washing machine motor: use it at very low speed? 2

[TayLow]

I have a working motor from my washing machine. I works at high or very high speeds, but I want to do some experiments with my students and need it to run VERY slowly (eg 2 rpm). I have now idea about electronics but I just hope you can point me in the right direction, is this possible? I don't need a long explanation, just want to know if it's a plausible idea or I should just forget about it.

I've seen videos where the guy decreases the input voltage in the motor and he gets lower speeds (for example 110V or 68V instead of 230V). He gets lower speeds by doing this (and also less power consumption) but those speeds are WAY to high for what I need. On the other hand, it doesn't seem sound to just keep decreasing the voltage input because then I would need ridiculous values like 500 mV (ie assuming the more I lower the voltage, the slower the motor runs).

Maybe I need an additional circuit/component to do what I intend, or it is not possible at all with this motor? One last thing: I will no put big load (much much less than the regular duties of the motor in the washing machine) so I don't expect the motor to develop its full torque.

Anny comments?
Thanks!

 

[jraef]

Impossible to tell without knowing exactly what type of motor you have, washing machines can have several different types, each with it's own issues and answers.

As a grossly generalized rule however, the answer is no. Assuming it is a single phase AC motor from a machine over 10 years old (because after that, this is no longer a safe assumption either), it could still be a Split Phase (SP), Capacitor Start - Capacitor Run (CSCR), or Permanent Split capacitor (PSC) design. If a lot older, it might even be a Universal Motor, which is an AC/DC hybrid. Of those types, only the Universal motor can be speed controlled without burning up. The others are all AC Induction Motors, which means that you cannot actually control the speed by just reducing the voltage, you can only reduce the TORQUE that the motor produces, which, given a consistent load, will RESULT in a drop in speed, because you are in essence flirting with stalling the motor. SP and CSCR motors have speed switches in them that allow them to start using special windings. Those windings are not rated to be used continuously, so if you drop the voltage and thus the speed, the Starting Windings kick in and then burn up, because you are trying to do something they were not designed for. A PCS motor can be used that way and will not have that problem because there is no speed switch inside. But the torque capability drops at the SQUARE of the voltage reduction, so to get it to run at an accurate speed that low is all but impossible. Then at the same time, the motor is going to be in extremely high slip, which means MOST of the current going into it will be making heat, not movement, and at the same time, it's internal cooling fans are no longer functioning. Death is just moments away. There are devices called "VFDs" that can be used with single phase PSC (only) motors, which reduced the voltage AND the frequency together, thus avoiding the high slip conditions. But a VFD like that will cost you more than just getting a slow speed motor.

Modern washing machines now have 3 phase motors and VFDs that convert the single phase supply into 3 phase for the motor, or they can have a new high efficiency type of system that is a motor and built-in drive that is called an "ECM", Electronically Commutated Motor. In those cases, you must have the entire system, not just the motor, or it will not work.

If by CHANCE you just happened to stumble across a very old Universal motor, then lowering the voltage will lower the speed, just like on your variable speed drill motor or your blender. But again, trying to run one at 6RPM is going to be iffy.


"Will work for (the memory of) salami"

 

[TayLow]

Wow that was fast and very detailed, MUCH appreciated!
The motor is like this:

http://www.youtube.com/watch?v=OgYZyZQbh-o

So, I think the safest will be to forget about it and try to find another alternative (probably a step by step one which is made for high torque and possibly low speed).
THANKS!!!

 

[jraef]

OK, did you see the sparks flying in that video? That means that is is indeed a "Universal" motor, AC/DC, and you can control the speed by controlling the voltage. So do you really need the MOTOR to spin at 6RPM, or a SHAFT to spin at 6RPM? Because if it's a shaft, you might be well served by just working out a suitable sheave systems for it. Run the motor as slow as you can and still maintain a reasonably stable speed (use a tach), then work out the ratios to get your work shaft down to 6RPM from that lowest speed.

By the way, on that video... gas can next to the electrical outlet? Sheesh...

"Will work for (the memory of) salami"

 

[itsmoked]

There are LOTs and LOTs of small gear motors you can hold in the palm of your hand designed to provide a shaft running at single digit RPM speeds.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TayLow]

jraef, thanks a lot again. Yes I need the shaft to spin at 06 RPM.
But as pointed out by itsmoked, I decided it will be better just to buy a small motor with included gearbox so I can have low RPMs and still good torque... in the end now I see the washing machine motor will not serve my purpose well...

 

[itsmoked]

Overpriced Grainger

Or

Mondo Selection of gear motors

Be sure to click on GEAR MOTORS on the left to see a hundred different ones.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

You know, if this is for students, it might be a valuable lesson for you do demonstrate BOTH methods just to let them come to the same conclusions. You already have the washing machine Universal motor, so you should be able to lower it's speed with a simple rheostat, an old fashioned dimmer if you will, as long as it is of suitable size. Then get the low speed gear motor as well, mounted side by side, and hooked up in such a way as to easily motiror the electrical side. Let them observe the precision, energy consumption and effectiveness of both methods.

"Will work for (the memory of) salami"

 

[roydm]

A windscreen wiper or window motor from an auto wrecker might fill the bill.