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# DC machines and Induction Machines Viva Questions & Answers

When DC Generator fails to build up the voltage, what are the reasons?

No residual magnetism, reversal of field connections and the resistance in the load circuit may be greater than critical resistance

What is field flashing?

In case when the dc machine fails to build up the voltage due to lack of residual magnetism, filed coils are connected to the dc source (battery) for short while for magnetizing the filed poles. Application of external source of direct current to the field of the dc machine is called field flashing

Why do we use starter for dc machine?

At the time of starting, no back emf is offered by the dc machine, hence huge starting currents flow in to the machine which damage the armature, in order to avoid the damage starter is employed. By using starter armature current is limited during starting the machine by providing external resistance and while starting this external resistance is removed in steps

Factor that has to be considered while choosing the resistor?

While choosing the resistor for connecting in filed or armature circuit care should be taken that the resistor current rating is more than the full load current rating of the circuit. For example, if rated field current is about 2Amps. Then resistor connecting in the field circuit should be chosen in such a manner that the current rating of the resistor is greater than 2Amps.

What are the different losses in dc machine?

• Armature and field copper loss
• Iron and magnetic losses (hysteresis and eddy current loss)
• Mechanical losses

Drawbacks of Brake test?

Some of the drawbacks of the brake test are:

• This test is performed with small motors only
• Internal losses cannot be determined

Induction Machine Viva Questions

Define Slip in Induction Machine?

Slip is the difference between the speed of the stator rotating magnetic field and the speed of the rotor of the induction motor expressed as a fraction of synchronous speed is known as slip of the induction motor.

Why Induction Machine always runs below synchronous speed ?

Induction motor works on the principle of induction. When both rotating magnetic field of stator and rotor of induction motor rotates with same speed no emf is induced in the rotor conductors resulting no torque is produced in the rotor of induction machine and motor will stop. Therefore in order to have torque for rotating the rotor induced emf is required. Hence Induction motor rotates with speeds less than synchronous speed.

How direction of 3 phase Induction Motor can be changed

Direction of 3 phase induction motor can be changed by changing any two supply phases of the induction motor. Changing any two phases will result in change in rotating magentic field to opposite direction. Therefore rotor direction is also changed.

What are the different starting methods of 3 phase induction motor?

Different starting methods employed while starting 3 phase induction motors are:

• Direct on line starting method
• Primary resistance starting method
• Auto transformer starting
• Star-Delta Starting method

When we can get Maximum Torque in Induction Motor while starting?

Starting torque for 3 phase induction motor is low. In order to increase the starting torque an external resistance should be inserted while starting of the 3 phase induction motor. It is to be remembered that starting torque of slip ring induction motor is high compared to squirrel cage rotor machine due to the provision of adding external resistance while starting the machine.

What are the different losses in 3 phase induction motor?

Different losses in the 3 phase induction motor are stator copper loss, stator iron loss, rotor copper loss, friction and windage loss and stray load loss.

What is the relation between torque and supply voltage in 3 phase induction motor?

Torque T = sV2

What is the direction of rotation of rotor with respect to the rotating magnetic field?

Rotating Magnetic Field is set up when 3 phase ac supply is provided to the stator windings of the induction motor. Rotating Magentic Field rotates at synchronous speed. Rotor rotates in the direction of the rotating magnetic field

What is Slip In Induction Motor?

Speed of the Induction Motor will be always less than synchronous speed and speed of the motor further reduces when load is applied on the motor. The difference between the speed of the stator (synchronous speed Ns) and the actual speed of the motor (N) is called slip of the induction motor and is designated by letter ‘s’. It is represented in RPM. However it is general tendency to represent as fraction or percentage of synchronous speed. What is the speed of the rotor magnetic field with respect to rotor?

Currents flowing through the rotor windings due to induction caused by stator will have a frequency of ( f ‘, which is equal to sf where f is the applied frequency to stator). This current flowing through the rotor sets up a magentic field equal to s x N  with respect to the rotor.

What is the relative speed between the fields set up by the stator and rotor?

Stationary. The speed of the stator field with respect to stator surface will be Ns and the speed of rotor with respect to the stator surface will be N. The speed of the magentic field of the rotor with respect to the rotor will be s x N. Therefore the speed of the rotor field with respect to stator surface is equal to rotor speed of the motor and rotor field with respect to rotor.

N +  s x N= N( 1-s ) + s x N=  N

Therefore the relative motion between stator field and rotor field will be stationary or zero.

What happens when Induction motor is loaded?

During no load operation the relative speed between stator magnetic field and rotor speed will be less (means slip is less). Hence small emf is generated which generates small currents and torque will be generated  in such a manner sufficient to overcome the friction and windage losses.

When motor is loaded (Load applied to rotor), motor torque reduces as the load torque increases. Hence motor slows down (slip increases). As the motor slows down, the relative speed between rotor and stator magnetic field increases. This results in greater motor currents and generates greater motor torque to overcome the load torque. Thus as the load on the induction motor increases, motor slows down until sufficient torque is generated to overcome the applied load torque.

Why Induction motors are widely used in Industries?

In Industries 80% of the motor drives used are induction motors. Induction Motors are widely used in Industries because of some features it possess.

They are:

• Low Cost
• Simple and Rugged Construction
• Absence of Commutator
• Good power factor
• Higher efficiency
• Good Speed Regulation

What is the relation between between  Induction motor and Transformer?

Induction motors are considered as transformers with secondary winding rotating. This is because both transformers and induction motor operates in such a manner that power is transferred to secondary windings (rotor) from primary windings (stator) by mutual induction principle.

Why the efficiency of the Induction motor is less compared to transformer?

Induction motor and Transformer operates on the same principle that power is transferred to secondary by mutual induction. But in transformer, flux will be traveled from primary to secondary winding through the core of the transformer. In induction motor flux travels from stator to rotor through the air gap. Therefore power factor will be less for induction motor compared to transformer which also results in less efficiency for induction motor compared to transformer.

Why Speed Control of Induction Motor is difficult compared to DC shunt motor?

Speed Control of DC shunt motor can be possible by providing resistance in the armature and field circuits and varying the resistors speed changes can be carried out over a wide range. On the other hand speed control of induction motor is costlier and bit tedious.

Speed of Induction motors = (120x f x (1-s))/p.

Therefore for speed control either supply frequency should change or number of poles should change and other methods can be employed. Care should be taken that in the process efficiency and power factor should be maintained.

Explain the operation of induction motor in simple words?

When 3 phase ac supply is given to stator of the induction motor rotating magnetic field will be generated. This magnetic field (flux) cuts the stationary conductors of the rotor of induction motor. Therefore an emf is induced by Faraday’s law of electro-magentism. As the rotor conductors are short circuited, current starts flowing in the windings of the rotor this produces a magnetic field. Due to interaction of both the magnetic fields a torque will be developed. If the torque generated is sufficient to overcome the load torque rotor starts rotating.