Switching on a small motor when a fan is running

[De Deitrich Fan]

Hi, I would appreciate some help with the following question. How can I configure a small motor to run whenever a 240v four speed kitchen extractor fan is running? It needs to be configured so that whenever the fan runs, power is supplied to the motor. The image attached shows the wiring schematic of the extractor (with added red text as the labels are so difficult to read). There are seven wires connected to the fan. Apart from the earth, I assume the black and red wires are neutral and line, respectively. There are also the following wires: orange, white, blue and brown which I assume are the connections for the four fan speeds, each wire being live when that fan speed is selected.

Would the following be a possibility? Make connections to each of the orange, white, blue and brown wires and run the four wires to four relays. Supply power to the relays and when the fan runs, each NO terminal on the relays supplies power to the motor. Would this work? Is there a better way?

A bit of background, the extractor is a De Deitrich HM2995E1 which was probably manufactured sometime between 2000 and 2004. I bought it second hand on eBay and apparently it was never installed – it looked brand new when I bought it. I installed it in 2019 and it works well. It contains two 250W fans (240V) and two 20W (12v) lamps.

I am planning to retrofit the house to the Passive House standard. Usually in a Passive House it is recommended to run a kitchen extractor fan in a recirculation mode so as not to pump warm air outside. I can’t buy a carbon filter kit for the extractor as it is obsolete and anyway I prefer to duct a kitchen extractor to the outside. So the plan is to fit a motorised damper in the exhaust ducting from the extractor so that when the fan runs the damper opens allowing the extractor to dump air to the outside. When the fan is not running, the damper would be closed maintaining the air tightness of the building. There will have to be another damper in inlet ducting so the extractor is not trying to suck air into an air tight building.

I am thinking of a damper like this one: https://www.ebay.co.uk/itm/382357016029 There are various types of damper available some with a spring return others with a motorised return. I will probably go for one with a spring return if available.

I would appreciate your thoughts on the above, I have never worked with a six wire fan before and I wanted to run the problem past those with more experience. Many thanks.

 

[Geoff1946]

I would GUESS that the coloured wires on the motor are tappings on a single winding.
If so you can think of it as like a transformer winding and if you connect a single relay across the winding the relay will operate whichever speed setting is selected. (I think!)
I would:
Check winding resistances to see if it is in fact a single winding with tappings.
Identify the neutral and fast speed one and check that it gets 230 volts when operating.
Connect a 230 relay across and see what happens.
If the fan speed control is more complex, and that won't work, then I suggest an airflow detector.

 

[binky]

You would have to be careful of loadings on the control box, which may take complete umbridge to having anything else connected to it.

I would take a different stance, it's unlikely you would have the fans on without the lights, so perhaps tap into the lights. My own hood has a light switch that turns it on and off, which I use rather than the buttons on the hood for the light. If you have something similar in a convenient position, then I would wire the dampers to that rather than try to bodge the control wiring. Other options could be a proximity sensor, so if standing next to hood, that opens the dampers, or just a separate switch altogether.

 

[ProDave]

I would start by measuring the voltage on each motor connection, with respect to neutral, at each motor speed. You might be lucky and find one of them is 240V at all speeds. In which case a small 240V relay will do what you want.

If you are not that lucky then some kind or relay with a little bit of electronics might be needed.

 

[poni]

this would work

 

[De Deitrich Fan]

Many thanks for your solutions to my problem, I now know what to do.

Geoff1946 & ProDave suggested checking for 240v across neutral and one of the fan speed wires. If I found it, that could control a relay which would switch the damper motor. This was certainly a better solution than my four relay idea but it does mean connecting to wires inside the extractor housing. Others have suggested a control system outside the extractor housing which would be much simpler to install.

I really liked blinky’s idea of an isolation switch in the supply to the extractor. When the switch is closed the fans and lights would turn on and similarly power could be taken to the damper motor. If I have a damper with a spring closing mechanism, that would work perfectly. If it was a damper with a motorised closing mechanism, it would need a relay before the damper. I suspect I will end up with a damper that has a motorised close hence I will need the relay before the damper motor. Nevertheless, it was a very effective solution.

In the end I think I will go with poni’s solution, effectively a CT clamp on the supply to the extractor. The advantage of this is that if I end up with a damper which has motorised closing, I would not need the additional relay before the damper motor. Looking around I found a current sensing switch that has a NO and NC output (https://www.aliexpress.com/item/4000984241239.html?spm=a2g0o.detail.0.0.3e24eUSpeUSpsZ&mp=1). This would be a very straightforward install with a 3 wire plus earth from the current sensing switch to the damper motor. The current sensing switch would control the damper in exactly the way I want to. A perfect soulution!

All your solutions would have worked better than mine, I wouldn’t have come up with any of them, thank you for your time and trouble.