Power Factor Correction at motor Terminals. In rush current. 1

[squeeky]

I was in a discussion:

http://www.eng-tips.com/viewthread.cfm?qid=368907

...and the consencus was that placing a capacitor at the motor terminals does not increase the inrush current. I can't find it so .... Here is a link:

http://ecmweb.com/content/correcting-power-factor-your-motor-terminals.

There are, however, limitations on capacitor size when connecting capacitors this close to the motor terminals. These limitations are products of self-excited motor overvoltage (of as high as two times source voltage) and excessive inrush current (of as much as 20 times normal full-load current). Subsequent transient torque caused by out-of-phase reclosing before the capacitor can discharge can also play a part in limiting capacitor size.

 

[Skogsgurra]

I added a simple presentation that covers a few aspects.

See

http://gke.org/presentationer/files/PFC and Motor Protection 1.pdf

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

Part of the different opinions may be caused by there being 2 different parts to the motor starting current. The first is the short duration transient inrush when first energized and the second is the longer duration accelerating transient.

 

[squeeky]

Thanks Skogsgurra for your drawing. Works for me.

 

[LionelHutz]

That drawing only covers what happens during acceleration and does not indicate what transients happen when first energized.

 

[Skogsgurra]

I will have a field day today. There are several little motors that I can do a quick check on. If customer says OK. I'll be back with some recordings showing at least starting transients. There are no capacitors, though.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

We get to the see the motor current but it won't answer the question.

I've never bothered measuring a capacitor/motor combination but I have measured capacitors by themselves and they typically have a high frequency ringing inrush. So, I would expect the very first answer from David provided in the other thread is exactly what happens. You'll get the 1/4 to 1/2 cycle capacitor ringing superimposed on top of the motor current. I'm not sure why there had to be this redundant thread about it.

 

[Skogsgurra]

Agree - the "Mother Thread" has given birth to a few more or less confused "Children". I just got back from the field. Yes, you are absolutely right. We both knew that. But I think that it could be of some value to those souls that think that the behaviour of physical systems is a conversation matter that one can have different opinions on.

One thing that I found is that the risk of self-excitation seems to be quite a bit exagerated. Large capacitors prolong the decay of induced voltage (keep rotor flux up), but there was no sign of overvoltage. At least not on the small motor. Haven't looked in detail at the "larger" one.

I'll sit down and collect a few zoomed recordings. Ready in a few hours.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

Here:

http://gke.org/presentationer/files/What happens during start and stop of an asynchronous motor.pdf

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[markstg]

I'm the OP of "Blinking Lights" and have been following resposes to all the children as I clearly misunderstood the transient behavior of the added capacitor to a starting motor circuit.

I haven't been able to stop my lights from blinking, but maybe by adding additional capacitance when the motor starts may have an effect. I'm wondering if adding the capacitance will reduce the duration of transient, as opposed to reducing the magnitude of the transient?

 

[electricpete]

One thing that I found is that the risk of self-excitation seems to be quite a bit exagerated

There are probably not a lot of people that actually test to see what happens when you disconnect a motor that is overcorrected. So it will be very interesting to see results. But I have to confess I am already prejudging the results. Small motors have high resistive damping and don’t show high-magnitude resonance characteristics as readily. Certainly the overexcitation scenario was not just “made up” out of thin air as I'm sure you'll agree but I guess maybe the question is how common would it be if people ignored the advice regarding overcorrection. Some related experience and discussion was here:
thread237-241267

I’ll be happy to be proven wrong (since that means I’ll learn something). Or maybe it won’t be a matter of right/wrong but just looking at things differently. I look forward to seeing your post.


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(2B)+(2B)' ?

 

[Skogsgurra]

I'm afraid that the motor that was available isn't the best one for this test. It is a Ziehl-Abegg with very high rotor resistance. And the power is only 0.55 kW. So the test probably doesn't say much.

I am getting curious now and will look for larger motors with capacitors connected to the terminal. The problem is that most people probably will get nervous when I mention that I want to do this kind of test. Time for some googling, I think.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[squeeky]

Our Irrigation pumps should be ready to run in November if you can wait. SOrry I restarted a thread but in Darkest Africa the internet is slow and I could not find it.

 

[electricpete]

It seems there are a lot of different interests in your experiments in this thread.

Regarding the overexcitation – I’ve got to admit I was surprised to see no peak in the decaying voltage after disconnecting the overcorrected motor. So I immediately looked for a reason to dismiss it and settled on resistance. But then again, if that was the whole story I’d still think at least there would be a at least a little blip in the voltage as passing through resonance. So maybe there will be some other lesson to be learned in studying the details.

Do you know if the motor stator was wye or delta wound?


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(2B)+(2B)' ?

 

[Skogsgurra]

Wye connected. BTW, the slip is quite high. Full load speed is 1380 at 50 Hz. So rotor resistance must be high.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.