Making an ac speed controler

[carbon dude oxide]

Hello, i have a friend who has asked me to help make him a speed controler for a specific motor. Its An AC motor and i would like to make sure that it is dont properly and safely.

This is th message he sent me with the info of the motor on it:

There's the usual mains-feed of 'earth, live, & neutral' to the Operating Switch. There are 4 wires coming from the Motor: 'earth, live, neutral & black(shows continuity between itself & 'blue' neutral). Between OSwitch & motor is an 'electro-magnetic-coil switch', this allows a bridge to connect the 'live from OSwitch' to 'live of Motor'. This Indicates 'change of rotation best sorted in Motor?
SEALEY MODEL NO. SM14/B
4" x 36" BELT & 6" DISC SANDER.

Disc Speed: 2670rpm (510m/min)

Motor: 1/3HP, 1ph (single phase)
220/240V  50Hz, 2P



He has also told me that he would only like to control it so it has speeds between 0-120 RPM instead of the fulll 2670RPM that the motor can handle. Is that possible with this motor or does this one need to be operated at full power (it was a belt sander motor)

[IonizedGears]

I would run it off an ac square wave. I am not sure if they exist but you could get a heavy duty op amp or amplifier to a 555 timer circuit and drive it that way.

[Kremmen]

No, we are talking mains powered equipment here. It would have to be an extremely heavy duty op amp.
From the motor description it looks obvious that you have an induction motor in your hands. Now there has been recent discussion on this forum about controlling induction motor powered fans using  a cheap phase angle control. That works for the kind of load you get from a fan prop, but it will not work very well at all for randomly variable load that you get in e.g. just a belt sander. Press the workpiece down a bit harder and the slow turning motor stalls, only to run away when you release the load.
There is no easy way - or let's say there is no shortcut. The proper way is not that difficult but it will cost a bit: replace the motor with a 3 phase equivalent and install a small VFD or variable frequency drive. For such a small motor you don't need a 2nd mortgage and that option cannot fail. Everything else will be a kludge.
OK, you _could_ replace the AC motor with a slow universal motor and run that under pwm control. But once done it won't likely be cheaper or better that a proper VFD thing.

[fcb]

Carbon dude: Only pain an suffering await you with this one.

It says 2p motor - you will need a VFD to drive it at  5% of speed - and it won't like it at all. VFD's are not easy to build - and they are cheap to buy. You'd be better off replacing the motor with an 8pole unit (1/4 the speed) and VFD'ing that.

I won't question the logic of spending a few hundred £ on speed control for a £150 belt sander.

[G7PSK]

Running the motor that slow with any form of loading is going to overheat it, the cooling fan will be mounted on the motor spindle you would need a separate fan in order to load the motor while running so slowly. I have seen lots of motors burnt out due to the addition of an external speed control to a motor that was not designed for speed control.

[VFD]

Carbon dude: Only pain an suffering await you with this one.

It says 2p motor - you will need a VFD to drive it at  5% of speed - and it won't like it at all. VFD's are not easy to build - and they are cheap to buy. You'd be better off replacing the motor with an 8pole unit (1/4 the speed) and VFD'ing that.

I won't question the logic of spending a few hundred £ on speed control for a £150 belt sander.

Right, it's much simple to buy a VFD to control the motor