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

Thread Starter

Oliver Klozoff

Joined Jan 31, 2018
7
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 each with each channel.

https://www.ebay.com/itm/Crunch-PX-...ash=item4649ce901b:g:DTYAAOSwyjBW6Wyd&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!
 
Last edited:

crutschow

Joined Mar 14, 2008
31,097
To get 90Vrms from an audio amp means it must have an output power of a least 90²/8Ω = 1000W into an 8Ω load.
Are you up to buying such a large amp?
If not you could use a smaller amp driving a transformer, such as a 120V-24V one.

Do you want to drive all the motors at the same time?

The problem with car audio amps is that you need to provide a separate 12V power supply for them.

Any audio amp will have sufficient fidelity to reproduce a sinewave from your generator.
 

MaxHeadRoom

Joined Jul 18, 2013
26,246
I think you may have a problem with lowering the frequency on a shaded pole motor.
Usually it is done with simple triac control (dimmer style).
The 1ph induction motor relies on a correct phase shift between the main winding and the phase shifted winding, in this case, the shaded pole, going too low can cause a drop out of run.
Max.
 

crutschow

Joined Mar 14, 2008
31,097
The 1ph induction motor relies on a correct phase shift between the main winding and the phase shifted winding, in this case, the shaded pole, going too low can cause a drop out of run.
Does the phase-shift in a shaded-pole motor really depend upon the frequency?
 

MaxHeadRoom

Joined Jul 18, 2013
26,246
Does the phase-shift in a shaded-pole motor really depend upon the frequency?
I have never put it to the test, may be needs some empirical testing!
The fact that a shaded pole motor is very low in efficiency and low starting torque, it does not take much to stall it.
Max.
 

nsaspook

Joined Aug 27, 2009
10,663
The shaded-pole motor depends on Lenz law (shade ring produces a opposing EMF to the applied EMF) to shift the flux across the pole face as the waveform rate of change increases, slows and reverses per frequency cycle so there is a frequency related effect on starting torque so it's better to start at full power/correct frequency and then slow it down if using AC power frequency control.
 

Thread Starter

Oliver Klozoff

Joined Jan 31, 2018
7
To get 90Vrms from an audio amp means it must have an output power of a least 90²/8Ω = 1000W into an 8Ω load.
Are you up to buying such a large amp?
If not you could use a smaller amp driving a transformer, such as a 120V-24V one.

Do you want to drive all the motors at the same time?

The problem with car audio amps is that you need to provide a separate 12V power supply for them.

Any audio amp will have sufficient fidelity to reproduce a sinewave from your generator.
 

Thread Starter

Oliver Klozoff

Joined Jan 31, 2018
7
To get 90Vrms from an audio amp means it must have an output power of a least 90²/8Ω = 1000W into an 8Ω load.
Are you up to buying such a large amp?
If not you could use a smaller amp driving a transformer, such as a 120V-24V one.

Do you want to drive all the motors at the same time?

The problem with car audio amps is that you need to provide a separate 12V power supply for them.

Any audio amp will have sufficient fidelity to reproduce a sinewave from your generator.





I need to drive 5 motors with one circuit and 1 with the other. Sorry, I misspoke in my initial post. This is for an air exchanger. 5 motors on one zone and a single on on the other. I'll buy as large of an amp as it requires for each circuit. I don't know how to do the math. And I don't know the ohms of the motor yet. Any idea on a ballpark number for a motor with the specs I had listed in the initial post?
 

MaxHeadRoom

Joined Jul 18, 2013
26,246
Normally air exchange radial fans and are driven by Triac control.
Is this to eliminate the L.F. 'buzz' at low RPM?
If so it is possible to Burst Fire the triac and this eliminates the noise due to random phase switching.
Max.
 

Thread Starter

Oliver Klozoff

Joined Jan 31, 2018
7
Normally air exchange radial fans and are driven by Triac control.
Is this to eliminate the L.F. 'buzz' at low RPM?
If so it is possible to Burst Fire the triac and this eliminates the noise due to random phase switching.
Max.
I'm doing it this way for no other reason than to be different and hopefully my son (and me) will learn a thing or two along the way about electricity. The inner workings of a Triac are both over our heads. I've read a bit about them though. The way I want to do it would be a better way to control RPM in the end anyways, right?
 

nsaspook

Joined Aug 27, 2009
10,663
Starting out with a shaded-pole motor puts you a strike down on reliable controllable speed control to begin with.

Like a dog walking on its hind legs - the marvel is not that it works
well, but that it can be made to work at all.
 

MaxHeadRoom

Joined Jul 18, 2013
26,246
I'm doing it this way for no other reason than to be different and hopefully my son (and me) will learn a thing or two along the way about electricity. The inner workings of a Triac are both over our heads.
Triacs are very simple devices, but let us know how it all turns out if you go the V.F. route!
Max.
 

Audioguru

Joined Dec 20, 2007
11,248
On the other website I mentioned that the cheap ebay "1000W" car amplifier made by Crunch might produce an undistorted output of only 22.4V RMS when two of its 4 channels are bridged and the car battery is being charged at 14.4V that will not be enough for the 120V motor.
 

Thread Starter

Oliver Klozoff

Joined Jan 31, 2018
7
On the other website I mentioned that the cheap ebay "1000W" car amplifier made by Crunch might produce an undistorted output of only 22.4V RMS when two of its 4 channels are bridged and the car battery is being charged at 14.4V that will not be enough for the 120V motor.
How many amps would I have if I ran the 22.4V RMS through a step up transformer to 120VAC?
 

Thread Starter

Oliver Klozoff

Joined Jan 31, 2018
7
Another thing I thought of.o_O Is there any gain in having the function generator generate a 3-phase sine wave and power a motor with each leg?
 
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