On Fri, 23 Feb 2018 13:09:18 -0800 (PST), wrote:

I use an ordinary cheap and dirty lamp dimmer to control the speed of a used sewing machine motor that is now used in a Van De Graf generator that I built for showing at science fairs. The dimmer does a reasonable job of controlling the motor speed, but the motor is running under a fairly light load, and I don't know how it would doo under a heavy load.
Speed regulation should be better with the dimmer under load than
the rheostat because it is not load dependent like a resistor. It
varies the power by time-slicing the torque instead of reducing it.

Normal dimmers are forward phase control.
A MLV dimmer (for magnetic transformer low foltage lighting) is
symetrical phase controland would likely be more suitable for the
inductive load of a universal motor. A reverse phase controlunit
(spec'd for electronic transformer low voltage lighting) might also be
better than a forward control as it switches on at the zero crossing
point.

Lutron DVLV 10p is symetrical, 1000va, 600p is 600va - add csa suffix
for canada.
DVRP 253P is reverse

The standard dv10p is forward 1000 va


(A PWM controller would be better yet, but is not part of this
discussion)

Also another question. Are you SURE it was a rheostat, and not a
variac??? The variac would be MUCH more suitable than a rheostat for
the application and kinda looks like a rheostat.

I have a 210va Powerstat 10B in my hands as I type this - about 2.75
inches in diameter and 2 inches deep. A WHOLE lot more efficient than
a rheostat.

A guy in Phoenix has one on Flea-bay for $76 USA (plus shipping - $96
to Canada - likely a LOT less within the CUSA.

I'll sell you mine for $75 Canadian Pesos but the shipping would
likely kill you. (The little blighters are not exactly light)