If a syn. motor run at less then syn. speed what will happening

Sandeep,

By definition a synchronous motor either runs at synchronous speed or it is running out-of-step, a very undesirable situation. The most common causes are either loss of excitation or a severely overloaded motor.

Alan

Yes Mr. Alan:- I wants to know what is the behavior of SM under these conditions?

* Under loss of exc. SM behave like as Induction Motor (Ns>Nr)

* if motor is over loaded it rotor speed is reduced from its syn. speed of revolving field due to mechanical stress I wants to know what will happening in machine due to which speed of the motor is decreases and what will its behavior.

But if we discus about Syn. Gen. it's behave like as motor under over loaded condition...!!

Slip will increase and motor will overheat significantly


RGDS

Sandeep,
It appears you have made up your mind already as to what answer you want from the likes of Mr. Alan and Mr. Norman, but maybe others would benefit from some slightly deeper explanation:

Let us assume we are discussing a synchronous motor on a fixed frequency network, which means that there is no way to get the motor to run over synchronous speed.

There are a multitude of different designs, from single-phase permanent magnet shadow-pole types, over larger permanent magnet machines, reluctance and hysteresis motors, to DC-excited motors. I will only consider the latter.

Following can happen for a motor with or without salient poles:
-1) If there is no damper winding on the motor and it is overloaded to the extent that it falls out of synchronism. it will initially pole-slip, and then come to a virtually complete stop, depending a little on the load it is driving. This phenomenon can have destructive effects on the motor and its foundation if it happens to a sizeable machine. There will further be induced voltages i the armature winding, and depending on any rectifiers in the circuit, this can lead to destructive overvoltages for the exciter and breakdown in the armature insulation.
-2) If there is a damper winding, this may take over the production of torque while the motor is running below synchronous speed. There will still be the dramatic effects as the motor falls out of synchronism, but depending on the dimension of the damper winding, this may also be damaged if the condition is sustained for a while. Due to the damper winding, the speed may not drop completely to standstill, but other destructive effects may be similar to a machine without damper winding.

Sandeep,

As a follow on to Ole's excellent advice, I'll offer this simplification, a synchronous motor can only carry load at one speed, if it is not at synchronous speed then the magnetic field from the rotor is not locked to the rotating magnetic field from the stator and there is no torque available.

The concept of slip is completely coincidental and of no load carrying consequence, the amortisseur windings are there to dampen out oscillations and for some forms of synchronous motors to provide a way to accelerate the unloaded motor to near synchronous speed at which time excitation is applied prior to the load.

Your observation about a generator taking reverse power ignores the fact that the generator never goes below synchronous speed while still attached to the grid. If it did the consequences would be swift, extremely dangerous, and costly.

If you are truly interested in this topic you will find plenty of information in any AC Machines textbook.
Alan