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I am building a small brewery and would like to heat a barrel of aprox. 280 liters of water with an three phase heating element. Its power would be 15 kW. That means that connected to 400V the full curent draw would be 37.5 A and current on each phase would be 12.5 A. Our house has main fuses 3x25A so i assume that wouldnt be a problem. I have asked a former coworker if there are any limitations and he said that current draw on start is around 5x higher than current while normal running. That means that current on startup would fry the main fuses.

So i have two questions

  1. Would i require to install a variable frequency drive for a soft start of the heating element.

  2. Which way is best for turning the heating element on or off apart from manually disconnecting the plug from the wall socket. I am thinking of a three phase relay controlled with a microcontroller such as Arduino.

Thanks.

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    \$\begingroup\$ "I am building a small brewery..." +1 for the attention-grabbing intro. \$\endgroup\$ Feb 14, 2017 at 7:51
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    \$\begingroup\$ Your current calculation is wrong. The total power is 15 kW, one phase is 5 kW. The voltage from one phase to the star point is 230 V and not 400 V, that is the voltage from phase to phase. The current of one phase is 5000 W/230 V = 21.7 A. This is less than the fuse current 25 A, but there not much current left for other purposes. \$\endgroup\$
    – Uwe
    Feb 14, 2017 at 9:47
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    \$\begingroup\$ As you mention your house I feel I should check that your house has a three phase supply. Are you certain you have a three phase supply to work with? This might change depending on where in the world you are but as far as I am aware in the UK (and probably the US) it is very unusual for a house to have a three phase supply as standard. Multiple fused circuits are common, multiple phases are not. \$\endgroup\$
    – TafT
    Feb 14, 2017 at 10:23
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    \$\begingroup\$ I come from Slovenia and three phase is quite common if you live in the countryside. :) \$\endgroup\$
    – Razmooo
    Feb 14, 2017 at 10:31
  • \$\begingroup\$ 15 kW will raise 280 l from 20C to 100C (non boiling) in under 2 hours. You could probably manage with lower wattage. \$\endgroup\$
    – Russell McMahon
    Feb 14, 2017 at 12:49

2 Answers 2

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No, startup is not five times nominal. For a resistive heater there is very little inrush going on and your fuses are time lag anyway. Also, "full curent draw would be 37.5 A" is not correct. You don't sum up the currents in a three phase system.

  1. No need. Forget it. A heating element is not frequecy dependent at all. The voltage ramp up is possible on some VFDs but again, you won't be needing it.

  2. One or more contactor(s). Also, you absolutely must wire it in series with a thermal cut out fuse/breaker. Contactor

EDIT: If you haven't bought the heater already, there is this very neat scheme of spacing your say three banks of heating elements as for example, 2, 4 and 8 kW and by binary addition, you have every combination of 2, 4, 6, 8, 10, 12 and 14 kW available to simplify fine regulation close to your steady state and also decrease the rise rate close to the set point. Also, if one element goes out, it will still work but with a bit more hysteresis. I'm missing the name for this scheme but I'll edit my answer when it turns up. For brewing beer, the temperature is crutial so you don't want to overshoot nor prolong you mash-in more than nessesary.

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  • \$\begingroup\$ you mean 14 kW instead of 16kW as the last value? \$\endgroup\$
    – masterX244
    Feb 14, 2017 at 12:53
  • \$\begingroup\$ @masterX244 Oh! Yes. I'll edit it. Any suggestion what this common control strategy is called? BCD? \$\endgroup\$
    – winny
    Feb 14, 2017 at 14:36
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    \$\begingroup\$ no :( i just saw this Question on the HNQ sidebar and while reading spotted that math error \$\endgroup\$
    – masterX244
    Feb 14, 2017 at 14:37
  • \$\begingroup\$ Thats a good idea. Would be great for controling the intensity of the boil itself. In my case, the heating element would just heat the starting water that would go into the process. \$\endgroup\$
    – Razmooo
    Feb 14, 2017 at 17:07
  • \$\begingroup\$ Just to make clear and credit where it's due, someone in the 70's cracked the idea and implemented it with two 7400-series logic circuits to reduce contactor wear. In our Braumeister, we adjust the recepie to accommodate for the slow rise time so it becomes Braimeister-specific. Unless you change anything, this isn't a problem but we have been discussed building our own with more power and solenoid activated cooling to eliminate varables. Hopping at different temperatures near the end made significant flavor differences. \$\endgroup\$
    – winny
    Feb 14, 2017 at 19:15
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VFD is used for induction motor speed, torque control, not for heater. Therefore, absolutely no VFD is needed. A heater element has a pure resistance, a simple contactor would do the job, there is no start current with heater element. Start current == steady state current.

schematic

simulate this circuit – Schematic created using CircuitLab

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