Leading power factor during crane operation? 1

[bacon4life]

Do container crane drives create leading kvars during operation? The data below is from a data logger on 4160V transformer serving a container crane. During idle periods, there was about -10.5 kvars measured per phase. During most of the operation the power factor goes to highly lagging as expected. However, during few of the period, the meter recorded large amounts leading kvars. Is this realistic? or is the meter being tricked? I don't have any details on meter, other than is a temporary meter at the 13.8/4.16kV transformer.

DATE_______TIME__	PF__	AMPS	KILOVARS
07-Feb-14 16:08:00	-0.60	 5.3	-10.1
07-Feb-14 16:08:30	 0.54	56.5	 113.7
07-Feb-14 16:09:00	 0.71	30.8	 52.0
07-Feb-14 16:09:30	-0.35	50.4	-113.2
07-Feb-14 16:10:00	-0.33	29.6	-67.1
07-Feb-14 16:10:30	 0.48	30.2	 63.6
07-Feb-14 16:11:00	-0.49	 7.7	-16.2
07-Feb-14 16:11:30	-0.56	 5.2	-10.4
07-Feb-14 16:12:00	-0.55	 5.3	-10.6
07-Feb-14 16:12:30	-0.58	 5.2	-10.2
07-Feb-14 16:13:00	-0.61	 5.3	-10.1

 

[itsmoked]

I would imagine whenever the crane is regenerating, (dropping and slowing the load), that would occur, since it's now diving power back to the electrical source.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

You may be correct Keith.
Going back to generator theory, If the voltage of a grid tied generator is raised above the grid voltage the voltage may not rise but the set will export VARs. I would expect that as the back EMF of a regenerating motor rises above the grid voltage, the motor, acting as an induction generator, will export VARs.
bacon4life, does the VARs out coincide with lowering heavy loads?

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

 

[mikekilroy]

missing data: is the crane motor on a vfd or softstart? IF so, perhaps listing model may help: someone may know that model and know if it has an active front end or not, and if it would even be able to regen to line and cause this.

http://www.KilroyWasHere<dot>com

 

[mikekilroy]

also, what is rating of the crane motor? Does it indeed pull 50amps when loaded or is that a combination of other things also? The crane motor is the only load where the dat recorder is?

http://www.KilroyWasHere<dot>com

 

[ScottyUK]

An induction machine absorbs magnetising vars from the source regardless of whether it is absorbing power (motor) or exporting power (generator). To export reactive power you need an over-excited synchronous machine.

 

[electricpete]

Good point Scotty.

It's a little bit of a head scratcher.

One thing that comes to mind. Sometimes the steady state induction motor model tricks us when we try to apply it to transient conditions such as acceleration, deceleration, plug reversing.

Would be interesting to try to look at currents and voltages during these transients in a transient model (a.k.a. state space model, examples in Krause's book) to try to see if anything resembling var reversal can be obtained.

I don't foresee having time to do that in the near future. I think some others on the forum have programs including ATP etc. Maybe they can try if they are interested.



=====================================
(2B)+(2B)' ?

 

[electricpete]

before that ...
obviously would help to know more about system (mike's questions)
And if more measurements are possibly like current and voltage waveforms or more details about operations going on during these periods, that would of course help everyone.

=====================================
(2B)+(2B)' ?

 

[waross]

Re my first post; I guess that the back EMF will not exceed the applied EMF in an induction machine.
It is probably the routine used by the meter.
If the meter is using the basic formula, PF = W/VA then when the motor regenerates, the sign of W will change. The meter may assume and show positive power in every case to simplify connection.
Rather than showing the KW as negative the meter may be showing the PF and KVAR as negative.
The electronic KWHr meters our little utility used converted exported power to imported power and then summed both exported and imported power under imported power. This defeated the old dodge of removing and reinstalling the meter upside down for part of every month to reduce the power charges.
These were residential meters and I never had the occasion to investigate PF and KVARHrs.
The National utility on the mainland used a similar default setting on industrial meters.
Anecdote: A wealthy businessman installed a wind turbine at one of his locations. He didn't bother with permissions or approvals, just had it connected and turned on.
At the end of the month he found that he had been billed for all the power that he had used and for all the power that he had imported., due to the default meter setting.

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

 

[crshears]

Hey Bill, at the very end of your thread you meant 'exported,' right?

CR

 

[LionelHutz]

Why is everyone so concerned about analyzing this as being an induction motor when no-one knows anything about what is connected downstream of this transformer? It could be a DC drive for all anyone knows. There could be a capacitor bank connected to the transformer.

I will say it is possible what is being measured is the true power factor. But, I won't make random guesses as to why it is possible or go beyond saying it can be possible. Post details on the motor and the drive operating the motor if you want to confirm this does happen and you want to know why it happens.

 

[bacon4life]

Thanks for all the feedback.
I found out that the meter was a Varcorder, which uses electric field sensor rather than a PT to calculate the power factor. It then uses the programmed nominal voltage to calculate kvars. Since it just uses the angle between current and voltage, I don't think it recorded that the real power flow changed direction during regeneration. Rather, it seems to have recorded real power flowing out and reactive power flowing to the crane as negative power factor.

 

[panter]

Bacon4life,
If you on a crane have asynchronous motors with frequency inverters with an additional option called regenerative mode without a resistor which would be spent on energy in generator mode, it must be returned to the grid .The question is what percentage of used energy should be returned.
Good luck