# SSC JE synchronous motor & synchronous generator question

Ques 61.  A 300 kW alternator is driven by a prime mover of speed regulation 4% while the prime mover of another 200 kW alternator has a speed regulation of 3%. When operating in parallel, the total load they can take without any of them being overloaded is (SSC-2013)

1. 500 kW
2. 567 kW
3. 425 kW
4. 257 kW

Ques 62. The emf induced per phase in a three-phase star connected synchronous generator having the following data (SSC-2013)

Distribution factor = 0.955

Coil-span factor = 0.966

Frequency = 50 Hz

Flux per pole = 25 mwb

Turns per phase = 240, then emf per phase is

1. 2128.36 Volts
2. 1228.81 Volt
3. 869.46 Volts
4. 1737.80 Volts

Explanation

E.M.F equation of an alternator is given as

E = Kc K√2π f Φ Np

Or E = 4.44 Kc K f Φ Np    ………….. (since √2π = 4.44)

Where

Kc = Coil span factor

Kd = Distribution factor

Φ = Flux per pole

f = frequency

Np = Turns per phase

E = 4.44 × 0.955 × 0.966 × 50 × 25 × 10-3 × 240

E = 1228.1 volts

Ques 63. Alternator used in hydel power station has more number of poles in it than used in thermal power station, because (SSC-2013)

1. Power generated by the alternator is less
2. Speed of the prime mover may be changed whenever required
3. Power generated by the alternator may be changed according to demand
4. Speed of its prime mover is less

Answer.4. Speed of its prime mover is less

Explanation

In hydraulic turbine in the hydroelectric plants operates at low speed, hence the speed of prime mover is low(as compared to steam in case of thermal plants). according to the relation, we have

f=(P x N)/120

P= no of poles

F= frequency

N= speed

So in order to have fixed frequency product of ‘p’ and ‘n’ should be a constant. Hence P should be more(as N is less). In order to accommodate more number of poles, the diameter of the rotor should be large.

Ques 64.  Which one of the following is correct? (SSC-2013)

1. The effect of field current on the main flux of a synchronous machine is called armature reaction
2. The effect of air gap flux on armature current of a synchronous machine is called armature reaction
3. The effect of armature current on main flux of a synchronous machine is called armature reaction
4. The effect of armature current on air-gap flux of a synchronous machine is called armature reaction

Answer.3. The effect of armature current on main flux of a synchronous machine is called armature reaction

Explanation

The effect of armature flux on the main field of a synchronous machine is called the armature reaction

Ques 65. Two alternators rated 40 MVA and 60 MVA respectively are working in parallel and supplying a total load of 80 MW. Speed regulation of both the alternator is 5%. The load sharing between them will be (SSC-2012)

1.  30 MW, 50 MW
2. 32 MW, 48 MW
3.  36 MW, 44 MW
4.  40 MW each

Answer.2. 32 MW, 48 MW

Explanation

Change in frequency from No load to full load

f = 50 x 0.05 = 2.5Hz

ffl1 = 50 – 2.5 = 47.5Hz

Since both alternators have same speed regulation then

ffl1 = ffl2

From the above diagram

For machine 1

(50 – f)/(80 – x) = (50 – 47.5)/40

x – 16f = 80 – 16 x 50

x- 16f = -720—–1

For machine 2

(50 – f)/(80 – x) = (50 – 47.5)/60

x – 24f = -1200 ——-2

From equation (1) and (2)

x= 48 MW

f = 60 Hz

So, machine A operates at a load of 48 MW While machine B will operate at a load of

80 – 48 = 32 MW

Ques 66. In a 3-phase synchronous generator, the stator winding is connected in star, because a delta connection would (SSC-2011)

1. Have circulating currents due to triple harmonics
2. Require more insulation and conductor material
3. Require larger conductor and more core material
4. Result in a short circuit

Answer.2. Require more insulation and conductor material

Explanation

Alternators are connected primarily in the star to achieve the following motives:

1. The phase voltages in star connection is 57.7 % of the line voltages, i.e. the armature winding in star connection is less exposed to voltage as compared to the delta connection which in turn prove more economic if we consider insulation, breakdown strength, the requirement of conductor material
2. Easy protection: Neutral grounding is necessary to allow zero sequence currents to flow to the ground in case of a fault.
3. Elimination of harmonics: Star connection facilitates a neutral connection which is instrumental in eliminating triple harmonics.
4. No circulating currents: In star connection, we don’t have circulating parasitic currents like in Delta which lead to heating losses.

