Alternator / Generator – MCQs with Explanatory Answers)

✅  Answer:     ( 1 )

Most of the alternators have rotating field and stationary armature. This is because of some of the advantages as given below

1. The stationary armature can be easily insulated
2. Stationary armature windings are safe from vibration and centrifugal forces and can be braced against high electromagnetic force
3. Number of slip rings required to provide direct current to the rotating field is less compared to number of slip rings required to provide direct current to the rotating armature.

✅  Answer:    ( 1 )

Salient pole rotors are basically used in low and medium speed Alternators. As speed is low, the desired frequency is obtained by having larger number of poles. Hence these machines have large diameters and short axial lengths.

Figure 1: Salient Pole Vs. Cylindrical Rotors

On the other hand, cylindrical rotors are used for turbo alternators running at high speed as 3600 RPM or 1800 RPM.

✅  Answer:   2

Let us consider an Alternator with P number of poles on the rotor revolving with N revolutions per minute speed. When a pair of poles pass through a conductor, EMF is induced in the conductor. The polarity of the induced EMF depends upon the polarity of the magnetic poles which cut through the conductor.

Number of cycles per revolution = P/2

Number of revolutions per second = N/60

Therefore, Number of cycles per second = PN /120

However, number of cycles per second is the frequency of generated voltage. Hence, frequency, f = PN /120

Since number of poles is usually fixed for an alternator, the speed of the rotor must be in synchronizing with the desired speed. Hence Alternators are also called as Synchronous Generators.

✅  Answer:    ( 1 )

Pole pitch is the angular distance between two poles on a machine. It is always 180 degrees electrical regardless of the number of poles in the machine. However, two sides of a coil are not always corresponding points under the adjacent poles and hence the coil has a span or pitch less than 180 degrees. This is known as a short pitch coil.  The pitch factor, K_c is the ratio of the voltage generated in the short pitch coil to the voltage generated in the full pitch coil.  Since EMF generated in the coil is always less than the EMF generated in the pole or full pitched coil. Hence for a short pitch coil, K_c is less than 1.

✅  Answer:    ( 2 )

• Given: Number of slots = 36
• Number of phases = 4
• Number of slots per phase = 36/4 = 12
• Number of poles = 4
• Number of poles per slot per pole, m = 12/4 = 3

Slot angle, α = 360/36 = 10 degree mechanical = 20 degree electrical

Now, Distribution Factor K_D, 

Substituting the corresponding values, we get: K_D = 0.5/0.52 = 0.96

✅   Answer:     ( 3 )

Given:

• Number of phase = 3
• Speed of the rotor, N = 800 RPM
• Line to line voltage, V_L = 430 Volts

Therefore, Phase voltage, V_ph = V_L / √3 = 248.26 Volts

Frequency of generated voltage, f = 60Hz

Therefore, number of poles, P = 120f /N = 9

Number of slots per pole per phase, m = 2

Total number of stator slots = 2 x Number of poles x Number of phase = 48

Therefore, slot angle, α = 360/48 = 7-1/2 degree mechanical = 33.75 degree electrical

Hence, Distribution Factor, K_D = sin [ ( 2 x 33.75 ) / 2 ] / 2sin ( 33.75 / 2 ) = 0.55 / 0.58 = 0.95

Pitch Factor, K_c = 1 ( Full pitch coil )

Number of slots per phase = 2 x 9 = 18

Number of turns per coil = 4

Number of turns per phase, T_ph = 4 x 9 = 36

Hence, RMS value of induced EMF, E_Ph = 4.44 K_cK_DT_phφf

As induced EMF is the generated phase voltage

Hence, flux per pole, φ = 248.26 / 261.072 = 0.027 Wb

✅  Answer  (3).

Given:

• Number of phase = 3
• Number of poles, P = 16
• Number of slots = 144
• Number of slots per pole = 9
• Speed of rotor = 400 RPM
• Number of slots per pole per phase, m = 3

Slot angle, α = 360/144 degree mechanical = 20 degree electrical

Distribution Factor, K_D = 0.5 / 0.52 = 0.96

Number of turns per phase, T_ph = ( 144 x 12 ) / ( 2 x 3 ) = 288

Assuming full pitch coil, K_c = 1

Flux per pole, φ = 0.03 Wb

Generated frequency, f = NP/120 = 53.33 Hz

Hence, induced EMF, E_ph = 4.44K_c K_DfT _phφ = 1943.53 Volts

The line voltage, V_L = √3 E_ph = 3366.29 Volts

✅  Answer:     ( 2 )

When the load connected to the Alternator varies during the latter’s operation, the generated voltage also varies. This is because of the voltage drops in armature resistance R_a, armature leakage reactance, X_L and voltage drop due to armature reactance, X_a. The armature reaction is due to the action of armature current flux and field flux.

✅  Answer:     ( 2 )

Given:

Effective resistance, R_a = 1.6 Ohms

Synchronous Reactance, X_s = 32 Ohms

Line Voltage, V_L = 13000 Volts

Power Factor, cosφ = 0.8

Hence, sinφ = 0.6

Three phase power = 1275 kW = √3 V_LI_Lcosφ

Therefore, line current, I_L = 70.78 Amperes

Phase voltage, V_ph = V_L / √3 = 7505.55 Volts

No load voltage for leading power factor of 0.8 is, E_p² = (V_pcosφ + I_LR_a)² + (-V_Psinφ + I_LX_s)² , or E_P = 6514.32 Volts

Therefore, Percentage regulation = ((E_P – V_ph)/E_P) x 100 = -13.20%

✅  Answer:    ( 2 )

Alternators can be connected in parallel with one another with common bus bars, a process known as Synchronization.