Motor Inrush Current

[ME27272727]

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.

 

[LionelHutz]

There are a couple of cycles of very high real current and then the magnetic field gets established and it's mostly reactive with a really low power factor until the motor reaches at least 60-70% speed.

 

[waross]

Graphs that I have seen of motor starting current show the very high current only in the first 1/4 cycle. The duration is so short that there is little real power consumed. This transient is seldom mentioned in discussions of motor starting current and seldom shown on motor speed/current curves. The magnitude depends on more than one factor and may vary from start to start by a ratio of more than 2:1. The I^2T is so little as to be ignored by virtually all thermal type over load devices. In very few instances the transient may cause nuisance tripping of instantaneous devices.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

If you remember the 60's, you weren't there? That now can be related to our age too I suppose...

"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.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ME27272727]

Thank you for the responses. With starting current being mostly reactive in the beginning, would it be correct to say that the engine of a backup generator would not necesarrily "feel" the reactive power from starting current? Rather the generator windings / circuits would be the main component that needs to be addressed when dealing with large starting current?

 

[waross]

Respectfully, I was under the impression that due to the alternating current, the motor was de-magnetized and re-magnetized each half cycle.
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