100 MCQ OF Synchronous Generator or Alternator with Explanation
21.Which type of rotor is best for a turbo-alternator?
Cylindrical rotor type
- In cylindrical Rotor type, the air gap between 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.
22.An alternator is generating power at 210 V per phase while running at 1500 RPM. If the speed of an alternator drops to 1000 rpm the generated voltage per phase will be
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 do the same. i.e. = (1000/1500) x 210V = (2/3) x 210 = 140V
23.An alternator is said to be over-excited when it is operating at
- Reactive power of the synchronous generator is given as
- When the excitation is increased i.e for the high value of If E CosδR > 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.
24.Armature reaction in alternator primarily affect
- As the load on the alternator vary it's terminal voltage also found to be 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.
25.The function of damper winding in an alternator is
- Damper windings are windings that are wound to the rotor poles of the 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.
26.If the armature reaction of an alternator produces magnetization effect on the main field then the power factor should be
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 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. [/bg_collapse]
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28.Two alternators are running in parallel if the excitation of the one of the alternator is changed then
- 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 power demand or load on the alternator is also constant.
- Since Input to prime mover is kept constant, real power generated remains unchanged (as per the load demand).
- When the excitation current is changed, magnetizing current in armature changes, in order to maintain the air gap flux constant.
- Thus the reactive power consumption/ generation changes for a machine(ie KVAr component).
29.When the power factor of an alternator is unity then the armature flux will be
- 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.
30.For 50 Hz and 2 pole, the maximum speed of an alternator is
Synchronous speed is given as Ns =120f/p = 120 x 50/2 = 3000
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Consider a System consisting of working at lagging power factor, a Synchronous motor working at an over-excited condition and a direct grid-connected to the connected to induction generator. Consider capacitive VAr to be a sink of reactive power. Which of the following statement is true?
- Given synchronous generator operating at lagging power factor i.e, an inductive load should be connected so that the machine will generate reactive power to 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.
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