# SSC JE synchronous motor & synchronous generator question

Ques 51. Regulation of an alternator supplying resistive or inductive load is (SSC-2015)

1. Infinity
2. Always Negative
3. Always Positive
4. Zero

Explanation:

The voltage regulation of an alternator is defined as the change in its terminal voltage when the full load is removed, keeping field excitation and speed constant, to the rated terminal voltage.

Where Vph = Rated terminal voltage

An increase in the load current in a pure resistive load causes a decrease in the output voltage. For an inductive load an increase in the load current causes a greater voltage drop as compared to the resistive load. Therefore for inductive and resistive load conditions there is always a drop in the terminal voltage hence regulation values are always positive.

In case of leading load that means capacitive load, the effect of armature flux on main field flux is magnetizing i.e, the armature flux is adding up with the main field flux. Since it is adding up, the total induced emf(Vph) will also be more than the induced emf at no load(Eph). Hence the regulation is negative.

Ques 52. The positive, negative and zero sequence impedances of 3-phase synchronous generator are j 0.5 pu, j 0.3 pu and j 0.2 pu respectively. When the symmetrical fault occurs on the machine terminals. Find the fault current. The generator neutral is grounded through reactance of j0.1 pu (SSC-2015)

1. -j 3.33 pu
2. -j 1.67 pu
3. -j2.0 pu
4. -j 2.5 pu

Explanation:

For symmetrical fault

If = E/(Zi +Zn)

Where E = Pre fault voltage Which is equal to 1

Zi = 0.5j & Zn = 0.1 j

If = 1/(0.5j + 0.1j)

If = -j 1.67

Ques 53. If the synchronous motor can be used as a synchronous condenser when it is (SSC-2015)

1. Overexcited
2. Under excited

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).
• In this case, a Synchronous motor will be operating at a leading power factor. 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 reactive power.  And it will use active power too for mechanical work.
• Therefore synchronous motor working on leading PF will work as a synchronous condenser.

Ques 54. Which of the following methods would give a higher than the actual value of the regulation of an alternator? (SSC-2015)

1.  ZPF method
2.  MMF method
3.  EMF method
4. ASA method

Explanation:

Compared to other methods, the value of voltage regulation obtained by the synchronous impedance method (EMF Method) is always higher than the actual value and therefore this method is called the pessimistic method.

Ques 55. If the excitation an alternator operating in parallel with other alternators is increased above the normal value of excitation, its. (SSC-2015)

1. Power factor becomes more lagging
2. Power factor becomes more leading
3. Output current decreases
4. Output kW decreases

Answer.1. Power factor becomes more lagging

Explanation:

If the excitation an alternator operating in parallel with other alternator is changed then it will change the power factor

• Suppose the excitation of the alternator is decreased below normal excitation then reactive power will change and the active power output (W or KW) of the alternator will remain unchanged.
• The under-excited alternator delivers leading current to the infinite bus bar.
• It is because the leading current produces an adding m.m.f to increase the under excitation.
• Similarly, an overexcited alternator operates at lagging power factor and supplies lagging reactive power to an infinite bus bar.

Ques 56. In an alternator, the effect of armature reaction is minimum at the power factor of (SSC-2015)

1. 0.5 Lagging
2. 0.866 Lagging
4. Unity

Explanation:

At unity p.f., the effect of armature reaction is merely to distort the main field; there is no weakening of the main field and the average flux practically remains the same.

At zero p.f. lagging, armature reaction is directly demagnetizing and the armature reaction weakens the main flux. This causes a reduction in the generated e.m.f.

At zero p.f. leading, armature flux is now in the same direction as the field flux and, therefore the armature reaction strengthens the main flux. This causes an increase in the generated voltage.

Ques 57. Damper winding in synchronous motors is used to (SSC-2015)

1. Suppress hunting
2. Improve power factor
3. Develop reluctance torque
4. Improve the efficiency

Explanation:

• Damper windings are windings that are wound to the rotor poles of the machine (winding it similar to that of an induction machine) which help in two ways.
• We all know that a synchronous machine is not self-starting. Thus providing damper windings help synchronous machines to act as 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.

Ques 58. Turbo alternators have rotors of (SSC-2014)

1. Small diameter and long axial length
2. Large diameter and long axial length
3. Large diameter and small axial length
4. Small diameter and axial length

Answer.1. Small diameter and long axial length

Explanation:

Smooth Cylindrical Type or Non-Salient Pole alternator rotor is used for steam-driven alternator i.e turbo alternator which runs at very high speed.Since the steam turbine runs at very high speed, therefore, they required fewer poles hence the diameter of the rotor is small and the axial or rotor length is large.

Ques 59.  A 3-phase synchronous motor is started by utilizing the torque developed in (SSC-2014)

1. The high-speed steam turbine
2. The damper winding on the rotor
3. The damper winding on the stator
4. The low-speed water-turbine

Answer.2. The damper winding on the rotor

Explanation:

• 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 help synchronous machines to act as an induction motor ( only at starting). Which helps the machine to self-start.
• 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.

Ques 60. Alternators are usually designed to generate which type of ac voltage? (SSC-2014)

1. With fixed frequency
2. With variable frequency
3. Fixed current
4. Fixed power factor