5 replies to this topic

[popa]

Hello, I'm new to the forum but would like to see if any one can help......the problem is that we have a very low power factor.....-.40 to -.70 when we have several 480 3 phase motors on line......we added three 3 phase 50 kvar capacitors to the system and the power factor didn't change...what did occur was that the bank of capacitors was creating a 200+ amp load.......i've never had a capacitor used to correct a power factor problem to draw that kind of current.....the motors are on staging VFD starters...HELP!!

[marke]

Hello popa

Welcome to the forum.

There are a number of issues here that need to be looked at.

1. Never apply power factor correction on the othput of VSDs, you will damage tha capacitor and the VSD. I do not expect that you have done this, but it needs to be said anyway.
2. There are two types of power factor, displacement power factor and distortion power factor. Displacement power factor can be corrected by capacitors, distortion power factor can be corrected by active or passive filters. - Capacitors will not correct distortion power factor.
3. Capacitors connected to a supply with high harmonic voltages will draw a very high current and will fail early.
4. Detuning reactors can be used to reduce the effect of harmonics on the current into power factor correction capacitors.
5. VSDs have a poor distortion power factor and can produce high harmonics on the supply which can cause high capacitive currents.
6. Power factor correction capacitors connected close to the input of VSDs can cause VSD rectifier failure due to high peak fault currents on transients.
7. When power factor correction is applied, the power factor into the motor will not change, the power factor improvement is only between the connection pont and the supply. You must measure on the supply side of the power factor correction.

How did you measure the power factor? can you differentiate between distortion power factor and displacement power factor?
Where did you make the measurement?

Have you looked at http://www.LMPhotonics.com/pwrfact.htm

Best regards,

[popa]

Hello popa

Welcome to the forum.

There are a number of issues here that need to be looked at.

1. Never apply power factor correction on the othput of VSDs, you will damage tha capacitor and the VSD. I do not expect that you have done this, but it needs to be said anyway.
2. There are two types of power factor, displacement power factor and distortion power factor. Displacement power factor can be corrected by capacitors, distortion power factor can be corrected by active or passive filters. - Capacitors will not correct distortion power factor.
3. Capacitors connected to a supply with high harmonic voltages will draw a very high current and will fail early.
4. Detuning reactors can be used to reduce the effect of harmonics on the current into power factor correction capacitors.
5. VSDs have a poor distortion power factor and can produce high harmonics on the supply which can cause high capacitive currents.
6. Power factor correction capacitors connected close to the input of VSDs can cause VSD rectifier failure due to high peak fault currents on transients.
7. When power factor correction is applied, the power factor into the motor will not change, the power factor improvement is only between the connection pont and the supply. You must measure on the supply side of the power factor correction.

How did you measure the power factor? can you differentiate between distortion power factor and displacement power factor?
Where did you make the measurement?

Have you looked at http://www.LMPhotonics.com/pwrfact.htm

Best regards,

[popa]

thanks mark, your right , I didn't install the capacitors at the output of the vfd's...I installed them at the service entrance...we have a square D power quality meter at the service entrance...also I have a clamp type meter that is designed to read power factor and the meters were very similiar in the readings...we made no changes between the vfd's and the motors....I didn't have a meter that would indicate the harmonics, I guess I should get one.... an additional 200+ amps on each of the 3 phase legs just kinda confused me a little... I sure appreciate your input and i'm glad that I ran across the forum....THANKS, Popa

[mariomaggi]

Dear popa,
if you will give more data we can help you better.
How big is your plant, how much nominal power you have installed at trafos?
What is the effective power used?
Regarding effective power, how much power is requested by VSD?

At first sight, I think that you have problems for harmonics.
Let me know.

Regards
Mario

[Rzn]

Dear Popa,

Mariomaggi is right. Please provide more information. However, here is the formula for calculating the required VAR of capacitor for improving power factor:

QC=P x (TanΦ1-TanΦ2)

Where

QC = required VAR of capacitor
P = running load
TanΦ1 = running load cosΦ
TanΦ2 = target cosΦ

This may help you.