# PWM Single-phase Induction Motor

Discussion in 'The Projects Forum' started by André Ferrato, Feb 12, 2016.

1. ### André Ferrato Thread Starter Member

Apr 5, 2015
206
1
Hello, i have and old fan here that has been with me for like 10 years. I plan to rework him entirely, give him a new look and some new functions, including some speed control.

I was googling something to control an AC circuit using PWM dc techniques and i found this:

At a first look it seens to work, but has anyone used it before ? The way i see i should face almost no problem with this arrangement. As it doesn't interfere in the frequency of the ac line or the voltage(Maybe a bit).

Also i have some other questions:

- By changing the frequency of the AC line to 120hz(It's 60hz) what would happen ?

- How does these variables relates to each other ? The RPM, the voltage and the frequency of the AC. Can anyone point me to a good link about single phase induction motors and it's characteristics ? Formulas, equations... that sort of things.

Jul 18, 2013
10,851
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Traditionally attempts to control the rpm of 1ph induction motors have not been that successful, if this is a fairly small fan then I would expect it to be a shaded pole variety, these can are typically be controlled by Triac type phase control.
If it is a capacitor run motor, you may find it dropping out of run on the lower rpm's.
Max.

3. ### André Ferrato Thread Starter Member

Apr 5, 2015
206
1
Why would it drop out of run?

Jul 18, 2013
10,851
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The capacitor is calculated to provide optimum phase angle (90°) control between the start and run winding, a 1ph induction motor without any phase shifted winding the field just oscillates back and forth across 180°, (single phase).
So by varying the frequency of the supply the phase angle between main and run winding changes, in some cases to the detriment of the motor performance, especially under load.
Max.

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5. ### André Ferrato Thread Starter Member

Apr 5, 2015
206
1
I see.. i thought the pwm in AC changed only the average level of AC that reached load.

Jul 18, 2013
10,851
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This is why you see extremely few 1ph VFD's, if at all, compared to 3ph versions available.
Max.

7. ### André Ferrato Thread Starter Member

Apr 5, 2015
206
1
Even if i use a zero crossing detector to put the pulses applied to the mosfet in the same phase as the AC motor? And then insert a very high frequency pwm into the AC wave, dimming it ?

8. ### tcmtech Well-Known Member

Nov 4, 2013
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I would try running your fan motor off of a simple variable AC power source like a variac transformer and see how it reacts to having it's voltage reduced. If the motor works fine and shows a fairly proportional speed to voltage applied ratio then a simple Triac based speed control similar to what is used in a common incandescent light dimmer will work.

No need to complicate what should be a simple process.

9. ### André Ferrato Thread Starter Member

Apr 5, 2015
206
1
I know i know that this is a rather different approach, but i would like to see if it works.

I have several optocouplers and opamps here, but no triac around, i would like to use the components i have around. That is an Arduíno or a clock signal generator IC( I have many of them), some of those that i said.

Jul 18, 2013
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You haven't confirmed as to whether the motor is shaded pole or capacitor type?
Max.

11. ### André Ferrato Thread Starter Member

Apr 5, 2015
206
1
Oh sorry, its a shaded pole 50w, 127v AC induction motor. Very weak one.

Jul 18, 2013
10,851
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Then the Triac would be the optimum, the effects of any variation in frequency would be unpredictable with the shaded pole, as it would with the cap motor.
Max.

Apr 5, 2015
206
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14. ### André Ferrato Thread Starter Member

Apr 5, 2015
206
1
@MaxHeadRoom I could not find any triac on the local electronic shops. I may try the pwm approach.. i have one last question, lets say i use a zero crossing detector to put the control signal(the one that is applied to the fet) and the AC line in phase.

Then when the AC line crosses the zero i initiate a series of pulses, like something around 10000... to chop the voltage that reaches the motor.

This would look like pwm right ? My concern is, am i changing the frequency of the line? Because i know the speed of the motor is dependent on frequency.

I would be doing the same thing from that image in the first one.