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I'm using a pair of IMO MB09 mini-contactors to switch current to an 230V AC motor. The contactors have 3 NO and 1 NC contact. Contact ratings are NO: 20A; NC: 3A

Of these I'm using two NO poles on each contactor to switch the motor current (expected to be 8A when running, startup peaks not known). The rest are used as interlocks for the switching control signals, <2A @24Vdc.

My colleague has told me that this is bad practice, that the load on the contacts is "unbalanced". I tried to look this up but can find no reference anywhere. He is very experienced and I like to trust him but could anyone confirm this or point me to a text on the topic? Will they be physically degraded in some way by the variation in thermal effects? Is it some inductive thing between the poles? I am feeling a bit ignorant.

I assume your motor is single phase, otherwise you should be switching all 3 phases.

I think your colleague is thinking of a contactor with integral overload, many of those will trip on out-of-balance if the current in one is zero. If thats the case run one phase through two O/L coils in series so they are all balanced.

If your contactor doesn't have an overload, it's OK to have an unbalance.

Hope this helps
Roy

Many thanks for this. I think the loads will be ok.

No problem with the contactor, each pole will carry it's rated current independantly of the others. I think your colleague is refering to the balancing of the overload. Single phase motors using a standard DOL starter will require all three poles to carry the same current. I do not know what motor protection you have?. If it is a standard 3 phase overload unit then this WILL require all poles to carry the same current.

Yes this is a single phase application.

Now I'm finding out what's at the bottom of this. I quizzed my colleague, asking, why do we need the same current on all the poles, and he replied it's how he was trained many years ago.

But thinking it through it seems that it might relate to keeping the wear even ... if you have greater current on two contacts but not the third, then that third one will degrade less and eventually the other two will be smaller. This larger contact might prevent full contact on the two worn poles.

Am I near the answer now? It seems to make sense in one way, but now I'm wondering why don't contactor designs comprise a way of taking up this potential play?

No, as I Jones and I pointed out most 3 phase overload units have an out of balance that will trip the contactor on loss of one phase. That's why you pass the current through all 3 in series. If the O/L unit is one that's an integral part of the contactor It's usually easier to jumper from the O/L back to the input terminal of the contactor.

If you have an O/L unit separate from the contactor or with jumpers between the two units you can jumper it there.

Your theory of uneven wear may have some merit, but I doubt it.
BTW, you say single phase motor, if it's phase and neutral of a 230/400 supply you only need one contact.

Regards
Roy