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Variac - The danger and careful use of variable autotransformers
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.
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#2
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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. |
#3
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Next 3 posts copied from another thread:
.................................. (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.) ......................... |
#4
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Apparently those Variacs are also risky from a safety viewpoint. Something about them not being fully isolated?
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#5
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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. |
#6
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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 |
#7
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Chuck, I think you will be okay, but I am not a licensed electrician.
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#8
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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). |
#9
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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 |
#10
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Chuck, I am about to run a machine for a couple of months with a variac 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. |
#11
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Quote:
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. |
#12
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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. |
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