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

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.

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

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.

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.

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.

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.

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

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.

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.