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Control of a washing machine universal motor

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Dept-74

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Hi,
I'd like to design a circuit able to control the torque and the direction of a universal motor.
I know the direction isn't supposed to be changed but the motor will run at very low speeds or even motionless.
To avoid overcurrent, i'll bridle it to 110v even if the mains will be 220v AC 50Hz.
I connect the two windings (armature and rotor) in series, and to reverse the direction, I must reverse the curent passing through one winding and not both. (H-bridge ?)

How would you do this ? Triacs, thyristors, mosfets, IGBTs ?
 
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chuckey

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Simple way :- use a relay as washing machine manufacturers do, else use four triacs. The other devices only work on DC. Though SCRs will work on unsmoothed DC, you still need four. The other will work but the circuits are more complex..
Frank
 

treez

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I remember at a showers place, they just used to switch a series triac on/off at a frequency kind of near mains frequency
 

Dept-74

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Treez, i think you are refering to phase angle control ;)
Chuckey, problem with relays is the lifespan. As I only need ~110v, I also thought about a derivative of your 4-triacs bridge using two thyristors and two diodes, so I could perform phase angle, and direction control, simply by choosing between the positive or negative portion of the sinusoid. thyristor.PNG
For those interested, I am designing a force feedback steering wheel ;-)
 

FvM

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The SCR bridge can work, but if you want to run the motor at low speed, you better use a variable DC source.

AC phase angle control will generate much noise and unwanted torque ripple, and you are running the motor with a small fraction of rated voltage for your low speed application. If you already have the motor, you should make some experiments with a lab power supply to get an idea of required current and voltage.

Torque of a series connected DC machine is rougly proportional to I², by the way, at least in the range below stator saturation.
 

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You're certainly right. 50Hz current impulsions could create undesirable effects. Problem with DC is the cost. MOSFETs are much more expensive than SCRs for the same voltage/current rating.
I didn't know about torque proportionnal to I² (that's interesting because I need the more torque I can have, and I measured at 12v, 1.7 amps a torque of 0.15N.m and PROPORTIONALLY estimated 1.38 N.m at 110v). I am a newbie. I used to work with low voltage DC motors. :roll:
Suppose I manage to rectify 220v AC in 110v DC (diode, low pass filter...) then with an H-bridge and PWM I control the average voltage at motor terminals.
Do you think it would be necessary to "close-loop" the current regulation ? Or the current will always be the same for a given voltage since the motor will always be working at full load (0 to 60 rpm) ?
 

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Your calculation is most likely wrong because torque is almost proportional to I² rather than I. You should also consider the motor maximum current rating, which is unlikely greater than 5 A, probably lower for 100% ED. You don't need 110V for your application.

Need to take care that the motor doesn't spin up to high speed without a load.
 

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While washing machine motors are cheap and if recovered fro a scrap machine are free they are not suitable for many applications. For their max H.P. they are run at 15-20 K RPM, which is too high for most applications unless a reduction gear box is used, or a 15:1 reduction via a belt drive such as in a washing machine.
I seem to think that they are run at about 50V for the washing cycle. This where they rotate the drum a couple of turns in one direction wait, then a couple of turns in the other direction. So in this mode they using 100W or so at a 50% duty cycle. Because the speed is so low their fan (if fitted) would not perform any useful cooling function.
A windscreen wiper motor might be a better starting place depending on your torque/speed requirements.
Frank
 

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