Burst firing control for inline Vent Fan

[mfacen]

Hello everyone...

I have this temperature controller in my basement that I designed, its an NodeMCU board ( ESP8266 Wifi module ) that I program with the Arduino software. The controller has temperature probes and controls an inline duct fan similar to the one in this picture:

The fan is rated at 80 Watts / 120 V , its a brushless single pole motor.
The NodeMCU drives the fan using a SSR ( Solid State Relay ) and at the moment its either on or off.
I have been wanting to make the system better by adjusting the speed of the fan depending on the temperature as it is loud and most of the time it doesn't need to run full speed to lower the temperature, with the setup I have I was thinking about driving the fan on/off in a PWM way but with very slow period of maybe 1 second or so, I did some experiments and I'm able to reduce the speed of the fan by switching on/off the relay, the fan has a lot of inertia so it kinds of averages its speed depending on the duration of the bursts.
Finally my question: is running the inline duct fan in this manner going to damage it in any way ? I did a trial and it seemed to work fine for a day, I tested different PWM ratios ( 1/2 second ON and 1/2 second OFF gives me about 1/3 of the speed ruffly ).
I did a lot of searches trying to find out if this manner of controlling a motor is feasible but couldn't find any literature other that burst firing PWM's for heavy resistive heating elements.
Thanks for your time...

 

[FvM]

How much is the motor peak current compared to average? There's surely some motor stress involved with this on/off pulsing method, but it must not necessarily be a problem. Overall power efficiency is surely reduced compared to continuous operation.

 

[mfacen]

Hi FvM thanks you for the response. I haven't measured the amperage, I'll do it next time I tinker with it, I'm working in the software side now trying to find out the better way to implement it, I'm not sure if its better to go for a lower PWM period. 500 ms seems to work better and the trials I did seemed to show that the lower the period the better it works, but I'm concerned about creating magnetic currents in the motor or some weird stuff happening in the windings the closer I get to mains frequency. I'm sure it puts more stress in the bearings too but the fan has an inertia period of 20 seconds from full on until it stops spinning so the stress is a lot less than a cold start.
I find it weird that I haven't seen any application where a ventilation fan is driven in this way and that makes me think there must be some kind of drawback on the electric side too. The other simple option of using true PWM with the ESP8266 controlling the firing of the triac in every cycle seems to be not so effective in controlling the speed of this kind of fans. Another idea I had is to use a triac and fire it once every two main cycles. Wouldn't it half the speed ? That would give me the option to run it at half speed, 1/3 speed, etc.