Powering an AC induction motor with a function generator and an audio amp.

[Oliver Klozoff]

I have 6 AC shaded pole induction motors that I'd like to power and control the speed of using a function generator on my smart phone or laptop for the input signal and using an amplifier to power the fans. The motors are 120v 60hz and approx. 40w each. I plan on controlling the speed by reducing the hz and v down to maybe 35hz 90v or so. I have an older component stereo receiver/amplifier I have been playing around with. I have a few questions, mainly:

-Am I better off using a component amplifier like I have been playing around with or a car audio type?
-If I go with a car audio type, am I better off with a two channel amp, or a bass style amp which would be more in the hz range of what I'm trying to do?
-Is there a better/cheaper amp available to do what I'm trying to do?
-Most amps are rated at 4 or 8 ohm (speaker rating) for the output watts. I do not know how to convert this to what I'd need to power motors.
-Will an amp accurately produce a sine wave generated from a function generator? If not, will it be accurate enough to power motors?

This is a 4 channel (bridgeable to 2 channel) amp I have considered. I'd probably try to power 3 motors of the 6 with each channel.

https://www.ebay.com/itm/Crunch-PX-...ash=item4649ce901b:gTYAAOSwyjBW6Wyd&vxp=mtr

This is a project my son and I have been playing around with and as you may or may net have been able to tell we're both learning as we go. I'll come up with a few more questions if anyone can chime in here. TIA!

 

[Audioguru]

The Crunch amplifier produces 250W into 4 ohms when two of its 4 channels are bridged without saying how much distortion. It might be clipping like crazy and producing squarewaves when in a car that is charging the battery with 14.4V, not 12V. Its output might be only 125W as a sinewave.
A 125W sinewave into 4 ohms has an RMS voltage of only 22.4V and a 120V motor will do nothing.

 

[Hopup]

Even 2000watt amp won't give even near that voltage at speaker ohms. Not really good with motors but I think you can buy Pulse width modulation motor driver very cheaply and you can adjust the power accuately with it.

 

[duke37]

You would need a transformer to boost the voltage. A big one is needed to run at low frequency.
Shaded pole motors have a phase shift due to the shading, I do nor know what the effect of low frequency would be, possibly a very low torque. However, the torque reqiured by a fan goes down sharpley as the speed is reduced.

 

[BobK]

An amp capable of 500W into 8 Ohms would give you 90V RMS. Seems a bit extreme to run a 40W fan.

An amp capable of 40W would output 18V. Coupled with a 5:1 transformer, you would get 90V.

Not sure what the effect of the lower frequency would be.

Bob

 

[Oliver Klozoff]

An amp capable of 500W into 8 Ohms would give you 90V RMS. Seems a bit extreme to run a 40W fan.

An amp capable of 40W would output 18V. Coupled with a 5:1 transformer, you would get 90V.

Not sure what the effect of the lower frequency would be.

Bob

"An amp capable of 40W would output 18V. Coupled with a 5:1 transformer, you would get 90V."

Yes, but then wouldn't you only wind up with 8W at 90V on the secondary side of the transformer?

 

[BobK]

No, a transformer preserves the power by raising the voltage and lowering the current, ore vice versa.

Bob

 

[duke37]

18V, 40W = 2.2A
90V, 40W = 0.44A

How do you get 8W, power (watts) = volts * amps

 

[BobK]

What do you not understand about “preserves?” For a transformer, the power out is the same as the power in.

As duke indicated, you get five times the voltage and one fifth the current. Miltiply them together and you can see that the power is the same.

Bob

 

[duke37]

You got Watts and Amps mixed up in #6.

Going back to the original requirement, you could put two or three fans in series to slow them down.

 

[Oliver Klozoff]

I misspoke. I meant to use/say amps instead of watts.

 

[Oliver Klozoff]

You got Watts and Amps mixed up in #6.

Going back to the original requirement, you could put two or three fans in series to slow them down.

Yup, I did. So to drive the 5 motors (40W each) I need about 2 amps @ 120VAC out of the secondary side of the transformer for full speed. So I'd need 12 amps @ 20VAC or 8 amps @ 30VAC (slightly more to make up for any inefficiency of the transformer) out of the amplifier to feed the primary side of the transformer. So I need 240W RMS out of the amplifier, correct?

 

[duke37]

My suggestion was to run the motors in series direct from the mains without any other clobber except some relays to alter the connections.

Each motor takes about 0.4A at full power but will take less at a lower votage. The maths are complicated but the torque is proportional to the speed squared and the current will mimic this but there will be an overhead due to the shaded pole.
You could try:-
six motors parallel (100% voltage)
three by two motors (50% voltage)
two by three motors (33% voltage)
one by six motors series (17% voltage)

If you have a Variac then you could measure the current versus the voltage and get an idea of the resulting breeze.

 

[Minder]

Shaded pole motors have miserable starting torque at the best, I don't see putting any number in series with good results!
M.