ELECTRICAL QUESTION & ANSWERS (10)

Q1) Can I identify a motor or generator by just seeing it?

Yes, you can identify whether it’s a motor or a generator. In terms of design of the machine, there is no difference between a motor and a generator. Difference lies in how a machine is connected and controlled.

IDENTIFICATION TECHNIQUE OF A GENERATOR:

If you see a prime mover (such as turbine, steam engine,another motor) connected to shaft, it’s a generator.

IDENTIFICATION TECHNIQUE OF A MOTOR:

If you see a mechanical load (such as pump, fan) connected to the shaft, then its most probably a motor

Q2) Working principle of dc motor?

The working principle of dc motor is “Whenever a current carrying conductor is placed in a magnetic field, it experiences a mechanical force”. The direction of this force is given by Flemings left hand rule.

When armature windings are connected to dc supply, current sets up in the winding. Magnetic field may be produced by field winding ( by electromagnetic induction) or by using a permanent magnet. In this case, current carrying armature conductor experiences force due to magnetic field.

Q3) Classification of dc machines?

Every dc machine can work as a motor or a generator. Hence this classification is valid for both. Dc machines are classified on the basis of field excitation method. This makes two categories:

1)     SEPERATELY EXCITED: field winding is fed by some external source

2)     SELF EXCITED: field magnet winding is supplied current from the output of the generator itself is called a self-excited generator

A)   SERIES WOUND : Field winding is connected in series with armature

B)    SHUNT WOUND : Field winding is connected in parallel with armature

C)   COMPOUND WOUND:

? SHORT SHUNT in which only shunt field winding is in parallel with the armature winding.

? LONG SHUNT in which shunt field winding is in parallel with both field and armature winding.

Q4) what is the difference between dc motor and dc generator?

ANS) Difference between dc motor and dc generator is that, in dc motor emf is created across its terminal by an external source where as in dc generator, emf is created across its terminals by principals of dynamically induced emf

Q5) Difference between synchronous and induction machine? What is slip?

ANS) SYNCHRONOUS MACHINES:

In synchronous machines, rotor( rotating part) moves in synchronism with the stator( stationary part) i.e. speed of the rotor is equal to the speed of the stator. The speed at which it runs is called as synchronous speed.

INDUCTION MACHINE:

In induction machines, rotor (rotating part) moves slightly less than stator ( stationary part) i.e. speed of the rotor is less than the speed of the stator. The difference between stator and rotor speed is called SLIP

Q6) Working of induction motor?

An induction motor always runs at the speed less than the synchronous speed. Supply is provided to the stator of IM and it produces a magnetic field. These magnetic field lines interact with the rotor and induces and EMF in it by principles of electromagnetic induction.

Since, lagging of flux current in rotor with the flux current in stator, hence induction motor can never runs at synchronous speed.

It is called as induction motor because electricity is induced in it by principles of electromagnetic induction rather than providing direct supply.

Single phase induction motor are not self-starting whereas 3 phase induction motor are self-starting

Q7) why single phase Induction motor is not self-starting and 3 phase is self-starting?

SELF STARTING:

Self-starting means, the rotor in an IM doesn’t requires any external push or external source to rotate. For self-starting we need a 3 phase magnetic field

3 PHASE IM:

In 3 phase IM, we provide 3 phase supply to the stator and it produces a 3 phase magnetic field. Rotating magnetic field is having the same phase difference of 120 degrees. These magnetic field lines interact with rotor and induces an EMF in it.

If we consider three phases than when phase A is magnetized than rotor will move towards phase A winding. Then phase B will magnetized then rotor will move towards phase B winding. Then phase C will be magnetized and rotor will move towards phase C winding. So rotor will continue to rotate

SINGLE PHASE IM:

Single phase IM produces alternating magnetic field hence they are not self starting and to make start them several methods are used such as:

·        Split Phase or Resistance Start.

·        Capacitor Start Motor.

·        Permanent Split Capacitor (PSC) Motor.

·        Capacitor Start Capacitor Run Motor.

·        Electronic Starter Circuiy.