Alternators MCQ Quiz - Objective Question with Answer for Alternators - Download Free PDF

Concept

A synchronous generator or alternator is capable of operating at all types of power factors, i.e. either UPF, leading, or lagging power factor.

• Leading power factor: If field excitation is such that E_b < V, the alternator is said to be under-excited and it has a leading power factor.
• Lagging power factor: If field excitation is such that E_b > V, the alternator is said to be over-excited and has a lagging power factor.
• Unity power factor: If field excitation is such that E_b = V, the alternator is said to be normally excited.

The V-curve for synchronous generator or alternator is shown below

• Synchronous generator or alternator is an electrical machine that converts the mechanical power from a prime mover into an AC electrical power at a particular voltage and frequency.
• The synchronous machine always runs at a constant speed called synchronous speed.
• To excite the field winding of the rotor of the synchronous machine, a direct current is required.
• Thus, an alternator is a polyphase synchronous machine in which excitation is provided by rotor winding connected to dc supply.
• With D.C excitation, there exists magnetic locking between stator poles and rotor pole. So that alternator rotates at synchronous speed.
• In a three-phase load, three different impedances are connected together in a star or delta fashion.
• The delta in a three-phase system is formed by connecting one end of the winding to the starting end of other winding and the connections are continued to form a closed loop.
• The star in the three-phase system is formed by connecting one end of all three impedances are connected together.

          The relationship between line and phase voltages and currents in star connected and delta connected networks are: 

The purpose of damper winding in an alternator is to prevent hunting, which is when a synchronous machine oscillates at a speed other than its synchronous speed.

Hunting: 
Oscillations of the rotor about its new equilibrium position, due to sudden application or removal of load is called swinging or hunting in synchronous machine.

Causes for Hunting:

• Sudden change in load.
• Sudden change in the supply system or in the field system.
• Load containing harmonic torque.

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Methods for eliminating hunting:

1. By designing the machine with a suitable synchronizing power coefficient.
2. By using the fly wheel.
3. By using damper winding.

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Damper winding:

• Damper winding is made with low resistance copper, aluminum, or brass.
•  They are inserted in the slots made under the pole shoes.
• With respect to damper winding, the rotor behaves like a squirrel cage rotor of an induction motor.

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Functions of damper winding:

1. In alternator to eliminate hunting and to suppress the negative sequence field.
2. In synchronous motor to eliminate hunting and for starting purpose.

Important points:

• When the rotor speed is less than synchronous speed, the inductor motor torque is developed in the same direction of the rotor rotation then the rotor will accelerate to reach synchronous speed.
• When the rotor speed is more than synchronous speed (super synchronous speed), the induction generator torque is developed in the opposite direction of rotor rotation then the rotor will decelerate to reach synchronous speed.
• When the rotor rotates at the synchronous speed, then there is no relative speed between armature flux and damper winding. Therefore no induced emf and no current in the damper winding and hence no torque. The machine is said to be under steady-state condition.

The correct answer is option 2.

The parallel operation of alternators is necessary because:

1.  During periods of high load, several alternators operating in parallel can supply a bigger load than a single alternator whereas during periods of light load, one or more alternators may be shut down and those remaining to operate at or near full load, thus enabling proper load sharing.
2. When there is maintenance or an inspection, one machine can be taken out of service and the other alternators can keep up for the continuity of supply. It increases the reliability of the system.
3. The losses in the system reduce, hence efficiency increases.
4. The operating cost is reduced.
5. It ensures the protection of supply and enables cost-effective generation.

Alternators are generally designed to supply electric power because of the following reasons.

• Constant Voltage
• Constant frequency
• Capability to deliver active as well as reactive power

An overexcited alternator always supplies lagging current to the connected load, which means that load is of lagging nature. Lagging load take active and reactive power from the supply or alternator. Therefore, reactive power flows outwards from an over-excited alternator.

Important Points:

• Under excited alternator works at the leading power factor
• The normal excited alternator works at the unity power factor
• The overexcited alternator works at lagging power factor

Mistake PointsFor Synchronous motor its opposite of alternator,

• Under excited synchronous motor works at lagging power factor
• The normal excited synchronous motor works at the unity power factor
• The overexcited synchronous motor works at a leading power factor

Construction of synchronous machine

• An alternator consists of two parts, the stator, and the rotor.
• The stator is the stationary part of the machine, and the rotor is the rotating part of the machine.
• The stator carries the armature winding in which the voltage is generated, and the output is taken from it.
• The rotor of the machine produces the main flux.
• In synchronous machines, the salient poles are rotating field winding.
• The poles are magnetized either by permanent magnets or by a dc current.
• The armature, normally containing a three-phase winding, is mounted on the shaft.

