control AC motor speed

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mr_byte31

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Hi All,

I would like to control washing machine AC motor to have speed of 80 RPM. I know it sounds weird but this is just a part of the project.

I was thinking to use the same concept of DC motors to control the speed of AC motors. the easiest thing was to use PWM technique.

I would use 220V source and the circuit should look like this :



I would use 220 V source and LM555 to be the trigger of the Relay.

would that work ?
 
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Washing machines often use AC motors with "shorted" rotors. Such motor cannot be controlled as you like. Only motors with wound rotors can be speed-controlled.
Reducing AC voltage by a SCR or triac rather controls motor moment and makes it difficult to start under load.
 

hmmm, so what about a thyrsitor ? :

 
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Depending if the mechanical load planned to spin is not too small if compared to the torque of the motor, I would recommend you perform a loop back control by just continuously monitoring the rotation speed and turning on/off the power accordingly.
 

andre_teprom said:
Depending if the mechanical load planned to spin is not too small if compared to the torque of the motor, I would recommend you perform a loop back control by just continuously monitoring the rotation speed and turning on/off the power accordingly.
Click to expand...

My motor is around 1/2 HP , the load is not high.
Feedback circuit would require a tachometer and micro-controller. I would prefer in the current time to make speed variation with basic components then improve later.
so what do you think about the circuit above that control the AC motor using thyrsitor ? do you have better recommendations ?
 

You can't run an AC motor with a single thyristor, the circuit is apparently intended for DC motors. Triac phase angle control works, but only for motors with low initial torque, e.g. fans. It most likely fails with a loaded washing machine motor.

Single phase motors need a means to generate a rotating field, either a capacitor or a special design of the motor ("shadow-pole"). A washing machine motor has considerable power and will usually use a motor capacitor which causes additional problems in triac operation.
 

mr_byte31 said:
do you have better recommendations ?
Click to expand...

It depends on the acceptable range required for the rotation speed, as well as how much of mechanical inertia (motor assembly+load) could maintain some steady spin without motor being driven.

A possibility whose feasibility depends on the assessment above, would be the injection of power on motor at consecutive entire cycles of the mains sinewave, what would subject TRIAC to a lesser stressing condition.
 

Check your motor. In the UK the motor is always a brushed " universal" motor and has an electronic control module. This is so the machine can do a couple of rotations in one direction, then a couple in the opposite direction on the wash cycle. Then runs up to high speed for the spin cycle.
Frank
 

andre_teprom said:
Depending if the mechanical load planned to spin is not too small if compared to the torque of the motor, I would recommend you perform a loop back control by just continuously monitoring the rotation speed and turning on/off the power accordingly.
Click to expand...

would it work using a relay control ?
 

would it work using a relay control
Click to expand...
Neither healthy for the relay nor the motor or load, I believe. Arcing causes short relay life, the jumpy torque shakes motor and load. A bumper car drive´(which is usually operated on/off) has a smooth characteristic compared to an AC motor.
 

FvM said:
Neither healthy for the relay nor the motor or load, I believe. Arcing causes short relay life, the jumpy torque shakes motor and load. A bumper car drive´(which is usually operated on/off) has a smooth characteristic compared to an AC motor.
Click to expand...

sounds very logic.....

but whats wrong about the thyristor circuit i provided above ? i dont see anything wrong with it !
 

but whats wrong about the thyristor circuit i provided above ? i dont see anything wrong with it !
Click to expand...

Induction type motor are controlled by frequency on speed. Just varying the ac available will just over heat the motors.

- - - Updated - - -

mr_byte31 said:
hmmm, so what about a thyrsitor ? :

View attachment 115804
Click to expand...

Would work fine for a light bulb or a series wound motor. But not an induction motor.
 
mr_byte31 said:
can any body provide a simple circuit to implement ?
Click to expand...

It would be simple (in theory) to:
(a) rectify mains AC,
(b) store 330 VDC on a large capacitor bank,
(c) run it through an H-bridge at 80 Hz, which turns it into 230 VAC.

This sounds straightforward. However it would be difficult to construct. It's a high voltage hazard. You'll need to use components which are rated for several Amperes, at high voltage.
 


Sounds very horrible for me !!! , I never deal with such circuits before.

I made some search and found a way but not sure if it would work probably or not :


I plan to add a LM555 timer between terminal 1 and 2.
 

Nope, that still won't because you are still just varying the ac available. Washing machine motors here in the USA are two speed motors. 1725 R.P.M. and 3450 R.P.M.or there abouts. The doubling of the rpm is caused by switching in more windings that effectively doubles the frequency. If you only require 80RPM a transmission would be in order. Otherwise if you do not rectify the ac voltage and build a variable frequency drive system you will not get the results you desire.

Companies like "Square D" and "Fanuc" do this all of the time in Industrial Applications.
 

It is not yet clear what is the level of precision of rotation speed that is required for this project. This also affects the control circuit to be selected. Once you already mentioned that the mechanic load is not too significant, the minor torque probably will quickly speed up the spin of motor, making it difficult to fine-tune to the exact 80rpm as you want. Keep in mind that 80rpm means few more than 1 turn per second, and it is not the full scale speed for these motors in normal operation. Therefore, you should consider to reduce the mains voltage ( supplying 110v instead of 220v, if available ) and should consider to deal with an encoder to take the instant speed, rather than using a tachometer kind.
 

Maybe the selection of a different motor? Bodine makes a group of motors that use a right angle worm screw drive. Some have permanent magnets in the motor and the armature has a commutator and the only voltage you need to control is to the brushes. Very much like radio controlled cars and boats that use 7.2 volt nicad batteries. Also there are motors for power windows that run with worm gear transmissions and supplies significant torque and the voltage can be controlled very easily.
I will find a source of those motors and post in this reply.

These have the planetary type of gears but should work fine.
 
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Thx all for the support.

I got the last image from a project on youtube : https://www.youtube.com/watch?v=_1rj-IgWQeM

It seems it works.

what I understand that I need to stick with the motor frequency (50-60 Hz) to get the max torque , the load is not that high but not even that low !
changing the firing angle of triac would generate a waveform with different frequncies, it looks for me like PWM !

I am sorry if i didnt get your points but I dont have that experience in motor control.

FYI - I use old washing machine motor which has no signals for speed indications.
 

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