Control current flowing in a coil

Other Hardware General Electronics
1

Hi everyone,

I am planning to salvage power from an old washing machine motor.

The motor model is u24502p13

It has :

  • 2 wires in the stator
  • 2 wires in the rotor (brushes)
  • 2 wires for speedometer

I would like to use an arduino to calculate the rotation speed (3rd pair of wires) and control a digital potentiometer to regulate current flow in the rotor and increase or lower magnetic field according to the current RMP and wanted RPM.

Am I doing it right ?

I currently don't know the voltage of the motor at 650 RPM but it's rated for 330W. Depending on the voltage it will generate X amps and I need to feed the rotor with same X amps (from what I understood, I'm not a god of inductive loads and magnetic fields). I guess it won't be higher than 2A if the voltages goes high enough.

Does such digipot exist ? Rated for ~5V up to 2A

Thanks

2

Motor power control is done with PWM, not with analog circuitry.

No, digitpots are signal level only, not for power.

These washing machine motors typically have an ac output tachometer for speed measurement,
so you'll need a circuit to convert that to something you can measure - the voltage will be > 5V at high speed,
note.

For such a motor you can control the field/stator current to regulate speed, more current = lower
top speed but more torque.

Regulating the armature is also possible, and gives speed control directly which might be more
sensible, but armature current is a lot higher than field windings.

In terms of hardware you need some sort of MOSFET switch or even a half-bridge, generously rated,
which can be tricky to get right at high-power - a pre-built motor controller is likely to be less
hassle.

What voltages are you expecting to need?

3

I have no idea at all.

I'm not at home until thrusday 27th but when I get there I'm going to put a dummy load on the (rotor ?) coil and measure voltage.

I will create an efficiency graph to see what voltage is the most efficient compared to rotation speed, and note the RPM and voltage so I can go further.

I think there are enough cheap components rated at 600V to build something correct.

From what I understood in your message, I would need to feed stator current to create torque and get the generated current through the brushes right ? In my initial post it was the other way around but I was not sure at all.

Thanks

4

Most of the power is in the armature current, the field winding is just to create a magnetic field.

Since this is likely a universal motor you're going to have to characterize it for DC, since the
field winding inductance will not have any effect at DC (ie won't be limiting field
current).

A field coil motor is always going to be more complicated to manage than a PMDC motor, as
you have to consider the losses in the field winding. You can of course get the benefit of
regulating via the field winding (lower power circuitry), but you have to understand about
motor control to do this well.

5

I had to read that 3 times since english is not my native language.

I'll have to stick with windings motor since I want this project to be the cheapest possible and PMDC are rarely found in dumpsters.

Also, working with that kind of motors will help me to understand how they work and expand my knowledge.

To sum up, I'll gather all information needed on this motor and then try to keep the RPM stable with a PWM controlled MOFSET.

I'll update this thread with new questions or results of my experiments when I get home.

Thanks