Ques 67. While starting synchronous motor its field winding should be (SSC-2010)

1. Kept Open
2. Connected to DC source
3. Connected to AC source
4. Kept short-circuited

Explanation

• In the case of an induction motor, the field winding is permanently short-circuited by end rings.
• In a Synchronous motor, the rotor winding is supplied with DC excitation. Now, if we eliminate the DC excitation, there are two ends of the wire available.
• Let short-circuit the two ends of the wire (or connect the two ends of variable resistors).
• Now the rotor winding of the synchronous motor is analogous to that of an induction motor.
• Therefore if we short-circuit the field winding of a synchronous motor then it will behave as an induction motor.

Ques 68. The angle between induced emf and terminal voltage on no-load for a single phase alternator is

1. 180°
2. 90°
3. 270°

Explanation

The generated voltage of the alternator is

V = E – Ia(Ra + JXs)

Where E = Induced EMF

Ia = Armature current

Ra = Armature resistance

Xs = leakage reactance of the armature winding

For no-load armature current Ia = 0

∴ V = E  (δ = 0°)

So at no load, the angle between induced emf and terminal voltage is 0°

Ques 69. A salient pole synchronous generator connected to an infinite bus power will deliver maximum power at power angle of (SSC-2010)

1.  δ = 0°
2. δ = 90°
3. δ = 45°
4. δ = 30°

Answer.2. δ = 90°

Explanation

Power delivered by synchronous generator

For Pmax  sinδ = 1

δ = 90°

Pmax = VE/X

Ques 70. Starting torque of synchronous motor is (SSC-2010)

1. Very low
2. Zero
3. Very high
4. Half-full load torque

Explanation

The starting torque of the synchronous Motor is zero.

Consider the rotor is at rest and the stator field is rotating at the synchronous speed at 100% slip. Suppose N pole of the rotating field in the stator is approaching at S pole of the rotor then the magnetic force will tend to turn the rotor in the direction opposite to the rotating field When the same N pole of the rotating field has just passed the rotor S pole, the magnet force will tend to pull the rotor in the same direction as the rotating field. These opposite interactions balance out, producing a zero net torque on the rotor.

Ques 71. The negative phase sequence in a 3-phase synchronous motor exists when the motor is (SSC-2010)

3. Supplied with Unbalanced voltage
4. Hot

Answer.3. Supplied with unbalanced voltage

Explanation

## Negative phase sequence current in Synchronous motor

• Negative sequence currents are produced because of the unbalanced voltage in the power system.
• The flow of negative sequence currents in electrical machines (generators and motors) are undesirable as these currents generate high and possibly dangerous temperatures in very short time.
• Phase current and voltage in the three-phase system can be represented in the form of three single-phase components.
• Positive sequence components, Negative sequence components, and Zero sequence components.
• Positive sequence currents exist during the balanced load condition.

Ques 72. For cooling of large size generators hydrogen is used because (SSC-2009)

1. It offers reduced fire risk
2. It is light in weight
3. It is of high thermal conductivity
4. All of the above

Answer.4. All of the above

Explanation

# Why is hydrogen used for Alternator cooling?

Hydrogen is the least expensive, with less weight, high thermal conductivity, less density, and less viscosity. Less weight, less density & less viscosity attributes to its flow rate. High thermal conductivity helps in better heat exchange. Least expensive helps in balance sheets, more power in fewer investments.

In order to reduce the high temperature of the alternator hydrogen gas is used as a coolant. The coolant, Hydrogen gas is allowed to flow in a closed cyclic path around the rotor. Heat exchange takes place and the temperature of hydrogen gas increases, for better cooling of the rotor in the next cycle it has to be cooled. Cooling of hydrogen gas is done by passing it through heat exchangers generally constituted with water. Now Hydrogen gas after cooling is allowed to pass through driers ( mainly silica gel which absorbs moisture) and allowed to pass again through the rotor.

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