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

Explanation:

• If the motor is squirrel cage induction motor, the number of rotor poles automatically gets adjusted to the number of stator poles
• But if the motor is wound rotor induction motor and number of poles on rotor and stator don’t match then the resultant torque will be zero. And the motor will not run.

Explanation:

• In 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 more the resistance, more the torque. When we add resistance to the rotor the torque is high, slip is high and the current is reduced.
• Starting torque T_st ∝ E x I x R/Z

Explanation:

At standstill rotor, conductors are being cut by rotating flux wave at synchronous speed n_s.

S = n_s -n_r/n_s

At standstill the rotor speed is zero, therefore

S = n_s -0/n_s
S = 1

Explanation:

• The induced torque is maximum when rotor resistance per phase is equal to the slip times the rotor reactance per phase under running condition. i.e R = S × X
• Note:  Maximum torque is inversely proportional to the standstill reactance of the motor therefore to achieve maximum torque X, and therefore inductance of the rotor should be kept as small as possible.

Explanation:

Since at starting period S = 1 therefore condition of maximum starting torque of an induction motor is that when the rotor resistance is equal to the standstill reactance. i.e. R= X .

Explanation:

For constant supply voltage Torque is given as

Low Slip Region of an induction Motor

• For Low slip region “S” is very small therefore the term S.X₂ can be neglected and R₂ is constant therefore

T ∝  S

• Hence in low slip region torque is directly proportional to the slip.
• So as the load increases speed decreases therefore to satisfy the load demand torque increases thereby increase in the slip.
• Hence the graph is in the straight line in nature and this reason is a stable region.

High Slip Region of an induction Motor

• For high slip region slip value approaches to 1 hence the value of R₂ is very small and can be neglected and the X₂ is constant, therefore

• Hence in high slip region torque is inversely proportional to the slip.
• Now when the load demand increases speed decreases. As speed decreases slip increases, therefore, torque decreases.
• But torque must increase to satisfy the newly increased load demand.
• As torque decreases due to extra loading effect its speed further decrease and slip further increases
• Hence same load act as an extra load due to the decrease in the torque produced therefore the speed will keep on decreasing and at last, the motor comes to standstill condition.
• Hence this reason is called an unstable region.

Explanation:

• In squirrel cage motor there is no provision made for adding external resistance because the end rings are permanently shorted with the rotor conductor, therefore, the starting torque of squirrel cage induction motor is very poor due to low rotor resistance.
• Therefore the rotor resistance speed control method is not applicable in squirrel cage induction motor.

Explanation:

• The direct online starting method is used for small motor up to 5 HP.
• The main purpose of any starter is to reduce the requirement of high starting current.
• Star delta operation means, first the motor will run in star after that when the motor is near to full speed then delta operation will be initiated.
• If the motor is started with star connection the phase voltage gets divided by √3 times i.e (phase voltage= line voltage/√3). since the voltage is reduced, the starting current is also reduced.
• Once the motor reaches the rated speed, the connection will be changed to the delta, but now the motor will not draw much current like at the time of starting.
• Note: Since the Torque is Proportional to Square of voltage. A high starting torque is not possible in the star-delta starting method.

Explanation:

• For an AC machine, the air gap flux is proportional to voltage and inversely proportional to frequency.
  flux ∝ V/f
• Maintenance of constant V/f is necessary to keep the load torque of the induction motor constant.
• The magnetic material of core of the stator gets saturated if the flux is increased beyond the limit. This is the reason to keep the V/f ratio constant so that magnetization of core remains the same.
• So, if we maintain a constant ratio of voltage to speed, we are maintaining a constant air gap flux. Because torque is proportional to air gap flux.
• So if the frequency is reduced by 10 % the voltage should also be reduced by 1o% to maintain the constant flux.

Explanation:

The current drawn by the motor is determined by the voltage applied across the terminals and stator impedance i.e air-gap impedance, and the rotor impedance.

So,

I=V/Z

where,
I=current drawn by the motor
V=voltage applied
Z=combined stator, air-gap, and rotor impedances

Z is constant for a given motor.
So, I is proportional to V applied.

When V decreases, I also decrease.

Explanation:

• The starting current of an induction motor is directly proportional to the applied voltage per phase to the motor.
• If low voltage will be applied to an induction motor than the starting current will be also low, lower starting current means lower starting torque.
• The best method to reduce the voltage starting is achieved by means of a star-delta starter.
• In this method, the motor windings are connected in star configuration during the starting period and then switched over to delta when the motor picks up the speed.
• By connecting the motor in the star configuration the reduction of voltage is 1/√3 = 0.577, therefore, the starting current also gets reduced to 0.577.