Electronics > Beginners
Universal Motor Speed Controller
BrianHG:
The easiest thing you can try is getting a 2kw 240v to 120v transformer. Yes, a real transformer, not the diode BS. The diode will not give you a true speed reduction, instead it will surge high current, full voltage into the motor at 50% the phase cycle. In this case, with no mechanical load on the motor, the motor will spin at around full speed. A real transformer will deliver a true half voltage. This does not guarantee half RPM, however, the speed will be reduced, but, also, the motor's strength will also be cut.
Check if you can adjust the brush orientation on the motor. In some old DC motors, I use to be able to slightly rotate the brushes giving a lower RPM, yet, a higher torque with the same voltage which may be what you are truly looking for. However, you need to observe the motor's idle draw current when doing this. Lowering the RPM too low like this will cause a gain in current draw eventually to a point where you will burn out the motor. (This means rotating the brushes axially (not turning the brush in it's socket) or rotating the outer electromagnets Mechanically, most motors don't allow this, for the old small circular DC motors used to almost allow it by rotating the rear cap of the motor which held the brush orientation, there was usually a peg or screws holding the motor's back plate in the optimum brush orientation.)
BrianHG:
Example, scratching out the protective plastic registry, rotating the illustrated motor's back plate, which holds the brush orientation, in the red direction slowed down the motor, but, increased it's torque. Rotating the brushes in the green direction increased the motor's RPM, but, lowered it's torque.
The same would happen if you instead rotated the position of the internal magnets.
Zero999:
--- Quote from: BrianHG on November 18, 2018, 10:45:05 am ---Example, scratching out the protective plastic registry, rotating the illustrated motor's back plate, which holds the brush orientation, in the red direction slowed down the motor, but, increased it's torque. Rotating the brushes in the green direction increased the motor's RPM, but, lowered it's torque.
The same would happen if you instead rotated the position of the internal magnets.
--- End quote ---
I've never tried that. Does it work in applications where the motor needs to be reversed?
BrianHG:
Yes, the motor slows down, and down, eventually drawing too much current at a stop (no back emf lowering the current here...), then accelerates in the opposite direction. At low RPM, the motor can gain so much torque, it is equivilant to having the motor on a geared down box, but efficiency eventually drops too much and heat is generated as the circuit begins to look like a short. You still get some amazing play in speed before the short circuit condition begins to take over.
Rotating that rear face-plate 180 degrees will spin the motor in reverse at the same speed. Or, you can just swap the wire polarity to do the exact same thing.
Careful on reversing large motors, the brushes may have an angle making them preferred to rotate in one direction. In my treadmill motor, when reversing direction, to prevent the brushes from scratching the commutator, I had to turn them around (They were rectangle in shape, the motor had removable/serviceable brushes)
BrianHG:
Remember what's happening here, rotating the brushes brings the charges magnetic poles on the core closer to the edge of the external magnet's north or south pole. And what happens if you have 2 magnets closer together, the attraction is that much stronger. The motor spins slower because the inductance of the change charge in the coils takes time to build current and a magnetic field instead of having the switch in magnetic field being around the optimum 45 degrees out of phase (electrically, this figure changes depending on the number of poles in the motor's design).
Careful, only around 10 degrees in rotation can double or half the RPM of the motor.
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