Answer.3. 425 kW

Answer.2. 1228.81 Volt

Explanation

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

E = K_c K_d √2π f Φ N_p

Or E = 4.44 K_c K_d  f Φ N_p    ………….. (since √2π = 4.44)

Where

K_c = Coil span factor

K_d = 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

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.

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

Answer.2. 32 MW, 48 MW

Explanation

Change in frequency from No load to full load

f = 50 x 0.05 = 2.5Hz

Full load frequency

f_fl1 = 50 – 2.5 = 47.5Hz

Since both alternators have same speed regulation then

f_fl1 = f_fl2

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

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.

Answer.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.

Answer.3. 0°

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°

Answer.2. δ = 90°

Explanation

Power delivered by synchronous generator

For Pmax  sinδ = 1

δ = 90°

Pmax = VE/X

Answer.2. Zero

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.

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.

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.