Dear Mr. Gandhi,

"Motor" or "Generator" simply refers to the direction of torque, assuming a certain direction the machie moves in.

There is no physical difference between motor and generator as the same machine can be used as both.

I assume you have something more detailed in mind but if so, you would have to ask a more precise question.

Dear Mr. Gandhi, I agree with Mr. Schulz, since the same machine can be used as motor or generator. The main criterion is the rotary speed compared to the synchronous. If the machine is slower than the synchronous speed, than it is a motor. If it is faster becames a generator. However, since a generator needs to be faster, I would try a lighter construction for that. As the equation for machines reveals:

P = C D2l n

P: power, C: Esson's factor

D: rotor's diameter         l: machine's lenght           n: rotor speed

For the same power, provided that C will be remain the same, a bit higher speed needs smaller dimensions. Smaller dimension means lighter construction. Especially D affects more. 

Again: The same machine can be motor or generator, but since the generator has to be faster, I would suggest a design with a bit smaller diameter. If a smaller machine targeted to be a generator, will be used as motor instead, then it wont give the same power, because as a motor will have less speed n, smaller dimensions, therefore the power will be lower. 

Synchronous machines,could be used, as motor or generator, similarly induction machines could be used as motor and generator. Hence first draw circle diagram of induction machine by performing O.C and S.C test, complete it to full circle, lower half is locus of  point, as generator, you could find losses(stator/rotor) power output at , the given negative slip, with better understanding as generator.,

I agree with the previous answers. There is no difference between the motor or generator, at least theoretically.

Looking at it the other way, an induction generator, once coupled to a wind mill, has to run continuously, 24 x 365, except for maintenance, where as that may not be the case with a motor. An induction generator should be made a little bit larger, (more dia) to enhance the heat dissipation. 

In Birla engineering collage(BVM),V.V.Nagar, Gujarat, India-388120., my students are performing practical to plot the complete Slip-Torque cherectristic of induction motor where the motor, under test working as induction motor in stable region, is supplied by another motor working as induction generator(both coupled to two D.C shunt machines, one with motor under test is dynamometer), in unstable region and adjust to load automatically even for unstable region as motor, to plot complete slip- torque cherectristic,  gives reading even for unstable region.......is very intrasting practical, in senior machine lab. do visit BVM.

When an induction motor is driven at a speed higher than the synchoronous speed, power flows from the rotor to the stator(slip is negative). When operating as a generator at a given percentage slip above synchronous speed, the torque, current, efficiency and power factor will not differ greatly from that when operating as a motor.