( SOLVED kinda...) Hi, everyone.

(william foster) #1

( SOLVED kinda…)

Hi, everyone.
i’m trying to PWM control a Big 240V AC fan via a Solid State relay.
and as expected when when my 50hz PWM frequency matches my 50hz power frequency weird things happen.
so i have been trying to slow it down to about 3hz or 320ms.

i’m running the latest smoothieware with the following config but it’s just not doing the trick, anyone got an idea why ?

switch.fan.enable true #
switch.fan.input_on_command M106 #
switch.fan.input_off_command M107 #
switch.fan.output_pin 2.5 #
switch.fan.output_type pwm #
switch.fan.pwm_period_ms 320 #

(Rob Taylor) #2

To control a AC fan you must have a zero crossing feedback circuit. That way you can control the phase.

(Paul Gross) #3

Typically you would have a PWM switching frequency much higher than the AC power frequency of 50Hz, something at least 1kHz for smoother control.

Also, as @Rob_Taylor1 ​​​ indicated, if your controller knows when the beginning of each 50Hz sine wave power cycle starts (the zero crossing), then you can vary the pulse width through the AC power cycle so that the resulting current is a fair approximation of a true sine wave: fatter pulses near the zero crossing and thinner pulses near the sine wave peaks.

Basically, you are aiming for a variable pluse width that is the integral if the 50Hz power sine wave. The higher your PWM switching frequency, the smoother the resulting current waveform through the motor coils.

The reasoning behind this is a bit complicated unless you are familiar with both calculus and the physics of pushing current through a coil of wire. I am happy to try to explain it if you want.

(william foster) #4

@Paul_Gross yeah i probably should be doing that way, but crudely pulsing it at 3-5hz should work also, i don’t really care if it’s not perfectly smooth.

(Rob Taylor) #5

You may want to check if there is a capacitor on the fan motor. Pulsing at that speed could significantly shorten it’s life span.

(Paul Gross) #6

@william_foster it all depends on what you are prepared to put up with.

The AC motor design expects a smooth 50Hz sine wave current, but anything too different can be very jarring - lots of vibration and noise, and a much reduced life span for the motor.

Chopping the drive voltage randomly, which is what your low frequency PWM controller is doing with respect to the power sine wave, is only going to shake the motor to bits with random current pulses.

(william foster) #7

@Paul_Gross oh i guess it is a quite big problem. thanks for all your help Paul & Rob, i’ll start looking around for a proper AC PWM type controller setup.

(ThantiK) #8

You have to have a zero cross circuit. What happens in that SSR is that a triac connects, and as long as there is power flowing it stays connected. It disconnects when the phase reverses polarity and crosses the zero point. You have to detect this, and time off of it in order to activate it again at the right time

(Jeremie Francois) #9

+1 for marking “solved” on your post! :slight_smile: