Synchronous Machines: 2 Marks Important Questions with Answers

Basic Electronics and Electrical Engineering

Chapter 4: Synchronous Machines

2 Marks Important Questions with Answers

1. Compare: Synchronous generator and DC generator?

DC generator

(i) The induced emf in the armature conductor is of alternating type. By using commutator and brush assembly it is converted to DC and made available to the external circuit.

(ii) The commutator and brush assembly arrangement are used to tap the DC current.

(iii) Armature must be a rotating member while field as a stationary.

Synchronous generator

(i) The induced emf in the armature conductors is of alternating type and is made directly available to the external circuit.

(ii) The slip ring and brush assembly arrangements are used to tap the AC current.

(iii) It has rotating armature and stationary field.

2. Which type of synchronous machine is used with hydraulic turbines? What is their operating speed range?

Salient pole type synchronous machine is used with hydraulic turbines. It has the speed range from 125 rpm to 500 rpm.

3. Why is the field system of an alternator made as a rotor?

(i) The output current can be led directly from fixed terminals on the stator (or armature windings) to the load circuit, without having to pass it through brush contacts.

(ii) It is easier to insulate stationary armature winding for high ac voltages, which may have as high a value as 30 kV (or) more.

(iii) The sliding contacts i.e., slip‒rings are transferred to the low‒voltage, low‒power DC field circuit which can, therefore, be easily insulted.

(iv) The armature windings can be more easily brazed to prevent any deformation, which could be produced by the mechanical stresses set up as a result of short‒circuit current and the high centrifugal forces brought into play.

4. Compare salient pole rotor and cylindrical pole rotor?

Salient pole type

(i) Diameter is high and axial length is small.

(ii) Poles are projecting out from the surface.

(iii) Air gap is non uniform.

(iv) Mechanically weak

(v) Preferred for low speed alternators.

(vi) Prime movers used are hydraulic turbines.

(vii) For same size, the rating is smaller than cylindrical type.

(viii) Separate damper winding is proposed.

Smooth cylindrical type

(i) Small diameter and large axial length.

(ii) Insulated portion of the cylinder acts as poles, hence poles are non projecting.

(iii) Air gap is uniform.

(iv) Mechanically robust.

(v) Preferred for high speed alternators.,

(vi) I.C. engine.

(vii) For same size, rating is higher than salient pole type.

(viii) Separate damper is not necessary.

5. Mention the two types of alternator.

The two, types of alternators are,

(i) Salient pole type

(ii) Smooth cylindrical type

6. Define pitch factor and distribution factor as applied to an alternator.

Pitch factor

Coil‒span factor K_p (or) Kc is defined as the ratio between vector sum of induced emfs per coil to the arithmetic sum of the induced emfs per coil.

Distribution factor

The distribution factor is defined as the ratio between emf with distributed winding to the emf with concentrated winding.

17. Write down the Emf equation of an alternator.

Actually available voltage/phase = 4.44 K_C K_df T ϕ  volts

Where,

K_C = pitch or coil span factor = cbs a/s

K_d = distribution factor  = (sin mβ/2) / (m sin β/2)

 f = frequency of induced emf in Hz

ϕ = flux / pole in weber.

T = No.of coils per phase.

8. Write down any four advantages of short‒pitch winding in synchronous machines.

Some of the advantages of short‒pitch winding in synchronous machines are

(i) They save copper of end connections.

(ii) They improve the wave‒form of the generated emf.

(iii) Due to elimination of high frequency harmonics, eddy current and hysteresis loss are reduced.

(iv) It increases the efficiency.

9. What are the factors affecting alternator size?

The factors affecting the alternator size are,

(i) In larger machines, power output per kilogram increases as the alternator power increases.

(ii) As alternator size increases, cooling problem becomes more serious, since large machines inherently produce high power loss per unit surface area (N/m²).

10. Define voltage regulation of alternator?

The voltage regulation of an alternator is defined as "the rise in voltage when full‒load is removed (field excitation and speed remaining the same) divided by the rated terminal voltage".

 % regulation “up” = [ (E₀‒V) / V ] × 100

 E₀ = No load induced emf

 V = Rated terminal voltage

11. What is synchronous impedance.

It is the ratio between open circuit voltage to short circuit current which are obtained from the OCC and SCC test.

Zs = E₁(open‒circuit voltage) / I₁(short‒circuit current)

Z_S = synchronous impedance

12. What is synchronous reactance?

Synchronous reactance is the combined effect of armature leakage reactance / phase and fictitious reactance / phase.

X_S = X_L + X_a

13. What are the reasons for drop in voltage from no load to full road?

When an alternator is loaded, the current flowing in the windings causes drop in the terminal voltage. The drop is due to the following reasons.

(i) voltage drop in the resistance of the winding.

(ii) voltage drop in the reactance of the winding.

(iii) voltage drop due to armature reaction.

14. What constructional features makes the flux distribution over the armature of the synchronous machine uniform in a salient pole type rotor?

The pole face is shaped such that the radial air gap length increases from the pole centre to pole tips. This results in uniform flux distribution.

15. What are the advantages of synchronous motor?

The advantages of synchronous motors are

(i) The speed is always synchronous irrespective of the load.

(ii) It can be used as synchronous condenser for power factor improvement.