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79 relay for motor protection

79 relay for motor protection

79 relay for motor protection

(OP)
I've read from ANSI DEVICE NUMBER that 79 relay is an AC Reclosing Relay. at this,I would ask some questions.
1. could you describe about the construction of 79 relay?
2. how does this relay work?
3. do you think this relay effective to prevent the motor from trip due to voltage dip due to short circuit fault?
4.which do you think is more effective? installing 79 relay or installing a UPS?
 

RE: 79 relay for motor protection

79 function typically refers to reclosing timer used on distribution feeder circuit reclosing.  Not really applicable to motors.  

If you are experiencing contactor drop-out or chatter due to voltage dip, use of UPS power for the contactor control power is one possible solution.  But this has its own drawbacks, especially when the UPS fails.  

 

David Castor
www.cvoes.com

RE: 79 relay for motor protection

There are also devices that can stablise short term voltage dips.

And dpc is right that a 79 relay isen't a correct application for a motor circuit.

RE: 79 relay for motor protection

Actually a motor circuit is the worst application for this relay.
Reclosing a motor circuit with the timings typical of a recloser has a high probability of causing severe mechanical damage. Bent or broken shafts, sheared keys in keyways, exploded couplings, stators turning in the housings, damage to machines by excess torque, either positive or negative and other damage related to sudden transient torque are possible with fast reclosing of a motor circuit. The bigger the motor the worse it gets.
A reclosing lockout would be more suitable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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