Single phase contactor

[Andy T]

Why is on single phase panels with 3 phase contactors is the comp wire looped in and out of each contact rather than across all the phase contacts to spread the load?

[Brian]

Andy,

Your worrying me... Don't touch the Panels!!!

You know the answer (if I've understood it correctly), but, were you drunk when you wrote the message?

Brian

[NoNickName]

Three phase contactors are more easily found than single phase, and for a matter of warehouse turnover, they are used also for single phase loads.
The wires are turned around to increase the insulation when contact is open, and for aesthetic reasons (three phase contactors with two unused poles are not good looking).
Loads are never split across poles, mainly because:
1) contactors must be sized for MRA and not one third of it. In fact in case of one-pole failure, the current across the remaining two poles will increase over rating.
2) two much labor splitting one wire in three. No bother.

[Dan]

I cannot fathom the original question and thus not any of the responses to it. How is it possible to have a single phase panel with 3 phase power? I think I am experiencing a language gap. Are we simply referring to 3 pole contactors with single phase wiring, then I woul expect we do not use the third pole. I feel I am misunderstanding something.

[Andy T]

Yes Three pole contactor on single phase wiring.NoNickName explains the reason for me thanks.And yes three glasses of wine can make me think that it was done for more than just looking good.

[Peter_1]

But if there's an overload connected to the relay (to protect the 2 phase compressor), then there's an additional technical reason for it.

[tsangep]

By using 3 poles all in series for a single phase load it is possible to use a undersized contactor for the job thus it is cheaper. The engineeering reason for this simple fact is on contact the arc energy is distributed into 1/3 of the orgional rating when the circuit makes and breaks.
if you paralell the contacts there are no gaurantees that all 3 poles will make at precisly the same time( in terms of mliseconds) thus one pole will weld up as opposed to seriesing the contacts to distribute the arc energy.

It also makes the contactor less prone to welding or it will make it easier to unweld the contactor if it ever closed on a dead short

[Peter_1]

I agree with your last statement but if you wire them in serie, teh current through each pole is the same as you should wire the load over 1 contact instead of 3.

[Electrocoolman]

Ok guys...a 3 pole contactor used on single phase.

When there is no overcurrent trip fitted...then only two poles need to be used, one for Phase one for Neutral...or if in the USA where they need to get 220v by using 2 phases, then both phases.

When there is an overcurrent trip fitted.....then the trip requires the same current value through ALL 3 POLES as it is designed for 3 phase use. (If you lose a phase the trip will operate to prevent 2 phasing). ....So to ensure that the same current is flowing through each pole of the overload we loop the output of one pole of the overload to the input of the next pole. The third pole can similarly be looped from the 2nd pole or (better) can be used for the neutral to the compressor. Doing this satisfies the overload which remains 'balanced'.

The rating of the contactor (ie each pole) will need to be able to carry the current. You will see an AC1 rating (Resistive load) and an AC3 rating (Inductive load)...you need to observe the AC3 rating.

The mention of helping in breaking the arc would be more appropriate on DC supplies. AC supply voltage falls to zero x2 every cycle which helps to reduce arcing, and the contactor also has x2 contacts in series on each pole anyway, which is why we use a contactor rather than a relay, which only has one contact breaking a circuit per pole.

Hope this helps everyone.....Adrian

[mickandlee]

At last Adrian has given us the correct answer

Many thanks - I was getting a bit worried