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Converting motor performance data into Eq Ckt parameters
4

Converting motor performance data into Eq Ckt parameters

Converting motor performance data into Eq Ckt parameters

(OP)
I  see there are a few people on the forum who do motor simulations on the forum.   ijl, zolka...  maybe more.    This forum seems to be a good place to discuss simulations (I don't know of any other forums where it is discussed ... does anyone?).   Also there was mention of a free software ATP to do simulations.

Unfortunately, a prerequisite for doing a simulation is determining the equivalent circuit parameters R1, R2, X1, X2, Xm.

I don't see a lot of discussion of this topic in textbooks.  The approach will vary depending on the available performance info that you are trying to match.    I tend to think it's easy to match a small number of performance data points  (we can throw in assumptions like X1=X2 if the problem is underspecified),  but the results are not too accurate.  As we try to match more and more performance data, it gets tougher, but the results are more accurate I believe.

Attached I have tried to do a match of data which iainuts posted 16 Jul 09 9:49 in thread237-249262: Low power factor, recip pump

That data sheet is included in tab "data sheet" of attached excel file.

In the attached excel file I have used the excel "solver" tool to find the equivalent circuit parameters which match the targets best (minimize the sum of the square of the errrors between model calculation and target).

I tried to make the spreadsheet self-documenting and self-explanatory.  The calcualtions are in the "main" tab.  I have provided an overview in the discussion tab along with discussion of results.

I will post the results here in the thread text, since I am not sure whether you guys will see any calculated values if you don't have the analysis tookpack loaded into excel.  

The solution of the circuit parameters was:
R_1    0.0298
X_1    0.1831
R_2    0.0326
X_2    0.1760
X_M    7.09
R_M    1.2116
FullLoadSlip    0.0125


Performance of equivalent circuit using these parameters is:
Performance Measure    ||    Calculated Value    ||    Units    ||    Target Value
FullLoadAmps    ||    111.12    ||    Amps    ||    113.00
FullLoadEff    ||    0.97    ||    none    ||    0.95
FullLoadPF    ||    0.90    ||    none    ||    0.87
FullLoadPower    ||    73203.09    ||    watts    ||    74600.00
HLEfficiency    ||    0.96    ||    none    ||    0.96
HLPowerFactor    ||    0.81    ||    none    ||    0.82
LRC    ||    737.40    ||    Amps    ||    675.00
LRT    ||    268.38    ||    N-m    ||    328.22
NoLoadCurrent    ||    36.00    ||    Amps    ||    28.00


As stated in discussion tab,  since deep bar effect is excluded we expect R2 to be higher during locked rotor than running conditions.  This is not included in the model and  I have "weighted" the locked rotor performance parameters low, so that the resulting equivalent circuit parameters better reflect running conditions than locked rotor conditions.  (I intend to use the parameters for simulation of transients during running conditions).

I am interested if anyone has any comments on this approach... suggested improvements... other suggested references or approaches for tackling the problem of converting motor performance data into equivalent circuit parameters.
 

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RE: Converting motor performance data into Eq Ckt parameters

(OP)
My discussion of variation of Rrotor was wrong in the "discussion" tab.  Rotor resistance is higher during start than run.  

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RE: Converting motor performance data into Eq Ckt parameters

(OP)

Quote:

My discussion of variation of Rrotor was wrong in the "discussion" tab.  Rotor resistance is higher during start than run.   
But the conclusion is correct. i.e. the discrepancy in the locked rotor performance results would tend to be cured if we had a higher resistance at locked rotor conditions (would cause LRC to decrease towards target and LRT to increase toward target).

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RE: Converting motor performance data into Eq Ckt parameters

(OP)
Results of simulation driving high inertia load are not very consistent.   Especially breakdown torque is much higher on my simulation.  Something is not right somewhere.

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RE: Converting motor performance data into Eq Ckt parameters

(OP)
I found one error.  I have inverted the conversion factor between ft-lbf and newton-meter.  This shows up in the locked rotor torque target which should be a lot higher.  

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RE: Converting motor performance data into Eq Ckt parameters

(OP)
As mentioned above, I did a simulation to try to check the equivalent circuit parameters.  Torque vs speed and current vs speed curves were generated by simulating the motor starting from rest to full speed while driving a no-torque high-inertia load. (the high inertia load should eliminate most transient effects)> Results are attached - roughly as expected except for breakdown torque which is easily 30% different than the data sheet.  

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RE: Converting motor performance data into Eq Ckt parameters

(OP)
I have made a few changes:
1 - use Thevinin approach to streamline the caluclation.
2 - fixed the torque target erro
3 - fixed an error in the half load calculation
4 - added breakdown torque.

This is still an error in there. If I look just at the full power results, they are not self-consistent as suggested in the check on rows 28-31. Still working on it.  If anyone sees the cause of that discrepancy let me know.

