Interview Question on Electrical Motor

1. What are the types of AC three phase motors?

Mainly there are two types.

1. A) Synchronous motors.
2. Plain synchronous motors.
3. Auto synchronous motors.
4. B) A-synchronous motors.
5. Induction motors.

Single phase motors

• Shaded pole motor.
• Capacitor start capacitors run motor.
• Capacitor start induction’s run motor.
• Split face motor.

Three phase motors.

• 3φ single squirrel cage motor.
• 3φ double squirrel cage motor.
• Squirrel deep bar induction motor.
• Slipring induction motor.

1. Commutator motors.

Single phase commutator motors.

• Plain repulsion motor.
• Repulsion start induction’s run motor.
• Repulsion induction motor.
• Series motor or universal motor.

3φ commutator motors.

• 3φ series motor
• Charge motor.
• Compensated motor.

2.What is the working principle of 3φ induction motor?

When 3φ supply is given to stator, a rotating magnetic field of constant magnitude is produced. This rotating magnetic field produces induced emf in the rotor winding as per faraday’s laws and this induced emf causes to circulate a heavy induced current in the rotor winding due to very small resistance of rotor. At the initial moment the frequency of induced emf is equal to the frequency of the stator supply voltage, when the rotor is stationary as in the case of secondary of a transformer. The rotor induced current according to lenz’s law flows in such a direction that it opposes the cause, which is inducing it. In this case the cause producing the rotor current is the relative speed between the rotating magnetic field if stator and the rotor and is maximum when the rotor is stationary. Hence to reduce this relative speed rotor conductor (rotor) starts to rotate in the same direction in which the stator field is rotating and tries to catch it up. The rotation of this rotor is developed due tog the torque developed in the rotor by interaction between the rotating magnetic field of stator and the field produced by the rotor current.

3.What is torque?

As said above torque is a turning or twisting moment of a force about an axis and it is measured by the product of force * radius at which the force acts. There are two types of torques. a. Starting torque: This is the torque, which is required to start the motor at load or no-load. b. Running torque: This is the torque, which is required to run the motor at normal speed and at normal load. The letter ‘T’ denotes torque in induction motor and torque is proportional to Ir φ cosϕr. That is T ∝ Ir φ cosϕr. Where Ir = rotor current. φ = Flux = stator flux per pole in Weber. Cosϕr = rotor power factor.

4.What is slip?

The difference in speed of stator magnetic speed ‘Ns’ (synchronous speed) and rotor speed ‘Nr’ is called slip or absolute slip and it is denoted by the letter ‘S’. ∴ S = Ns – Nr / Ns. Slip has no unit. Percentage of slip of induction motor varies from 4 to 5% in small motors and 1.5 to 2.5% in big motors. In other words slip ‘S’ = fr / f. Where fr is rotor frequency and f is stator frequency.

5.What is the working principle of double squirrel cage induction motor?

In double squirrel cage motor outer cage rotor winding is of high resistance and low reactance. Inner cage winding is of high reactance and low resistance. At the time of starting rotor frequency is equal to the stator frequency and there by the reactance of the inner cage winding is comparatively high (XL = 2πfL) because it is linking more inner winding than the outer winding. So the impedance of inner cage winding is very high. Hence the current flow through inner cage winding is very less comparing to the outer cage winding. That is a very high ratio of current is passing through the outer cage winding at the time of starting and there by produces very high starting torque. When the rotor starts running the speed of the motor can be increased and the slip will be decreased and there by the rotor frequency (‘S’ = fr / f). So that in the running condition the reactance of the inner cage decreases to the lowest value and hence the Impedance (XL = 2πfL). So the current in inner cage winding will be comparatively more than the outer cage winding at the time of running. So now inner cage winding produces more torque than outer cage at the time of running and the motor running torque is good enough.

6.Why starter is necessary to start the AC motor?

At the time of starting motor starting current is high (4 to 5 times). Therefore if motor is directly started the supply voltage may be disturb.

By the help of starters starting and stopping of motors can be made easily as we required. Because starters provides overload tripping difficulties.

The help of starters can protect motor against the single phasing by the action of overload arrangements.

Protect the motor from no-voltage and its difficulties.

