Motor transient simulation û basis for assumption that Vn is 0?

[electricpete]

I see across a wide range of literature that motor transient simulations are performed under the assumption that (as long as the supply voltage is balanced), the machine neutral voltage is equal to the supply system neutral voltage. Specifically the balanced phase-to-neutral voltages of the supply (Van, Vbn, Vcn) are assumed to appear individually across each leg of the wye connected machine winding. But I don’t see any discussion of the basis for the assumption (Krause’s book uses the assumption throughout without ever once addressing the basis for the assumption).

Under sinusoidal steady state conditions, we can easily verify the assumption is correct by impedance analysis.

But under transient conditions, we have a non-linear system (torque is the product of multiple state variables). No such linear impedance analysis is possible. What form of proof or verification is available for that assumption that the machine neutral voltage remains equal to supply system neutral voltage. What are the prerequisite conditions necessary to assure this assumption is satisfied?


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[waross]

Hi Pete;
Remember that an induction motor is also an induction generator. That is the source of the back EMF.
The back EMF is what holds the neutral at close to zero volts.
One of the reasons for the high current unbalance that results from a much lower voltage unbalance may be the action of the inherent induction generator trying to the best of it's capacity to balance the terminal voltages.

Bill
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"Why not the best?"
Jimmy Carter

 

[electricpete]

Thanks Bill.

I like that description of the reason for unbalanced currents in presence of unbalanced voltages – makes good sense. And provides one framework for viewing the induced voltages.

I would certainly welcome any further intuitive explanation of the basis for the assumption but I am leery of using intuition to support assumptions regarding transient analysis. I do think most of our intuition about induction motors comes from the steady state equivalent circuit or associated sinusoidal steady state concepts. There were several examples of things in the DOL start thread where the transient behavior completely defies our intuition because it is not predicted by the induction motor steady state model.


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[electricpete]

It would be interesting to monitor the neutral to ground voltage during a dol start. Does anyone have easy ability to make such a measurement?

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[electricpete]

... to clarify of course I meant the ungrounded machine neutral / wye point, not any system neutral.

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[waross]

If you connect a 9 lead wye motor for lower voltage operation, one of the two wye points is easily available for monitoring. The other wye point should be the same potential.

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