×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Evaluating Retrofit Energy Saving Devices, (PF improvement type)

Evaluating Retrofit Energy Saving Devices, (PF improvement type)

Evaluating Retrofit Energy Saving Devices, (PF improvement type)

(OP)
Dear All,

I'm evaluating a Energy Saving Device (powerliner) based on power factor improvement. This unit manly consists of capacitors which are installed near the motor.

Already in this forum there is a lot mentioned about such units. Most of the savings consist of power line savings which is logical and can be explained.

The manufactor claims to have high savings, up to 5 or 6%. From my point of view, that's not realistic.

To do further investigation, we've done some measurements on a motor (application: water pump)

We've installed 2 measuring points. 1 at the power cabinet, 1 near the motor, just before the energy saver connection.
Normal measured load is appr. 150 kW. As expected we see a large variation in Q (kvar) but we see also a slight decrease in P (kW) power when the unit is switched on. Even near the motor we see a decrease of approximately 8 kW = 5%.

At this point we can not explain this variation. It was not possible to do torque measurment on the motor shaft. But in generally, such a passive device can nog influence the motor output power, can it?

Does anyone know how to explain this variation, or has some experience with these type of devices?

Thanks for your answer,

Kind regards,

Rob van Heur

(The Netherlands)  
 

RE: Evaluating Retrofit Energy Saving Devices, (PF improvement type)

The C near the motor is reducing the current due to lagging power factor at the motor. This would be the reduction in power caused by increased heating of the lines due to that lagging PF current.  It's exactly as it should be.

If you had bigger wire you would see a smaller improvement.

The real savings of this type of device is if you are penalized for PF. If you aren't then the payback is much longer.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Evaluating Retrofit Energy Saving Devices, (PF improvement type)

Because wire is expensive, installing electricians (especially contractors) use as small of wire as they can possibly get away with and maintain an acceptable level of voltage drop. So if you have a high line to start with, and you end up with only a 5% VD at the motor, the motor is OK with anything less than 10% and they will consider that acceptable. But that 5% VD signifies a power loss in conductor heating. So by lowering the overall current the conductor carries by placing the Caps at the motor, you are lowering both the VD and the total conductor heating losses (being a percentage of total current). In some cases, such as this one, it shows up as a measurable savings. As Keith says, larger wire would have had a similar effect and would probably have cost less had it been thought of at the initial installation. But if it is already installed, that device might, in this SPECIFIC instance, be of value.

The problem I have with these things is more in the way the purveyors sell them, basically implying that they will ALWAYS get that much, or more! It's way too situational to make those kinds of generalizations. They find and document applications like this and use that as "proof" that it works, then allow potential customers to believe it will happen for them too. It's just not that simple, but simple sells.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Evaluating Retrofit Energy Saving Devices, (PF improvement type)

How "near" to the capacitor connection point. Measuring 5% less power very close to where the caps are connected is strange.

RE: Evaluating Retrofit Energy Saving Devices, (PF improvement type)

Do not fall for this. Use your own calibrated meters. Only kW savings comes from reduction in I^2R losses, and 5% would be a too large a loss.

Imho, it is not worth the time and energy spent to "evaluate" such gizmos. And yes what is the device "near" the motor? Where is the "far" from motor meter?

Rafiq Bulsara
http://www.srengineersct.com

RE: Evaluating Retrofit Energy Saving Devices, (PF improvement type)

Yes, I should have commented on that. It's generally well recognized that power factor correction capacitors save you money by avoiding utility demand charges. Capacitors don't save any energy and will never pay back if they are installed for the purpose of saving energy.

I've been thinking about this a bit more. If your metering equipment is out a bit - say a phase shift error between current and voltage - then it would see a slight portion of the reactive current change as a change in real power.

The power into the motor can not go down if the load hasn't changed. The capacitors are not perfect and they have some losses, typically up to 0.5W per kVAR. You probably installed 30kVAR of capacitor which could have a 15W loss. I would expect this loss to easily counter any gain due to lower current in the conductors.

So, it really makes no sense that the power input could go down.
 

RE: Evaluating Retrofit Energy Saving Devices, (PF improvement type)

(OP)
Dear All,

thanks for your answers so far! The answers seem compelteley logical to me. What is strange in tis case, that we measure the kW power decrease near the motor. The cable length between RESD (retrofit energy saving device) connection point and motor terminals is only a few meters. Therefore it's really strange because at this measuring point the cable losses should me really small. The power difference between both measuring points is about 1 kW which can be explainded by cable losses. (Appr. 200m cable, but not fully loaded)


I've uploaded some measuring data / graphs. See:

http://files.engineering.com/getfile.aspx?folder=43541e67-6dc4-410e-9208-3b647d6e9ff7&file=RESD_project_EngTipsForum.pdf


LioneHutz, can you explaine a bit more about the meting equipment? If the euipment is out a bit, but the connection or meter isn't changed when the RESD is switched on/off, will there still be a measuring fault?   


Kind regards,

Rob
 

RE: Evaluating Retrofit Energy Saving Devices, (PF improvement type)

The current has two components, the real current and the reactive current. The real current contributes to kW and the reactive current does not contribute to kW. The capacitors will reduce the reactive current but not the real current. There is a 90 degree phase separation between the reactive and real currents. If there is a phase angle error in the method used to separate the real current from the reactive current in the kW meter, the kW indication may include a portion of the reactive current and so show a false drop in kW.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Evaluating Retrofit Energy Saving Devices, (PF improvement type)

I see Bill has already got the explanation covered.

The comment I'll add today is that if you're involved with a company that is selling a power factor correction capacitor as a "retrofit energy saving device" then these particular capacitors also likely cost way too much and can be sourced through normal channels much cheaper.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources