How to control RPM of an AC motor with minimal parts

Question

I purchased a fan (https://www.acinfinity.com/smart-cooling/axial-fans-accessories/115v-ac-axial-hs1238a-muffin-fan-120mm-x-38mm-high-speed/#product-warranty) and I was wondering what the simplest method would be to reduce the RPM of the motor in order to quiet it down if need be.

I've read there are certain ways to control AC motors unlike DC motors and if not done properly you can damage the motor and/or it's windings.

From what I understand is that most AC motor controllers chop the AC waveform up to either reduce or increase the HZ provided. Is this correct?

Here are my thoughts on ways to control the motor, and I was wondering if anyone could tell me if they are the correct way. If not provide a better simpler way.

1) Simply add a resistor in series. This will drop the voltage supplied, but likely increase current draw on the windings?

2) Add some capacitance some how to adjust the power factor of the motor to increase/reduce torque?

3) Triacs to chop up the wave form to reduce the ON time the motor receives.

Are any of these correct? If not what is the simplest solution to drop the RPM. I am not looking for variable adjustment, just a simple reduction when inserted into the circuit.

If any of the above are correct, what would the schematic look like for 2 or 3.

Thanks!

Answer 1

1) Simply add a resistor in series. This will drop the voltage supplied, but likely increase current draw on the windings?

When driving a fan with a shaded pole or permanent split capacitor (PSC) motor a variable resistor is the simplest way to reduce speed. It will reduce the voltage and reduce current slightly rather than increase it. It will reduce the power used, but not as much as other methods.

The capacitor switching method is explained in more detail in my answer to the following question: varying run capacitor for speed control of single phase motor

2) Add some capacitance some how to adjust the power factor of the motor to increase/reduce torque?

That may work somewhat like a resistor, but selecting a value will be problematic. The speed will not be adjustable. It has been done commercially with PSC motors, but I don't think with shaded pole motors. There is a three-speed switch offered for this fan. That is probably a capacitor switch for a PCS motor. If this is a PSC motor, its capacitor(s) could be quite small. With capacitor switching, you typically get three speeds.

For the fan you have, you probably need to use either a resistor or triac voltage reduction method or buy what the manufacturer offers. When the fan is sold without the speed control, the capacitor is probably buried inside the motor housing with no external connection for switching capacitors.

3) Triacs to chop up the wave form to reduce the ON time the motor receives.

True. That provides a continuous range of speed adjustment. It provides more power reduction than a series resistor.

Answer 2

I have had success in slowing down different AC blower motors by simply adding in series with the 115V AC power cord circuit a "motor starting capacitor". The needed capacitance value will vary depending on the motor's design. But a starting place would be in the 30 to 45 microfarads(uF) with a 120 volt or higher rating. The smaller the capacitor size in uF the more the motor speed will be reduced. CAUTION!!! When working with these sizes of capacitors be aware that when voltage is applied to then removed from a circuit containing the capacitor there can be a lethal charge contained in the capacitor. A short across the capacitor terminals using an insulated tool will discharge it with a startling arch.