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TexanAv8R
Joined: 26 Oct 2016 Posts: 13 Location: Dallas, TX
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Link Posted: Wed Oct 18, 2017 7:06 pm Post subject: Any benefit to switchable 110/220 V to heater element |
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First off, my gooogle-fu seems to be off today so if this has been discussed ad-nauseum, please tell me as such and I'll try to figure out where.
I was talking to someone about my setup, and they were trying to convince me that I should have a way to switch one leg of the power going to the heating element between HOT and NEUTRAL.
Essentially, I would connect the thermocontroller to the two SSR's (SSR1 / SSR2) but have a secondary DPDT switch that would swap between the output line of SSR2 and the NEUTRAL Bar.
According to the person trying to explain it, it would give the thermocontroller more fine control of the temperature once it's near the target... essentially it wouldn't be dumping the full 5500W of power into the element and causing spikes in the temperature.
I feel like this is overkill, but would be interested if anyone else has done something like this.
Comments?
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kal Forum Administrator
Joined: 12 Dec 2010 Posts: 11116 Location: Ottawa, Canada
Drinking: Pub Ale, Electric Creamsicle, Mild, Pliny the Younger, Belgian Dark Strong, Weizen, Russian Imperial Stout, Black Butte Porter
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Link Posted: Wed Oct 18, 2017 7:34 pm Post subject: Re: Any benefit to switchable 110/220 V to heater element |
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FYI - This would toggle the element from running at the full 5500W (when connected to 240V across two HOT lines) and 1375W (when connected to 120V across a HOT and NEUTRAL). When running at 120V instead of 240V, a 5500W element will output 1/4 the power or 1375W.
TexanAv8R wrote: | According to the person trying to explain it, it would give the thermocontroller more fine control of the temperature once it's near the target... essentially it wouldn't be dumping the full 5500W of power into the element and causing spikes in the temperature. |
I'm afraid this person does not understand how PIDs or closed looped feedback systems work. How PIDs work: https://en.wikipedia.org/wiki/PID_controller
Less accurate control is actually what would happen if you were able to randomly switch between having 5500W or 1375W, as it would confuse the system.
The PID that controls the heating element 'learns' the system dynamics including how much power it has to work with and will throttle or turn on/off the element to compensate. It will not overshoot or undershoot once it understands the system through auto-tuning as described here: http://www.theelectricbrewery.com/control-panel-setup
Take a look at some of the videos in my BREW DAY STEP BY STEP that show this in action: www.theelectricbrewery.com/brew-day-step-by-step
You'll notice how in the videos the temperature always lands exactly right. It doesn't overshoot, it doesn't undershoot. I use 5500W heating elements.
Having a switch that allows you to manually cut the available power randomly by 1/4 (without the PID knowing) would only confuse the PID and will cause it to overshoot or undershoot in certain situations.
To use an analogy: Think of you driving your car and using the gas and brake pedals. Over time you've learnt the response of your specific car. You know how hard to hit the gas or break to get the results you want. You know how hard to press the gas to get the acceleration you want and when to ease off. You know how soon and how hard to hit the brakes if you want to stop at a stop line within a certain distance. Now imagine someone sitting in the passenger seat has a hidden switch that is able to randomly (without telling you) cut your engine power by 75%, or make your breaks 75% less effective. It would confuse you, as the vehicle wouldn't behave how you expected. Your car would not accelerate how you expected when you hit the gas, and you'd probably overshoot the stop line when you use the brakes. You'd have to retrain yourself, but the problem is, this person is randomly turning the switch on and off without telling you so you have no way of knowing how the vehicle behaves at any given moment as you can't even see how the switch is set. They may turn the engine power switch from 25% to 100% while you're accelerating causing you to overshoot your target speed.
A PID works the same way.
In fact, if you watch a PID at work, you'll that as it's approaching a target temp from below it'll turn on the element less and less, firing the element all the time at the beginning but then only firing it for a few small split seconds at a time as it gets close, to ensure it lands exactly on the target temp. It can do this because it knows exactly what power it has to work with. Doing what you propose would break that and make the system less accurate.
So long story short, not a good idea. Not to mention it makes the system more confusing to use and creates work for the brewer. It would also take longer, as it takes 4 times longer to heat with 1375W than 5500W.
Kal
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Last edited by kal on Wed Oct 18, 2017 7:56 pm; edited 2 times in total
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TexanAv8R
Joined: 26 Oct 2016 Posts: 13 Location: Dallas, TX
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Link Posted: Wed Oct 18, 2017 7:52 pm Post subject: |
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Thanks Kal,
I figured as much and was trying to frame that argument, but the links you provided do a MUCH better job of it.
Thanks!
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kal Forum Administrator
Joined: 12 Dec 2010 Posts: 11116 Location: Ottawa, Canada
Drinking: Pub Ale, Electric Creamsicle, Mild, Pliny the Younger, Belgian Dark Strong, Weizen, Russian Imperial Stout, Black Butte Porter
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JSB
Joined: 17 Oct 2016 Posts: 125 Location: NE Ohio
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Link Posted: Thu Oct 19, 2017 10:08 pm Post subject: |
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Come on Kal... break out the Root Locus and Nyquist plots!
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kal Forum Administrator
Joined: 12 Dec 2010 Posts: 11116 Location: Ottawa, Canada
Drinking: Pub Ale, Electric Creamsicle, Mild, Pliny the Younger, Belgian Dark Strong, Weizen, Russian Imperial Stout, Black Butte Porter
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