Author Topic: Cheap AC motor speed controllers?  (Read 6399 times)

Offline loply

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Cheap AC motor speed controllers?
« on: July 03, 2013, 03:41:12 AM »
Hi folks,

After looking for nearly a year I finally grabbed myself a Biax scraper on eBay for an (almost!) reasonable price the other day.

The one I have is the single speed version so, as seems to be the norm, I want to add a variable speed controller to it. Most of our American friends seem to refer to these as 'router speed controllers' but I couldn't find that item anywhere, so I ordered one of these cheap widgets off ebay:

http://www.ebay.co.uk/itm/High-torque-50V-220V-10A-AC-Motor-Speed-SCR-Controller-Power-2000W-/230897430868

The tool is only 250w and this widget claims to be good for 2000w but I wonder if anybody knows how it works? Does it vary the frequency somehow, or the voltage? Is it too cheap to be true?

Looks very small for handling 2000w! Not that I'll need to put that much into it...

Cheers,
Rich

Offline awemawson

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Re: Cheap AC motor speed controllers?
« Reply #1 on: July 03, 2013, 04:24:52 AM »
They use an SCR (Silicon Controlled Rectifier) which is a device that doesn't pass current until you 'fire' it using a third gating electrode. The control knob varies the point in the sine wave that the scr is triggered, hence controlling the power. Suitable for 'universal' motors with a commutator like mains drills and routers but not suitable for synchronous ac motors.

Andrew Mawson
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Offline loply

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Re: Cheap AC motor speed controllers?
« Reply #2 on: July 03, 2013, 06:35:47 AM »
Thanks for that awemawson. Just doing some more research, I presume the Biax has a universal motor though to be honest I don't entirely know what that means, is it a DC motor?

In any case, I can't decide whether to use this cheap widget or buy a proper variac and make myself a little 'variac box' with a 3 pin plug receptacle on the front which can be used for various tools. I'm not sure if there's any advantage to the smooth wave of a variac over the choppy wave of a cheap SCR thing?

Thanks,
Rich

Offline vtsteam

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Re: Cheap AC motor speed controllers?
« Reply #3 on: July 03, 2013, 07:38:11 AM »
Universal motors have brushes. As awemawson said, a router speed controller (and a Variac, as well) will work for only these types of motors. They are found in routers, drills, circular saws etc.  I don't know what a Biax scraper is. If it's a hand tool, most likely it uses a brushed motor. Brushed motors are usually pretty loud by comparison with others. Check on the side of the motor and see if there are two circular plugs for the brushes.
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Steve
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Offline John Rudd

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Re: Cheap AC motor speed controllers?
« Reply #4 on: July 03, 2013, 07:39:48 AM »


Suitable for 'universal' motors with a commutator or permanent magnet motors...
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Offline John Swift

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Re: Cheap AC motor speed controllers?
« Reply #5 on: July 03, 2013, 08:00:08 AM »
Hi Rich

the universal motor works on either a DC or AC supply
http://en.wikipedia.org/wiki/Universal_motor

you will need to use an over size variac transformer to ensure you don't overload the winding
at low speeds

the ebay controller should work but going by the pictures of the minimal components on the  PCB and the lack of an control IC 
I expect its more of a lamp dimmer
( like figure 4 - http://sound.westhost.com/lamps/dimmers.html )

I would not expect the ebay speed control  has any way to compensate for changes in mechanical load on the motor
but may be OK for use with the biax tool

as an example this speed control circuit -  http://www.aaroncake.net/circuits/acmotcon.asp
has a feedback circuit to measure the motor current and compensate for variations in load
the voltdrop across R13 is a measure of the motor current
to run of 240V some component values will need changing

another example - http://www.velleman.co.uk/manuals/k2636.pdf


    John
« Last Edit: July 03, 2013, 09:20:03 AM by John Swift »