Variac - The danger and careful use of variable autotransformers

[Håvard]

Would it be possible to use a variac as the transformer? Would still need the rectifier and capacitor. However, the voltage would be adjustable. If I understand this correctly, the fuse on these things are kind of on the input, so the 4A fuse on mine would not blow until you pull 4A*220V, well over 800VA.

[Mike Richards]

I use a Variac and a transformer. The Variac adjusts the voltage to the transformer. When I find the voltage that works best for the circuit, I replace both the Variac and the transformer with a transformer of the proper size.

The danger with a Variac is that the input side and the output side are not really separate, which can and does cause major problems.

[Richards]

Next 3 posts copied from another thread:

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(To convert your present power supply to one giving 1/2 that voltage, you will have to change the toroidal transformer. If you can find a Variac transformer at a good price, you could dial in almost any output voltage, but a new Variac transformer will cost more than the power supply.)

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[Gerald D]

Apparently those Variacs are also risky from a safety viewpoint. Something about them not being fully isolated?

[Richards]

Gerald is totally correct. You MUST NOT use a Variac as a replacement for a transformer. You place the Variac before the transformer to vary the voltage that the transformer receives. (My Variac has a range of 0 to 140% of normal.)

So, for instance, if I have a 70VDC power supply that I want to test with 35V motors, I would reduce the voltage, with the Variac, going to the toroidal transformer by about 50% so that the toroidal transformer would give me 35VDC. As soon as I had verified the correct voltage for the motor, I would buy a toroidal transformer that gave me the desired voltage - so that the Variac would not be needed.

[skypoke]

I've got the a2a motors, which I intend to wire bipolar. I'm thinking of getting a dual output toroid xfmr which will give me either 25 or 50 volt output depending on how it's wired. So...70V dc would be the likely output which, if I understand this correctly, is a little on the high side for when motors are operated at low speeds. Here's the Q....

What if I feed the toroid with a variac? Would I maintain required isolation?

Chuck

[Gerald D]

Chuck, I think you will be okay, but I am not a licensed electrician.

[Richards]

If the Variac feeds the toroidal transformer, the circuit will work. I do exactly that when I'm testing various motors. NEVER use a variac in place of a transformer. Also, make sure that the variac only affects the toroidal transformer and not any other auxiliary power supplies.

Seventy volts is a little high for an A2A wired bipolar series, but within the permissible range (77 VDC is the computed maximum voltage).

[skypoke]

Thanks Mike and Gerald,

I'll start off with the variac and see what my optimal voltage is. Using the variac on a permanent basis is not that appealing.

Could I trim the output dc voltage, either with series diodes or using half a bridge rect.? Possibly a circuit that could be selected as needed?

Chuck

[Gerald D]

Chuck, I am about to run a machine for a couple of months with a variac (http://en.wikipedia.org/wiki/Autotransformer) before its toroid (Our hottest month is February). I don't think it is dangerous if one respects the output as mains supply with a distinct live/hot and neutral, and there is a RCD breaker feeding it.

Trimming the DC voltage side is unheard of in my book, but then my book on this is very thin.

[Gerald D]

If I understand this correctly, the fuse on these things are kind of on the input, so the 4A fuse on mine would not blow until you pull 4A*220V, well over 800VA.

A variac is rated by the number of amps the coil can carry before damage. Since it only has one coil, both the input and output are limited by the same current value.

My variac is rated 2 Ampere. If I want to power a 300VA toroid transformer with a maximum of 2 Amps, the lowest output voltage that the variac can be dialled to is 150V.

[Richards]

Gerald has made a very good point. A variac that is large enough for this kind of testing is expensive. I have one that I bought almost thirty years ago that cost more than $100 back then. It is only rated at 10A, so I use it with caution when I'm playing with stepper motors.

Also, a variac should NEVER be used in place of a transformer. It is used in series (before) the transformer to modify the voltage going to the transformer. A variac is wired differently from a transformer and will cause serious problems if it is used in place of a transformer.

Personally, if you're concerned about getting the highest performance from your stepper motors with acceptable heat, I would buy two or three toroidal transformer that were rated to give the proper voltage, or a little less, and simply swap them in and out of the circuit to see which one worked the best. I've done that very thing. My main test toroidal transformer is a 25/50VAC unit (depending on how it is wired, parallel or series). I found the voltage to be just a little higher than I like, so I next tried a 18/36VAC unit (again, depending on how it is wired). The lower voltage unit works perfectly. The motors turn as fast as I need them to turn and the heat is reduced just enough to make me feel comfortable with that transformer.

I still use a variac when I'm experimenting, but I've been using a variac for thirty years and have only fried one - so I'm careful with the way that I wire things together.