PWM control for cheap power-tools?

[medved]

I am not sure whether this is the right forum, but I'll ask anyway.

I would like to use drilling-machine as a motor SOURCE for mill spindle, it would only power spindle by belt, it wouldnt touch workpiece itself. There would be enough place to mount encoder wheel and sensor from some old mouse.
As far as I know, cheap power-tools dont use closed loop to control their speed, so I would like to add this feature by myself.


My question is, how can this be done -and by that I mean the power section, not the pic/avr programm section.

I suppose AC can not be simply pulse width modulated the way DC can be. Instead one solution would be to use digitally controled resitsor to contol AC current going through the motor. Are there chips capable to withstand 220V and few Amperes, and dont cost too much?


Another solution I can think of would work in case there is a universal motor(series AC motor) which works also with DC. The problem is then, where to get 220V DC and transistor/mosfet that can handle such voltage? Can 4-bridge-rectifier and condenser be connected directly to 220V electic plug(without any transformer) and be used as a 220V DC source for power-tool, or will there be a short-circuit and 4-bridge-rectifier will fry?



But I am affraid that power-tools use shunt-wound motors. Is there a way to PWM control shunt-wound motor, or can it only be controled by warying resistor placed in series with winding coils of stator electromagnet? Maybe such motors cannot be precisely controlled, but for me it would be enough to electronically prevent motor from standing still when cutting force gets higher.


Please tell me any suggestions, or how is speed/torque control usually done.

[H500]

If you have an induction motor, you can't pwm it without it overheating. You should have no trouble controlling the speed of a universal motor using PWM. A better choice might be a surplus treadmill motor.

http://www.surpluscenter.com/sort.as...c&keyword=MBDD

You need to rectify the AC, but smoothing with a condensor is not required for simple speed control. The irfp460 mosfet will easily handle the voltage you're using. Make sure you use a proper gate driver. Simply connecting it to the o/p of a mcu might not be good enough. Take a look at the ir2183.

If you just need simple torque compensation, you can use a thyristor circuit.

[Al_The_Man]

There are IC's out there you can use to provide feedback for universal motors (ac/dc). These use a triac and a full wave bridge for phase control, these circuits are typically found in treadmill and washing machine circuits. They use a simple coarse type of tach using a prox or hall sensor on a tabbed metal wheel.
Here are some links to similar projects http://www.siliconchip.com.au/cms/A_...ntArticle.html
Al.

[ViperTX]

So, in 1982 when I designed the first Sears electronic router I used the predecessor to the PIC to fire a Triac, a Sprague Hall Effect sensor which was activated by a ring magnet with pairs of poles provided the speed feedback info. The triac is self-commutating...which basically means that it will turn-off at every zero crossing....well close depending on the type of load, so you only have to turn it on at the right time.

As Al stated you need to have a universal motor.

[greybeard]

If it's any help, I've used light dimmer switches to control the speed on low power cheap drills.

[abasir]

For universal motor, just use zero crossing detector, random-phase triac driver (MOC3022 or equivalent) and a triac (BTA41, etc). The PIC/AVR need to trigger the triac driver at appropriate time after the zero crossing to control amount of power delivered to the motor.

[PaulH]

Just to ask a dumb question: This discussion has revolved around universal motors. Is there any way to control the speed of an induction motor?

[Al_The_Man]

In the case of a 3ph motor the VFD is the best way to go, single phase motors present a problem, first they should be permanent-capacitor start/run type but even then they tend to drop out of run or stall when used with varying loads or less than medium speeds, when used on triac style controllers.
Al.