# Have variac - now what?



## sparky961 (Jun 13, 2008)

I have a 10A autotransformer variac that goes from a few volts to just over line voltage. It was given to me a few years back, and have never found the right use for it. It's missing the knob on top, and since I never had a use for it yet, I never fashioned a knob. I'm pretty sure the adjustment shaft is isolated - because I've adjusted it while it's powered, and I'm still alive to talk about it - but even still, I'd be more comfortable with an insulated knob.

A few months ago, I fried some circuitry with it while I was attempting to make a high current DC power supply using it. It was about 3am when this happened and I should have gone to bed long before that, instead of playing with potentially lethal currents and voltages.

Can anyone give some theories or experience with using it as a motor speed control for a 3/4 HP universal motor? The current is adequate, but I'm a bit rusty on my AC motor theory... 

Any other creative uses for this?

-Brian


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## Alphawolf45 (Jun 13, 2008)

I made a motorized potters wheel used a 20 amp autotransformer and a 3/8ths inch Milwaukee AC drillmotor ..That was just what stuff I had laying around. The thing worked for lots of use, never failed and now resides on my storage deck..Will gut it for parts someday.
.
 There too much about ac motors that I dont know so I cant actually address your questions..For myself , when I think variac I think about rectifying and filtering the output for a variable DC power supply..Varying the voltage to a dc motor always works far as I know but I am aware that some AC motors would rather burn up than operate at significantly lower voltage..buts that the little bit that I know..


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## cfellows (Jun 13, 2008)

A variac can be used to control the speed of a universal ac motor without much danger of hurting the motor. The problem with universal motors is that they develop high horsepower by virtue of their high speed. When you reduce the voltage with a variac, the torque falls off pretty fast. 

As Alphawolf said, variac's do work very well with permanent magnet DC motors. But then, you do have to rectify the current. That's pretty easy to do with bridge rectifiers which are cheap and easy to connect. I've never tried filtering the current for DC motor use. Don't know if it really helps or not.

Chuck


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## Alphawolf45 (Jun 13, 2008)

Yeah no, youre right Chuck, bit of line cycle ripple wont bother a dc motor..I was thinking for general power supply use I use electrolytics to get the ripple down low enough to use with audio circuits..wasnt really thinking motors...

As Alphawolf said, variac's do work very well with permanent magnet DC motors. But then, you do have to rectify the current. That's pretty easy to do with bridge rectifiers which are cheap and easy to connect. I've never tried filtering the current for DC motor use. Don't know if it really helps or not.

Chuck


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## Bernd (Jun 13, 2008)

I've powered a treadmill motor using a variac and 50 amp bridge diode. Works nice.

Bernd


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## sparky961 (Jun 15, 2008)

Thanks for the input. I guess it's not time for this one to go in the yard sale pile yet. 

-Brian


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## BrianS (Jun 30, 2008)

How about using your variac like I do: For a hot wire foam cutter. Of course then you can do some lost-foam metal casting. (Which I plan to do in the future) ;D I got the idea from http://wolfstone.halloweenhost.com/HalloweenTech/fotmak_MakingFoamTools.html They use their foam cutters to make Halloween decorations. Its a neat site and worth checking out.

My setup is a foot switch, Variac, 120V primary/12V secondary transformer (blue box) and then the cutting tool. I made a couple of different sized hot-wire "bow saws". Here's a picture with the smallest "saw":






Hope this helps.
Biran


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## Steam4ian (Jun 30, 2008)

G'day Brian

1/ keep the variac out of the skip.
2/ remember it is an auto transformer so the output terminals are at mains potential as is anything connected to them. You need an isolation transformer for things like hot wire cutters.

Uses. Only on ac universal motors or any dc motor with a bridge rectifier. Universal motors are series wound to give high torques but the speed falls with load. On ac the variac will be less satisfactory than many seemingly simple solid state controllers because the later have an elementary speed compensation function built in.
But with a dc shunt field or PM field motor the variac and a rectifier will give good control with only a few percent speed droop on load.

Good luck, keep one hand in you pocket (an old rule!)

Regards,
Ian


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