# Speed Control of single phase AC induction motor - Possible options?

## Can the speed of an 230V 50Hz single phase Induction Motor be controlled by varying supply phase?

• Total voters
3
• Poll closed .

#### Vedu2611

Joined Aug 3, 2017
26
Hi all,

So I'm working on a project titled "Controlling the speed of a single phase induction motor".

Supply to motor is a 230V,50Hz single phaseAC.

While researching on the internet I came across this one circuit that proposes a model that uses A microcontroller and a triac circuit and a zero crossing detector circuit to "adjust the speed".

Now I also read elsewhere on this website that a single phase induction motor can only be controlled by changing it's frequency - voltage changes won't result in a change in speed; and that for the you require a VFD that is usually quite expensive. Best use a 3Ph Induction Motor.

Now, herein lies my question: What if the voltage of the supply to motor is changed by controlling the phase angle? Using the microcontroller for a custom delay value and firing the triac after every zero-crossing)

Shouldn't this not control the speed, since the frequency remains the same?

I'm attaching the circuit and a report that was proposed on the other website.

Best Regards,
Vedant

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#### Picbuster

Joined Dec 2, 2013
1,027
Voltage will never change speed the frequency does. ( for that type of motor)
The current will affect the torque. ( voltage, current and load will also effect the speed.)
If you want to build your own you should build a sinus generator with a variable frequency.
Change input voltage to DC to feed the generator.
All other systems (like triac) will produce spikes and will violate EMC spec's.

Picbuster

#### crutschow

Joined Mar 14, 2008
26,999
Now I also read elsewhere on this website that a single phase induction motor can only be controlled by changing it's frequency - voltage changes won't result in a change in speed; and that for the you require a VFD that is usually quite expensive. Best use a 3Ph Induction Motor.
That is correct.
So reducing the voltage using an SCR phase-control circuit will not work.

You have to change the frequency (and for frequencies below the motor's rating, you must also reduce the voltage in the same proportion to avoid saturating the motor magnetics).

#### Vedu2611

Joined Aug 3, 2017
26
That is correct.
So reducing the voltage using an SCR phase-control circuit will not work.

You have to change the frequency (and for frequencies below the motor's rating, you must also reduce the voltage in the same proportion to avoid saturating the motor magnetics).

Could I just request you to go through that report I have attached - they have explained how a change in phase angle of supplied voltage leads to a change in voltage and hence a change in speed of motor.

Please refer to that and reaffirm if they are incorrect, or it is me who has interpreted it wrongly... in which case, what would be the correct interpretation? (They have users commenting that the circuit has worked for them)

Edit: This site says something otherwise.... http://www.orientalmotor.com/brushl...-control-methods-of-speed-control-motors.html

Regards,
Vedant

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#### recklessrog

Joined May 23, 2013
985
Without looking it up I'm can't see how varying the phase (???) in the supply to a single phase induction motor will work due to the spacing of the flux rings in the rotor which are determined by the ac frequency and are not movable, are you mixing up Phase control of single phase supply by chopping the voltage at a point in the cycle, (firing angle switching) with phase difference between two supplies? Two phase motors use a capacitor to delay the phase from a single phase supply to a mechanically spaced second winding which locks the rotation speed to the line frequency and also have fixed flux rings. "phase" is a relative term referenced to some point in the cycle, so single phase means exactly that.

Try googling induction motors for a better description, it's been a long time since I last dealt with the subject and my memory is not that great lol.

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Joined Jul 18, 2013
22,855
You need to specify the type of 1ph induction motor, anything but shaded pole are a little iffy to control, especially when using phase angle control, Cap start/run motors tend to drop out of run at low rpm/under load when controlled this way and also when frequency controlled.
This is one reason that you rarely see 1ph VFD's, just about all of the hobby CNC community that use a induction motor spindle, use a 3ph motor with a 240v 1ph supply version VFD.
I am experimenting (when I get the chance) with a pic based control of shaded pole motors using a burst method instead of phase angle control as is done with a dimmer style Triac control, which tends to create buzz at low rpm.
Max.

#### Vedu2611

Joined Aug 3, 2017
26
@recklessrog Yes I think I have now identified the misconception -

All the links and material that I have posted are methods of STATER Voltage control and not of the rotor - and the torque is proportional to the square of the stater voltage, thereby making speed also affected by change in voltage.

The triac in the circuit is fired after certain delays that are predefined and trigger signal is provided by the microcontroller.

"Are you mixing up Phase control of single phase supply by chopping the voltage at a point in the cycle, (firing angle switching) with phase difference between two supplies?."
Ah, I had used the correct terminology in my question description... but yes I was confused Nevermind.

Problem solved, thank you so much indeed!

Best Regards,
Vedant.

#### cmartinez

Joined Jan 17, 2007
7,313
am experimenting (when I get the chance) with a pic based control of shaded pole motors using a burst method instead of phase angle control as is done with a dimmer style Triac control, which tends to create buzz at low rpm.
Now you've caught my attention.... please do tag me when you open a thread on that subject.

As for the OP's original question. Yes, crutschow's given the most accurate answer. A single phase brushed motor is easy enough to control by just varying the voltage reaching it, or through phase-angle control, or PWM (which in the end amounts to the same thing)... but an induction motor is an entirely different animal. They're designed to work in sync with AC frequency, like a surfer riding a wave. If you were to change the wave's height (amplitude) things won't work the way you want them to.

