Running an AC Mains powered fan at different speeds

[Edwina Lee]

TL;DR Summary

Mother has a mains A/C extractor fan. Can the fan run at different speeds with a light dimmer switch? Any pitfalls?

My mother has an alternating current extractor fan plugged into the mains.
I want to run it at different speeds.
I am thinking of using a light dimmer switch to deliver variable voltage to control the fan speed.
(An electrical engineer friend told me that it is possible only if the fan is designed for variable speed, but I lost contact with him after that.)
So, is what he said true? Why can I not run the fan with a variable voltage?
Should I try? Might it damage the fan?

 

[jrmichler]

There are different types of fan motors, some of which can have their speed varied, and some that cannot. Can you post a photo of the motor nameplate? A photo of the entire fan would also help.

 

[Edwina Lee]

Thanks jrmichler. Unfortunately, I cannot access it as the motor is on the outside of the high rise.

 

[Edwina Lee]

So, the next question is curiosity. Why do some fan motors cannot run at different voltages?
And would it get damaged if a lower voltage is used to try to run it?

 

[berkeman]

Thanks jrmichler. Unfortunately, I cannot access it as the motor is on the outside of the high rise.

Why is it outside? You mentioned "extractor fan", so does that mean it is wall mounted? If you can get the make and model of the unit, that may help us to figure out more about it. Maybe ask the high-rise building owner or maintenance person if they know the make and model of the extractor unit?

 

[essenmein]

So, the next question is curiosity. Why do some fan motors cannot run at different voltages?
And would it get damaged if a lower voltage is used to try to run it?

I guess this is not quite so simple!

Technically any electric machine can be operated with a lower voltage, the question arises how that lower voltage is generated, and if there are already any electronics between the AC mains and the machine itself.

For example a single phase induction machine will rotate at slower speed (for a given load) with a lower voltage (even if still 50/60Hz), mind you for very different reasons than say a DC motor, the problem that occurs with a light dimmer is that the sine wave from the wall turns into a chopped sine wave, ie much more harmonic content that something like an induction machine won't like, but a universal (brushed) motor won't care about.

So the issue with a dimmer is less about the lower voltage and more about how it changes the shape of the waveform. Machines like induction, or any synchronous machines really do not like harmonics, ie waves that are not sine shaped, because this can cause counter rotating torques (ie the machine is fighting itself) this creates extra losses and could burn the machine at worst case.

 

[jrmichler]

Induction motors can be run at reduced voltage IF they:

1) Have a high resistance rotor (similar to Nema Design D).
and
2) Have a load where the torque increases with speed squared, such as a fan load.
and
3) Have zero friction at zero speed such as a fan or blower directly attached to the motor shaft (no belt drives or shaft seals).

Both my ceiling fan and the two motors in my heat recovery ventilator (HRV) are two phase motors running on single phase power. A capacitor creates the lag for the second phase.

All three of these motors are multiple speed. The speed change is by adding series capacitors to reduce the voltage at the motors. The ceiling fan came with a remote that controlled a circuit board in the fan to switch capacitors to control speed. I removed that board, and replaced it (and the remote) with a wall mounted switched capacitor fan speed control. The HRV was purchased as a single speed unit. That single speed was too fast and too noisy, so I wired a switched capacitor fan speed control into the wire common to the two motors. It has been running very nicely on low speed for over six years now.

Do you have a make and model for the fan? A photo of it?

Unfortunately, I cannot access it as the motor is on the outside of the high rise.

Camera on a stick, start it recording video, then do a frame grab?

 

[Edwina Lee]

Short of unscrewing the motor or do a photograph from the outside, I discovered that it is a KDK 25AUH (25 cm extractor fan). It has an HP condenser motor, so I guess it can only run on a narrow range of voltage - i.e. single speed.

 

[jrmichler]

Your fan is rated 1125 RPM, and your country has 50 Hz power frequency.

The synchronous frequency for a 50 Hz motor would be 1500 RPM, so your fan is running at 75% of the synchronous frequency. It appears that your fan meets all three of the conditions listed in post #7, so you should be able to reduce the speed with a speed controller.

Use a speed controller that switches capacitors, not one that uses SCR's, for the reasons mentioned in post #6. These are sold in the U.S. as "no-hum" fan speed controllers for ceiling fans. Fan speed controllers have 3 or 4 discrete speeds. Light dimmers, where the output is continuously variable, will not work.

I do not see any way that reducing the speed would harm the motor. It draws only 29 watts at full speed, and the power and voltage will both decrease at reduced speed.