Contactors problem, please help me solve this

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kulp

New member
Location
sri lanka
Can I use a 4 Pole contactor for a 3 phase motor?

same as can I use a 3P contactor instead of 2P contactor by only connecting 2 terminals in it?
 

controlled

Senior Member
If using a 4 pole contactor, make sure all the contacts are rated for the motor load. Most contactors also have auxillary contacts used for controls or monitoring. The contacts are usually rated for much lower current.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Can I use a 4 Pole contactor for a 3 phase motor?

same as can I use a 3P contactor instead of 2P contactor by only connecting 2 terminals in it?
Being that you are in Sri Lanka and probably IEC based, there are certain conditions and rules that might require that you break the neutral conductor. Those generally would not apply to motors I don't think, but I am not familiar with all of your rules, just that the existence of 4pole contactors for 3 phase systems is based on these rules. Over here, there are more rules that PREVENT us from breaking the neutral conductors rather than require it and at best we ALLOW it. My point is, asking this question in a forum based mostly on North American installations may not serve you best, you should be asking this question of people familiar with your local codes and regulations.

But aside from that, there is no reason why you cannot only use 3 of the 4, or 2 of the 3 poles, if appropriate for your installation.

That said, IF you are controlling a single phase motor with a 3phase (3 pole) motor starter, meaning it has an OL relay attached to it, then there is an issue when using IEC OL relays. You can get nuisance tripping on an IEC OL relay if current is not flowing through all 3 poles. So the practice is, for a single phase motor, to "loop back" one pole through the third unused pole so tht there is current flowing through it. This technically only relates to the OL relay, but if it is close coupled to the contactor, the only practical way to do it is to loop back through the contactor too. This issue never applies to a 4 pole starter, because the 4th pole is for Neutral and never goes through the OL relay anyway.
 
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