100 Most Important 3-phase Induction Motor MCQ with Explanation | Polyphase induction motor MCQ With Explanation

Explanation:

Cogging Effect ( Magnetic Locking) in an induction motor

• During the starting of squirrel cage induction motor if the number of stator slots is equal to the number of rotor slots or an integral multiple of rotor slot then it causes magnetic locking between stator and rotor slot.
• The upper and lower attracting force between the stator and rotor slot becomes more prominent than the tangential force, as a result, the rotation of the motor is stopped.

Explanation:

P = 120f/N

= 120 x 50/ 1400

= 4.28

The number of poles should be even and the whole number. Therefore the number of poles must be 4.

Explanation:

In the above question i.e Ques 12 we already find out number of poles i.e 4
N_s = 120f/P
= 120 × 50/4
= 1500
and slip is equal to
s = N_s − N_r/N_s
= 1500 − 1440/1500
= 4%

Explanation:

• The air gap is one of the main concerns for designing any type of electrical motor. The efficient operation of electromagnetic devices requires low resistance passage of magnetic flux so that it can minimize the amount of electrical energy needed to create the magnetic field.
• However, gaps in the magnetic circuit are normally unavoidable. The air gap needed to separate the revolving rotor from the stator should be as small as possible to reduce the magnetizing power requirement.
• If the air gap is increased, it will offer high resistance to magnetic flux and the stator will draw more magnetizing current from the supply to maintain the required flux in the air gap to the rotor. This will reduce the power factor of an induction motor or any other electrical motor.

Explanation:

• An induction motor is a single excited machine. The crawling word it self-suggest crawl means moving with low speed.
• This characteristic is the result of improper functioning of the motor that means either motor is running at very slow speed or it is not taking the load.
• The resultant speed is nearly 1/7th of its synchronous speed. This action is due to the harmonics fluxes produced in the gap of the stator winding of odd harmonics like 3rd, 5th, 7th, etc.

Detail Explanation:

• The harmonics fluxes produced in the gap of the stator winding generate odd harmonics like 3rd, 5th, 7th, etc.
• These harmonics create additional torque fields in addition to the main torque.
• The torque produced by these harmonics rotates in the forward or backward direction at Ns/3, Ns/5, Ns/7 speed respectively.
• The torque produced by the 5th harmonic rotates in the backward direction. The torque produced by fifth harmonic works as a braking action in small quantity which can be neglected.
• Now the seventh harmonic produces a forward rotating torque at synchronous speed Ns/7.
• Hence, the net forward torque is equal to the sum of the torque produced by the 7th harmonic and main torque.
• The torque produced by 7th harmonic reaches its maximum positive value just below 1/7 of Ns and at this point slip is high.
• At this stage, the motor does not reach up to its normal speed and continues to rotate at a speed that is much lower than its normal speed. This causes crawling of the motor at just below 1/7 of normal speed.

Explanation:

Auto-transformer starting of an Induction Motor.

• The auto-transformer starting method is also known as the reduced-voltage reactor method.
• The auto-transformer method reduces the initial voltage applied to the motor and therefore the starting current and torque also get reduced.
• The motor can be connected permanently in delta or in the star, is switched first on the reduced voltage from a 3-phase tapped auto-transformer and when it has accelerated sufficiently, it is switched to the running (full voltage) position.
• The principle is similar to star/delta starting and has similar limitations.
• The advantage of this method is that the current and torque can be adjusted to the required value, by taking the correct tapping on the autotransformer.
• This method is more expensive because of the additional autotransformer.

Explanation:

• If we interchange any two phases, you can reverse the motor running direction. i.e. say R-Y-B motor running clockwise, change to R-B-Y, the motor will rotate counter-clockwise.
• However, the motor should be stopped before phase interchange is done. If it is done during running conditions, a braking torque will be produced before the change in direction of the supply is prolonged.

Explanation:

Why is an induction motor self-starting?

• 3 phase induction motor is self-starting because it consists of cage windings of parallel conductors (made up of copper or aluminum) short-circuited at the ends by end rings, hence forming a closed path for current flow.
• And for SLIP RING induction motor the ends of the rotor windings are externally connected by a variable rheostat (resistance is varied in order to give it proper starting and running current).
• So when 3 phase ac supply is given to the stator windings of the induction motor, RMF (revolving magnetic field ) is produced, that links with the conductor of cage rotor and produces emf, so a current start flowing through the conductors, hence torque is developed which causes the rotor to rotate.

Explanation:

• When 3 phase supply is given to the stator of a 3 phase induction motor, rotating stator flux will be produced, which will induce emf in the rotor windings, according to Faraday’s law of electromagnetic induction.
• Now if the rotor windings are kept open-circuited, no current will flow in these windings. The question is why we need rotor currents?
• This is because to run the motor and produce torque from it. The magnitude of torque produced depends on the product of stator flux and current in the rotor coils.
• Due to short-circuited rotor windings, short circuit rotor currents will flow in them, giving rise to their own magnetic field linking with the stator flux. And here we get a running motor.

Explanation:

• An induction motor is a single excited machine. The crawling word it self-suggest crawl means moving with low speed.
• This characteristic is the result of improper functioning of the motor that means either motor is running at very slow speed or it is not taking the load.
• The resultant speed is nearly 1/7th of its synchronous speed. This action is due to the harmonics fluxes produced in the gap of the stator winding of odd harmonics like 3rd, 5th, 7th, etc.