Motor Inrush Current
Motor Inrush Current
(OP)
Hello. I'm trying to gain a better understand of the moments when a motor is started (no soft start or VFDs). Is the power delivered at startup mainly real or reactive? Thanks.
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RE: Motor Inrush Current
RE: Motor Inrush Current
I seem to remember an issue with some motors tripping breakers on instantaneous trip on some starts in the late 60's but you know what they say about remembering the 60's. grin
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor Inrush Current
"Inrush" is technically that initial magnetizing current spoke of, a few cycles in duration but potentially from 7-20x the FLC of the motor. "Starting Current" is what begins AFTER the windings are magnetized, and ranges from 500-700% of FLC depending on motor design. But yes, Starting Current is mostly reactive in the beginning, then becomes mostly active as you approach full speed. I usually consider that to be 70-80% of full slip speed, but it too varies by motor design.
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RE: Motor Inrush Current
RE: Motor Inrush Current
When the initial energization is at wrong point on wave or when there is residual magnetism in the wrong polarity relative to initial energizeation, there may be a first cycle transient. When these effects coincide the transient may be very high but of very short duration. (Less than 1/4 cycle.)
Transformer energization may result in a DC component causing an offset to the magnetizing current. This may take a tme measured in cycles, or in the worst cases, measured in seconds. I am not aware of this effect in motors. I have not seen this effect on graphs which show the first cycle transient.
Although there are similarities between motor windings and transformer windings, when the sizes relative to the systems are compared between large unloaded transformers and motors being connected to a grid, any offset current may be so slight as to be un-noticable.
But I have a lot of respect for your knowledge, Jeff, and I'm keeping an open mind.
Bill
--------------------
"Why not the best?"
Jimmy Carter