What kind of rheostat? What kind of motor?
A rheostat, of the type used in an old sewing machine pedal, will only
properly control the speed of a universal (series-parallel wound) motor (AKA
AC/DC motor) or a straight DC motor (the kind with brushes).
Beyond that, you need to select a rheostat with current capacity greater
than that of your motor, and ideally, several times larger (a slower running
motor will consume more current than a motor running at full speed).
Like Don said, it depends. I'll mention that using a rheostat to vary
the speed of a 12 VDC motor isn't going to work very well if you have a
varying load on the motor.
Use a DC motor speed control instead, it will give you much more torque
at lower speeds where a rheostat would stall the motor.
I'm not affiliated with the below:
See ebay item number: 130268566396
or cut and paste this link:
Or do a search on DC motor speed controllers, I think they have
virtually the same unit good for up to 30 amps.
A wire-wound rheostat of the appropriate wattage will do
fine. An electronic speed control mentioned by another
poster is more efficient because it can lower the speed
but keep the torque fairly constant. You may obtain a
speed control from a broken cordless 12 volt drill and
it would work. You could check pawn shops or even call a
couple of construction companies to see if they have any
worn out cordless drills. You could also drop by your
local Dewalt or Skill repair depot and ask them if they
have any in their trash heap.
That's what it is. It give much much more torque at low speeds vs
lowering the voltage. I think that one also allows you to vary the
frequency of the PWM, but I'm not sure why you would want to do that?
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