Induction Motor Inrush Current DC Component
Induction Motor Inrush Current DC Component
(OP)
Is there a dc component in motor inrush current similiar to a transformer inrush current?
We are advised that circuit breaker contacts are possibly being worn due to it tripping on the inrush current periodically. And that it is the inrush current with dc component is possibly activating the short time element.
We are advised that circuit breaker contacts are possibly being worn due to it tripping on the inrush current periodically. And that it is the inrush current with dc component is possibly activating the short time element.





RE: Induction Motor Inrush Current DC Component
Keith Cress
kcress - http://www.flaminsystems.com
RE: Induction Motor Inrush Current DC Component
RE: Induction Motor Inrush Current DC Component
More effective would be to make adjustments to prevent chronic trips during startup. Can you say more about the protection? What is it?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Induction Motor Inrush Current DC Component
200kW motor, 440V.
400A MCCB with electronic LSI trip unit (awaiting confirmed settings). Don't know details of the contactor yet, but it is the MCCB that is failing not contactor.
I wondered if the operator may be stopping and starting the motor quickly, I could understand some dc component then.
RE: Induction Motor Inrush Current DC Component
The scenario of breaker tripping on inrush is certainly possible. The variability of the inrush explains why it seems to happen somewhat randomly.
NEC allows instantaneous setting to be increased to avoid nuisance tripping, up to a max of 13x FLC, I believe.
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
RE: Induction Motor Inrush Current DC Component
Since you mentioned transformer, we should mention that for a motor the current decays much faster than for a transformer.
Your problem statement is not exactly clear.
Possbily.. possibly... what you need to tell are the facts. You are seeing instantaneous trips I assume (I'm not familiar with short time trip for motor protection)? Is that the problem or the problem is some condition of contacts? Were contacts insepcted? New installation? How long has it been doing it?
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RE: Induction Motor Inrush Current DC Component
For all excel enthusiasts, I uploading two files which explain this phenomenon.
The solution for this problem is : Increase the setting of the Breaker to max 17 times FLA. If the breaker has fixed instantaneous characteristic, replace the breaker with an MCP with a dial setting feature. Better still, install a breaker with a feature to overlook the first 1/2 cycle inrush.
RE: Induction Motor Inrush Current DC Component