Need help controlling ac motor speed.

Thread Starter

SPE

Joined Mar 7, 2017
8
Hello all, not sure if this is posted in the correct place or not but I'm working on a project I could really use some help with.

I have built a circuit designed to do ac phase control. The circuit will dim a light bulb but I can not for the life of me get it to control a single pole motor's speed. When I connect it to the motor the motor just turns very slowly while humming. Any input would be greatly appreciated.

A picture of the circuit is attached, it uses an H11AA1 to detect zero cross then triggers a BT137 triac through an moc3010 optocoupler. The yellow box is part number CBBX2 and to my understanding is a snubber but I may be mistaken.

The motor is a single pole shaded AC motor 115v, 1.4 LRA and I have previously controlled it's speed with an off the shelf light dimmer. I only really need two speeds controlled by a microcontroller so I am also open to other ideas.
 

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MaxHeadRoom

Joined Jul 18, 2013
19,181
What is the circuit you have at the moment?
There is a couple of circuits in the Picmicro site AN958 etc.
I am working on a small pic ramp up app for Universal motors at present, and then may look at phase angle alternatives for shaded pole motors.
Max.
 

Thread Starter

SPE

Joined Mar 7, 2017
8
What is the circuit you have at the moment?
There is a couple of circuits in the Picmicro site AN958 etc.
I am working on a small pic ramp up app for Universal motors at present, and then may look at phase angle alternatives for shaded pole motors.
Max.
The circuit we are currently using is the one pictured and detailed in the post is there something else you were asking for? Thank you very much for the reply.
 

Glenn Holland

Joined Dec 26, 2014
667
The speed of an AC motor is proportional to frequency so the speed of your shaded pole motor cannot be reliably controlled by varying just the voltage with a phase control device such as a light dimmer.

For an AC motor, you need to use an inverter operating in accordance with the Variable Voltage Variable Frequency (VVVF) theorem which says the voltage and frequency must be varied in a constant ratio. For example, if you want to change the speed by half, the frequency will be changed from 60 Hz. to 30 Hz. and the voltage must also be changed from 110 to 55.

I'm also not sure that a shaded pole motor will respond to a VVVF inverter like a conventional induction motor. Furthermore, the motor must have insulation compatible with an IGBT inverter. Otherwise, the sharp edged pulse width modulation (PWM) will cause the insulation to fail.
 

MaxHeadRoom

Joined Jul 18, 2013
19,181
The speed of an AC motor is proportional to frequency so the speed of your shaded pole motor cannot be reliably controlled by varying just the voltage with a phase control device such as a light dimmer.
.
Unfortunately the (shaded pole) ceiling fans I have don't know this and they rotate fine with a Triac dimmer anyway!o_O
Max.
 

Thread Starter

SPE

Joined Mar 7, 2017
8
The speed of an AC motor is proportional to frequency so the speed of your shaded pole motor cannot be reliably controlled by varying just the voltage with a phase control device such as a light dimmer.

For an AC motor, you need to use an inverter operating in accordance with the Variable Voltage Variable Frequency (VVVF) theorem which says the voltage and frequency must be varied in a constant ratio. For example, if you want to change the speed by half, the frequency will be changed from 60 Hz. to 30 Hz. and the voltage must also be changed from 110 to 55.

I'm also not sure that a shaded pole motor will respond to a VVVF inverter like a conventional induction motor. Furthermore, the motor must have insulation compatible with an IGBT inverter. Otherwise, the sharp edged pulse width modulation (PWM) will cause the insulation to fail.
I have considered this but since the motor does in fact respond to a light dimmer I assume it will also work with the phase control here, just trying to figure out what the issue is with my current circuit.

Max I will draw the circuit out tomorrow.
 

Glenn Holland

Joined Dec 26, 2014
667
I have considered this but since the motor does in fact respond to a light dimmer I assume it will also work with the phase control here, just trying to figure out what the issue is with my current circuit.

Max I will draw the circuit out tomorrow.
If the voltage is varied and the frictional load is constant, the motor speed will vary because the torque varies. This is a mechanical relationship like what happens to motors during a moderate to severe power brown out.
 

Thread Starter

SPE

Joined Mar 7, 2017
8
If the voltage is varied and the frictional load is constant, the motor speed will vary because the torque varies. This is a mechanical relationship like what happens to motors during a moderate to severe power brown out.
Would it not be possible to exploit this to get the two speeds I need? Also could you refer me to a schematic for a variable frequency controller?
 

MaxHeadRoom

Joined Jul 18, 2013
19,181
You mention you want control via a Micro, if not there is ample Triac dimmer circuits out there.
For a micro phase control you need to detect the zero crossing point as in Fairchild AN-3006 etc.
There are a few way out there that use transformer or transformer-less detection.

Max.
 
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