Motor starting calculation

[victa1288]

I am doing motor startng study. I have two sets of data from motor manufacturer.One is motor circuit data, which is impedance data. The other is motor characteristic data, which is speed-torque curve. ETAP software accepts both. I am wondering which set of data is more accurate for the motor starting study.

I was told by a senior guy that I should use the speed-torque curve. But I personally think the motor circuit data is more accuarate for the study. Because the motor circuit data determine the speed-torque curve and speed-torque curve shape is based on defferent test conditions.

But I am not sure if my understanding is right. I have to convince that guy to accept my idea. Who have a better understanding of motor starting.

I apprecite it for your great idea.

 

[Alex68]

I will try to convince you that the senior guy was right!

All the calculation prg use more or less the same motor model. But only "more or less".
Starting from the measured values, that means voltages, currents, pf, torque, etc, you can obtain your model with some formulas. These aren't exactly the same in all books. For instance, some books reports the rotor resistance as a slip function.
Thus it is not sure that you can use the proposed model parameters in your prg obtaining the correct motor behaviour.

The aim of your study is to reproduce, to simulate the motor behaviour. So you should input the measured parameters and let your prg to reproduce them as better as possible.

 

[davidbeach]

I would say that if you have the motor impedances, and the program (I've never used ETAP) can accept motor data as impedances, there is more information in the impedances than there is in a speed-torque curve. The speed-torque curve can be derived from the impedances, but not the other way around. Also, having the impedances means you don't have to try to pick points from the speed-torque curve.

 

[jraef]

In my opinion it would depend on the connected load. I use SKM when analysing reduced voltage starting and have never used anything BUT the torque-speed curve. These software programs are best when matching the motor's capabilities to the load's torque-speed requirements, which is not necessarily linear. The motor equivalent circuit is good for analyzing the effect on the electrical circuit, but if the motor comes close to stalling or stalls as a result of a reduced voltage start, the circuit analysis is pointless. If on the other hand you absolutely know for a fact that the motor will start successfully, then I suppose the equivalent circuit analysis would be OK, but I don't know if it would be better.

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

 

[electricpete]

Usually when you get a torque speed curve you get a current speed curve as well. If you have both of those I would use them (ask for current speed if you didn't get it). Torque-speed and current speed are directly what your program needs.

Equivalent circuit parameters are a little further removed from what you need. You can certainly calculate torque/speed and current speed from equivalent circuit parameters but then you have to make sure that the conventions your ETAP program uses are exactly the same as what the manufacturer is supplying you.

I think that by and large manufacturers estimate equivalent circuit parameters and derive torque speed curve from there. But there may be more to it than just that. Consider this fact:
You can not ever get a curve which has a local minimum between locked rotor torque and breakdown torque using the standard linear equivalent circuit. Yet many motor torque speed curves from the manufacturer have this feature. That leads me to believe there may sometimes be more than equivalent circuit parameters which goes into developing the torque speed curve.

Based on these factors I would be more comfortable using torque-speed and current speed directly rather than going to some more abstract equivalent circuit parameters and trying to infer the correct torque-speed and current-speed curves.

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