The Shop > Electronics & IC Programing

Cheap AC motor speed controllers?

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loply:
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

awemawson:
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.

loply:
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

vtsteam:
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.

John Rudd:


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

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