Induction Motor Rotor/Stator Time Constants 3

[goldenon]

I'm doing some research on the time constants associated with the induction motor. Currently I'm using a simple model of the induction motor to determine the time constants associated with the rotor and stator windings and was looking for some general information.

In general, does the rotor have a smaller or larger inductance than the stator? What about the resistance?

I'm specifically interested in a motor between 75-100HP, 380-420V. From what I have found the inductance can range from a few mH to hundreds of mH. Also the resistances tend to be fairly small, 0.1-10 Ohms. Am I in the ball park here?

Any input on these parameters or general information about induction motors would be appreciated! I'm a newbie.

 

[davidbeach]

A good electric machines text will answer that and the myriad questions those answers will generate. And then another iteration or two.

 

[goldenon]

Well I've search a lot of texts online for several days and have not come across an answer. I figured it was elementary. I also figured if it was common knowledge, someone would help point me towards a text or an answer.

 

[davidbeach]

Check out faq238-1287 and look at some of the motors/drives books. Lots of variations between motors. The basic motor equation is fairly simple to write out, but it will take a whole lot more to describe all the implications.

On the other hand there are some folks here that quite happy to go into great gory detail; you'll just have to wait for them.

 

[goldenon]

Those introductory books do have a few examples, but they seem to be specialized cases where the rotor and stator have the exact same windings. I'm not sure that reflects real world motors. So I was wondering if anyone knows of general trends (larger rotor inductance than stator). Also, Is my intuition wrong here, it seems that the inductance decreases with the size of the motor, is that true? I know that these design parameters can differ wildly from motor to motor, but are there any general characteristics?

I welcome specific or general examples.

Thanks

 

[electricpete]

If you don't mind, I'm interested to know the purpose of your query.
Trying to estimate motor parameters for purpose of simulation?
Evaluating minimum off time before reenergization?
something else?


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(2B)+(2B)' ?

 

[waross]

When you are looking at the rotor you may wish to consider inductive reactance and effective voltage as well as inductance and line voltage.
At standstill, the rotor frequency, rotor inductive reactance and effective voltage are maximum.
For a 60 Hz, 1760 RPM motor, the rotor frequency at standstill will be 60 Hz and the effective voltage will be dependant on applied voltage.
When running at 1760 RPM the rotor frequency will be related to the slip frequency which may be calculate by 1800 RPM - 1760 RPM.
That works out to a rotor frequency of 1.33 Hz.
The inductive reactance will be reduced by a factor of 40RPM/1800 RPM. or 0.0222 of the inductive reactance at standstill.
The effective voltage driving current through the rotor impedance will be applied voltage minus back EMF.
Losses are neglected.
Note also that these conditions apply at full load on the motor. With lighter loads or with overloads the speed will change and rotor frequency,(slip frequency), and back EMF will change. Hysteresis and I²R losses will introduce non-linearities that may be most apparent at light or no load conditions.

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

 

[goldenon]

I'm doing simulations. I am interested in the transients associated with scalar control method (constant Volt/Hz). I'm trying to determine the performance degradation for certain loads. I believe the transients associated with constant Voltz/Hz is determined by these parameters. However, I'm currently using a very simple, DC model, of the induction motor. So I'm not concerned with the reactance or frequency dependence. Just the DC transients of the motor.

 

[electricpete]

I have a spreadsheet that I use to estimate induction motor equivalent circuit parameters
It was discussed here to some extent as I was beginning to develop it.
thread237-250022

I have attached a more recent version to my post here in your thread.

This basic approach is to use excel solver / optimizer to try to find (guess) the set of equivalent circuit parameters that best match the information that you know about the motor (targets).

After each optimization you can view sum of squares comparison of model predictions against targets to see how good is the match.

You can also adjust the weight of each target within the optimization. For example in your case I don't think you are interested in anything related to DOL starting, so I would weight all of the starting performance targets as 0 (and by the way these starting targets can influence the model heavily because the effective L2 and R2 change between running and starting).

I think it's a pretty good approach and I've used it many times and validated it against other data when needed enough to satisfy myelf. Sometimes the information on the nameplate of a motor is more than enough to get a reasonable model. If you have a motor data sheet with tests at no-load, 25%, 50%, 75%, 100% it is even better. More inputs are usually more robust. Unfortunately I'd also say it requires a little TLC to get a feel for how it acts. Some sets of inputs (targets) will converge easily to a single (and presumably correct) result. Others sets produce highly variable results depending on initial conditions or small changes in targets. Explore the sensitivity of the model.

It may not be explained well enough, I really don't know. I also don't know if something like this is vaguely in line with what you're looking for.

By the way for your question about L1 vs L2. I often use a thumbrule L2/L1 ~ 1 (all parameters referred to the stator). It doesn't have a strong basis so often I weight it low. For each of the thumbrules you can test to see what impact it has on the model. I only impose strong weight on the thumbrules when I don't have enough other info to get a good model.


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(2B)+(2B)' ?

 

[7anoter4]

Very interesting program! Thanks, electricpete.