Re: AC fan speed control by periodic on/off switching

On May 2, 11:45 pm, ectoplasm <e_c_...@xxxxxxxxxxx> wrote:
I have a fan (110 V AC) that I want to control the speed of.

I want to do this by periodically switching it on/off. Say, period of
about 5 seconds; and the 'on' period can be controlled as 10-100% of
this time. I already have a fan that has this built in, and it works
great, silent too. Basically it's variable pulse width control of an
AC appliance.

I probably need a thyristor / triac circuit for this, switching on the
null-crossings. Would be nice if the period time could also be
variable.

Searches on Google failed because I don't know what keywords to look
for. Can someone help me out?

Thanks,
E.


There is something that I have wanted to try out for quite a while.
It is a variation on dropping cycles that I think would lead to lower
losses in the motor. I will explain it and others can shoot it down
if I am wrong.

To decrease the operating speed of an induction motor, it is best to
lower both its voltage and frequency. A device like a triac can be
fired to allow an alternation through or not.

The triac it fired on the positive alternation.
The negative alternation is skipped
The positive alternation is skipped
The triac is fired on the negative alternation
The positive alternation is skipped
The negative alternation is skipped
.... repeat

This lowers the frequency to 1/3rd that of the mains frequency.

The RMS voltage on the motor would be 1/sqrt(3) of the normal line
voltage. However, the frequency content will be a bit funny. If I
did my math right, only 0.4 of that will be at F/3.




.

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