Negative Phase Sequence and effect on Motor Current unbalance

[gtmack]

Information and explanation of the unbalance from incomming supply voltage that causes the phantom phase shift causing motor coil currents to be excessive at different times of the day, obviously due to reticulated supply unbalances, like 2 phase loads to electric trains from a 3 phase supply, single phase loads excessively on 1 of the phase supply lines.

 

[davidbeach]

Is there a question in there somewhere? I'm not sure there is a statement or a question.

 

[waross]

Do you have unbalanced supply voltages causing excessive motor currents?
yours

 

[jraef]

Is there a question in there somewhere? I'm not sure there is a statement or a question.

I'm with you. It sounds like a statement, not a question.

I have a question though. What the heck is a "reticulated supply imbalance"?

reticulated
adj : resembling or forming a network; "the reticulated veins of a leaf"; "a reticulated highway system"

How does that apply to a voltage imbalance?

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Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

It sounds like gtmack is struggling with English and doesn't have the best bi-lingual dictionary at hand.
With a little patience and some judicious questions we can probably rise to the challenge of communicating a little better with gtmack.
I agree with your comments, davidbeach and jraef, but let's remember the International aspect of this forum and cut gtmack a little slack as he struggles to translate his problem into English.
A question, jraef, do you know off hand if delta connected motors are any more or less prone to high currents as a result of supply imbalance than are wye connected motors?
Respectfully

 

[gtmack]

Well people !!!!!

I may not be as old and knowledgeable as some of you people.

But I certainly don't try to belittle someone because I dont understand their question.

Thanks **"waross"** for your input.

I took the meaning of the green questionmark in front of the subject text to mean exactly that !

I could not intemperate that as a statement was following.

Fellas *** All I am asking , is there anybody that has constructive explanation of the effect of unbalanced incomming supply causing this problem.

It's a real event that happens not a ski fi story.

I'm sure other pepole would have heard the word RETICULATED to refer to the supply and distribution of utility services such as electricity ,water, etc.

Thanks people

Does anyone use SKYPE

 

[davidbeach]

Never heard of reticulated used for utility systems. I still don't know what you are asking. What problem is "this problem"? What is "Negative Phase Sequence" in the thread title? Does it refer to negative sequence currents, negative sequence voltages, or something else?

While any of us can guess at what you might be asking, there are people here from all over the world, using slightly different terminology, and having to guess at the meaning of a vague, poorly worded question won't generate a good answer.

What is the problem you are seeing? What is the system doing at that time? What are the voltages and currents? What is the load, what happens to the load? The more information you can provide, the more likely you will get an answer that applies to your situation.

 

[jraef]

OK I'll step up and apologize for my sarcastic tone. I actually didn't intend it to be as bad as it came out sounding, but I must have been harboring some frustration at that moment and it came out in my post. Sorry.

However, I still think davidbeach has a point. Please read this faq, because it still seems that in order to offer any meaningful help we must "interpret" your meaning. faq569-1083

A question, jraef, do you know off hand if delta connected motors are any more or less prone to high currents as a result of supply imbalance than are wye connected motors?

Good question and I must say I never pondered it before. I don't see why (pun intended). In general, voltage imbalance causes current imbalance. Current imbalance causes negative sequence currents to flow between the windings, which in turn create counter-rotating torque in the rotor. This causes either a loss of net torque or, depending on the nature of the load, an increase in current for the same slip speed, which translates to higher operating temperature. I can't see how the motor being Wye or Delta would make a difference in that.

Maybe in the magnitude of the imbalance though because of all 3 phases sharing a common point in Wye? Would that distribute the voltage imbalance more evenly among the 3 phases instead of just 2 in delta? if so, It could mean that a Wye connected motor could have less deleterious effects from a voltage imbalance compared to a delta connected motor of the same voltage. Interesting thought. There are some other motor design people here with a better understanding than me, maybe they can chime in.

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http://www.jraef.com

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[jraef]

Nope, probably not after reading this, faq238-314. It reminded me that neg. seq. currents must flow equally in all 3 phases given the same impedence in the circuit, so wye or delta, it is what it is.

http:/

http://www.jraef.com

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[swn1]

Keep in mind that "negative sequence" voltage and current are a descriptive or analytical fiction. Convenient in context, but in reality what you have are unequal phase currents or voltages.

Also, the "symmetric components" treatment (pos, neg, and zero sequence analysis of three-phase power) assumes single-frequency system. Harmonics are a serious problem in many places and are not dealt with at all in that framework.

To address the original question: you can understand and quantify the effect by obtaining from a textbook or similar the equivalent circuit for a motor. To a first approximation, a motor is a voltage source in series with an inductor.

In a healthy, running, three-phase induction motor, the voltage source is the generator effect of the spinning rotor and its magnitude is a large fraction of the average phase voltage but its phase angle lags the line voltage phase. The current in each lead can be predicted by complex version of Ohm's law.