Three Phase Induction Motor - Electrical Engineering (EE) MCQ

Concept:
In an induction motor,

• The stator is stationary and the stator field rotates with synchronous speed.
• The rotor field rotates with synchronous speed and the rotor rotates at a speed less than the synchronous speed.

Application:

• Given that, the speed of the rotor is 960 rpm
• The synchronous speed will be more than the rotor speed and the difference is very less and hence the speed will be 1000 rpm.

Important Points:

• In a synchronous motor, the rotor and rotating magnetic field of stator rotate at the same speed and this speed is known as synchronous speed.
• In an induction motor, the rotor rotates at a speed less than the rotating magnetic field of the stator and the difference between these two speeds is called slip speed.

Slots in Induction motor:

• The speed of the induction motor is inversely proportional to the load torque. In semi-closed and closed slots, the air gap between the stator and rotor is small as compared to open slots. As the air gap is small, the requirement of magnetizing current to establish the flux in the air gap is less.
• It results in improved power factor In order to semi-closed slots or totally closed slots are used in induction motors.
• Among all the three types of slots, semi-closed type slots are preferred for induction machines as semi-closed slots having the partial advantages of open type and partial advantages of closed type slots.
• Open-type slots are generally preferred for synchronous and dc machines.
• In general, closed type slots are used in low hp motors, to control the starting current, as the leakage reactance offered by closed type slots is very high compared to other types of slots.
• Large size induction motors use open slots so that already prepared and properly insulated coils can be easily inserted in open slots.
• In order to improve the power factor semi-closed slots or totally closed slots are used in induction motors.

Induction generator always works with leading power factor since it will take large amount of reactive power to produce sufficient amount of working flux so that armature reaction is always magnetizing hence it will work always with leading pf

Important Point

• induction generator is basically an induction motor, which runs above the synchronous speed
• when it acts as a generator it will supply the active power back to source, but for this supply of active power it needs reactive power as input to keep its winding excited
• in case if the induction motor is connected to the grid, it will draw the required reactive power for the excitation of windings, but if it is standalone system (ie. not connected to grid) then a capacitor bank will be always connected, and this will provide leading reactive power to keep the winding excited for the process of mechanical to electrical energy conversion
• since the reactive power is supplied by the capacitor, the induction generator is operating in leading power factor

• Starting torque is directly proportional to rotor resistance
• Squirrel cage induction motor has very low starting torque due to its rotor resistance of very low value
• We can’t add any external resistance as its rotor is short circuited through end rings
• But in case of slip ring motor, we can add external rotor resistance
• By increasing rotor resistance, we can shift the slip for max torque towards 1 slip, so, for a particular value of external resistance it is possible to achieve max torque during starting
• Slip ring motor is preferred over Squirrel Cage motor when high starting torque is required
• Squirrel cage motor has better running torque compared to slip ring motor

Concept:

• If a balanced three-phase voltage is supplied to balanced three-phase windings in the stator of an Induction motor, the resultant flux remains constant in magnitude but rotates at the synchronous speed.
• Synchronous speed is related to the supply frequency and number of poles, for which the winding (stator) has been designed. This is termed as a rotating magnetic field formed in the air gap of the motor.

Where N_s = Synchronous speed in rpm
f = Supply frequency
P = Number of poles

• The direction of rotation of resultant flux in the air gap depends upon phase sequence. It rotates from the leading phase to the lagging phase.
• Frequency of rotating magnetic field is same as that of the supply frequency, while rotor frequency is slip times of supply frequency. 

Concept:
Rotor resistance method of speed control:
Under load condition torque approximately
T ∝ (sV₁²) / (R₂ + R_e)

• In this method, some external resistance is inserted under the load conditions.
• Then the slip of the induction motor increases to maintain the load torque constant.
• As slip is increased, the speed of the motor will be reduced to below the rated speed.
• In this method, the motor acts as a constant torque variable power drive.
   

For torque constant(T = k)

For induction motor, the torque in rotor resistance control methods is given as

For full load torque s α R₂

= 0.1055

The torque in a three-phase induction motor is given by,

Observations:
1. At standstill, 
Torque is directly proportional to the square of the supply voltage.
2. For the low values of slip, i.e. for the speeds close to the synchronous speed
 i.e. the effective rotor circuit resistance is very large compared to the rotor reactance.

T ∝ s
Torque is directly proportional to slip. Thus, torque-slip characteristics are linear in the lower slip region.
3. For the high values of slip, i.e. for the speeds away from the synchronous speed
 i.e. the effective rotor circuit resistance is very small compared to the rotor reactance.

T ∝ 1/s
Torque is inversely proportional to slip. Thus, torque-slip characteristics are rectangular hyperbola at higher slip region.

Concept:
Single phasing:

• A three-phase motor must be connected to the rated load voltage and load for proper working. If due to some reason, one phase of the motor gets disconnected, the motor will continue to run from the active 2-phase supply. This is called single phasing.
• The motor will continue to run with vibration and reduced speed. However, depending upon the loading condition, the motor may/may not start on two phases.
• It will work satisfactorily if the motor carrying the load not more than 0.5 times rated load
• Single phasing is not desirable for the proper operation of the induction motors and appropriate measures should be taken to protect the machine.

Causes of single phasing:

• One of the three back up fuses blow (or fuse melts)
• One of the conductors of the motor is open-circuited.
• Wrong setting of the protection device provided on the motor.
• Relay contacts may be damaged or broken.

Effect of single phasing:

• Motor runs with reduced speed.
• It operates with uneven torque and produces a humming noise.
• Due to loss of current from one phase the current flowing through the remaining two phases increases. The winding insulation, at times, may not be designed to withstand the increased current and heat thereby damaging the insulation and causing short-circuited between the winding and the motor burns out.
• It may cause an overloading of the Generator.
• If the motor is arranged for stand by and automatic starting than the motor will not start and if the over relay provided fails to function the motor may burn.

Concept:
Slip in Induction Motor (s)

Where,
N_s is the stator frequency
N_r is the rotor frequency
Also, fr = s fs     ---(2)
Where,
f_r is the rotor frequency
f_s is the supply frequency

Explanation:
The rotor is locked means Nr = 0
So, from equation (1), s = 1
From equation (2),
f_r = f_s
Therefore, the rotor frequency of the induction motor = supply frequency.

When 3-ϕ induction machine working as an induction generator, then 
Slip (s) = 
Where, 
N_s = Synchronous speed
N_r = Rotor speed
Frequency of rotor current = s × f
Where s is the slip
f is the supply frequency

Calculation:
Given that,
Supply frequency (f_s) = 60 Hz
Rotor current frequency (f_r) = 5 Hz
Number of poles = 4
Synchronous speed, 
We know that,
f_r = (s) f_s
⇒ 5 = (s) (60)
⇒ s = 1/12
To work as an induction generator, rotor speed should be slip speed greater than synchronous speed, therefore 

⇒ N_r = 1950 rpm