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Can PWM be used to avoid tripping circuit breakers?

Posted: Tue Mar 10, 2015 3:44 am
by Chad
Hi,
I'd like to install heating elements with excess wattage, then run them at a lower duty cycle in order to avoid scorching of wort.

For example, I would like to run 2x5500 W elements on a 25A circuit breaker, but use a duty cycle of only 40%.

Obviously run at full capacity these two elements will draw about 50A of current, but what I'm wondering is whether the rapid switching of PWM (combined with SSR relays) will smooth the load enough that the circuit breaker will only "see" the average of 20A?
I realise that an alternative would be to create a more complicated circuit using a combination of NO and NC wirings to alternate the current between the two elements, but purely using PWM has its own benefits (for example, if I were to upgrade to 35A I could increase the duty cycle to 70%)

Thanks,
Chad

Re: Can PWM be used to avoid tripping circuit breakers?

Posted: Tue Mar 10, 2015 7:30 am
by oakbarn
50 % Duty means ON 100% for half the Time OFF for half the time, not 50% POWER.

I suspect they would draw the full wattage when turned on.

Re: Can PWM be used to avoid tripping circuit breakers?

Posted: Tue Mar 10, 2015 12:35 pm
by Chad
Yeah, that's why I'm asking - obviously if for example they are on for 2 seconds then off for 2 seconds it's without a doubt going to overload the breaker, but if the PWM is sufficiently rapid then the load will in effect become smoothed out to an average load.

The question then is how often such switching would need to happen, and whether the BCS is capable of that type of rapid PWM switching. For example, this Arduino tutorial https://prototyperobotics.com/tutorials ... modulation mentions default PWM rates of 500 Hz (i.e., it switches on and off 500 times per second).

Re: Can PWM be used to avoid tripping circuit breakers?

Posted: Tue Mar 10, 2015 9:52 pm
by oakbarn
I thought I read where the minimum inteveral is 0.1 second. Maybe someone with more knowledge could help as I do not know. I am not sure I would put anything on a circuit breaker that would potentially overload it regardless. My luck is that I would somehow full power it on by accident.

Re: Can PWM be used to avoid tripping circuit breakers?

Posted: Wed Mar 11, 2015 1:06 am
by Chad
Yes I think you're right - I've hunted around on the online support some more and found that the minimum ON time is 0.1 second, so then it's just a question of how responsive the circuit breaker is I guess...

You're right though - this approach would create the possibility of accidentally turning it on at 100% and overloading things, although presumably all that would happen is that the circuit breaker would trip. There is apparently the option of setting a maximum duty cycle % though - still only a software-based protection but presumably enough to stop someone (me) from accidentally overloading it.

Re: Can PWM be used to avoid tripping circuit breakers?

Posted: Thu Aug 18, 2016 5:18 pm
by Sparkologist
A 25 amp breaker is too small for one 5,500 watt element. (5,500 / 230v = 23.9 amps) A 30 amp circuit should be the minimum to maintain a load of less than 80% of the over current protection.
How hard is it for you to run a 60 amp circuit to your control panel and feed two separate 30 amp breakers in the panel that each feed an SSR relay?
You do not want burning wires or tripping breakers in the middle of your brew.

Sorry.... I see this is an old post.

Re: Can PWM be used to avoid tripping circuit breakers?

Posted: Mon Sep 12, 2016 6:06 am
by clearwaterbrewer
even though it is an older post, I think this even older post covers it:

http://forum.embeddedcc.com/viewtopic.php?f=6&t=591