Hi all,
I need a variable speed mains AC motor for a project I'm working on. Ideally, I'd like to be able to vary the speed using a variac. It needs to go up to ~4000 RPM, which is beyond most mains drills. Any ideas on where I can get one, or what bits of equipment I can cannibalise?
TIA
Thinking about it, can anyone think of a reason why I can't use a motor from say a circular saw? I'm thinking:
What's the STARTING torque ? And as a secondary question, what's the total power ?
If you don't have to start against a load, then you can use most sorts of motor. If it must start "in gear", then you probably need a "universal" motor, i.e. with brushes, rather than an induction motor. These are easiest to find anyway.
I don't think an electric drill or saw is a good idea for starters, but a cheap router will do 4,000 rpm easily and comes with built-in speed control. Some saws will get you to almost 4,000 and you can add the speed control.
And forget using a variac for speed control - save it for the "electric gherkin" demonstration (Google for "electric pickle"). The way to do it is by phase control (light dimmer, with a few extra bells and whistles on). Light dimmers are prety rubbish (400W max, most are only
250W and they don't take kindly to motor loads either). However someone like RS sell a wide range of ready-made phase control modules (just add a variable resistor) and some of them are cheap (even from RS). If you have a big induction motor you can't do this, but if you need something that big then find a cheap 3 S/H phase motor and buy a cheap(ish) VFD (variable frequency drive) from eBay.
The motor in our Hoover w/m looks a serious bit of kit. 1-2 HP, integrated speed sensor, smooth closed-loop electronic speed control, in either direction. The pulley ratio suggests that, at 1200rpm on the drum, the motor shaft must be doing over 6000rpm.
But you need a circuit diagram to see how to wire one up.
Pretty low.
I can only really guess at this. It will be spinning a tungsten rod in a rotary spark gap, so I reckon anything more than 300W will be more than enough.
Yes, I thought of a router after posting about the saw - I'd forgotten how cheap they've become. My only question is will it go slow enough? I need quite a wide range of speeds, from a few tens RPM to ~4000 RPM.
But I love variacs! And I have several of them...
Found some on RS - surprisingly cheap.
Thanks.
I think that's probably way bigger than I need. I really should have specified the power requirement in my original post :-)
If it helps, the speed of a drill motor is something like 10,000 rpm before the gearbox.
If you're on a tight budget, a decent variac is likely to sell on Ebay for more than the cost of a phase control module.
c. 8k rpm at the low end - so you would have to gear it a fair bit.
CPC have some as well - I posted a link a few days back - 3 models upto
1.5kW
Found it - great, thanks.
It's not so much that - I wouldn't want to sell any of my variacs - but it's the kind of project where I try to use whatever I have available.
The CPC modules at £12 seem like a very good deal.
There's a thought - and you get speed control as part of the deal. Maybe it's time for a PP drill.
A Strimmer or Flymo motor then?
Don't run any universal motor on no-load though, because of runaway.
You have to be careful reusing washing machine motors. Off-load speed with no feedback control is often significantly faster than the speed at which the motor armature flies into pieces, rather explosively. I am also aware of cases where the microprocessor servo control has failed to limit the speed when there's no load too, so again be careful, even if you think you have the servo control connected up correctly when the load isn't the expected washing machine drum. (The case I came across, the microprocessor crashes if the speed increase is as fast as it is with no load, and leaves the motor powered and running much faster than its top speed.)
Washing machine motors need to have chickens sacrificed to them before you can understand their arcana.
Any idea what would happen if I used the output from one of these to feed a device which already has its own speed controller (like a router)? Would it work?
You would end up with cascaded PWM controllers... The switching rates would be unsynchronised so you would get aliasing between them. Net result sounds like lots of edges (hence broadband noise) and not alot of power. That is assuming you do not kill one of the controllers.
All in all I think I would avoid that one ;-)
Cool!
Ok.
I've just hooked up my router to a variac, and that seemed to work pretty well. Any reason to avoid this arrangement?
No, that should be better (if you don't need torque at low speed).... there may be a voltage threshold where the PWM speed controller stops working though.
Yes, I found that threshold. :-)
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