A Synchronous machine can generate electricity even if the speed of the shaft is less than synchronous speed. But an induction machine starts generating only when its shaft is rotated with a speed greater than synchronous speed.
What makes the difference in these two types of machines, that one machine can generate electricity at any speed of shaft(but frequency of output power varies with the variation of speed of shaft), whereas the other one (induction machine) doesn't give any output power before a certain value of speed of shaft??
When IM is operating on the speed below syn speed the slip of the machine is positive, and this will be termed as Induction Motor. But when the speed of rotor is above the syn speed then the slip become negative and induction machine works as Generator. This is the advantage IM that we can use it as a Motor as well as Generator.
Stator and winding of both induction and synchronous machines are same(induction machine can be used as synchronous machine by changing only, the rotor assembly). Both work on rotating magnetic field developed by stator winding. Connected with mains, induction machine work as motor with positive slip, and as generator with negative slip. Synchronous machine work as synchronous motor with trailing(-ve) torque angle delta, and as synchronous alternator with leading(+ve) torque angle delta, with respect to pole axis of rotating field, with rotor pole axis . When not connected with mains, as there is no magnetizing current, induction principle fails, while with synchronous machine (if pole/rotor magnetic field exist(parmanant magnet) or field applied) depending on rate of cutting of flux by stator winding EMF will induced in winding, frequency will depends on speed of rotor, and magnitude on speed and pole strength like D.C generator.
@ Mr.Desai
If an induction machine is used as motor for a long time, If it develops a residual magnetic field in its stator/rotor windings , can it now generate electricity, with its shaft rotating at speed less than synchronous speed?
Also an induction machine works as a generator (with -ve slip),when the shaft is rotated at speed higher than sync. speed, how is the current induced in the stator windings, without any excitation source?
For better understanding, please draw the full circle diagram of the induction machine you used, find power out put for different negative slip, and compare it, with practical results, will clear your concept. Normally induction generator are used in wind power generation, working continuously without trouble.
Perhaps the following might help.
1. Generate electricity is different from generator/ motor. a voltage
is generated if there is a change of flux linkage in a coil or there is a
relative motion of a conductor in an appropriately directed magnetic field.
You can observe this in alternators as the field which is fixed to say the
rotor sweeps past the stator conductors . The same is true of an Induction machine
, the difference is the rotating field is produced by spatial and temporal postions
of windings so in a slip ring motor you can see this "induced" voltage even when the
rotor is at standstill (with windings open). Now if the rotor is rotated by external means till it reaches the synchronous speed the relative motion becomes zero so we get zero induced voltage. We can continue to rotate the rotor at a higher speed then again we have caused a relative motion so we do get induced voltage in the rotor.
This explains generation of electricity , so obviously if there is a remanent field attached to the rotor there will always be a relative motion between the
rotor field and stator conductors (exactly like an alternator ) so we get a
voltage at the stator.
2. Generator in normal understanding means it is able to supply power
as opposed to motor which consumes power. In machines this means
as a motor a torque is produced to drive external loads such as pumps etc.
However if an external torque drives the rotor and energy is being put
into the machine, for example a turbine, then the machine can generate
electrical power. So in our context here generator = Mech energy converted to
electrical energy and motor =Electrical energy converted to mechanical energy.
For an intuitive explanation see 3. below.
3. Think of two cars A in front towing a car B coupled by a spring.
In this situation the spring stretches and A pulls B . The spring is analogus
to the interaction of two magnetic fields(with angular separation) of the stator and rotor (and as all things electrical it is invisible that is the wonder of electrical engineering). Now if B is able to move at the same speed as A then there is no stretching of the spring and force is not transmitted . This is what happens at synchronous speed in the induction machine , there is no induced voltage, no rotor current . no spring!!.so no conversion of energy. Next imagine B moves faster , now the spring compresses and B is pushing A, B is supplying energy to A , exactly a
similar situation occurs at super synchronous speed the rotor field moves
ahead of the stator field and the rotor mechanical energy is converted to
electrical power. Of course as in all analogies there are limitations . Here
the stator produces currents while supplying energy electrically to balance the
external driving torque.
4. To have a strong basis for understanding machines(or anything electrical ) basic laws of Ampere and Faraday put together in the well known Maxwell equations is a must.
hope this helps
Cheers
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