VARIAC as DC motor speed controller from DC power source
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.