popa Posted May 5, 2006 Report Share Posted May 5, 2006 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!! Link to comment Share on other sites More sharing options...
marke Posted May 5, 2006 Report Share Posted May 5, 2006 Hello popa Welcome to the forum. There are a number of issues here that need to be looked at.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.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.Capacitors connected to a supply with high harmonic voltages will draw a very high current and will fail early.Detuning reactors can be used to reduce the effect of harmonics on the current into power factor correction capacitors.VSDs have a poor distortion power factor and can produce high harmonics on the supply which can cause high capacitive currents.Power factor correction capacitors connected close to the input of VSDs can cause VSD rectifier failure due to high peak fault currents on transients.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, Mark Empson | administratorSkype Contact = markempson | phone +64 274 363 067LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters Link to comment Share on other sites More sharing options...
popa Posted May 5, 2006 Author Report Share Posted May 5, 2006 Hello popa Welcome to the forum. There are a number of issues here that need to be looked at.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.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.Capacitors connected to a supply with high harmonic voltages will draw a very high current and will fail early.Detuning reactors can be used to reduce the effect of harmonics on the current into power factor correction capacitors.VSDs have a poor distortion power factor and can produce high harmonics on the supply which can cause high capacitive currents.Power factor correction capacitors connected close to the input of VSDs can cause VSD rectifier failure due to high peak fault currents on transients.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, Link to comment Share on other sites More sharing options...
popa Posted May 5, 2006 Author Report Share Posted May 5, 2006 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 Link to comment Share on other sites More sharing options...
mariomaggi Posted May 5, 2006 Report Share Posted May 5, 2006 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 Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it Link to comment Share on other sites More sharing options...
Rzn Posted October 14, 2006 Report Share Posted October 14, 2006 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. Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now