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RE: Converting motor performance data into Eq Ckt parameters

(OP)
Attached is the same question posed in a more readable form. For the specified circuit parameters and slip, I calculate two different values of Pout depending on calculation method.  Which method is wrong?

The circuit parameters:
R_1    0.024621
X_1    0.297346
R_2    0.029392
X_2    0.287261
X_M    7.668602
R_M    1.849573
Slip    0.011928

One calculations gives Pout ~73000watts and the other gives Pout~78,000 watts.  There has got to be an error in there somewhere.  

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RE: Converting motor performance data into Eq Ckt parameters

(OP)
I wondered if RM might be causing a problem.  I set RM to 0 and the discrepancy decreased, but not nearly to 0 - it does not resolve the discrepancy.

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RE: Converting motor performance data into Eq Ckt parameters

(OP)
I've got to multiply the losses by 3.

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RE: Converting motor performance data into Eq Ckt parameters

(OP)
The loss calculation is corrected in the attached.

There is now very good agreement for the performance data near full and half load.  I think I am done with it.  

... unless there are comments/suggestions on the approach.

 

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RE: Converting motor performance data into Eq Ckt parameters

If this is as worthwhile as you appear to think it is, perhaps you should post it as a FAQ, in which case, you can edit the FAQ, and not have a string of replies that you'd have to bump anyway.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Converting motor performance data into Eq Ckt parameters

Great suggestion IRstuff.

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

RE: Converting motor performance data into Eq Ckt parameters

(OP)
I stated that this is a prerequisite for motor simulation (transient).      The importance depends on your interest in that subject.

My apologies if the forum was inconvenienced.   The bumps occurred over a relatively short time on a slow time (weekend)  when there were not many other active threads, and I assumed most people would not re-open the thread if they had initially determined the subject matter was not of interest to them.

Next time I will keep the number of posts lower.
 

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RE: Converting motor performance data into Eq Ckt parameters

I was merely intimating that you can edit your own FAQs, so you could keep a clean posting, and bumping wouldn't be necessary, because FAQs are way easier to find than a random thread...

TTFN

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RE: Converting motor performance data into Eq Ckt parameters

(OP)
Thanks IRStuff.    I'll take your suggestion at face value and consider posting this as a FAQ when I have it finalized.

I will mention this estimation of equivalent circuit parameters has a variety of other uses beyond simulation.... Over the weekend we had a small fan motor that tripped on instantaneous when it was stopped and restarted after an unknown period of time around 1 second or less.  The question was asked how long is the minimum off time before restart.  NEMG MG-1 provides an answer that the motor should be off for at least > 1.5 times the open circuit time constant (slow/transfer reclose) to allow the residual voltage to decay.   As it turns out based on troubleshooting over the weekend we had captured data including voltage decay recording on load-side of contactor after disconnect for this particular motor which allowed to determine the open circuit time constant very well.  But if we didn't have this data available, we could have  estimated the open circuit time constant from the equivalent circuit parameters (Tau_oc = [L2+LM]/R2), which in turn could be roughly estimated from nameplate info and torque-speed/current-speed if available.  

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RE: Converting motor performance data into Eq Ckt parameters

2
Interesting and useful approach to an important problem. As electricpete mentioned, it seems that the "standard" equivalent circuit is not accurate enough. Another problem may be the (un)availability of a detailed data sheet.

I have used a little program motdatpr (see http://pp.kpnet.fi/ijl/prog.htm) for my simple simulations. It calculates the parameters of the equivalent circuit from the values at full power, plus the locked rotor current ratio. ATP, or actually the drawing program ATPDraw includes a utility, WindSyn for the same task.  

RE: Converting motor performance data into Eq Ckt parameters

(OP)
Thanks ijl
I looked at motdatpr
The problem is underspecified from a standpoint of unknowns and equations.
There are 5 unknowns: L1, L2, LM, R1, R2
There are 4 equations:  1 - power at full-load slip,  2 - power factor at full-load slip, 3 - efficiency at full load slip, 4- current at slip=1 (locked rotor).

Their resolution was to arbitrarily declare X1=X2 to get the last equation which seems a fairly common assumption.

I was able to use my spreadsheet and get very close to the same results as motdatpr by assigning a weight to thumbrule X1=X2.   At least it's a double check there don't seem to be any large errors in my calculations related to  LRC and to full load power, power factor, efficiency (compared to motordapr).

I think I need to work on the interface a little bit.  I should have just the inputs and outputs (no calcs) on one sheet.  Should have a button to trigger a macro to set up solver.  Should put voltage and synchronous speed in more obvious place.  Should put slip range in a more obvious place.  If was really energetic I would add torque speed curve points perhaps at slip 0.25, 0.5, 0.75, 1.   Same thing for currents.  Any parameters which are not available to match or not desired to match can be turned off by setting the weight to 0.   

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RE: Converting motor performance data into Eq Ckt parameters

ijl - a lps for that useful link. need to try it when I find some time.

pete - look forward to your modelling. your math skills are impressive. good luck.

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