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)
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)