control the speed of 2000W motor?

JoeB-

Hi

I'm wondering can anyone help with this problem I've been having for a while...

I have a 220V 2000W universal motor that spins at 18,000rpm!!! It's a motor for a woodworking router. I would like to be able to use this motor at a speed of only 9,000 rpm or thereabouts but with the same power and torque, I wonder can anyone advise on a cheap method to do this?

I used to think there were two ways to do this, now I think it might be three...

The first method would be how dimmer switches work (I think), these would vary the voltage from 220V to 0V and this results in less light... I don't think it's good for an electric motor...

The second method would be the way it's done in commercial speed controllers... a 'frequency drive' which always applies 220V but varies the 50Hz of the supply, apparently the motor can't tell the difference and just runs with reduced speed. These are expensive (>€200) and are hard to get for 220V.

The third method would be to alter the 'duty cycle' of the motor and just supply 220V at 50Hz for small periods, in my case I'd use a 50/50 cycle with the power being supplied to the motor for only half the time. Some people say the startup forces cause problems but the motor is running at speed once started and so this shouldn't affect it. This seems to be the cheapest method as it only requires a very fast switch that can turn on and off....

I could be totally wrong with all that, any advice would be dead handy... in particular I'd need a method that doesn't wreck the motor and allows the motor to have full power or torque even at slower speeds.

Cheers
Joe

david powell

JoeBallantine wrote:

Hi

I'm wondering can anyone help with this problem I've been having for a while...

I have a 220V 2000W universal motor that spins at 18,000rpm!!! It's a motor for a woodworking router. I would like to be able to use this motor at a speed of only 9,000 rpm or thereabouts but with the same power and torque, I wonder can anyone advise on a cheap method to do this?

I used to think there were two ways to do this, now I think it might be three...

The first method would be how dimmer switches work (I think), these would vary the voltage from 220V to 0V and this results in less light... I don't think it's good for an electric motor...

The second method would be the way it's done in commercial speed controllers... a 'frequency drive' which always applies 220V but varies the 50Hz of the supply, apparently the motor can't tell the difference and just runs with reduced speed. These are expensive (>€200) and are hard to get for 220V.

The third method would be to alter the 'duty cycle' of the motor and just supply 220V at 50Hz for small periods, in my case I'd use a 50/50 cycle with the power being supplied to the motor for only half the time. Some people say the startup forces cause problems but the motor is running at speed once started and so this shouldn't affect it. This seems to be the cheapest method as it only requires a very fast switch that can turn on and off....

I could be totally wrong with all that, any advice would be dead handy... in particular I'd need a method that doesn't wreck the motor and allows the motor to have full power or torque even at slower speeds.

Cheers
Joe

Hi Joe,

A universal motor, is baiscally very similar to a DC series motor, where the field winding is placed in series with the amature, they give very high start up torques, and can run at quite high speeds ( they can even distroy them selves, as they can run away due to field weakening ).... As the field winding is in series with the amature, when the AC voltage cycles, the current in both the field and amature windings reverse, but as both currents reverse the induced force is in the same direction, which is key to their operation.

1) Yea in theory the "dimmer" approach should work except that you are trying to pass 10A, most dimmer switches would only be rated for a few hundred watts, ie a few Amps... Alot of the vacume clean speed controls operate like this. These "Dimmer" type circuits use a triac, which from memory has an on state voltage drop of about 1.3V. As you are passing 10A, the triac would have to dissipate 13W of energy, so it would need a biggish heat sink. :mad:

2) Comercial speed controlers are designed for 3 phase AC motors. If you change the frequency on a universal motor, it will not change in speed, in fact a universal motor will actually run of DC power. So if you apply 220V at 25Hz the motor will run at the same speed.

3) The term duty cycle is normally applied to DC systems, and alot of series DC motors are controlled in this way. Normally if you wanted to run at 50% speed you would ramp the PWM from zero % to 50% over a period of time. At low speeds the motor could take a good bit of current. Under point 3, you did kinda mentioning about "turning on" the power to the motor, for part of the mains cycle, this is in effect what a dimmer switch does, the triac will conduct from the time it is turned on, untill the device naturally turns off ( comutates ) at the end of the mains AC cycle.


I would look at some of the vacume cleaner speed controls, and look at the sort of power they are switching, ie how many watts the motor is. Typicall the vacume cleaner speed controlers i've seen do have a heat sinking on them, but they still might be a bit "weak" for your 10A load.

Hope this is of some help... .

JoeB-

thanks for the reply, very good.

That's a good idea about vacuum controllers, I was also thinking about fan controllers and other controllers. Modern power tools all come with variable speed controllers nowadays, I don't think I can buy one but it should be possible to make something up with a few transistors now that I know about triacs... :rolleyes:

link to dimmer switch, triac circuit design

I had come across the duty cycle and pulse width modulation stuff in relation to DC motors alright... wasn't sure if it worked with AC. It's like reasonance in some ways, just give the motor little re-inforcing pushes now and then....

I was thinking at one time that I could just put a diode on the AC supply, this would cut out 50% of the power, would this have the desired effect? and is this feasible? And is it possible to just cut open the power cord, insert a big diode into the live wire, tape it all up and go to work??? Or am I mad!


Cheers