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I'm trying to reduce a PSC electro-motor rpm to about 80% to 90% of it's nominal speed. It is attached directly to a centrifugal fan and a mechanical approach is not possible. The ratings are: 450 W, 2.2 A , 220 VAC, 50 Hz, 700 rpm, winding insulation class B.

I tried placing a series 15 Ω 150 W resistance on the current path. And it worked fine but the resistance got too hot, adding a heat sink helped but didn't completely solved the problem.

I also tried using a dimmer circuit, and it caused a 15 degree Celsius temperature rise from the normal temperature (I think because of the added harmonics), which is 60 degrees in 23 degree environment (all skin temperature). Since the working environment of this fan is 45 degree Celsius and the insulation class is B, I think dimmer solution would fail.

I'm now thinking about placing a series inductance on the current path. But I haven't seen a single page about this approach on the internet and I don't know if this would work. Inductor in this size is not readily available and making it is a lot of work. I would like to know why this approach may not work.

I would also appreciate any other solution or idea which cost less than 10 to 20$.

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4 Answers 4

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A motor with class B insulation should be able to withstand 80C rise by resistance in a 40C ambient. That would be 75C in a 45C ambient. It is difficult to estimate what the rise in skin temperature would need to be would need to be.

A series inductor should work similarly to a series resistor except that the phase angle of the voltage drop will be nearly 90 degrees with respect to the supply voltage. As a result, 150 ohms inductive impedance will not provide the same motor voltage as was provided using the resistor. If you measure the voltages across the 150 ohm resistor and across the motor, you should be able to calculate the inductance needed for an equivalent result.

Reducing the speed of the motor by reducing the voltage will increase the motor slip. That will increase the rotor losses, but the reduction in load gained by reducing the fan speed compensates for that. However if the motor is overloaded when running the fan at the rated motor speed, the motor could still overheat at the reduced speed.

Another alternative that you might consider is to reduce the capacitance of the auxiliary winding capacitor. The value will need to be determined by trial and error.

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  • \$\begingroup\$ Thanks for your very informative response. I tried lowering the capacitor from 15uF to 10uF and 5uF. with first having no significant effect on the rpm and in 5uF case, although the speed of the fan was very desirable the current went up too high, the rated current is 3.3A on fast winding and I read 4.8A on multi-meter. and the motor temperature also went up very fast. \$\endgroup\$ Jul 26, 2017 at 14:23
  • \$\begingroup\$ You should check the current when the motor is running with the original capacitor at full speed. The fan may require too much power for the motor. The motor current vs. slip may be too high for any slip-increasing method to work. You could check the current with the resistor method. \$\endgroup\$
    – user80875
    Jul 26, 2017 at 14:45
  • \$\begingroup\$ I've done that. the motor has 3 winding, low , medium, high. the current of which respectively is 2.4A, 2.8A and 3.3A. these currents are measured with nothing attached to the motor and the original 15uF capacitor. when I connected the lower value cap (5uF), the motor got hot quickly and I measured 4.8A on high speed winding. the current with resistor or dimmer attached was normal, but with dimmer, the motor got hotter than usual while with the resistor attached it's temperature was normal(the resistor on the other hand rose up to 180C after an hour). \$\endgroup\$ Jul 29, 2017 at 5:04
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The only alternative that would not increase the heat in the motor is a single phase VFD. They exist, although few and far between, and they are expensive; generally more expensive than just replacing the motor. Single phase VFDs (for running SINGLE phase motors) can ONLY work on PSC (or Shaded Pole) motors, but since that is what you have, that is an option in your case. The most widely known version is made by a UK company called Invertek, sold as the OptiDrive E2 Single Phase. That exact same product is sold under 5 or 6 other brand label agreements in various countries, but Invertek is the actual mfr. I'm not recommending it, I happen to think it's too expensive of an option, but you should at least know it's an option.

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Adding heat from an inefficient motor may not seem like a good approach but it depends how much ambient heat there is and the humidity.

THis is an example of how ceiling fans work with line rated plastic caps.

enter image description here

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For low cost: Buy a 500VA transformer with taps on the primary for 240 and 208 V. You can find a used one with 24 VAC output for low voltage stuff for less than $20. Tie your power to the 240 VAC taps, and put your motor on the 208 taps (one tap will be shared - this should be ground or neutral). Leave the secondary unconnected.

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