# Switching CT's : VFD-Motor

Discussion in 'Homework Help' started by zezizou, Jun 23, 2014.

1. ### zezizou Thread Starter New Member

Sep 26, 2013
8
0
Okay so here's my problem...

I'm working with a problem about a electric drive (VFD) input and a motor output. This is a three phase power system connected wye-wye.

There are two CT's (current transformers) placed on two of the lines. They measure the phase angle and current of the lines and simulate the third phase angle.

The output data is fine:
Po = 3,052 kW
Vo = 7,298 V
Io = 245 A

However, the input data (current, power, and pf) are wrong:
Pi = -3,037 kW
Vi = 13,497 V
Ii = 133 A
Pf = -0.990
This would yield an angle of Phi = 171.89 degrees.

Now, I know the efficiency is 97%. So I solved for efficiency = Po/Pi
So my correct value for Pi:

Pi = 3,146.39 kW

However, the problem states that I have to explain and prove mathematically why the power factor is negative. I'm pretty positive that one of the CT's is on the wrong line, or they both are on the wrong lines, reading data from (for example) phase A when it should be phase B.

Now a normal power factor for this application is 0.95-0.97. So I tried working from that and I still can't figure out how to solve this problem from the phases.

The angle with a power factor of 0.96 is Phi = 16.26 degrees.

I have tried shifting a certain phase by 180 or 120 degrees but I can't seem to get an angle that would correspond with 0.95-0.97 power factor.

If anyone has any experience with CT's and pf error/correction it would be a massive help.