three pole contactors

[333rocky333]

Came across a panel
Inside it was a three pole contactor For a single phase motor
The contactor had overloads fitted

The live went in on L1
Then from the overload output L1 linking back to feed back in to L2
Finally the live load connected To the overload output of L2

The neutral went in on L3 and out of overload on L3

Anyone know reason why they do this , looping through twice.
I have seen it several times now in older panels

 

[crystal ball]

I always thought it was so the overload elements were all in circuit for fault detection, but it should only need one element to see the fault and trip, spreading the load maybe , but if the contactor is correctly rated surely no need.

 

[ColJack]

it's to ballance the current / heat through the overload so it doesn't trip because of one "phase" overheating..

it's also a good idea when useing motors and such so that if the contacts weld shut, there's a second set that can open..

 

[Pensdown]

3 pole overloads are designed to sense the current in all 3 poles. So when they're used for single phase loads the current must still run through all poles for them to work correctly.

 

[crystal ball]

it's to ballance the current / heat through the overload so it doesn't trip because of one "phase" overheating..

it's also a good idea when useing motors and such so that if the contacts weld shut, there's a second set that can open..

But if the o/l is set correctly it should not trip, not sure what you are saying about welded contacts, can you explain further

 

[333rocky333]

it's also a good idea when useing motors and such so that if the contacts weld shut,

Only last week found one pole welded in at another site, kept the other two in also, three phases on constant to a 12 kw water element.
Is that possible for one to weld in with out holding the other two in
Or do you mean one of the overload sections fail

This load is less than 3 amp single phase pump, so would the 3 pole overloads, still work the same as a two pole version.

I would have thought so, but cannot find confirmation

 

[333rocky333]

I always thought it was so the overload elements were all in circuit for fault detection, but it should only need one element to see the fault and trip, spreading the load maybe , but if the contactor is correctly rated surely no need.

As the two are in series not really spreading the load either.

 

[Spark123]

If you wire 2 of the coils in series theres going to be the same amount of current flowing in all 3 of the overload coils, (ignoring things like earth faults). One problem I have come across using this sort of arrangement is the resistance of the overload coil can have an effect on the voltage to the motor.

 

[333rocky333]

If you wire 2 of the coils in series theres going to be the same amount of current flowing in all 3 of the overload coils, (ignoring things like earth faults). One problem I have come across using this sort of arrangement is the resistance of the overload coil can have an effect on the voltage to the motor.

So would it better wired using just two poles only,or would that cause more probs.
Would like to know why, all the panels seem about 20 years old , maybe I will contact the makers

 

[Spark123]

Are they causing a problem?

 

[333rocky333]

Are they causing a problem?

No

Just that last year, one loose terminal screw connection, of one wired via two poles in series burnt out , so We changed it for a two pole.

Just wondered if we could have just left that slightly damaged pole unused, if the other too were not damaged.
And whether the o/l would function the same

Unless Using just two of the three poles would create a New problem or stop the overload system functioning as should

 

[Spark123]

It would depend on the unit, tho I'd prefer to replace it if it was damaged anyway.

 

[crystal ball]

Came across a panel
Inside it was a three pole contactor For a single phase motor
The contactor had overloads fitted

The live went in on L1
Then from the overload output L1 linking back to feed back in to L2
Finally the live load connected To the overload output of L2

The neutral went in on L3 and out of overload on L3

Anyone know reason why they do this , looping through twice.
I have seen it several times now in older panels

I seem to remeber this is the preferred method of starter manufacturers like crabtree, memera etc

 

[ColJack]

here is a PDF that explains about contactor overloads..


apparently, there are 2 types of overload, bimetalic thermal ones, and electronic ones..

with the bimetalic thermal ones there is something called a differential trip bar that causes it to trip faster if it loses a phase..

with the electronic ones there is phase monitoring which trips it if it loses a phase..

so there's your reason.. it's so that current runs in all 3 poles to stop it tripping / tripping faster..

 

[Spark123]

Not to forget the dashpot oil overloads, found in old equipment. You need to be careful as the mineral oil can be carcinogenic.