Responding to earlier comments on "PROC: Different Ampere Readings" ( "http://www.control.com/1026152969/index_html":http://www.control.com/1026152969/index_html ):

Different current readings are caused by an unbalanced voltage at the motor terminals. Furthermore, NEMA (National Electrical Manufacturers Association) standards cover this problem in greater detail. Note, I called it a "problem" because it has a deleterious impact on motor performance, as well as life-expectancy.

Now, the details. Percent unbalance is defined as the ratio of the max phase deviation to average phase value, in %. NEMA standards limit it to
< 1.0 %. Beyond that a derating factor is required. Doesn't seem like much, but typically, current deviation is related to the square of the
voltage unbalance. In other words a 3% voltage unbalance will result in a current increase of 1.03^2, or 6%.

Discussion related to rms, harmonics, or meter-type is not the point. Start with the premise, that unless the instrumentation is very old, the
readings are probably rms... certainly the case for modern instrumentation technology. Also, input and output differences are not covered in this discussion.

Problem severity is, of course, related to motor load, so I will restrict further discussion to full-load operation. So then, how does the problem manifest itself. Visualize, that the motor is comprised of two motors. The normal one that we all "see" rotates in a positive direction due to the balanced voltage. However, there is a teeny-weeny one that responds to the unbalance, and it that rotates backwards. The consequence is a moderate increase in stator current, as well as reduced torque! For the example above, the temperature-rise will be 1.06^2 or 1.12 times the rated value. And, I'm quite sure that many of you have heard that insulation life-expectancy is halved for each 10 degrees C increase in temperature rise.

Fortunately, the real world is not (excuse the pun) as "negative". First, most motors operate well below full-load. Second, the problem is
more severe for medium to large motors. As for small motors, the most severe unbalance condition, i.e., single-phasing, is likely to be
detected because today's drives (should) have 3 overload devices.

And now the caveat: NEMA standards do state: "Operation of the motor with more than 5 % voltage unbalance is not recommended."

Regards,
Phil Corso, PE
(Boca Raton, FL)

 

Further to my earlier response (Mon, Jul 15, 7:04pm):

Add the following real-world "saving-grace" condition to the penultimate paragraph.

"Third, few motors operate at or near rated ambient temperature."

Regards,
Phil Corso, PE
(Boca Raton, FL)

 

Dear Mr Phil,
Your theory of two motors is new to me.
Would you mind explaining some more.
Thank you very much and best regards,
sekar

 

Obviously we do not see unbalanced currents in a VSD+Motor system.
The reason is:
1) If the motor draws an unbalanced current, the VSD will not let it (it will trip), provided that the VSD and motor ratings match.
2) If the motor does not draw an unbalanced current, the VSD by itself does not draw it (The VSD supplies the balanced DC-link).

Therefore, the case mentioned before is not related to the unbalanced motor, but wrong readings.

What you have written may be applied to unbalanced motors WITHOUT VSD, but this was not the case the author asked.

 

Responding to Suseka's query (Wed, Jul 17, 4:39pm) as well as several off-list queries concerning two-motor theory:

First, I apologize for my failed attempt at humor. I just realized, that the pun on the word "negative" in my penultimate paragraph, probably went un-noticed because there was no earlier mention of the word "negative!" Secondly, I should have used the analogy of two motors on
the same shaft... one large having a positive phase sequence, the other, smaller, having a reverse, or "negative", phase sequence.

And now, the rest of the theory.
To analyse unsymmetrical faults, such as voltage unbalance in polyphase circuits, EE's resort to a powerful mathematical tool developed in 1918,
called "Symmetrical Components." Unbalanced voltage and current vectors are replaced with 3-sets of balanced components called:
positive-sequence; negative-sequence, and zero-sequence. The first two react to positive and negative rotating voltage vectors, respectively.
All electrical components... generators, motors, transformers, cables, etc... are assigned impedance values for each sequence-type. (At this
point, I will ignore zero-sequence because they are used only for unbalanced faults involving ground/earth.)

For analysis of unbalanced supply voltages, the equivalent circuit is that of a 3-phase normal size motor, rotating in the positive sense. The
developed current and torque are influenced by the motor's positive-sequence impedance values. Now here is where the second motor come in. Smaller in size, its represented as a single-phase motor. Its stator winding, characterized by negative-sequence impedances, is connected in series with the large motor, but in reverse (negative) phase-sequence. Then, the smaller motor produces a counter torque to that produced by the large motor. Physically, think of it as it a single-phase motor that is reverse-wired in series with one of the larger motor's stator windings.

Now that additional details have been provided, hopefully the "pun" is meaningful. If you, or others, want more info, let me know.

Regards,
Phil Corso, PE
(Boca Raton, FL)

 

Responding to H. Ozevin's comments (Thu, Jul 18):

I agree, that, from your viewpoint, I provided more info than was requested. And, it is certainly true that your experience and mine are
different. For example, I have seen VFD's with unbalanced motor currents and you haven't.

My interpretation of the original thread was that there were two queries, The first asked, while there was significant current unbalance in the supply to the AC/DC converter, the motor current unbalance was virtually nil. I believe Mr. Hergenhahn provided a plausible response. The second query addressed the existence of standards that relate to Input / Output current unbalance. I am not aware of one. But, I am aware of the NEMA Standard covering motors. So I provided details.

I also agree, that, your statement regarding instrument validity is correct. A method was presented to the A-List, under the thread titled "ENGR: Power Consumption for Motor w/VFD". Simply, it illustrates how the 2-wattmeter method will yield measurement authenticity.

BTW, if, as you state, the VFD does not allow unbalances, then, what is the "trip" setting.

Regards,
Phil Corso, PE
(Boca Raton, FL)