Question

Questions 7 - 10

Answer 1

Q.7

Synchronous Machine=SM

Induction Machine =IM

The Synchronus and Induction machines are AC machines.Both have a stator and rotor.The Stator in both SM and IM is alike.Bothe machines depend upon the production of rotating magnetic field for their operation.And this rotating magnetic field is produced because of balanced 3 phase current in the stator windings.Therefore the stator construction is similar. Stators of both SM and IM are made of magnetic core, three-phase AC balanced windings, the base to fixing core, the terminal cover and other things are same.

The major difference comes in the Rotor.Rotor of SM needs excitation and this excitation is provided by an external DC source,hence the presence of slip rings is a must in SM.The rotor core   is laminated by the steel sheets that are punched into by steel plates.Some SM have permanent magnet rotors that distinguishes them from IM.On the other hand IM have a rotor in which the Rotor windings are short circuited.We can have a wound rotor or a cage rotor.The cage rotor is just a cage of shorted bars.This makes it totally different from a SM.

If we are using a wound rotor then it also has slip rings which are then shorted or to which resistors can be connected for speed control.

Q.8 The operational differences are as under

A SM works on synchronous speed.Its speed determines the

Frequency of supply if it is a generator or its speed is determined by the supply frequency if it is a motor.

It follows the realtion Ns=120f/P

Important point to remember is that a Synchronous motor is not self starting,while an induction motor is self starting.

Another difference is that an induction machine always works at lagging pf.It always consumes lagging vars from the grid.While a synchronous machine can operate at lagging ,leading pf.That is it can act as both source and sink of reactive power.

The speed of Synchronous machine doesn't vary with load torque.It always rotate at Ns.While an induction machine's speed can vary and can be controlled easily.The work for maintaining synchronous machine (specifically a motor)is large. Therefore, as a motor, the induction motor is the choice at the most time.

An induction machine doesn't run on synchronous speed,rather it slips behind (motor ) or above (generator) the synchronous speed.The IM doesn't produce torque on Synchronous speed as there would be no relative motion between the rotor windings and the rotating magnetic field.After the power is switched on, the stator produces a rotating magnetic field which rotates at a synchronous speed. The rotor bar is cut by the rotating magnetic field which produce an induced current,and hence a torque is produced which drives the rotor to catch the rotating magnetic field.

The slip is given by s= Ns-N/Ns

Q.9

The emf equation of An alternator is given by E=4.44 N (phi) f

The value is in rms.N is number of turns.Phi is the flux.f is the frequency.As the field current increases the flux, phi increases which basically means that the Induced EMF will increase.Not only this increasing the excitation causes an increase in the reactive power.If alternators are in parallel then the load sharing of reactive power also changes .The alternator whose Excitation increases shares more reactive power.

Q.10

As the speed is increased the frequency of the supply also imcreases as Ns=120f/P. Now as the frequency increases the induced emf also increases and hence the terminal voltage of the alternator also increases.

Hope this helps.