Variable AC speeds for a grinder motor ? - how...

Hello,

I have a 2200w grinder motor (240v, 50Hz) here which I would like to be able to run at different speeds - i.e. from zero up to its full supply rpm...

If this were a DC motor I'd simply tap some of the voltage off elsewhere or get more sophisticated with PWM etc... But with AC I dont know what is going on... does the motor run off the frequency of the supply and lowering the voltage would simply reduce its torque ? or ?

Basically its a one off application with time constraints, what simple method could I use ? I'm hoping a simple AC 'dimmer' circuit will do it

...there are two wires coming out of it now that I have removed the trigger

any help appreciated,
Nick

An Ac dimmer would do the job

you can buy cheap comercial units but they are normaly up to 1000W

if you change the triac for a bigger version and ad a heat sink to it you have wahat you want and pretty cheap

Robert-Jan

Hi Nick,

to control a grinder motor you need a phase control circuit (like a dimmer).

You might try to use a commercial light dimmer if the grinder has a power demand of less than 300W.

However the dimmer circuits are normally made for resistive loads, but a few can also control motors of small to medium power demand.

I'd like to introduce my µC controlled phase control circuit for inductive loads up to 5KW, but it's too early for that. The device is still under test.

Boncuk

Boncuk said:

I'd like to introduce my µC controlled phase control circuit for inductive loads up to 5KW, but it's too early for that. The device is still under test.

Boncuk

Click to expand...

I am intrested in that one

Robert-Jan

rjvh said:

I am intrested in that one

Robert-Jan

Click to expand...

Depends on payment amount.

It depends on the motor, if it's a universal motor (i.e. one with brushes) it will work, if it's a tre AC motor (i.e and induction motor) then a lamp dimmer circuit is useless.

To control the speed of a true AC motor you need a variable speed drive which is an inverter that changes the frequency of the applied AC voltage.

Since you're running a grinding wheel (a flywheel) this motor shaft has some inertia.
You could use PWM for the AC supply. 10 Hz would probably be a high enough freq. for something this massive, and just adjust the duty cycle.

Put a FWB in series with the motor input to have your PWM circuit work on DC.

Willbe said:

Since you're running a grinding wheel (a flywheel) this motor shaft has some inertia.
You could use PWM for the AC supply. 10 Hz would probably be a high enough freq. for something this massive, and just adjust the duty cycle.

Put a FWB in series with the motor input to have your PWM circuit work on DC.

Click to expand...

From the grinders that I have used, when a grinder is running at slower speeds...has very little power, and is practically useless.

That would put a lot of stress on the motor. The starting current is much higher than the running current. It's possible that the motor could overheat when run at a certain duty cycle.