But nowadays non-vector controlled VFD are not that expensive anymore... Most of the time they'll cost the same as the motor you're trying to control, which is less than $200.00 dlls. #### MaxHeadRoom Joined Jul 18, 2013 22,855 , crutschow's given the most accurate answer. or through phase-angle control, or PWM (which in the end amounts to the same thing)... but an induction motor is an entirely different animal. They're designed to work in sync with AC frequency, like a surfer riding a wave. If you were to change the wave's height (amplitude) things won't work the way you want them to. But nowadays non-vector controlled VFD are not that expensive anymore... Most of the time they'll cost the same as the motor you're trying to control, which is less than$200.00 dlls.
Not too clear on your answer as I have a few (shaded pole) induction motors running on the Triac based controllers, also some PSC (< 1/2hp) motors will run on it also if used in a fan type applications etc, evidently there are a couple of suppliers that claim VFD control of 1ph induction motors, but personally would rather use the 1ph in, 3ph motor solution for over 1/2 HP.
I will let you know when I have the burst one done, just finishing up on a slow start for Universal motors.
Max.

#### strantor

Joined Oct 3, 2010
5,534
an induction motor is an entirely different animal. They're designed to work in sync with AC frequency, like a surfer riding a wave. If you were to change the wave's height (amplitude) things won't work the way you want them to.
That's an excellent way of explaining it. Thanks; I'll add this to my mental library for the next time I have to explain it.

#### strantor

Joined Oct 3, 2010
5,534

Could I just request you to go through that report I have attached - they have explained how a change in phase angle of supplied voltage leads to a change in voltage and hence a change in speed of motor.

Please refer to that and reaffirm if they are incorrect, or it is me who has interpreted it wrongly... in which case, what would be the correct interpretation? (They have users commenting that the circuit has worked for them)

Edit: This site says something otherwise.... http://www.orientalmotor.com/brushl...-control-methods-of-speed-control-motors.html

Regards,
Vedant
I skimmed to the end your report and the motor type discussed was a blender motor, which is a series wound motor (AKA "universal motor") - not an induction motor. The speed of a universal motor is governed by the load and has no correlation to AC frequency. These motors can actually be run on AC or DC. If you run one unloaded, theoretically its speed would be infinite, practically it might fly apart (always run with a load). These motors can be speed controlled by a zero-crossing SCR controller no problem, but the speed is hard to control because it also depends on load. if your load is variable, speed will be variable also. If your load is constant (like a fan), then speed regulation by SCR is OK.

The article from orientalmotor should be ignored. It isn't technically wrong about anything but it isn't very clear either. Sometimes they're talking about induction motors, other times not, and they don't specify which is which, or even which induction motor type they are talking about. It is hard to learn anything valueable from that page.

If you want to control induction motor speed, save yourself a lot of grief and get a 3-phase motor. Single phase motors are not good candidates.

Joined Jul 18, 2013
22,855
Again, I would assert than it depends on what version of 1ph induction motor that requires control.
Max.

#### strantor

Joined Oct 3, 2010
5,534
Again, I would assert than it depends on what version of 1ph induction motor that requires control.
Max.
ok, fine. But it's easier to steer clear altogether.

Joined Jul 18, 2013
22,855
Tell that to all the ceiling fan users!

#### cmartinez

Joined Jan 17, 2007
7,313
Again, I would assert than it depends on what version of 1ph induction motor that requires control.
Max.
Yeah, now that you've mentioned it, I've built phase-angle speed controls for shaded pole motors. Specifically for sump pumps. But their speed is very hard to control that way, and they do warm up. Perhaps applying PWM instead would be an improvement?

Joined Jul 18, 2013
22,855
Not a method I would use for a sump pump, I would tend to throttle output for control, plus current goes down.
Max.

#### cmartinez

Joined Jan 17, 2007
7,313
Not a method I would use for a sump pump, I would tend to throttle output for control, plus current goes down.
Max.
Yeah... I confirmed what you're saying the hard way...

#### PUAR

Joined Jun 28, 2018
11
Hello everyone.
I'm new to this forum, i hope i can find the help i need.

I'm in a project similar to the OP, and also need to control the speed of a single phase motor. Actually what I need to control is air flow through motor speed.

The motor plate is in the attached file, following is a summary of the most relevant data
Single phase, 220v AC, 60hz, ~3.25A
The motor is from an old Buchi 710 fluid bed dryer

We bought a VFD (stratus II from contrlresources http://controlresources.com/pdf/stratus2.pdf) and though i wired and programmed it correcly, the VFD won't make the motor reach it's max rpm. I'll explain it better.

When I connect the motor directly to the power supply, the motor starts and reaches max rpm almost instantly, but when I connect the VFD the motor takes approximately 30 secs to reach the "max speed", which is noticeably lower than the true max speed (the one it reaches without the VFD)

Can anyone please give me some advise about how to approach this issue? the only parameter i have not messed with is the holding register 40029 which corresponds to PWM carrier frequency. I left it alone simply because i don't know what that is and how is it related to what i want to do.

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Joined Jul 18, 2013
22,855
Normally a typical VFD will require the pole count (RPM) etc and other data from the motor to be entered into certain parameters, I have heard of very few people that have successfully ran a 1ph motor on a VFD.
This one reason why a ECM, Electronically Commutated motors are used in HVAC applications etc.
These are essentially 3ph motors with a form of VFD internal.
Max.

#### cmartinez

Joined Jan 17, 2007
7,313
I have heard of very few people that have successfully ran a 1ph motor on a VFD.
... actually, I have heard of none? ... how did they go around it?