Hi friends,
One of my old friend has got a problem related to AC Motor...and so am I. Here it is:

This motor is used in textile mills where weaving is done. This motor is having mechanics that hold the yarn and supplies it to the weaving machine. This motor maintains the amount of yarn on the this mechanics. If it is empty, the motor moves at full speed. When the yarn is half full, the motor moves at half speed. When the yarn is full, the motor stops. Other than this there is nothing printed on the motor.

This motor has 3 wires on a connector. It is a 3 phase motor. There is a control card for this motor.
This card has 5 Nos IRF840 and one IRF740, with 3Nos of CD4001 3 Nos of TLP531 and 2 Nos of LM393.
I think it is a standard circuit using MOSFETS for driving an AC Motor. The supply to this card is 220VDC for MOSFETS and 9VDC for TTL circuit(through regulator 7805).
The control card has a PIC16C54 on it, and that IC is damaged. Now he wants to rebuild this whole board using an easily available CPU. I suggested the MCS51 family. And that is agreed upon.
If I give 3 phase supply voltage from the mains, the motor runs at full speed.

I will be firing the MOSFETS using PWM so that the motor gets 3 phase 50Hz.

What I need to know is:
1.What should be the frequency of the PWM signal?
2. If I have to reduce the speed, then do I have to change the frequency of firing so that the supply to the motor is 3phase @25Hz, but the PWM frequency remains the same.

Some links regarding point 2 will be really helpful.

I have read the AN984 from Microchip. Link was posted by Kai Klass on this forum in another thread...thanks for it.
On page 5 of this AN984, it says:"Assuming that a 16kHz PWM carrier is selected and the maximum modulation frequency is 60Hz."
Can the 16kHz carrier frequency for PWM be used for 50 Hz system? If not then what should be the PWM carrier frequency?