Permits automatic control when required.

7.What are the types starters used for starting of induction motor?

Direct on line starter (air break) mechanically.

Direct on line starter (air break or oil immersed) electrically.

Star delta starter.

Slipring motor starter.

Auto transformer starter.

8.What are the speed control methods of induction motor?

By controlling the supply voltage.

By controlling the supply frequency (Ns = 120f / P).

By varying the number of poles (Ns = 120f / P). d. By rotor rheostatic control (for small speed variation).

9.What is magnetic locking or cogging effect of induction motor?

In squirrel cage induction motor some times the rotor and stator care teeth or slots are comes face to face or parallel at stationary condition. If we are starting the motor at this condition the motor get hesitated to start or run due to the attraction developed between those rotor and stator teeth or slots. This is known as the magnetic locking or cogging effect of a squirrel cage induction motor. This type of magnetic locking in squirrel cage induction motor can be avoided either by skewing the rotor slot or by selecting the rotor slot, such that there is no common factor between the rotor slot and stator slots.

10.What is skewing?

Skewing can be done by turning the rotor slots about 15° from the parallel position of slots with the shaft. That is rotor slots are not in parallel with the shaft but there is an angle of about 15° with the shaft.

11.What are the losses in induction motor?

Stator losses (stator copper losses, stator iron losses).

Rotor losses (rotor copper losses, rotor iron losses).

Windage and friction losses.

12.What is synchronous motor?

An alternator, which is running as a motor can be called as synchronous motor and it runs at synchronous speed while it converts electrical energy into mechanical energy. It requires both AC for armature and DC supply for field.

13.What are the advantages and dis-advantages of synchronous motor?

Advantages

1. It’s a constant speed motor and is equal to synchronous speed from no load to full load.
2. It has good efficiency higher than induction motor.
3. It can be run as a motor and also as an alternator as per the requirement. More over it can be used as synchronous condenser.

Dis-advantages

1. It can not be used as a varying speed motor. Because its speed can not be varied.
2. As a motor it is not self-starting type and it can not be started on load.
3. It requires both AC and DC supply.
4. Hunting is also produced in this motor.

14.What are the applications of synchronous motor?

These motors are used in powerhouses, in sub stations for the improvement of power factor by connecting it in parallel to the supply and it is run without load under over excitation of field.

Used in big industries where many induction motors are installed to improve the power factor.

Used for constant mechanical loads.

15.What is hunting effect?

When the load is varied to the motor the oscillation being setup in the rotor about the position of equilibrium corresponding to change of load condition. So the damper winding acts the magnetic lines of force and causes to create the opposite torque, which keeps the rotor in the same position of the particular load. This oscillation of the rotor is known as Hunting or Phase swinging. To reduce this hunting damper winding is helpful.

16.What is synchronous condenser or phase advancer?

An over excited synchronous motor takes leading current just like a condenser and gives leading power factor. A synchronous motor, which I used only for the purpose of improving power factor, can be called as synchronous condenser or phase advancer.

17.Why single-phase motors are not self-starting?

When a 1φ supply is given to the single winding of the single phase motor, the field produced by it changes in magnitude and direction sinusoidal (pulsating flux).Such and alternating field is equivalent to two fields of equal magnitude and speed rotating in opposite direction. Such rotating magnetic fields produces two torque’s on the rotor. So the rotor can not rotate in any direction. Because the net torque developed by the motor is equal to zero. So a single-phase motor is not self-starting.

18.What are the methods to self-starting of single-phase motor?

Splitting one phase into two phases.

By using capacitor.

By using repulsion method.

By shading the poles. e. By connecting the field in series with the rotor having winding with commutator (AC series motor or universal motor).

19.What are the methods to control the speed of single phase motor?

In AC single-phase motors speed control can not be achieve as smooth as in DC motor. There are following few methods of speed control.

1. By changing the number of poles of stator.
2. By changing the applied voltage to the stator.
3. Frequency control method.
4. Rotor rheostat control.
5. By operating two motors in concatenation or cascade or tandem method.
6. By injecting an emf in the rotor circuit.
7. By changing slip.

20.What is the effect of frequency and high voltage at the start of motor.

