Can motor instantaneous trip be caused by contact bounce < + cycle?

[electricpete]

We have had intermittent trips of the instantaneous only breaker (MCP) when starting a 60 hp motor (occurred twice in approx 20 starts).

It was discussed some here thread237-225193 although I will repeat some details:
motor: 60hp, 3600rpm 460vac, KVA Code G induction motor started DOL in combination starter

breaker: Cutler Hammer HMCP 100R3 instantaneous only breaker, set on tap setting G (900A rms).

In between these two trips, we have replaced the breaker, and the starter. (the two trips occurred with two different breakers installed and two different starters installed).

Following both occurences, we have checked the motor breaker and monitored motor starting current. We have probably a total of 15 monitored starts of the motor. Motor current never exceeded approx 1100Amps peak during the start. Breaker trip point was never below approx 900A rms (which corresponds to about 1300A peak).

One thing we noted during monitoring after the 2nd start was that the starter contacts did not close simultaneously, and sometimes bounced. Worst case time from first contact closing on any phase to last closing on any phase was ½ cycle – most < ¼ cycle. This did not occur in a sister motor. We did not see any increase in current associated with this phenomenon, but wondering if it could (depending on timing). Example of non-simultaneous closing is shown on first slide here where unit 2 is the motor that tripped and unit 1 is a sister motor. (we measured Vab and Vac).

Question: Do you think this type of contact bounce and non-simultaneous closing (<1/2 cycle total) can significantly increase the current during starting of a motor ?

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[electricpete]

Another question: If I were trying to build a simple model which captured the "relevant" aspects of the problem, what would you suggest?

I am thinking about modeling it as a wye-wye transformer with secondary shorted and varying the closing times... looking at effects on primary current. I'm not sure how realistic or representative this would be since of course the rotor doesn't resemble anything like a wye.

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[waross]

Hi Pete;
The only code that I have handy is the 1998 Canadian Code.
The setting maximum for an instantaneous breaker is Either 1300% of Full load current OR 215% of the motor locked rotor current. This address the issue of motors with higher than normal starting currents.
If your motor was at the top end of the code G envelope, 6.29 KVA/Hp, the 215% setting would be 1020 Amps. Will your codes allow a setting above 900 Amps?

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

 

[alehman]

I wouldn't think so bounce or non-simultaneous closure would significantly increase the current.

NEC allows HMCP's to be set up to 800% of the full load current as specified in the NEC (60hp/460V is 77 amps). If that is insufficient, the code permits increasing to a maximum of 1300%.