SPDT Relay on controllers

[GiorgioC]

Hi, new here.
Have you ever used the NC connection on the SPDT relay often present on refrigeration controllers?
In case, what for?
Thanks in advance.

[Brian_UK]

Depending on the size of the relay they can be used for all sorts of things.

They are often used as signal contacts and if capable of carrying the current could be used to power a compressor crankcase heater for example.

[monkey spanners]

Had one where the normally open contact failed so used the NC contact to control another relay and wired the fans through the NC contact on the second relay.

[GiorgioC]

@Brian_UK
Could you please explain me better about the crankcase application you mention?
I mean, if you connect the crankcase resistor to the NC terminal of the SPDT relay, what you would connect to the NO terminal of the same relè?

@monkey spanners
Smart workaround!
BTW, as you surely understand, I'm searching for original system application.

[chemi-cool]

Controllers are like their name, controlling systems.
I never connect controller to compressor or fans.
For that purpose we have contactors and currant relays.
Crankcase heater should be connected to a NC contact on a contactor.

Any currant you pass through an electronic controller will heat it and shorten its life.

[GiorgioC]

@chemi-cool

"Controller" doesn't mean "No power actuator".
The greatest part of the refrigeration controllers are designed for direct driving compressors, defrosting power resistors, fans, and so on. Specifications about maximum load for each one of the different relays present in the controller is one of the most detailed part of their datasheet.

Other controllers (the ones for big jobs) are equipped with small relays only (usually 2-5 A max) and used to energize power contactors.

My question is about the first type of controllers (i.e. IR33 and/or MPX by Carel).

Why do you write the crankcase heater should be connected to NC and not to NO? Do you mean there is another load connected to NO on the same contactor?

Thanks in advance for your answer.

[chemi-cool]

IR33 is a small controller with miniature relays. If you apply the nominal output currant, you will shorten its life.
For example, if a compressor that draw 10 amps will be connected to a relay output that says 16 amps, the compressor motor will draw about 25 amps at start. That will heat the miniature relay contacts and might burn them. It takes some time for this to happened but its no good for the controller.
When you use the controller outputs to energies contactor, the currant is a few milliamps and the controller will last many years.
The crankcase heater is keeping the oil in the sump hot to avoid refrigerant liquid accumulating and boil it while the compressor is at rest. When the compressor starts, its motor will keep the oil hot so no need of extra heating.
When I connect the heater to the NC contacts on the compressor contactor, it disconnect it when the contactor is energies and connect it again when the compressor stops.