Motor PF & Resultant Capacitive Equivalents 1

[Fraser21]

Ok guys, little rusty over here..project management too long and not enough engineering in my life I guess.

We recently surveyed at Pump 150 H.P. 480V 3P. Initial Current was 175A @ .79PF We have a guy who claims he makes a "little black box" that will change the intial PF to .96 reducing the amperage to 144A. I'm assuming this little black box simply has a capacitor in it to counter the inductive load by magnetizing the coils in the motor. Anywho, i'm just working what little info I have to try and figure the reactive power. And what capacitance will bring the reactive power close to 0 leaving almost a totally resistive load. Just for fun. This guy claims his "little black box" will reduce power losses in motors 10-200 H.P. by 6-12%. I'm going to install a Dranetz PX5 before and after this box is installed to see if this will work.

I figure the from what we monitored (175A @ 480 3P)
Your Real or True Power (P) would be (175*480*1.73*.79)/(1000) = 114.8kW

Angle = cos^-1 (.79) = 37.8 Deg.

From this your Apparent power (S) would be the following: 114.8kW / (cos 37.8) = 145kVA

And your Reactive Power (Q) would be 114.8kW(tan 37.8) = 89kVARs

First off, am I right so far?

 

[rbulsara]

Here is my own newsletter of May 2009 on the subject.

http://www.srengineersct.com/Newsletters/May 2009-News Letter-PFCC.pdf

Rafiq Bulsara

http://www.srengineersct.com

 

[Fraser21]

I believe they are getting penalized for power factor, i will request their electric bills and do some research. This may be fixed in a much easier manner as you guys say. Are we all in agreement that the survey I attached performed by this guy is a little over zealous with his savings per month.

I'm gettting the feeling that this black box is a type of sinosoidal motor controller as PQ Doctor said above. Its too small for a capacitor set-up.

And again, the motor is fully loaded deeming it useless in this application?

Guys I appreciate the help on this stuff, i'm still pretty green on the engineering side of things...I know enough to get me in trouble haha.

 

[rbulsara]

Anyone who says he has a "black box" that does magic, are usually selling snake oil. They do not survive too long on this website. Those who fall for it are not engineers.

Rafiq Bulsara

http://www.srengineersct.com

 

[wareagle]

Fraser21 Quote "Guys attached are the survey results from a couple of motors in the building."

This guy is doing the same thing you were doing in your calculations. He takes the Amps and volts before the caps and gets KWHs. Then he uses the new Amps x volts to again calculate KWHs. Adding the caps does not reduce the KWHs used by the motor. It only reduces the KVAH used by the motor. As previously stated your only gain will be a reduction of the I²R losses. However, if you are being billed a penalty for PF, adding capacitors can be a big help in reducing you monthly bill. Again you don't need this guy to get this done. By adding the correct size capacitors to each motor you will reduce the KVAR demand and I²R losses. Do it yourself. You know more that this salesman.

 

[wareagle]

How about getting amps load for the 150 hp motor with and with out caps and the resistance of the feeder to the motor and calculate the approximate I²R losses for the hours you stated and let us know what you find. You know the present PF. Assume you correct it to 0.90 and see what happens.

 

[Electic]

Beware of little black boxes that do not disclose what is going on inside. I was asked to verify results of such an installation. The vendor had a cute demonstration showing massive current reduction on an UNLOADED motor.

Installation was completed on a sewer plant and several of the local power monitors indicated load reduction as the mystery boxes were connected, however; the power company primary metering located about 1000' away indicated no change. Eventually the very expensive little black boxes were removed.

I was not able to verify why the local meters showed a reduction. The units might have been manufactured in Isreal, and from photographs looked like they had capacitors in them, but it was never clearly specified. I thought there might have been some circuit imposing a harmonic to frustrate the local CT's or metering, but once again this is speculation. If the vendor cannot clearly communicate what is going to happen in terms understandable to other engineers it might be a hoax.

 

[edison123]

Simple solution. Ask him to connect his magic box and show the reduction in KW-Hrs (not current, which will be lower).

Muthu

http://www.edison.co.in

 

[KJLRSA]

Correct me if I am wrong, but when we design individual motor compensation, you cannot just look at the PF of the motor. We look at the no load(Magnetising) current and size the capacitors to about 85% of this value. If you start just adding caps to the motors, you risk self-synchronization and damage to your system. I dont have any technical documentation to this regard, but got this rule of thumb from my director.

We always suggest bulk correction for clients with high kVA demand charges or PF penalties, mainly due to reduced maintenance and installation costs.

 

[wareagle]

Fraser
If you don't give us some feedback we will have to ban you from the forum for an unspecified time period yet to be detemined.

 

[LionelHutz]

Around here, utilities have 2 different groups of charges. They charge 3 or 4 items based on the kWh and they charge 3 or 4 items based on the kVA demand. The capacitors can lower the kVA demand which will lower those kVA demand charges.

You can not lower the kWh used by the motor with a capacitor. You would need a more efficient motor to lower the kWh used by the motor.

The capacitors will have about 0.3W of losses per kVAR. So, 75kVAR will have about 22.5W of losses. This would eat up just over half of that 41W of savings calculated above, meaning the savings in that example would really be about $10 a year.