Controlling AC Motor Speeds

This is good up to a point. One thing I would mention is if you already have a wound rotor motor that you wish to attach an inverter to, you can do this in two ways. You can just short the brush inputs together outside the motor, in this case you still need to perform maintenance on the brushes. OR.... you can put shorting buss bars on the slip rings in order to get rid of the brushes entirely.. You will have to put at least three shorting bars on the motor in order that you don't unbalance the motor, but I have successfully done this with great results.

WARNING!

THIS IS INCORRECT!

TAKING THIS ADVICE WILL RUIN YOUR MOTOR!

Yours is NOT a Wound Rotor Induction Motor. You posted, as an apparent minor side note after your "blah blah blah", that it has DC excitation. That makes it a Synchronous motor, not a Wound Rotor Induction Motor (WRIM) as Shawn V assumed. Shorting the rotor windings will cause it to attempt starting without ever pulling in to synchronicity, and the subsequent rotor field voltage will get extremely high, which can start a fire.

Speed control of 3 phase AC asynchronous induction motors, which includes WRIMs and Squirrel Cage Induction Motors (SCIM) can be accomplished by using pre-made devices called Variable Frequency Drives, or VFDs for short. No need to go trying to make one from scratch with a crystal oscillator etc etc., that will likely cost you 10 times as much as it would to buy one off the shelf. They are very common and very inexpensive now.

However, speed control of a Synchronous motor is MUCH more tricky and requires very specialized knowledge of their application, use and protection. From the tone of your initial post, you do not possess these necessities. Better to hire a professional with experience in this specific field, it will cost less than replacing the motor and whatever else the fire may damage.

I wonder -

why stator is having exciter ? when rotor is having DC (excitation) ?

is it a DC motor ?

but the OP calls it AC motor.

Am I confused?

I took the "stator" statement as further evidence that the OP was unfamiliar with motor terminologies at all, and he meant "rotor". A Stator is excited by simply applying power...

You are not clear in stating the problem .You say DC armature then slip ring and then commutator.What kind of motor are u talking about?

An older Synchronous AC motor fed by slip rings with a shaft connected DC exciter supply generator would have all of those elements. Here is a picture of an extreme version of that.

The device that looks like a small motor connected to the shaft of the larger one is actually a DC generator. As the motor turns, the DC generator supplies excitation power for the rotor of the synchronous motor. So the DC generator has a commutator and brushes, the synch. motor has slip rings for its rotor. In smaller versions these are more closely coupled to where someone may not be able to differentiate them as separate if they are unfamliar with the technology.

If you closely read the problem stated there is no mention of excitor.Even to day on large turbo generators we have a dc generator coupled to the machine for excitation.He doe s not state it that way.All I am trying is to understand what is the question.Thanks for intrepreting the same.