Power in 3 Phase Electric Motors 2

[rdrago]

How do I measure power in a 3 phase electric motor? I am trying to justify the installation of a drive on a motor based on the power savings obtained through the use of a drive. The motor is running a fan load. The problem lies in the fact that the power factor at different motor speeds varies, thereby varying the reactive componet of the power. Although I can certainly measure the resistive power used by the motor, I don't know how to measure the reactive power as the motor speed varies. The power factor listed on the nameplate of the motor is the power factor obtained at the full speed of the motor only. Since the power factor varies at lower speeds, it cannot be used to calculate the reactive power. Can anyone help me with this?

 

[rdrago]

Thanks to all for the information.

To wwilson: You offer good advice. The improvement in your power factor while using line reactors is impressive. The ABB ACS400 has a 3% line reactor feature although I'm not sure what 3% means at this point. Thanks again.

 

[wwilson]

The data I supplied was for a 5% line reactor. I think the difference between a 5% and 3% would be fairly small but I don't have any data to support that. We tested MTE, Hammond, and Trans-coil line reactors. Let us all know how things go. wwilson

 

[jbartos]

Suggestion: Many VFD procures look at the VFD price mainly. Obviously, many inexpensive VFDs may cause a very big power quality problem. Therefore, the current state of the art of VFDs should not be overlooked, even though the AFE VFDs may cost about 60% more than the basic "6-pulse front VFD."

 

[rdrago]

I plan on using line reactors on the input side of the VFD and dv/dt filters on the output side. My existing cable runs average 300' from the proposed inverter to the motor. The existing motors are 1972 vintage 20 and 30hp models. Upon reading some of the historical postings on this forum about installing VFD's on non inverter duty motors, I am beginning to wonder if this project can be safely done without destroying the motors even with the use of filters. When the VFD's are installed, the control can possibly run the motors from 0 hz to 60 hz. At the low speeds, I am becoming very concerned about the old motors overheating.
rdrago

 

[cbarn24050]

Hi, with so many vfds you might consider a star/delta transformer 12 pulse rectifier to supply all the drives, you would save on the filters and line reactors. Try to put the inverter near the motor. Cooling should not be an issue with motors driving fans as the load drops dramaticaly as the speed drops.

 

[rbulsara]

I would keep VFDs within 50 feet of the motors, to avoid ill effects of voltage spikes, especially for old motors.

300 feet is too long even for new motors.

 

[jbartos]

Suggestion: Visit

http://www.mirusinternational.com

etc. for more info on passive filters that are advertised as the better solution than the isolation transformer. However, the passive harmonic filter preserves the galvanic connection between power supply and load. The isolation transformers are isolating power supply from the VFD and load, which is may be viewed as the better solution.

 

[wwilson]

All,
I just received this article that helped me sort out what rdrago is shooting for by converting to a VFDs.

Power Factor Improved by Variable Speed AC Drives

http://www.drivesurvey.com/index_library.cfm?feature_id=70

wwilson

 

[rdrago]

The jbartos site contains an excellent article on non-linear loads and their effect on the transformers that supply them. Since VFD's are non-linear as viewed from the power source, harmonic frequencies are generated that lower the power factor of the source at the drive input.

However, the wwilson site contains a technical sales article that claims the power factor to be near unity at the drive input. Is the article talking about the fundamental only? What does the source really see? Is the claim about power factor improvement real or is it sales propaganda?

Thanks for a great discussion.

rdrago