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star delta motor protection 1

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DanThomas

Electrical
Jan 15, 2004
3
We just had a 50HP compressor motor burn up. It has a star / delta starter. One of the delta contacts failed, causing that part of the winding to single phase. The overload relay never tripped. Motor FLA is 60 Amps. Overloads rated at 63 Amps. Overload relay is connected to T1, T2, and T3. The contact that failed was one that goes to T10, T11, and T12. Should there be another overload relay for leads T10, T11, and T12? The existing overload relay may work for an overload condition, but it certainly provided NO protection when one of the contacts failed. Any recommendations would be appreciated. Thank you.
 
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Suggestions to the previous posting marked ///\\Not to deviate from Dan's question, but I have a general question about the starter being used.
The purpose of the S relay and contacts?
///Normally, S relay is an electromagnetic contactor. Its purpose is in the motor starting sequence to connect the motor winding into y-connection to reduce the motor starting current to one third. After a specific predetermined time for the specific motor expires in the time delay relay, S opens and 2M contactor closes, thus connecting the motor in delta-connection for its ongoing duty. Some manufacturers include this schematic in their catalogs, e.g. Square D in the Digest. More information is available from the manufacturer.\\ Is it for making the Delta connection for the motor windings?
///Actually, it is for y-connection. See the above explanation.\\When the S relay drops out and the 2M contacts close, does that somehow increase the voltage if indeed T1 and T5, T2 and T4....(etc.) are connected?
///Yes, the 2M contactor increases the voltage to line to line across each motor winding set.\\
 
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