Electrical Machines Interview questions

A list of top frequently asked Electrical Machines interview questions and answers are given below.

1) What is a single phase Autotransformer?

A Single-phase autotransformer is a single winding transformer in which a part of the winding is common to both high-voltage and low-voltage sides.

2) What are the advantages of Autotransformer?

The advantages of autotransformer are as follows:

1. Less winding material is required in case of the autotransformer.
2. These transformers are not costly.
3. These transformers are smaller in size.
4. In Autotransformer ohmic losses and core losses are less.
5. These transformers have high efficiency.

3) What are the disadvantages of Autotransformer?

The disadvantages of autotransformer are as follows:

1. In case of Autotransformer effective impedance is smaller.
2. In the case of open circuit winding, full voltage is transferred from the primary side to the secondary, and this can burn the equipment.

4) What are the applications of Autotransformers?

The applications of autotransformer are as follows:

1. These are used for the interconnection of the power system at different voltage levels like 132KV and 230KV.
2. In the distribution system, autotransformers are used to boost the supply voltage.
3. Autotransformers can be used for starting induction motors and synchronous motors if it has many tappings.
4. Autotransformers can be used as variac (variable a.c).

5) What is the general system requirement of Alternator?

There is a requirement of two types of system for the generation of e.m.f.

1. The Magnetic system is required for the production of the magnetic field.
2. Armature system is required for the houses of conductors on which the EMF is to be induced.

6) What are the advantages of Stationary Armature and Rotating Field System?

The advantages of the stationary armature and rotating field system are as follows:

1. The coils of the stationary armature can be insulated easily.
2. The cooling of the winding is more efficient.
3. In the motor high peripheral speed can be achieved.
4. In this Slip rings and brushes are not necessary, and we can easily supply the output current to the load circuit.

7) What is meant by Stator? What is meant by Rotor?

In AC/DC motor or Generator, the stationary part of the system is called the Stator, and the rotating components of the system, i.e., either in generator or motor is called the Rotor.

8) What are the Advantages of the three-phase motor over Single phase motor?

The advantages of three Phase motor over single phase motor are as follows:

1. Starting Torques are higher.
2. Improved speed regulation is there.
3. Less vibration is there.
4. The operation is quite.

9) What is the basic principle of operation of Alternators/DC generators?

Alternators/DC generators work on the fundamental principle of Faraday?s law of Electromagnetic Induction. This law states that when we move any current carrying conductor in a magnetic field, then an EMF is induced in it.

10) How to minimize the Eddy Current Losses?

Eddy current losses can be minimized by laminating the core and insulating the core from each other with paper or varnish.

11) What is meant by Turbo Alternators?

High-speed alternators are called Turbo Alternators. Because of the high speed of rotation, the diameter of the rotor is reduced, and the axial length is increased. In this two or four poles machine are used, and steam turbines are used as prime movers.

12) What are the various types of Rotor used in the Alternators?

The various types of a rotor are:

1. Salient pole Rotor.
2. Squirrel Cage Rotor.
3. Wound Rotor.
4. Cylindrical Rotor.

13) Define Pole Pitch?

Pole pitch is defined as the distance between the Centres of two adjacent poles. In which one pole pitch is equal to 180 electrical degrees. We can also describe it as the number of slots per pole.

14) Define Coil Span?

Coil Span is defined as the distance between the two coil sides of a coil. We can also express it in electrical degrees or many slots.

15) What is meant by full pitched winding?

Full pitched winding are those winding in which the coil span is equal to the pole pitch.

16) What is meant by short-pitched winding?

Short pitched winding are those in which the coil span is less than the pole pitch. These winding are also called as short-chorded winding.

17) What are the advantages and disadvantages of Short-pitched winding?

The advantages of the short-pitched winding are:

1. Copper is saved for end connections.
2. The Waveform of the generated EMF is improved.
3. The generated EMF in the winding can be made approximate to a sine wave, and the distorting harmonics can also be reduced in these winding.

The disadvantages of the short-pitched winding are:

1. In these winding the total voltage around the coil reduces. Because the voltage that induces in the two coils is out of phase.

18) Why the efficiency of a three-phase induction is the motor is less than that of a three-phase transformer?

The efficiency of a three-phase induction motor is less than three-phase transformer because three-phase induction motor is like a rotating transformer and because of the rotating nature of induction motor there is friction and winding losses in it. Three phase transformer is a static device; therefore, there are no rotational losses.

19) Why is the air gap between the pole pieces and the armature kept very small?

The air gap between the pole pieces and the armature is kept very small because of the following reasons:

1. To maintain the field strength at a high value.
2. To generate the high voltage.
3. If the air gap will be large than stronger air gap is needed to produce the required voltage; therefore, the air gap is kept very small.
4. Because of the reason that flux lines can pass through iron very quickly than the air gap.

20) Why does curving the pole faces in a DC machine contributes to a smoother DC output voltage from it?

The commutation of the DC machine must be as ideal as possible for having smoother output voltage. For good commutation, the flux density in the air gap must decrease gradually from maximum value under the center of the pole to zero.

21) The series field winding has low resistance while the shunt field winding has high resistance. Why?

In the case of the generator, the series winding takes the full load current so, it requires very few turns to produce the necessary magnetic field, and therefore the series field winding has low resistance. But in the case of shunt winding the shunt resistance has full load voltage across it and requires a large number of turns to limit the current in it; therefore, the shunt field windings has high resistance.

22) What are the conditions to be satisfied for parallel operation of DC generators?

