Stator/Rotor Resistance/Leakage Information

[SunRunner]

Does anyone know of a good site (or other publication) that contains typical motor data including Rotor/Stator Resistance/Reactance values as a function of HP?

I have to calculate motor torque/HP and corresponding pump performance when a induction motor is driven (by a VFD)beyond rated speed. I have been using Cochran's book Polyphase induction motors as the reference/methodology.

 

[BigLe]

Parameters of the motor are not a function of hp, but its intrinsic properties. There are ways to measure/calculate the parameters you mentioned and it involves usage of power meter and running the locked-rotor test, no load test, etc. There is an IEEE procedure described in some thread before may be you can find the doc number, I forgot here.. for an easier start, refer to

http://courses.ece.uiuc.edu/ece431/

. there is a lab manual link at the end of the page and you can find one experiment talking about how to measure the parameters step by step.. I believe there are other resources better than this if you search the web carefully.

 

[laundry]

hello sunrunner
the torque above base speed as no bearing on this matter it is your power that you need to look at, as your speed increses your power fauls linearly.so in general if you were to run a 5kw motor at 100hz then you would need a 10kw motor, all depends on what type of pump,and what you are doing with it hope this small bit of info will help.

 

[Skogsgurra]

Done some "laundrying" of the text above.

The torque above base speed has no bearing on this matter. It is your power that you need to look at, as your speed increases your power falls linearly. So, in general, if you were to run a 5kw motor at 100Hz, then you would need a 10kw motor. All depends on what type of pump and what you are doing with it. Hope this small bit of info will help.

Yes laundry. That is almost correct. But that part about how power falls linearly when speed is increased above base speed needs to be expanded on. For practical purposes, the power is considered constant above base speed. It is true, however, that the peak torque falls with speed squared and hence the peak available power falls linearly with speed.

But, since cooling normally gets better as speed increases, it is often possible to have a constant power output between base speed and sometimes up to twice that speed. It all depends on what rotor class (peak torque/rated torque) the motor has.

The concern about rotor resistance, stray reactance etcetera is not necessary here. (That is what text-books do to you when you are off guard). Just use published motor data. Permissible load as a function of speed is very often available. Have seen that for AB, ABB, Siemens and more. AB and ABB have particularly good sites. Visit them and look for products/AC motors/data sheet or technical guides.





Gunnar Englund

http://www.gke.org

 

[laundry]

hi gunner
Just to reply, operation above base speed is easily achieved by increasing the output frequency of the inverter above the normal mains frequency;two or three times base speed. the output voltage of an inverter cannot usually be made higher than its input voltage therefore the v/f characteristic is typically since v is constant above base speed, the flux will fall as the frequency is increased after the output voltage limit is reached. the motor flux falls in direct proportion to the actual v/f ratio. this greatly reduces the core losses, the ability of the motor to produce torque is impaired and less loadis needed to draw full load current from the inverter.thi is why i said an increase in motor kw would help.

 

[SunRunner]

Thanks guys for the timely replies. Let me clarify my question.

I work in the toxicological waste remediation industry where we are continually asked the preverbal “what if?” scenario. Given a motor/pump application where the information provided is just the motor HP. There are no motor torque curves available, no meter data, no Xd’, Xd”, stator/rotor resistances/reactances, magnetizing reactance, etc. Basically there is no motor information and I cannot enter the contaminated area to get meter data. My challenge is calculating the speed at which a given motor/pump application will stall.

I understand simple mathematical relationship that HP= Motor Torque(less F&W) * rotational speed and typical induction motors driven by a VFD operate in constant T up to rated voltage and speed, and constant power above that.

Please excuse the following analogy but it clearly illustrates the point. Let’s say that I want to follow the American trend of buying a travel trailer. I have two cars (which I don’t), one is a Mazda Miata, and the other is a Ford F350. I know the Mazda can go really fast, but has little torque, and the Ford has a lot of torque but cannot go really fast. Let’s assume for illustration purposes only, that both have same rated HP (Mazda has low torque and high speed, and the Ford-high torque but low speed). If I hook my trailer to the Ford, it will be able to pull it all day long at a rated speed. But if I connect it to my Mazda, there will be point where motor will stall due to the low torque.

I clearly understand the mathematical relationship for a typical induction motor (which is the most common motor we have installed in these high radiation areas). If voltage is proportional to frequency (as is the case when this motor is driven by a VFD) this would imply that torque means constantly applied flux, dependent only on absolute slip. The torque-speed curve is constant, dependent only on the difference between synchronous speed and actual rotor speed. This theory is valid for volts/hz up to rated voltage. Above rated voltage, speed and torque have an inverse square dependence on frequency governed by the equation (or a common deviation thereof) T=((3p/ns)(V^2*(R2/s))/((X1+X2)^2 + (R2/s)^2) (Beaty et. al, pg. 124).

Cochran in his book “Polyhphase Induction Motors” implies the slip at maximum torque is directly proportional to rotor resistance, that is, the greater the rotor resistance, the greater slip. And the R2/s ratio is constant at the speed where maximum torque occurs, and the other winding parameters determine the value of maximum torque.

So here I sit trying to calculate the frequency at which my 60hp motor, connected to a 60hp vertical turbine pump, inserted into a waste pool with a specific gravity of 1-2.5, will stall.

