Transient torque calculation 1

[Gyo]

I have read some online references on this topic and the method of calculating the torque and current under short circuit, reconnecting and starting consist of loads of calculus and numerical methods. Is there an easier way to do it so that I could use Excel to display the numbers and plots? I am not brilliant in maths.

 

[electricpete]

These are inherently complex phenomena. A dynamic model with 5 state variables is pretty common to represent the motor. That asssumes the entire mechanical system acts as a single inertia... if mechanical system has a relevant spring (like coupling between motor and machine) then additional state variables are required.

There is a thumbrule, something like 20 - 30 times full load torque is max expected for most transients if I'm remembering right. I don't know how universal that is.

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(2B)+(2B)' ?

 

[electricpete]

clarification

something like 20 - 30 times full load torque is max expected for most transients

should have been

something like 20 - 30 times full load torque is max expected /worst case electrical torque for most transients

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(2B)+(2B)' ?

 

[7anoter4]

You can try this:

http://www.measurlogic.com/Resources/Paper3_book.pdf

 

[electricpete]

LPS for that - I hadn't seen that type of analysis. It sort of makes sense we could try to estimate a worst case from the equivalent circuit, although I'd have to give some more thought to understand what the did.

Let's try some worst-case numbers for reclosing using equation 10

let’s say Es = 1, Em = 0.8 (residual), X1 = 0.05,
M = 1*0.8/0.05*(0.866+sqrt(1+0.8+0.8^2) = 38.
I guess that's believable for a worst case (although we still made some assumptions).

let’s say Es = 1, Em = 0.3 (residual), X1 = 0.05,
M = 1*0.3/0.05*(0.866+sqrt(1+0.3+0.3^2) = 13.5

So even if limit the resiudal voltage to 0.3, still get possibly 13 times rated torque?
I wouldn't have guessed that high.


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[electricpete]

X1 = 0.05 was my guess at a reasonable motor stator leakage reactance. I think values 0.02 - 0.10 are typical for large motors.

It strikes me that the stator leakage reactance plays a role in limiting the post-closure current (because it is the impedance connected between the terminal voltage and the magnetizing branch in the transient equivalent circuit). From that standpoint, it seems like it should probably include some power supply impedance also....i.e. what is the impedance between the motor and what would be the infinite bus at voltage Es. For a motor whose terminal voltage drops by 20% during start (~5*FLA) that might be in the neighborhood of 0.04 pu reactance. I'm assuming this reclosure resembles a balanced three phase fault (like a motor start) in the sense that the positive sequence source impedance is the one of interest... not completely sure if that would be the right assumption.

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[7anoter4]

Thank you electricpete for your attention. Always I thought retirement is a period you will have plenty of time of doing nothing. But ,in the contrary, the free time is shrinking dramatically! So, all my intentions to study new theory and to deep the old one are vanishing.
I am thankful to you comments and I'll try to find other source of "inspiration", also.

 

[Gyo]

Many thanks all for giving your thoughts and comments.

 

[electricpete]

7anoter4 - You must have had a long rewarding career in electrical engineering. As long as you keep finding time to come to eng-tips to share your knowledge/experience, I'll be grateful.

I'm curious, what did you do before you retired?

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[7anoter4]

It will sound as a "curriculum vitae" nevertheless I don't intend to find- never- another job.
I graduated with–what in Europe it is defined as "engineer diploma" and it is equivalent to MS according Bologna Accords-in Construction of Electrical Machines and Apparatus –including transformers , low and high voltage apparatus[breakers, disconnectors and other].
I started to design low-voltage ac and dc machines as induction motors, dc generator for galvanizing and other-non standard-for special applications- including technology and materials.
After a while I started in a cable company –equipment purchasing and installation, putting into operation, technology and all other technical problems.
But, as you know, in industry the salary was low and I got a proposal to work in
a electrical design department of a utility company.
I remember my first project: it was to choose an induction motor for a big installation.My principal order: "you have to design an induction motor for this installation" and I ask why it cannot be a standard one. The answer was "no". So after 3 days of intensive work I presented a thick file:3 alternatives of complete design-electrical and mechanical- included technology and tools to make a prototype. The chief was very surprised :"You have to find only a suitable one from a catalogue[not from our already prepared list-what we call it "standard"].
"Sorry chief ", I said "in my world it is called "choose a motor" not "design a motor". I chose it in the first half an hour".
Few years later I got a new job: a small utility field department chief. However, after a while I got another job as Manager of Electrical Department in a large steel complex-plenty of 110 kV paper-low-pressure oil cable, 40-65 MVA transformers, 4 gas-turbine and 2 diesel-generators and 10 square km of stuffy installation-power and -all around- analogue control.
The last 20 years I participate in a large power station design company –mainly electrical physical design for fossil power units 150-650 MW and combined cycle gas turbines.
Many thanks again, electricpete, and I apologize for the late answer and long explanation.