The conditions for parallel operation are as follows:

1. The generator's polarity must be the same, or the connections must be interchanged until the polarity becomes the same.
2. Both the generators must have equal voltage.
3. The change of voltage with the change of load should be of the same character.
4. The positive and negative of the bus bar must be appropriately connected with positive and negative terminals of the generator.
5. There must be stable and similar rotational speed characteristics for the prime movers driving the generator.

23) What are the reasons for the parallel operation of transformers?

The main reasons for operating transformers in parallel are as follows:

1. This is an economical method for large loads.
2. Parallel operation of transformer allows us to add extra load if required in future.
3. The space capacity of the substation can be reduced by parallel operation if we connect transformers of standard size.
4. The parallel connection maximizes the electrical power system availability as we can shut down any system for maintenance without affecting other system performance.

24) What are the significant parts of DC Generator?

The significant parts of DC generator are:

1. Yoke Poles.
2. Armature.
3. Field winding.
4. Armature Winding.
5. Commutator brushes.

25) What is a Commutator?

The device which converts the alternating induced emf of a generator to the DC emf is called Commutator. In the case of the motor, it produces unidirectional torque.

26) What is meant by reactance voltage?

When a coil undergoes Commutation in a coil then an EMF is induced, and this induced EMF is called reactance voltage.

An armature coil has certain self-inductance, and when the coil undergoes commutation then there is a change in current (di) in that coil, and the commutation takes certain time (dt) thus the coil undergoing commutation has self-induced EMF which is given by L(di/dt).

27) What are Electric Motors?

An electromechanical device that converts electrical energy into mechanical energy is called an electric motor.

Almost all the motors operate by the interaction between the magnetic field and current carrying conductors to generate force.

28) On what principle Electric motor works?

When a current carrying conductor is placed in an external magnetic field perpendicular to the conductor, the conductor experiences a perpendicular force to itself and the external magnetic field.

The main principle behind the production of mechanical force is the interaction between the electric current and magnetic field.

To find the direction of the force experienced on a conductor can find out by the right-hand rule for the force.

29) How many types of electric motors are there?

There are mainly two types of electric motors:

1. A.C Motors.
2. D.C Motors.

30) What is an AC motor?

AC motors are electric motors that are driven by an alternating current. It consists of two parts an outside stationary stator and inside the rotor. The outer stators have coils that are supplied by an alternating current to produce a rotating magnetic field and inside rotor is attached by output shaft, and the torque is provided to it by the rotating field.

31) What is a DC motor?

The machine that transforms electrical energy into mechanical energy in the form of rotation is called DC motor. Its movement is produced by the physical behavior of the electromagnetism. The magnetic field used to generate movement is produced by the inductors inside them, or we can say that DC motors are mechanically commutated electric motors that are driven by Direct Current (DC).

32) What are the applications of the Electric motors?

Applications of Electric motors are:

1. Industrial fans.
2. Machine tools.
3. Household Applications.
4. Record Players.
5. Disk Drives.
6. Power tools.
7. Blowers and pumps etc.

33) What is the Universal motor?

The motor which is designed to operate on AC and DC power both is known as Universal motor or Series wound motor. It performs very well on AC because in case of AC the current in both the field and the armature alternates in synchronism manner and hence, the resulting mechanical force occurs in the constant direction of rotation.

34) What are the advantages and disadvantages of the AC motors?

Advantages of AC motor are as follows:

1. AC motors have a low cost.
2. These motors have a long life.
3. These motors have high efficiency.
4. AC motors are available in high rating.
5. It is available in standardized types.

Disadvantages of AC motor are as follows:

1. Variable frequency source is required for speed control.
2. High starting inrush current is there.

35) What are the advantages and disadvantages of the DC motors?

Advantages of DC motor are as follows:

1. DC motors have long Lifespan.
2. These motors require low maintenance.
3. DC motors have high efficiency.
4. DC motors have high holding torque.
5. Precision positioning is there.

Disadvantages of DC motor are as follows:

1. Its initial cost is higher.
2. They require a controller for its operation.
3. DC motors are costlier.

36) What is the slip speed?

Slip speed is the difference between the synchronous speed and the actual speed of the rotor. The formula for Slip is:

S ≜  (NS-Nr)/NS    Per unit (p.u)

37) What are the advantages of skewing of cage rotor conductors?

The advantages of skewing of cage rotor conductors are as follows:

• Skewing of cage rotor conductors helps to reduce the noise during operation.
• The locking of the stator and rotor teeth is reduced because of skewing of cage rotor conductors.

38) How many types of losses occur in a transformer?

Two types of losses occur in a transformer:

• Iron loss or Core loss P_i
• Copper loss or I²R loss P_c

39) What is an iron loss or core loss Pi?

Iron loss is the sum of hysteresis loss (Ph) and eddy current loss (Pe). This type of loss mainly occurs in the magnetic core of the transformer.

P_i = P_h + P_e

40) What is a Copper loss or I²R loss P_c?

The loss which takes place in the primary and secondary winding of the transformer because of the winding resistance is called the Copper loss or I²R loss.

The Total copper loss in the transformer = Primary winding copper loss + Secondary winding copper loss

41) What is Dielectric Loss?

The losses that occur in insulating materials, that is, in the transformer oil and the solid insulation of transformer are known as a dielectric loss.

42) Why do we perform an Open-circuit test and Short-circuit test on the transformer?

The Open-Circuit test and Short-Circuit test are performed to determine the circuit constant, efficiency and regulation of a transformer, without loading the transformer. The power consumption of these loads is very less.