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.


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!

  • \$\begingroup\$ An audio amp isn't built for efficiency but for extremely low distortion. It would be a great waste of energy to drive a motor with it, even if you have one motor with matching impedance and voltage. \$\endgroup\$ – Janka Feb 1 '18 at 1:22
  • \$\begingroup\$ So any ideas what I can use for a better amplifier? \$\endgroup\$ – user174849 Feb 1 '18 at 2:06
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    \$\begingroup\$ Let's start with this. Not all induction motor types can be speed controlled with a variable frequency drive. The best case (basically, the only case) is if your motors are three phase induction motors. If not, you can basically just stop right there. \$\endgroup\$ – mkeith Feb 1 '18 at 4:11
  • \$\begingroup\$ If they are three phase induction motors, you can buy a product called a VFD (variable frequency drive) which can drive the motor using switching techniques similar to class D audio amplifiers. \$\endgroup\$ – mkeith Feb 1 '18 at 4:12
  • \$\begingroup\$ @mkieth I revised my answer to cover VFD. \$\endgroup\$ – Charles Cowie Feb 1 '18 at 12:08

You should apply 2 volts per Hz, 70 volts for 35 Hz. You shouldn't apply a higher voltage than 2 X Hz. You should apply power at a reduced frequency and then increase the frequency and voltage to accelerate the motor rather than just switching on the motor. You don't need a very good sine wave.

Since you are working with 40 W, 120 V, 60 Hz shaded pole motors, they are undoubtedly single-phase motors. Variable frequency operation should be possible. Since the loads are fans, the load torque diminishes drastically as speed is reduced. That helps with variable frequency operation.

You can probably buy a small variable frequency drive (VFD) for not much more than the price of your example power amplifiers. You can buy VFDs designed for 120 volt, single-phase input, but the output will be stepped up to 240 V, 3 phase. The output voltage can probably be programmed to 120 volts. That will cut the power rating in half, so you will need a 500 W, 3/4 Hp VFD for 2 motors per phase.

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