100 MCQ OF Synchronous Generator or Alternator with Explanation

Answer.2. Cylindrical rotor type

Explanation

Cylindrical rotor type

• In the cylindrical Rotor type, the air gap between the stator and rotor is uniform.
• Cylindrical Rotor type machine is generally used for the high-speed operation.
• Commercially cylindrical rotor type synchronous generator is also called as turbo-alternator or turbo-generator.

Salient-Pole type

• In the salient pole type synchronous generator the air gap between the stator and rotor is not uniform.
• They are used for low and medium-speed operation and for a small synchronous machine.
• Commercially Salient pole type synchronous generator is also called as the hydro alternator and hydro generator.

Answer.1. 140

Explanation

The voltage of an alternator is almost directly proportional to speed. Since the speed has reduced to two-thirds of what it was, so the voltage will also reduce to 2/3. i.e.

= (1000/1500) × 210V

= (2/3) × 210 = 140V

Answer.2. Lagging Power Factor

Explanation

The reactive power of the synchronous generator is given as

• When the excitation is increased i.e for the high value of If  E Cosδ_R > V. Therefore the Reactive power Q is positive.
• This means that the synchronous generator is delivering the reactive power to the infinite bus and its power factor is lagging.
• In that condition, the alternator is said to be over-excited.

Answer.2. Terminal voltage per phase

Explanation

• As the load on the alternator vary it’s terminal voltage also varied as in the D.C generator.
• When there is no load across the generator stator (Armature), there is no current in the armature winding hence field flux around the winding will be zero.
• Thus, as soon as the load is connected with armature terminals, its current will also increase proportionally causing its flux to increase.
  • As we know that every current-carrying conductor its own flux so armature of the alternator also produces its own flux.
  • The field flux ( main flux) produce by the field winding of an alternator.
  • The flux produced by the armature is called armature flux.
  • So the armature flux will distort the value of main field flux. This distortion is known as armature reaction.
  • The effect of armature flux depends upon the magnitude of the current flowing through the armature winding and nature of the power factor of the load connected to an alternator.
• In case if the load is at unity power factor, the effect of armature reaction is of cross magnetizing type.
•  If the load is at a lagging power factor, the effect of armature reaction is of demagnetizing type.
• In case if the load is at the leading power factor, the effect of armature reaction is of magnetizing type.

Note:- In magnetizing effect armature flux assist field flux hence greater EMF is induced in the armature. Hence there is an increase in the terminal voltage for leading power factor loads.

Answer.4. All option are correct

Explanation

• Damper windings are wound to the rotor poles of the Synchronous machine (winding is similar to that of an induction machine) which helps in two ways.
• We all know that a synchronous machine is not self-starting. Thus providing damper windings helps synchronous machines to act like an induction motor ( only at starting). Which helps the machine to self-start.
• We all know that hunting is a persistent phenomenon when it comes to synchronous machines.
• We can reduce hunting to a great extent by damping it. They don’t let the motor to oscillate abruptly, they damp the oscillations thus increasing the stability of the machine.

Answer.3. Zero Leading

Explanation

Zero Power factor means that either the circuit is purely resistive or purely capacitive.

If the load is purely capacitive or at the leading power factor, the effect of armature reaction is of magnetizing type.

Note:- In the magnetizing effect, armature flux assists main field flux hence greater EMF is induced in the armature. Hence there is an increase in the terminal voltage for leading power factor loads.

Answer.4. The Wattless component will change

Explanation

In an a.c. (alternating current) circuit, there are two components to impedance. There is a resistive component and a reactive component. The reactive component is the algebraic sum of the total inductive and capacitive reactances.

• • The reactive component stores power in a magnetic field (for inductors) and an electric field (for capacitors).
  • When the power source is turned off, the magnetic and electric fields collapse and return the stored energy back to the circuit in the form of induced current.
  • Therefore the current flow in the circuit which does not dissipate any power in the circuit is called “wattless current or component“.
• Now coming to the point when the prime-mover of an alternator is rotating at a constant power basis, i.e. its power and speed remain constant, now if you make variations in the exciting coils, then the output power from the alternator will vary in accordance.
• The output power of an alternator is proportional to the speed, and excitation voltage.
• Now if the excitation varies then the power factor at which load is delivered is also changes, but the speed is constant and the power demand or load on the alternator is also constant.
• Since Input to prime mover is kept constant, the real power generated remains unchanged (as per the load demand).
• When the excitation current is changed, the magnetizing current in armature changes, in order to maintain the air gap flux constant.
• Thus the reactive power consumption/ generation changes for a machine (i.e KVAr component).

Answer.2. Cross-magnetization

Explanation

In case if the load is at unity power factor, the effect of armature reaction is of cross magnetizing type.

• • In the case of cross-magnetization, the flux at the leading pole tip is reduced while the flux at the trailing pole tip tends to increase.
  • Hence average flux in the air gap remains constant but its distribution gets distorted.
  • Such distorting effect of armature reaction at unity p.f of the load is called the cross-magnetization effect of armature reaction.

Answer.4. 3000

Explanation

Synchronous speed is given as

N_s = 120f/p
= 120 x 50/2
= 3000

Answer.1. Synchronous motor and Synchronous generators are sources and the induction generator is a sink of reactive power.

Explanation

Given a synchronous generator operating at a lagging power factor i.e, an inductive load should be connected so that the machine will generate reactive power to the lagging load.

• Synchronous Motor is over-excited therefore it is acting as a synchronous condenser.
• An induction motor will absorb the reactive power for the development of a magnetic field.