Electrical Machines MCQ Quiz - Objective Question with Answer for Electrical Machines - 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

Torque in the 3-phase induction motor

The torque of a 3-phase induction motor is given by:

\(T={3\over ω_s}{V^2\over ({R_2\over s}^2)+X_2^2}{R_2\over s}\)

where, V = Supply

ωs = Synchronous speed in radian/sec

R2 = Rotor resistance

X2 = Rotor reactance

s = Slip

Condition for maximum torque

The maximum torque of a 3-phase induction motor occurs at:

\(s={R_2\over X_1+X_2}\)

If the maximum torque has to occur in starting, the value of the slip (s)=1

\(1={R_2\over X_1+X_2}\)

\(R_2=X_1+X_2\)

The maximum torque of a 3ϕ induction motor occurs when rotor resistance is equal to leakage reactance (X1 + X2).

Lap winding

Lap winding is a type of armature winding in which the coil terminals are connected in such a way that the number of parallel paths is equal to the number of poles (for simplex winding) or a multiple of it (for multiplex windings).

This type of winding is commonly used in high-current, low-voltage applications like large DC generators and motors.

The number of parallel paths in a lap winding is given by:

A = P × m

where, A = Number of parallel paths

P = Number of poles

m = Multiplicity of winding (1 for simplex, 2 for duplex, 3 for triplex, etc.)

Calculation

Given, P = 4 

m = 2

The number of parallel paths is given by:

A = 4 × 2

A = 8

Additional Information

• Nature of load: The type of connection also depends on the nature of the load. For example, different connections may be suitable for balanced and unbalanced loads.
• Voltage levels: Different voltage levels may require different types of connections.
• Safety: The type of connection may also affect the safety of the transformer and associated equipment.

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

The speed of the induction motor can be controlled by the following methods:

1) V/f control (or) frequency control

2) Changing the number of stator poles

3) Controlling supply voltage

4) Changing winding resistance by adding rheostat in the stator circuit

Given figure shows the symbol of DC shunt motor

Temperature rises in electrical Machines are due to power losses occurring in various parts of the machines.

The various power losses occurring are rotational loss, friction and windage loss, and electrical and magnetic losses.

The permissible temperature rise of electrical machines is determined by the type of insulation materials used for winding.​

Properties of insulating materials for electrical machine

A perfect insulating material ought to possess the subsequent properties:

• High dielectric strength.
• It should withstand high temperatures.
• Good thermal conductivity.
• It should not undergo thermal oxidation.
• It should withstand vibration, abrasion, and bending.
• Not to absorb moisture.

• Delta – star connection type three-phase transformer is used for both large and low voltage rating transformers.
• Delta – star transformer is used at the generator side to step up the voltage levels and Star – delta transformer is used at the load side of distribution systems to step down the voltage levels.
• Star – star connection transformers are used for small, high voltage transformers.
• Delta – delta connection transformers are used for large, low voltage transformers.

Additional InformationAs the line voltage is √3 times of the phase voltage, hence star connection is used to give the output.

• When the load on the motor shaft increases, the load torque becomes higher than the motor torque. So that there is deceleration of rotor, due to which rotor speed is reduced, which increases the slip of rotor w.r.t RMF.
• Due to increased slip induces more emf in the rotor and hence more current in the rotor circuit.
• The air gap power being transferred to the rotor is now higher than before and hence more current is drawn in from supply at stator terminals. Hence, the motor torque is increased.
• So, this increase in slip, rotor induced emf, rotor current, air gap power, motor torque goes on until the motor torque becomes equal to the required load torque. The increased load is now being driven at a lower speed.
• At no load, slip is quite small because the only amount of motor torque required is to overcome friction and windage.

• Single phase induction motor is used in household refrigerator.

• An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source
• Isolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected
• Isolation transformers block transmission of the DC component in signals from one circuit to the other but allow AC components in signals to pass
• Transformers that have a ratio of 1 to 1 between the primary and secondary windings are often used to protect secondary circuits and individuals from electrical shocks between energized conductors and earth ground

Delta – star connection type three-phase transformer is used for both large and low voltage rating transformers.

Delta – star transformer is used at the load side of distribution systems to step down the voltage levels to provide a neutral connection.

Star – star connection transformers are used for small, high voltage transformers.