Speed control of induction motor

[mr_byte31]

Hi All,

I have single phase induction motor(1/2 HP , 220V). I got it from very old washing machine.

I want to control the speed of the motor to be around 80 RPM. the precision is not a mater for me but the motor will work for days!

I made a simple tacho circuit to read the speed of the motor using micro-controller and now i need to control the speed.

I was planning to use PWM technique to control the speed but many articles on internet didn't recommend it. They stated that variable frequency driver is the best and most power efficient. I found VFD very expensive

I made some searches and found this video : https://www.youtube.com/watch?v=_1rj-IgWQeM

The main concept depends on this circuit :


please share your advises and experience.

[mris99]

Hello,

- Is your induction motor a split-phase asynchronous motor with a main windings and a starter windings?
- If it is a split-phase than is it a Capacitor start motor or Permanent-split capacitor motor?

Check the the differences here:
http://en.wikipedia.org/wiki/AC_motor#Single-phase_induction_motor

- Sometimes 3 phase (3x230V) motors are used in single phase configuration, e.g. I know such a type of washing machine motor. It has 6 windings instead of 4.

1. Capacitor start motor is not suitable for changing the speed.

2. Permanent-split capacitor motor.
    Phase control maybe works if you drive the starter winding serial with the capacitor directly from the mains and control the main windings only with a tirac. But is it not a proper solution only works with constant loads. The speed stability is very poor without feedback.
    VFD works, but not directly. You have to remove the capacitor, and drive the main and starter windings with 90 degree difference. The voltage on the main windings has to be proportional with the speed (220V at nominal speed). The voltage on the starter windings has to be also proportional to the speed but higher with a factor about 1.2-1.4. (e.g: 1.3*220V at nominal speed.)

3. 3 phase motor in single phase configuration
    It is the best with VFD. You just need to remove the capacitor, and use the 3 terminal as a 3 phase motor with standard VFD.


- What is the nominal speed of your motor. About 1440 1/min ?

   Your desired speed is only a small fraction of the nominal. It means that the power of the motor will be also a small fraction of the nominal. (VFD can provide constant monument, in that case the power is proportional to the speed, in case of phase control it is even worse.)
   You need also external cooling fan.

It is much better if you use gears, and drive the motor near to the nominal speed.
You get much higher monument and the power is also near to the nominal.

Best regards,
Mris

[mr_byte31]

Oh , a lot of info !
Thanks a lot.

Ok , I am planning to use pulleys to get the desired RPM.

it should be something like that (just skip the ratios on the figure):



pulley A is driver pulley.

This mean the motor would not have any electronic speed control. it will run with maximum speed and the pulleys will take care of speed control due to the diameter ratios.

[Vineyards]

Mains amplitude may change significantly depending on the general load conditions. I would say (though not sure) this might be as much as 20% up and down.
If it is a three phase motor it should be immune to speed changes but if it is a mono phase motor, the speed will change According to amplitude. You can use a VFD circuit for this but for three phase ones speed control comes with sacrifices. At least, this is how I remember.

[j0k3r]

Take a look at this magazine article, EPE March 2015.
They have a "10A/ 230V Intelligent Speed Controller for Universal Motors"

[Magnox]

They have a "10A/ 230V Intelligent Speed Controller for Universal Motors"

It's not a universal motor though - it's an induction motor. If it was a univeral motor it would be trivial.

The gear/pulley solution is the only vialble solution for speed control here. I have a grinder I use for knife sharpening that is single phase induction, and went through a similar research process to discover electronic speed control here was a waste of time. I found a three-speed pully on ebay instead and hammered it onto the shaft.