Brows

Ask toyodenki siezo, http://www.toyodenki.co.jp/en/html/sangyo.html

Allow me to try to clean up your post:

I am working in a sugar mill. There is a machine called "batch refine". In this machine, a variable motor is installed. It is made by Toyodenki Siezo, Japan, which is operated by whose power supply is a cycloconverter. The output of this cycloconverter is has a variable voltage and frequency an AC supply. While the rotor of this motor is a claw pole type of solid iron and not a laminated core; for field weakening purposes, the motor had two field coils energized by sepearately excited DC supply. My question is, how is it possible to operate a motor having an ac supply if the rotor is made of solid iron? What is function of field weakening?

I know English is a difficult language, but you must try harder!

You missed "the" out of the last sentence.

Writing in English is more difficult even for the people have English their mother language spare the people having it as THE 2nd language.

The Cycloconverter is converting 3 phase Ac to DC with variable Voltage. Your motor seems to be a DC motor since the fields are DC Supplied....(?!)

I've held a claw type rotor in my hands, but for the life of me I can't remember what it was from. It was 30 something years ago!
Have a look for Cycloconverter on Google, I've had a fascinating read, but nothing related to what I remember.

Tony goes of in to corner of rest room sucking his thumb.

Cycloconverters were used to directly convert AC to AC. They gave a much lower frequency than the supply line. The motors had a large number of poles and a very large diameters. As far as I remember, train traction was the main market. There wasn't any DC circuit involved in the power section other than the supply for the gate drivers.

If you do get DC out of your cycloconverter, it isn't one. It is simply a DC thyristor drive.

If your motor is really an AC synchronous motor, you could have a solid iron rotor as most of the magnetic field is DC. Only the exterior of the rotor sees a small AC magnetic flux. More efficient motors do have a small layer of laminated steel at the periphery of the rotor. The motor excitation would be a variable DC current. The two coils strategy might have been used instead of a continously variable DC source.