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)
24 poles
48 poles
12 Poles
None of the above
Answer.1. 24 poles
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)
Run at super-synchronous Speed
Stop Running
Run at sub-synchronous Speed
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)
300 kW
100 kW
150 kW
200 kW
Answer.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)
Changing the alternator speed
Changing the field Excitation
Changing the terminal Voltage
Changing the prime mover input
Answer.2. Changing the field Excitation
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)
DC series motor
Slip ring Induction motor
Synchronous motor
Squirrel cage induction motor
Answer.3. Synchronous 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)
It is simplest to perform and compute
Armature reaction is wholly magnetizing
It gives regulation value lower than its actual found by direct loading
It gives the regulation value higher than its actual found by direct loading
Answer.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)
Reduce eddy current losses
Reduce distortion of the waveform
Cool the machine
Strengthen the magnetic field
Answer.3. Cool the machine
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)
Mechanical Synchronizer
Voltage Booster
Phase Advancer
Noise Generator
Answer.3. Phase Advancer
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)
Armature current and power factor
Field current and armature current
Terminal voltage and load factor
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)
The alternators must have the same phase sequence
The terminal voltage of each machine must be the same
The machines must have equal kVA ratings
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.
Equal line voltage
Same frequency
Same phase sequence
Same phase angle
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.