AC motor control

We have decided to provide a cheap solution for the pottery makers in south region of India. We are trying to build a speed control for a 1 HP AC motor which runs at 1440 RPM. The initial plan was to use a thyristor, which was dropped due to difficulties we faced. We are now trying our hands with a triac. Can anyone suggest some suitable circuits? We found one using U208 but the IC is not available in India. The due date for submission is approaching and we have NO CLUE... Hoping I get some replies.

Hi,

Try approaching one of the 'big boys' - the prices of stripped down, simple inverters are tiny - appeal to their "good story" side.

Marc

Generally there is no difference in control if you use triac instead of thyristor pairs, in DIY device... so it is nearly impossible to control AC motor in wery simple way by using triacs because of inductance of motor. It makes high voltage (>2000 V) spikes on triacs and they always go out of order.

Would you like to use power resistors? They can work. Would you like to to use transformer for reducing voltage on motor - you can make several voltage steps at single transformer?

The bad news is following: if you reduce voltage and if the load of the motor change value, the motor speed will not be stable because of bad speed-torque characteristics of motor. Without load you will give the same 1400 RPM. M1/M2=U1*U1/U2*U2. It means if you reduce voltage 2 times the torque (Mmax) of motor reduces 4 times.

I see two good solutions:
1. Buy a SOFT STARTER with slow speed capability or
2. AC drive (frequency convertor)

Thanks a lot for the reply...

We did buy a soft start IC... U2008B from ATMEL. The only problem is the submission is due in 2 days and the IC has not yet arrived for whatever reasons.

I will look into power resistors and stepping down seems a good option too... Commercially available AC drives are very costly.

Hi Kameshwari
Please try with ST thyristors for your application, these are available in open market with comparatively low cost.

I though triac turned off when the current pass by zero, so why the spike ? Thank you

Fred Talbot

Ahhh, that wouldn't be a single phase motor by any chance?

Roy