Same load has different power factor when fed from different sources

A

acolella

Member
Occupation
Engineering Technician
I have some ECM fan motors that are running different power factors when fed from different sources. The motors are 230V single phase motors. The first source that they were fed from is a 600V/240V single phase step down transformer. When fed from this transformer I got the following measurements:

233.7 VAC
4.0 amps
653 watts
0.702 power factor
934 VA
-660 VAR
D 45.2° phase angle
6.06% THD voltage
94.3% THD current

The second source that they were fed from is a 480V delta primary 230V wye secondary three phase transformer. That isn't a typo either. This transformer has a 230V wye secondary. The exact same motors were tested and the exact same power analyzer was used to obtain the measurements. When fed from this transformer I got the following measurements:

236 VAC
5.5 amps
663 watts
0.51 power factor
1.296 kVA
-1.117 kVAR
D 59.2° phase angle
1.63% THD voltage
165% THD current

Has anybody ever seen this before? Why would the motors run a lower power factor/draw higher current at essentially the same voltage and power when connected to the three phase transformer?
 
Have you looked at the sine wave with an oscilloscope? Are there any harmonics on the 230 WYE trans that aren't on the other? What else is on each transformer? Do you have a lot of drives with 6 pulse front ends or some other rectification going on?
 
Those ECM motors likely have diode rectifiers on their inputs and draw current only near the peaks of the voltage waveform. That leads to the very high THD (Total Harmonic Distortion) on the measured current in both cases. Also, the harmonic content will be sensitive to shape of the voltage waveform near the voltage peaks.
 
I'm just speculating, but if the higher 6% voltage THD in the first case is due to flattened peaks in the voltage waveform, that could conceivably result in having a lower current THD than in the second case that has a lower 1.6% voltage THD. That would be because the flattened peaks allow longer diode conduction times allowing less peak current. As suggested above, only scope measurements of the waveforms could isolate the real differences.
 
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