole cycle will result in zero torque. Thus due to a cancellation of torque during positive and negative half cycle synchronous motor can’t be self-starting.
• For the synchronous motor to be self-starting we prefer damper winding. or mechanical input for starting.

Ques.46. The synchronous impedance method of finding voltage regulation of an alternator is called the pessimistic method because (SSC-2015)

1. It is simplest to perform and compute
2. Armature reaction is wholly magnetizing
3. It gives regulation value lower than its actual found by direct loading
4. It gives the regulation value higher than its actual found by direct loading

Explanation:

The regulation calculated from the synchronous impedance method is higher than the actual value found by direct loading hence this method is called as the pessimistic method.

Ques 47. Hydrogen is used in large alternator mainly to (SSC-2015)

1. Reduce eddy current losses
2. Reduce distortion of the waveform
3. Cool the machine
4. Strengthen the magnetic field

Explanation:

# Why is hydrogen used for Alternator cooling?

Hydrogen is the least expensive, with less weight, high thermal conductivity, less density, and less viscosity. Less weight, less density & less viscosity attributes to its flow rate. High thermal conductivity helps in better heat exchange. Least expensive helps in balance sheets, more power in fewer investments.

In order to reduce the high temperature of the alternator hydrogen gas is used as a coolant. The coolant, Hydrogen gas is allowed to flow in a closed cyclic path around the rotor. Heat exchange takes place and the temperature of hydrogen gas increases, for better cooling of the rotor in the next cycle it has to be cooled. Cooling of hydrogen gas is done by passing it through heat exchangers generally constituted with water. Now Hydrogen gas after cooling is allowed to pass through driers ( mainly silica gel which absorbs moisture) and allowed to pass again through the rotor.

Ques 48. If a synchronous motor working at leading power factor can be used as (SSC-2015)

1. Mechanical Synchronizer
2. Voltage Booster
4. Noise Generator

Explanation:

• When the synchronous motor operates at the leading power factor then Rotor is overexcited in such a way that back emf (Eb which is generated in stator due to dc excitation of the rotor ) is greater than the supply voltage (V).
• At this time resultant flux is greater than that is required for the unity power factor,  then this extra flux will generate reactive power so the motor will generate additional reactive power.  And it will also use active power for mechanical work.
• Therefore synchronous motor working on leading PF will work as a synchronous condenser or phase advance

Ques 49. For V-curve for synchronous motor, the graph is drawn between (SSC-2015)

1. Armature current and power factor
2. Field current and armature current
3. Terminal voltage and load factor
4. Power factor and field current

Answer.2. Field current and armature current

Explanation:

The Graph plotted between the armature current Ia and field current If at no load the curve is obtained known as V Curve. Since the shape of these curves is similar to the letter “V”, thus they are called V curve of the synchronous motor.

Ques 50. Which of the following condition is NOT mandatory for alternators working in parallel? (SSC-2015)

1. The alternators must have the same phase sequence
2. The terminal voltage of each machine must be the same
3. The machines must have equal kVA ratings
4. The alternators must operate at the same frequency

Answer.3. The machines must have equal kVA ratings

Explanation:

• There are five conditions that must be met before when two alternators running in parallel.
1. Equal line voltage
2. Same frequency
3. Same phase sequence
4. Same phase angle
5. Same waveform
• We can use 2 alternators of 6 MVA and 4 MVA each instead of using single 10 MVA alternator because it is economical than using a single alternator of the same rating.
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