PWM for AC Motors?

[vacpress]

Hey guys. I have a question. I know that DC motors can be controlled via PWm with an H-bridge circuit. This is usefull if you want to do closed-loop control of a DC motor, or if you want to have a better control of the motors speed.. (we all know this stuff i think..)

anyways, how is this done with an AC motor?

Anyone know what sort of AC motor is in most power tools(like a porter cable router for instance?)

Thanks!

[Al_The_Man]

Routers are typically universal motors, these are brushed series connected AC/DC motors, Generally the control is by phase angle control, these motors do not offer very reliable speed control, unless a semi-sophisticated control is used with a simple pulse tach feedback.
The high priced - high speed spindles are 3 phase motors and use Variable frequency drives for control.
Al.

[vacpress]

Ok. Cool.. So, let me ask if this would work, in theory...

A Basic stamp or equivelant senses a 1 or 2 'hole' encoder disk on a porter cable router. The stamp knows what speed is desired, and pulses a high-amperage solid state relay or other device(triac?) with its PWM output.

Very simple?

Another option would be a potentiometer with a small stepper on it, controlling the speed, with the position being driven in closed loop by the stamp...

Any comments?

What is phase angle control?

[vacpress]

http://www.freescale.com/files/micro...TORPACTUT.html

I found that, but it dosent really explain how to do this phase angle control..

[Al_The_Man]

Devices for phase angle control consist usually of SCR's or TRIAC's, A search of these should explain.
Al.

[vacpress]

So, let me ask, is the idea of trying to do this absurd, or something? It seems easy and inexpensive... and a usefull tool....

Thanks for the replies.

[ViperTX]

vacpress.......not all AC motors are created equal.....so, narrow it down a bit....

[vacpress]

Ok. I want to control the speed of a Porter Cable 6902 Router. I beleive it is an Single Phase AC Induction Motor. But, I am not certain. I Cannot find any data on this. However, I think it would be neat if the project could work on most common shop tools... maybe even on my enco knee mill..

I wish this thing was DC, then I wouldnt have any questions! Hbridge, PWM.. Bang..

But I dont know much at all about AC motors! I need someone to say.. Ok, you need to use a VFD(variable frequncy drive) to set one up you.... ... ..... just a quick synopsis. I think someone here knows all about this. All my research is helping, but I cannot find much specifically about digital control of single phase AC motors.

Here is a diagram to help you understand what I am thinking. It is simplified.

I have found a article on controlling an AC motor, but it uses a transformer in its design., I would prefer not to use a transformer.

See it! http://www.ai.uga.edu/mc/alcor/Alcor...-Published.pdf

Please help guys! The payoff will be a schematic and code to make a +/- $15 LCD interface RPM controller for your toys. The real machines Ive used all have this, and it is very handy. It allows for greater repeatability from dayto day, and a wider range of automation if you have an automatic tool changer.

The price breaker on this would be controlling phase angle costs alot. But because I havent found this specific bit of info yet, I blunder on!

Thanks for any help!

[vacpress]

If I cannot figure it out soon, I am going to give in and use a stepper motor driving a Harbor Freight - type Router Motor Speed Control. I really would prefer not to, but I may... It would cause the cost to go above $15, but I bet the circuit inside that thing is really easy to reverse engineer..

Thanks!

[vacpress]

What is inside this?

http://www.harborfreight.com/cpi/cta...emnumber=43060

[Al_The_Man]

Probabally SCR or Triac, like a light dimmer.
BTW, The 6902 appears to be a AC/DC ( brushed) universal style motor.
Al.

[vacpress]

So, if this motor is indeed a universal AC\DC type, I can go with a 'normal' H-bridge and PWM circuit?

What makes you beleive this motor may be a universal ac\dc type? It is a shame there is not more information about this.

At this point I think I would rather figure out how to modify phase angle to do AC control... I like to learn new things.

All the same, if it turns out the porter cable router is capable of DC operation, I should have this thing done in 2-6 hours of work..

Thanks for the help so far.