followup - motor starting current waveforms 3

[electricpete]

A followup related to my question: "motor starting current waveforms… contact bounce?" located at thread237-49867

Here is a single slide of motor starting current which should be much easier to view:

http://reliability-magazine.com/pub/21b_start.ppt

My questions are similar to the two that I had posed before:

#1 - There is what appears to be an unbalance between the peak-to-peak magnitude of the currents (ignoring dc components) visible at approx 7 cycles after the start. When looking closer it is seen that the red and green phase currents start with a lower magnitude (~36A pk-to-pk) than the blue phase (~44A peak-to-peak). This last until about half-way through the window, at which point all three currents go to 44A peak-to-peak and remain there for 2 more seconds (beyond this one slide). Why would the red and green phases start out at lower magnitude and then grow?

#2 - When viewing the dc component of the blue phase current, there appears to be some slow oscillation of that dc component as opposed to the expected pure simple exponential decay. Has anyone seen this before? Any explanations?

 

[jbartos]

Suggestion: When transients are involved, there are usually many causes to be analyzed.

 

[DougMSOE]

I have seen this before and it is not contact bounce, or at least very very unlikely.
What this appears to be, from my experience 30+ years is a rotor circuit problem, i.e. rotor bar or a cage problem.
The standard test for this problem is to single phase the stator with a low voltage ac and with an inline amp meter watch the current. The change in reflected impedance will cause the amp meter to dip. If no problem, no dip. The only trouble is that this method does not see all that is going on in the stator during startup.
What I have used is a wide band Hall effect clamp on amp probe into an oscilloscope and capture the wave form (as you have done). As stated above, your wave form is that of rotor problem not contact bounce.

 

[electricpete]

Hi Doug.

Which characteristic of the waveform do you think indicates a rotor bar problem? The current "unbalance"? The "oscillating" dc component?

 

[Shortstub]

Electricpete,
By the very nature of 3-phase, there will always be asymmetry in the phases. The highest peak will be displayed in the one having been subjected to the highest phase-voltage magnitude at t=0. The other two phases magnitudes will be proportionately less due to the fact that their voltage magnitudes are different at t=0. In your case were the red and green phase current-magnitudes negative?

 

[electricpete]

Thx Shortstub - If I understand you correctly, you are describing the effect of what I call the "dc offset". (exponentially decaying dc offset). That portion is always expected to be different for three phases. But I am used to seeing the ac component (seen by examining the peak to peak) be the same for all three phases.

 

[Shortstub]

Electricpete,

Let me clarify my position:
We agree that each phase-current is made up of two components.. one DC the other AC. But, here is where I disagree... the DC components are the same for the three phases. Therefore, as you observed, their AC components must be different, because their driving voltage nmagnitudes at t=0 are different. The sine term describing the AC component contains three elements, sin(wt+a-d), where:

w = omega.
t = time.
a = alpha (the phase-voltage time-displacement angle.
d = arctan(X/R)

For the blue-phase, a=0. If the phase sequence is blue-red-green, then the corresponding time-displacement angles for the red and green-phases are -120deg and +120deg, repectively! Thus, their rms, hence, their peaks, will be less than the blue-phase, they will be equal, and their peak-curve envelope intercepts with the t=0 axis should both be below the time-axis. The latter assumes that the peak-curve envelope of the blue-phase is above the time-axis!

 

[Shortstub]

Electricpete,
My statement about the DC components was not correct. Whie the DC decay rate, or decrements, are the same, the initial magnitude of Idc, or the t=0 intercepts for the thee phases, are different. Also, while the blue-phase Idc(max) is positive, the corresponding red and green-phases values will be negative.

Thus, the total I(t) for the red and green-phase currents are less than the blue-phase, and then as their respective decrements approach zero, they become equal to the symmetrical blue-phase current value.

You should observe that the DC decrement of the red and green-phases become zero in less time than does the blue-phase.

 

[jbartos]

Suggestion: The waveform deviation from symmetry could also be caused by the measuring devices/methods/setups error(errors or influences). Is the transient interval waveform irregularity at this motor only or does it appear at other motors?

 

[jOmega]

ElectricPete,

And what are the phase voltages doing during the starting sequence ? Of interest would be both[/] P-P and P-N.

Just to eleminate voltage imbalance from consideration.

Oh, and not having seen previous thread... what is power rating of motor ?

Thanks.

 

[Shortstub]

Because all phase-currents eventually reach identical steady-state values, i.e., 44A pk-pk, unbalanced terminal voltages are highly unlikely!

 

[electricpete]

The motor is 2500hp. Shortstub's logic is correct. I have the voltage waveforms (I believe they are included in links from first thread), and there is a few percent change in voltage during starting but it remains balanced throughout.

to jbartos - we have measured several other motor starts using similar setup and have not seen this particular behavior before.

 

[jOmega]

Shortstub & EP, I have seen a phase go unbalanced during the DOL start of a motor.... when one phase is considerably more loaded by single phase loads... In such a case, the starting current is sufficient to pull that phase down, and then, when the motor is at speed... the balance returns.. .. It is not uncommon to find that a system that was reasonably balanced when new... gets to this state over time when additional loads are added without giving consideration to balance...

It does happen.... a lot more than you would think.

Be that as it may, glad to know that phase-phase and phase-neutral voltages were measured and were all well balanced. Good to rule this out of the consideration...

 

[electricpete]

Thx fyi - the quasi-steady state values reached on the right side of this particularl slide are in fact locked rotor current. A drop to running current occurs aprox 2 seconds later.