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I'm not sure if this is is where I should ask this but is it possible to use the VARIAC as a DC motor controller from a DC battery source? The thing goes like this: The 48V, 3kW DC motor is connected to the VARIAC. The VARIAC is then connected to the 48 V battery source.

The idea was actually from a Chinese guy and I'm not sure if it's possible. I have consulted with my professors and they don't believe it would work. I am also researching about this but there are not enough articles and forums to support this.

Thanks for anyone with an answer.

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A Variac is actually an adjustable autotransformer, and is designed to adjust incoming AC voltage up or down. It is also not an isolation device.

DC can't be transformed with a transformer, so it would be rather pointless to use a variac as speed control for a DC motor and a DC power supply. You could get some crude control, if you were feeding the variac with 48VAC or so, ran the output to a rectifier, and then to the DC motor, but I suspect you would be much better control and performance with an off the shelf PWM controller for 48VDC.

It is likely to be much less expensive then a 3KVA variac as well.

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I have consulted with my professors and they don't believe it would work.

Professors are very, very clever people*. If they say it is not possible I would drop it.

In fact I also believe it is not possible. Variacs are transformers and transformers are relying on inductance. That is : they need a continuous changing current. With DC you only get the resistance of the copper wire which is very low.

Maybe your Chinese friend is using the Variac as a giant potentiometer. You don't want that as it will heat up!

*Which also occasionally make errors.

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No. A variac is a variable transformer, and transformers need AC for their function. If you hook a variac up to DC, it's just a very low-ohmic variable resistor.

schematic

simulate this circuit – Schematic created using CircuitLab

With a variable resistor, you can create a lower DC voltage from a higher one, but at the price of having a non-neglible loop current across it. For 10VDC and a typical variac of 1Ω, it's 10A, and will drop 100W on the variac alone.

So, this is a waste of electrical energy and may overload the variac.

If you instead only connected the variac in series to the DC motor, it won't have any effect due to its low-ohmic nature. If you put a higher-ohmic (and high-power) variable resistor in series with the motor, you could indeed limit the torque and thus, the speed. Again at the price of wasting energy.

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