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Member

Join Date: Jan 2012
Posts: 9

### Meaning of Negative Sequence Currents

08/18/2012 2:48 AM

I see some trouble with how symmetrical components are treated. In lots of texts, I have heard something along these lines.

When the current in a 3 phase motor is unbalanced, we can resolve it into three sets of balanced currents.

1. Positive sequence currents.
These produce rotating magnetic fields in usual direction and produces +ve torque and power

2. Negative sequence currents
There produce rotating magnetic fields in opposite direction and produce -ve torque

3. Zero sequence currents
These produce stationary and pulsating magnetic fields and produce no torque.

I fully agree that mathematically, 3 phase unbalanced set of currents (or any phasors) can be thought off as sum of 3 sequence components. What I disagree is when people talk about the effects of these currents. For example I don't think we can say the losses in stator winding = (I_positive^2*r + I_negative^2*r + I_zero^2*r).
Mostly, I find texts talking about negative sequence currents.

"Since negative sequence currents produce rotating magnetic fields in opposite direction, it will have large relative velocity with rotor (nearly 2*Ns , Ns is synchronous speed). It will create large induced currents in rotor and huge eddy losses."

But I don't think we are allowed to find out the induced currents and eddy losses due to the 3 sequence currents individually, just like we can't find the I^2*R losses individually.
For example, consider that a DC current of 2A is flowing in one of the winding of a motor. Or even better, lets consider a motor with no current in any winding.We can think of 0 current as 0 = 100*Sin(wt) + (-100*Sin(wt)) , as composed of two AC currents. Now Clearly, we can't individually calculate the losses (eddy and I^2*R) due to each current and Add.

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Guru

Join Date: Apr 2010
Posts: 537
#1

### Re: Meaning of Negative Sequence Currents

08/19/2012 7:16 AM

I think the point you are missing is that these three induced currents are different frequencies, so you can add the power components.

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Power-User

Join Date: Sep 2011
Location: Near the New Madrid Fault. USA
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#2
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### Re: Meaning of Negative Sequence Currents

08/20/2012 10:53 AM

Rixter, can you give me any information on Spectral analysis (FFT) in 3 phase motor noise? I have all the Vibe stuff on motor bearings and not referring to that, just what I am seeing on windings. I am using a UE Systems ultrasonic gun at 30KHz to collect data. I gave you a GA on the post. Thanks

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Guru

Join Date: Apr 2010
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#3
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### Re: Meaning of Negative Sequence Currents

08/21/2012 9:09 PM

Hi Rockyscience,

Thanks for the GA. I'm not sure what I can do to help. I haven't really done any motor vibration work, and I'm not familiar with the ultrasonic gun other than to know that it is usually used to detect gas leaks. I assume you are trying to capture vibration that is related to the rotation rate. If you can capture some time samples and get them into a file, and have access to a program like MATLAB, you can compute the power spectrum. You might see fundamental and harmonics of the rotation frequency. It sounds like an interesting project.

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Power-User

Join Date: Sep 2011
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#4
In reply to #3

### Re: Meaning of Negative Sequence Currents

08/22/2012 8:37 AM

Thanks Rixter, I have such programs and am actually touching the motors with a contact probe at 30KHz.. not airborne (too much background noise).

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