Motor Neutral

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broadgage

Senior Member
Location
London, England
Correct, no standard type of the three phase motor needs a neutral. If the motor is wired in Y then no external connection should be made to this point.
Be aware however that many motor control panels DO need a neutral connection in order to power relays, contactors and other ancillary equipment.
Likewise many machines that incorporate a 3 phase motor DO need a neutral, not for the motor itself but for ancillary equipment.
 

paulengr

Senior Member
There is one “exception”. With wye connected large motors with differential relay protection the wye is grounded. It is not a neutral but depending on how it is wired the ground may be insulated.


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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
There is one “exception”. With wye connected large motors with differential relay protection the wye is grounded. It is not a neutral but depending on how it is wired the ground may be insulated.
To be clear, by “large”, he means typically 2,000HP and up, medium voltage motors. Not something the average electrician will come across very often in their career and when they do, it will likely be in a detailed engineered project with clear drawings and specifications telling you exactly what to do. People only implement differential protection on very high value assets where high speed detection of winding faults can mean the difference between thousands of dollars in down time versus millions.
 

paulengr

Senior Member
Hey I resemble that remark about large motors. Our shop tops out at rebuilding them up to around 25,000 HP including the clean room for the 11 kV stuff. Big toys are fun until they break.

I’ve only seen differential motor protection once in a plywood plant in South Carolina. It was 4160 but it was only around 500-800 HP. The whole arrangement was goofy. Probably a contract engineer design if I ever saw one.

About the only thing you might save with differential protection is maybe, and this is a stretch, maybe core steel losses. A 2000 HP motor is still going to cost around a million give or take to rewind but could be triple that if the core has to be restocked.



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