As the field current rotation does not affect the phase sequence of an alternator so with reversing the direction of field current the phase sequence remains same i.e. RYB

Note:

If the direction of field current is reversed, then there is no change in its phase sequence. usually, the phase sequence of the alternator can be changed by changing/reversing the direction of rotor rotation. But not depends on field's current direction

• Waveform and phase sequence is fixed by the construction of the generator and its connections to the system.
• During the installation of a generator, careful checks are made to ensure that the generator terminals and all control wiring are correct so that the order of phases (phase sequence) matches the system.
• Connecting a generator with the wrong phase sequence will result in a short circuit as the system voltages are opposite to those of the generator terminal voltages.
• The voltage, frequency, and phase angle must be controlled each time a generator is to be connected to a grid.

The correct answer is option 1): 600 RPM

Concept:

Mechanically coupled alternator means the rotor of these alternators is coupled so that both machines will run at the same speed. We know that, synchronous speed

\(N_s = \frac{120 \times f}{P}\)

i.e., Ns ∝ f and Ns ∝\(\frac{1}{P}\)

One machine is delivered at 40 Hz. Assume the number of poles in that machine will be P₄₀. Another machine is delivered at 50 Hz. Assume the number of poles in that machine will be P₅₀.

\(\frac{40}{50} = \frac{P_{40}}{P_{50}} \Rightarrow \frac{P_{40}}{P_{50}} = \frac{4}{5}\)

Since magnetic monopole cannot exist we will multiply denominator and numerator with 2

\( \frac{P_{40}}{P_{50}} = \frac{4 \times 2}{5 \times 2}\) = \(\frac{8}{10}\)

Hence, Highest possible speed =\(\rm \frac{120 \times 50}{10} \ or \ \frac{120 \times 40}{8} \)

= 600 RPM

The correct answer is option 4): (92.60 percent)

Concept:

The efficiency of the alternator is the ratio between the output to the Input

Efficiency = \(\frac{Output}{Input} \times 100\)

Output  = Input - losses

Calculation:

Output = 25 kW

Loss = 2 kW

Input = Output + loss

= 25+2

= 27 kW

Efficiency = \(\frac{Output}{Input} \times 100\)

= \(\frac{25}{27} \times 100\)

= 92.60 %

Synchronization of alternator means connecting an alternator into the grid in parallel with many other alternators, that is in a live system of constant voltage and constant frequency.

Before connecting an alternator into the grid, the following conditions must be satisfied:

Equal voltage: The terminal voltage of incoming alternator must be equal to the bus-bar voltage.

Same frequency: The frequency of generated voltage must be equal to the frequency of the bus-bar voltage.

Phase sequence: The phase sequence of the three phases of alternator must be similar to that of the grid or busbars.

Phase angle: The phase angle between the generated voltage and the voltage of the grid must be zero.

The first condition of voltage equality can be satisfied by a voltmeter. To satisfy the conditions of equal frequency and identical phases, one of the following two methods can be used:​

• Three Dark Lamps Method
• Two Bright, One Dark Method
• Synchroscope Method

• In the case of alternators, a field rheostat may be used to change the excitation or its field current.
• If alternators are running in parallel, a change in the field current will not change the active power shared significantly but will change the operating power factor.
• With the change in the excitation, the armature current will change which will change the active power by a small amount.
• If the excitation decreased, its power factor becomes more lagging.

The correct answer is option 2): \(\rm\frac {Phasor\ sum\ of\ emfs \ induced \ in\ that\ coil} {Arithmatic \ sum \ of\ induced \ emfs \ in\ a\ coil}\)

Concept:

• The coil span factor or pitch factor is defined as the ratio of the voltage generated in the short-pitch coil to the voltage generated in the full-pitch coil. The coil span factor is also known as the chording factor
•  It can also be defined as the ratio of the vector sum of induced emf per coil to the arithmetic sum of induced emf per coil.
• Pitch factor = \(\rm\frac {Phasor\ sum\ of\ emfs \ induced \ in\ that\ coil} {Arithmatic \ sum \ of\ induced \ emfs \ in\ a\ coil}\)

An alternator power output is directly proportional to the sine of its load angle.

\(P = \frac{{EV}}{X}sin\theta\)

Where,

P = Power output of the alternator

E = Sending voltage of the alternator

V = Receiving voltage of the alternator

X = Rectance of alternator

θ = Load angle

• The maximum power output from the alternator would occur when the load angle is 90 degrees.
• But still, the load angle is maintained between 20 and 30 degrees.
• This is done to ensure the stability of the power system in which the alternator is connected.
• The load angle of 90 degrees is known as the point of critical stability, if the load angle exceeds this value then the alternator will become unstable and its speed can go out of control.
• To ensure that this situation never occurs the alternator is always operated at a load angle of 20-30 degrees.