Negative phase sequence
Negative phase sequence
(OP)
When referring to the NEMA curves regarding derating of machines due to negative phase sequence, what formula is used to determine the voltage unbalance ?
Vub = (Max - Ave)/Ave x 100
or
Vub = V2/V1 x 100 (neg seq/pos seq)
Both would give different answers the way I see it. Seems to me the sequence solution must be more accurate.
When calculating Iub the sequence formula gives about half the ub of the max/ave formula in a study I am busy with.
Vub = (Max - Ave)/Ave x 100
or
Vub = V2/V1 x 100 (neg seq/pos seq)
Both would give different answers the way I see it. Seems to me the sequence solution must be more accurate.
When calculating Iub the sequence formula gives about half the ub of the max/ave formula in a study I am busy with.





RE: Negative phase sequence
14.36.1 Effect on Performance-General
“negative sequence voltage” having a rotation opposite to that occurring with balanced voltages. This negative sequence voltage produces in the air gap a flux rotating against the rotation of the rotor, tending to produce high currents. A small negative-sequence voltage may produce in the windings currents considerably in excess of those present under balanced voltage conditions.
14.36.2 Unbalance Defined
The voltage unbalance in percent may be defined as follows:
percent voltage unbalance=100 x max imum voltage deviation from average voltage / average voltage
My interpretation: negative sequence (V2/V1) is the more relevant factor in determining heating and derating, however is not normally available without detailed calculation. Therefore the simpler unbalance formula based on max deviation from average is used and is expected to give a close approximation.
That approximation is the same as you (Max-Ave)/100 provided that you don't take Max literally... it is the max or the min, depending on which is further away from the average.
RE: Negative phase sequence
If this does not resolve your comments, can you post your data?
RE: Negative phase sequence
RE: Negative phase sequence
However imprecise the NEMA definition may be, the points on the derating curve for induction-motor power are based on NEMA absolute average deviation, so isn’t the difference somewhat academic so far as MG-1/C84.1 are concerned?
RE: Negative phase sequence
RE: Negative phase sequence
However, 99% of the users will have no phase angle information necessary to calculate negative phase sequence. So NEMA estimates it from the unbalance which is supposed to track negative sequence pretty well.
My question is why didn't the % unbalance track close to the % negative sequence in this case. I would like to see the magntidues and angles.
RE: Negative phase sequence
RE: Negative phase sequence
http://www.elongo.com/pdfs/voltages.PDF
for: the voltage unbalance and impact on the motor in terms of increased heating and increased motor losses
http://www.eee.metu.edu.tr/~hava/jp6.pdf
for: the matrix conveter and negative sequence related mathematical relationships
etc. for more info
RE: Negative phase sequence
http://www.siemp.co.zw/Acrobat%20Files/module06.PDF
for Figure 6.7 depicting the motor percent increase in losses and temperature due to percent phase to phase voltage imbalance.
RE: Negative phase sequence
R=176A
Y=197A
B=174A
The average method gives 8,1% and the sequence method should give:
I1=182,7A
I2=7,38A
Iub=4,04%
The above seq. method result ties up with what we recorded.
Have I got it right?
RE: Negative phase sequence