For a constant load if frequency decreases motor current will increase and at the start if voltage is more motor current also increases.

21. What is the minimum voltage required for starting of 6.6 kV motors?

Minimum 80% of rated voltage.

22. What are the limits of vibration measurement for motors?

50 microns for displacement and 5 mm/second for velocity.

23. What you mean by SPDP?

Screen protected drip proof.

24. What is the current in single phasing?

2 times of rated current.

25. What is the impedance per phase of delta connected motor?

1.5 times the total impedance

26.What are the effects of unbalanced voltage on induction motor?

There will be negative sequence current, which will heat up the stator winding and weakens the insulation. These currents will induce emf in rotor and heat up the rotor bars and cause breakage in them. Also due to high leakage fluxes due to negative phase sequence current the end parts heating will be more.

27. What are the reasons for high current in motor? High frequency – at 51 Hz current will be 105% of the normal current.Low frequency – at 47.8 Hz current will be 102% of the normal current., High voltage, Under voltage, Mechanical over loading, Process requirement.

28. What are the reasons for unbalanced current in motor?

Loose power cable connection.

Voltage unbalance.

Short-circuited turns of coils of winding.

29.What are the reasons for vibration in the motor?

Vibration could be because of mechanical faults and electrical faults.

Mechanical faults.

1. Wrong alignment of the motor on foundation.
2. Wrong installation.
3. Improper fitting of bearing and cooling fans.
4. Periodic impulse loads such as reciprocating compressors.
5. Pulley of heavy weight which cause bending of motor shaft resulting in non uniform air gap.
6. Damage of bearing or bad bearing.
7. Bad coupling.
8. If the axial alignment of the motor and the driven machine is incorrect and rotor is allowed to contact its axial stops, high axial vibrations may occur, together with high bearing temperature high and even bearing failure.
9. Machine base and foundation problem. j. Under sized bearing.
10.  Electrical faults.
11. Air gas dissymetry.
12. Broken rotor bars.
13. Slackened stator core.
14. Slackened rotor core.
15. Interturn short in the rotor winding in the two-pole machine.
16. Unbalance in rotor winding.
17. Unbalance power supply voltages. If the vibration is because of electrical fault, de-energise the machine and watch the vibration as it runs down

30.What are the reasons for winding temperature high in the motor?

For motors having class – B insulation the temperature should not be more than 110°C and for motors having class – F insulation the temperature should not be more than 130°C. In case temperature is more, then the following could be the possible reasons.

1. Electrical overloads.
2. Over and under voltage.
3. Over and under frequency.
4. Voltage unbalance. Voltage unbalance create unbalance of currents and increase in temperature which will be 2*(% voltage unbalance)* (% voltage unbalance)*.
5. Loose connection at motor terminals.
6. Unbalance current.
7. Single phasing (if OLR protection is not working).
8. Long acceleration cycle.
9. Unusual system grounding conditions.

31.What are the reasons for bearing temperature high?

Temperature of bearing should not be more than 90°C. In case temperature is higher than the 90°C the following could be the possible reasons. a. Inadequate lubricants inside the bearing. b. Faulty bearing. c. Bearing is jammed. d. Over greasing. e. Improper grade of lubricant

32.What are the reasons for harmonics in the motor?

Generally even harmonics are not expected to be present in three phase motors. Triple-n harmonics like 3rd, 9th, 15th etc. are also not expected. The dominant odd harmonics expected are 5th, 7th, 11th and 13th etc. Presence of strong 2nd harmonics indicates unbalance voltage, unbalance winding impedance, rotor defects, magnetic imbalance, faulty rotor skewing etc. Very strong 3rd harmonics indicates magnetic saturation, ground leak currents, overloads etc. Overloading causes overheating, resulting in non-linear magnetization which gives high 3rd harmonic winding faults, short circuits. Hot spots in rotor or stator also may indicate higher harmonics.

33.What are the possible reasons for motor to take long acceleration time?

Following may the possible reasons for motor to take long acceleration time. a. Excess loading. b. May be rewound motor with poor quality of winding conductor having high resistance. c. Defective squirrel cage rotor. d. Applied voltage is too low.