All that being said, my question was simply asking of a technical source that would be able to provide a range of values winding parameters for a 60 HP motor (or any other size motor)? Similar to the generic information found in NEC, IEEE 242, IEEE 141, IEEE 399, etc for other common electrical distribution components (ie. A 500 kcmil copper cable has an ampacity of 430 at 90 degrees C, resistance of approx 0.0258 ohms/1000 ft, etc).

I appreciate you guys taking the time to help me solve this problem.

-Sun Runner

 

[Skogsgurra]

Tricky situation.

I would go to the manufacturer and have him give me the data needed. Anything else would be guesswork - and that doesn't sound to be a good idea in your case.

One way out would be to connect an intelligent inverter and let it identify the motor. You will need some input data, though. Like HP, nominal voltage, frequency and speed. The inverter will then give you the "sigmas and the R:s".

If you can't go there. How do you do maintenance?

And - if this is a hazardous workplace, there should be documentation on everything - or someone will shut the place down. Have a look in your bosses book-shelves. She might have some information that she doesn't know about.

Gunnar Englund

http://www.gke.org

 

[SunRunner]

Gunnar,

You have arrived at my problem. Our company, for some bizzare reason, does not see the inherent value in engineering configuration control rather relies on its engineering staff to derive technical basis for continued O&M. I do not like it, but that is just reality. Oh, I forgot, its a government facility....

Motor Maintenance? What is that? I do not believe anyone here has done any maintenance - the thought process (due to the extreme hazardous environment) is run to failure because its cheaper (so they think)to replace the failed component than to periodic maintenance. We recently won the battle to get switchgear maintenance done! It has taken people getting killed at LANL and other to change things.

I've contacted GE, ABB, US Motors, Reliance, ect to get some generic motor data, but most are pretty tight lipped about the winding parameters. I think it is proprietary.

Now I am seeing help for others...

One more clarification of why I am having to calculate such a bizzare situation. The ultimate reason for the calc is to try to figure out the maximum spray leak of a toxic effluent if an operator was to inadvertantly drive the VFD beyond 60hz. So I am challenged to calculate the max Torque,frequency, speed (just below motor stall)as a function of the load (pump inertia, flow, head, speed, etc.). I would have never thought that as a EE I would have to do this. Its a challenge, and fun, in a sick sort of way.

-Sun Runner

 

[itsmoked]

Geesh sounds like an interesting job SunRunner. Where the govment has ponds of radioactive sludge with motors running to their noble deaths... Would this be in southern WA? Or does this type stuff go on at LANL too?

 

[SunRunner]

Man you're good! I knew that sooner of later someone would figure the location out.

Any ideas on where I may be able to get motor winding parameter data?

 

[electricpete]

It seems like you can make a lot of inferences from existing speed torque curves without knowing the exact parameters.

From your discussion I took it you are interested in slip at maximum torque (slip at breakdown torque). Call it s_Tmax

You can find s_Tmax from an existing curve at 60hz.
Many manufacturers pulbish their torque speed curves for new motors
For example Reliance you can search here

http://www.reliance.com/cgi-bin/mtrquery.pl

Reliance sells a 60 hp 460vac 3560 RPM base speed general purpose motor with performance characteristics given at

http://www.reliance.com/pdf/pdf/aced/W01027-A-A001.pdf

On page 2 from torque speed curve you can see max torque at 3350 rpm. You can calculate the corresponding slip (call it s_Tmax i.e. slip at max torque) as (3600-3350)/3600

Now can we correct this value to other frequencies? Yes. As you probably know, s_Tmax can be derived from the equivalent circuit.

s_Tmax = R2/sqrt(R1^2+(X1+X2)^2)

Now, since R1 << X1+X2 and also R1 shows up in quadrature to X1+X2, we can neglect R1 with very little error
s_Tmax ~ R2/sqrt((X1+X2)^2)
s_Tmax ~ R2/[2*Pi*f*sqrt(L1+L2)^2]

Now we can see that s_Tmax is proportional to 1/f.

You can calculate s_Tmax (and speed at T_max) at any other frequency from the 60hz value using this relationship.

If you are also interested in the value of max torque, similar approximation as above (neglecting resistance) will lead you to the conclusion that it is proportional to V^2 / Fsync. So you can correct the 60hz value to other frequencies using this relationship.

Important to note that characteristics (at 60hz) can vary widely among motors. For Tmax at least you know the minimum for NEMA motors from NEMA MG-1.

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[electricpete]

Whoops. On the torque you should correct it as (V/F)^2 (not V^2/F).

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[SunRunner]

Thanks Electricpete.

I'll give the information you provided a try.

I appreciate the time you spent in explaining the relationship. I know that there are many people out there who appreciate your efforts.

 

[acmotorengineer]

BE CAREFUL - published speed torques may not be actual motor torque - rather NEMA minimums or statistically factored.

AND: slip and torque at max breakdown is highly variable, difficult to measure due to rotor heating at this high rotor current. Consequently breakdown torque information is usually generalized or calculated.

Also: Some manufacturers publish equivalent circuit data for Vector motors. This data is required to properly program Vector drives.

AND: if you actually got the equivalent circuit parameters, they are actually converted to a standard criteria (particularly accelerating speed data) and are significantly different to any real operating point.