4 replies to this topic

[Gorf]

I've got a potential requirement for a device which will control a typical 3 kW fan heater, it will need to be in one of four states, off, 1 kW, 2 kW and 3 kW output. Can anyone point me to a resource which will show me the sort of things I will need to consider in order to do it with an ND+? I'm assuming I need to have some sort of relays to replace the switches on the heater?

[Paul Newton]

Random thoughts: You might consider using triacs with opto-isolators (LED) built in. I think I recall there was another topic using these. I like the idea of using opto-isolators with LEDs because you can avoid the Netduino Classic "turn on" issue by connecting both LED legs to the Netduino. With relays you might have one big relay to control current to the fan and to the other (smaller) relays for each heater - that way the heaters could only recive current if the fan has current. Paul

[Gorf]

Thanks, Paul.

I had to Google "TRIAC" - just so you know the level of competence you're dealing with, here.

The sort of heater I had in mind has two rocker switches, the combination of which dictate the four consumption states of the heater. I thought I'd just need to replace these two switches.

The bizarre reason for doing this (rather than simply using the built-in thermostat that these heaters always possess) is that the heater will be an energy sink. A customer has a Solar PV system whose inverter is potentially capable of exporting 6 amps more than the transformer of the grid can cope with. My planned solution is to read the flashing LEDs on the various meters involved, and switch on the heater (only needs to be 1 kW or 2kW setting) if the threshold is breached.

Obviously, the problem with this approach is that this will only happen on a nice, sunny day. And my solution is to switch on a heater! I'm open to alternative ideas...

[Paul Newton]

Hi, Relays will be a simple way to replace the existing switches. But they may wear out over a log time. (e.g. dirty contacts) It sounds like you don't have the normal worries about safety - e.g. it won't matter very much if the fan heater starts when it is not required. (as long as the fan is running) Generating too much would not be a problem in the UK! A neighbour has just installed five solar PV panels opposite me. His system can export anything it makes. How big is the array, and what does/can the array generate? (volts, amps, watts) Its ashame the energy is just being wasted. Is there anything else it can be used for - like heating domestic hot water, or driving a fountain. Paul

[Gorf]

It's a 10 kW array of panels, attached to a 4 kW (16 A) inverter and a 6 kW (22 A) inverter. They dropped the ball anyway with the 6 kW inverter - it's not compliant with single phase export in the UK. Not wanting to take this too far off topic, but my main reason for dumping the energy is because most of the alternatives (charging a battery, heating a tank of water) will get us back to square one when the battery is charged/tank is hot. The demand will stop at some point, and the potential to damage through exporting too much will return. As for the peak output - I'm in Lancashire, and the 10 kW system on the roof of the building where I work has peaked at 10.02 kW this month. That's on a three-phase supply, though, so it's fine.