Induction Motor Inrush Current DC Component 1

[jimmy2times]

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.

 

[itsmoked]

Is this a wye-delta started motor?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jimmy2times]

No, DOL start

 

[itsmoked]

As you probably know there is a very large current associated with starting a motor. It doesn't really matter whether there is a DC component or not. You can't alter it much without an entirely different starting method. Interrupting that large current takes a large bite out of the contactor's lifetime.

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

 

[jimmy2times]

I have some sketchy details at the moment but awaiting more info.

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.

 

[dpc]

Yes, there is a dc transient over and above the symmetrical locked-rotor current. The magnitude depends on the voltage phase angle at the moment of energization. Similar to transformer inrush.

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

 

[electricpete]

If it is a high efficiency motor (most new motors are - search old posts for relevant quote from NEC and NEMA MG-1 that defines a high efficiency motor), I think it is up around 17*FLC that you can go to. Using the setpoint strategy for older motors gets you in trouble with new high efficiency motors.

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.

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

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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[cherry2000]

This has been a topic of discussion lately in the Project that I am working on. Several internet sites discuss this with detail. The simple matter is a sinusoidal excitation of a L-R circuit. On application of a waveform Vsinwt, the response in the L-R circuit, is always with a DC component. Now the DC component depends on X/R ratio of the circuit & also the instant of switching. Now, if one runs a calculation, it is seen that DC component is maximum at time t, when V=0. This inrush is a half cycle phenomenon, and rapidly dies out. However, unfortunately, the modern day Current Limiting breakers with instantaneous feature, rapidly opens on a 1/2 cycle inrush. Some manufacturer's have cleverly developed MCPs (motor circuit protectors) with a slight delay on the Instantaneous feature, to allow the 1/2 cycle transient.Others, who do not have this feature, have to fall back on NEC Exception to MCP settings, increasing setting to max of 17 X FLA.

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.

 

[cherry2000]

Here's the more interesting second file. Try to change the instant of switching. and see the magnitude of DC component changing.