Me and some friends are expanding our beer brewing setup with an extra kettle and heater. We want to use both of these at the same time (one for the mash step and one for the boiling step) to brew two batches at the same time. The problem however is that we now have two 3000W elements on a single 16A 230V breaker, which will trip the breaker in no time.
The good news is that we do not need the full 3000W in the mash step, so we are looking for a way to control the current flow to this second heater. I have read a lot about people controlling electric heaters with slow on-off cycles, but these would still trip the circuit breaker since both elements would be on full power for a couple of seconds.
TLDR: is there a way to electronically (and variably) control the current flowing through a 3000W 230V AC heater?
TLDR: is there a way to electronically (and variably) control the current flowing through a 3000W 230V AC heater?
No, but there is a solution to your problem.
In your research did you read about burst control or burst firing for controlling heaters?
One way to control an AC heater is to control each half cycle of the mains, or each pair of half cycles, so for example if you turn on 5 cycles followed by 15 cycles off then the heater will get 1/4 power. What I suggest is you do this an interleave the cycles for the 2 heaters, so taking the above example if heater 1 happens to be on for 5 cycles and off for 15 cycles then heater 2 can be on for up to 15 of the cycles that heater 1 is off for.
To do this you need triacs for switching the mains and zero crossing detection to time the switching correctly.
++Karma; // For asking a clear, easy to understand question on your first post.
Putting a 6kW load on a 230V 16A circuit is never going to be good, you either need to downgrade
the heaters or upgrade the circuit. Overloading circuits is the way to start fires.
Heating in the wiring goes with current squared, so that you risk quadrupling the temperarure
rise in the wires by doubling the load, which will then start to char and degrade, eventually
becoming carbonized and conductive, then the fireworks show and the building fire. I wouldn't
risk this even if the breaker is tested regularly!
Thank you for the quick and helpful reply PerryBebbington!
I have a couple more questions if you don't mind:
So, building a circuit like this one (I would increase the resistor value because of the higher voltage):
One thing I would like to verify about this diagram is that the ground on the left would probably be the AC earth ground? And the ground on the right the arduino ground? Otherwise the use of the opto-coupler to isolate the two circuits would be overruled right? I also see many circuits without the GND connection at the AC side, so I'm wondering if that is strictly required or not?
Then I would probably have to connect the previous circuit to an interrupt pin on the arduino and time my TRIAC switching based on that?
MarkT:
Putting a 6kW load on a 230V 16A circuit is never going to be good, you either need to downgrade
the heaters or upgrade the circuit. Overloading circuits is the way to start fires.
Heating in the wiring goes with current squared, so that you risk quadrupling the temperarure
rise in the wires by doubling the load, which will then start to char and degrade, eventually
becoming carbonized and conductive, then the fireworks show and the building fire. I wouldn't
risk this even if the breaker is tested regularly!
We are planning to insert a redundancy fuse right before our circuit to counter this problem.
Your concerns are also our concerns, that is why we are looking for a way to limit the current flow or, as per suggestion of PerryBebbington, to switch only one of the heaters on at a time.
As a way of redundancy for the switching, we are going to insert a current detection system as well. This way we can verify if current is flowing and only turn on the other heater when the first one is off.
MarkT:
Putting a 6kW load on a 230V 16A circuit is never going to be good.
My understanding of rubendenolf's question is he wants a way to manage 2 * 3kW heaters on a supply that can only supply a little over 3kW, such that he can use both heaters but not exceed the capability of the supply.
Heating in the wiring goes with current squared, so that you risk quadrupling the temperature
rise in the wires by doubling the load
That is a misunderstanding of how heat output and current are related. For a fixed value of resistance if you double the current you also double the voltage making the power dissipated in the resistance 4 times as much (2* current x 2 times voltage). Having 2 heaters doubles the current but the total resistance is half and the voltage remains the same, so 2 heaters draw twice the current an emit twice the heat, not 4 times the heat.
So, building a circuit like this one (I would increase the resistor value because of the higher voltage)
Circuit looks reasonable but I fell sure there are simpler circuits. The only zero crossing detection I have used recently was not isolated from the mains and just used resistors to drop the mains into the detector. For obvious reasons I am not recommending that! Someone else might post a better circuit.
One thing I would like to verify about this diagram is that the ground on the left would probably be the AC earth ground? And the ground on the right the arduino ground? Otherwise the use of the opto-coupler to isolate the two circuits would be overruled right? I also see many circuits without the GND connection at the AC side, so I'm wondering if that is strictly required or not?
Everything to the right of the transformer is low voltage and safe anyway. It's fine.
Then I would probably have to connect the previous circuit to an interrupt pin on the arduino and time my TRIAC switching based on that?
Yes, you want to trigger one, and only one, of the triacs at or very slightly after zero crossing. You might investigate solid state relays for AC (DC ones are different), which might have zero crossing built in, don't know, never used one. If you use a SSR with built in zero crossing then you don't need to worry about it.
PerryBebbington:
My understanding of rubendenolf's question is he wants a way to manage 2 * 3kW heaters on a supply that can only supply a little over 3kW, such that he can use both heaters but not exceed the capability of the supply.
That was indeed my question.
All my questions are answered, I will also look into the SSR with zero crossing detection.
Thank you for your time!
The only zero crossing detection I have used recently was not isolated from the mains and just used resistors to drop the mains into the detector. For obvious reasons I am not recommending that!
Why not? If you built one without an optocoupler, I’d agree but an optocoupler provides the necessary isolation. Placing a transformer in the circuit is not good as it will introduce phase shift and affect the very thing you’re trying to measure, the zero crossing point of the mains.
Proper pc board layout, resistor and optocoupler selection are key in achieving an accurate and safe zero crossing detector.
Just curious, are you planing on using a micro to control this? I would think it would be pretty easy to switch one on for say 5 seconds and then off for 15 seconds and the other one the opposite. You could also put in a small delay between the switching to make sure that they are never on at the same time.
PS. the 5 second and 15 second figures are indicative. Could be any period.
WattsThat:
Why not? If you built one without an optocoupler, I’d agree but an optocoupler provides the necessary isolation. Placing a transformer in the circuit is not good as it will introduce phase shift and affect the very thing you’re trying to measure, the zero crossing point of the mains.
Maybe I should have mentioned that I was not planning on adding the transform, seemed unnecessary to me And indeed I had also looked it up and found that it phase shifts, so would totally mess up my detection.
windoze_killa:
Just curious, are you planing on using a micro to control this? I would think it would be pretty easy to switch one on for say 5 seconds and then off for 15 seconds and the other one the opposite. You could also put in a small delay between the switching to make sure that they are never on at the same time.
I'm planning on using an UNO/MEGA, depending on the amount of GPIO I will need in the project. I have some boards laying around with a built-in ESP8266, so I can create a web interface to control the heating, pumps etc. I will do some tests later when I have all the necessary parts and see what kind of delay works best for me.
WattsThat:
Why not? If you built one without an opto-coupler, I’d agree
It doesn't have an opto-coupler on the input, the micro-controller is live. It is in an earthed aluminium box. The output is opto-coupled. I don't publish on a public forum details of potentially dangerous circuits as I have no control over the expertise of those who build them.
PerryBebbington:
It doesn't have an opto-coupler on the input, the micro-controller is live. It is in an earthed aluminium box. The output is opto-coupled. I don't publish on a public forum details of potentially dangerous circuits as I have no control over the expertise of those who build them.
And indeed I had also looked it up and found that it phase shifts, so would totally mess up my detection.