wye/delta overload calculation

[jesmond]

I would like to know how to come up with the correct overload rating for a 3 phase/600V/100hp wye/delta motor and how you came up with the answer thank you

 

[dpc]

The nameplate should give you a full load amp rating.

 

[Shortstub]

Responding to Jesmond's Jan 23, 2003 query:

Are you asking:

1) How the overload settings are calculated?
or
2) How are the cable sizes calculated?

 

[Marke]

Hello jesmond

Normally, the overload protection is connected in series with the motor windings. (You get the greatest level of protection with it connected there!!) In this situation, the relay should be rated for root 3 of the motor rating. i.e. multiply the motor rating by 0.577

Best regards, Mark Empson

http://www.lmphotonics.com

 

[DanDel]

I hate to assume your question wasn't answered correctly yet, but a wye-start, delta-run motor starter usually has one set of overloads, for the delta-run only.
If it is this type of motor, then you must have six leads. You should have one nameplate FLA for the delta connection. The overloads are based on the nameplate FLA at the correct voltage (delta) connection. Typically, most people end up using values slightly higher for the overloads to avoid nuisance tripping.
If you're interested in the actual current, the wye connection has a voltage applied to each winding which is 0.577 times the delta connection voltage, which allows a starting current of about 1/3 of the current when the full voltage is applied.
Per the NEC, for your motor, 3 phase/600V/100hp, FLA is 99A(actual table 430.150 voltage is 575V). Starting current varies depending on the Design letter, but design B,C, and D are 580A, meaning the wye-start current would be about 193A. Hope this helps.

 

[jbartos]

Suggestions:
1. Follow some major overload relay manufacturer heater selection procedure. E.g.

http://www.ab.com

There might be CTs involved, and various types of overload relays.
2. The star-delta starter standard circuits appear in the manufacturers' catalogs, e.g.

http://www.squared.com

http://ecatalog.squared.com

Digest, that is posted on the Square D web.