How to measure induction motor efficiency? 1

[shenyee]

Dear All,
Recently, I try to collect a set of data from PWM controller that control the speed of a three-phase induction motor. The problem is how to measure the efficiency of the motor. It's related with the electrical power input and mechanical power output. That's no clear description on how to measure.
I'm still not clear with this. Wish anyone here with experience can tell me in more detail about any equipment or alternative I can get the measurement. Thanks a lot!!

 

[rbulsara]

Take the motor and controller to a test lab with dynamometer! There is not other scientific way that can be rigged up in field that I know of.

 

[UETS]

The method of motor efficiency calculation utilized by PdMA Corporation is a
unique and accurate proprietary method of efficiency calculation tested to
within 2% of torque cell measured efficiency at Advanced Energy Labs in
North Carolina. Utilizing nameplate information we determine the slope of
the relationship between the input power to, and output power from the
motor. From there we identify the input running characteristics at the time
of the test and calculate the output values. Once the output and input
values are known, the efficiency can be determined. PdMA motor testers Emax or MCEmax can determine the efficiency of any motor.

Visit

http://www.pdma.com

 

[jbartos]

Suggestion: Visit

http://www.reliance.com/pdf/brochures/raps_913.pdf

for a Motor Efficiency Vizard

 

[UKpete]

see recent discussion in:

thread237-67726

 

[electricpete]

Estimating output power from speed using linear relationship with slip is a simple and crude method which can be done by anyone with a stobe or other means to determine speed. Hardly proprietary nor sophisticated no matter what some vendor may suggest. And hardly accurate. UETS forgot to mention that NEMA allows motor nameplates rounded to nearest 5rpm... figure out what that does to your accuracy and you can toss that 2% number out the window.

 

[electricpete]

I double checked my facts in NEMA MG-1 1998. Didn't see the 5rpm but I found this:

"12.46 VARIATION FROM RATED SPEED
The variation from the nameplate or published data speed of alternating-current, single-phase and
polyphase, medium motors shall not exceed 20 percent of the difference between synchronous speed and
rated speed when measured at rated voltage, frequency, and load and with an ambient temperature of
25°C."

So using a 20% accurate parameter (slip), we expect to get a 2% accurate measurement? Hmmm. Makes you wonder about all those other claims those guys spew.

 

[jbartos]

Questions to the previous posting: What about

http://www.reliance.com/pdf/brochures/raps_913.pdf

a Motor Efficiency Vizard?
Have you checked a source code?

 

[electricpete]

jb - are you saying that you believe that slip which is required to be no more than 20% accurate can be used to accurately predict power or efficiency to 1 or 2%

uets - I didn't mean to be disrespectful. If you are associated with a product you should identify that. If you can show where I am mistaken I would be interested to hear.

 

[jbartos]

Clarification to the previous post marked ///\\///Please, notice that the original posting does not address the slip variable. The slip was introduced in:\\electricpete (Electrical) Sep 13, 2003
Estimating output power from speed using linear relationship with slip is a simple and crude method which can be done by anyone with a stobe or other means to determine speed.
///I am sorry, I do not mean to be disrespectful. However, to try to reduce the original posting topic that appears to have an answer and solution to a specific variable, i.e. slip, seems to be rather limited or misleading.\\

 

[electricpete]

jb - I was not responding to the original post, I was questioning the discussion of the pdma device, which I believe determines output power based upon slip.

Your link:
"All electrical measurements can be made at the motor
disconnect point, without the need to open the motor conduit box. Lab tested accuracy to
within 1%....
By measuring the input power, voltage, current, speed, temperature, frequency, and
resistance at two motor load points, the Motor Efficiency Wizard provides accurate
efficiency data.."

I don't see any way that this device can estimate output power except by using speed (slip), with corrections for voltage etc. If there is a more detailed description of this or the pdma box which explains how it can be done, I would be interested to see it.

Out of curiosity, if they don't use slip to estimate output power, then why do they bother to determine speed?

 

[jbartos]

Suggestion: The measured Performance Set of Parameters includes:
Output Power HP, Input Current, Power factor, Efficiency, Speed RPM, Output Torque.
Verifying the first line speed, hp and torque:
5.5HP x 746 Watt/HP~1199RPM x 0.105 (rad/sec)/RPM x 24.2 ft-lb x 1.356 x N-m/ft-lb
4103 Watts ~ 4131.3 N-m/sec
which is about right with .007%=100%x(4131.3-4103)/4131.3 error
This is where the calculated speed is also needed.
The nameplate speed at rated load is 1185RPM
and the calculated speed at 5.5HP is 1199RPM. This is off by 14RPM, and at about rated load, 77.5HP, the calculated speed is 1185.5RPM. This is very accurate result with respect to 1185RPM motor nameplate speed. There is no slip mentioned, and expressed explicitly. Also, I do not see any acknowledgement to EPRI there.

Generally, the efficiency EFF may also be calculated over electrical losses in the motor since:
EFF=Pout,watt/Pin,watt=1-Ploss,watt/Pin,watt
If Ploss,watt is calculated over the motor parameters, not necessarily over the motor speed, then the EFF is known.

 

[electricpete]

jb - The purpose of your analysis seems to be to show that the torque and horsepower which are calculated by the electrical box are consistent with the measured speeds and nameplate speed. I see no reason to be surprised at this. If speed is a part of their calculation then their results will be consistent with speed. You will notice that nothing on the sample report relates to an actual measured torque or speed. It is only mentioned that the device is 1% accurate based on dyno tests, without any supporting data (how many motors did they test?).

UETS - If the claims are true, I believe this would be a truly relevant device for the purposes of measuring efficiency. As we know, even the best dyno labs have been shown to differ up to 1 or 2 efficiency points when measuring efficiency during round robin tests. But it is logical to be skeptcial about this method. Why is it not addressed in IEEE 112 standard methods for calculating efficiency? Does the pdma device require measuring electrical parameters at two different current levels as well as shutdown (like the Reliance?)

 

[electricpete]

I think I can see how the Reliance machine gets close.

Assume for simplicity balanced system which can be analysed from single-phase equivalent circuit, we have unknown parameters R1,R2,L1,L2,Lmagnetizing,Rcore (Rmag represents core loss).

We measure R1 directly, so it is a known quantity.

Now we have two sets of measurements of V, I, s under two different loading conditions.

Each set of measurements gives one complex (phasor) equation, equivalent to two scalar equations.

Two complex/phasor equations are equivalent to four scalar equations... almost enough to analyse our 5 remaining unknowns (R2,L1,L2,Lmag,Rcore). Perhaps there is an assumption L1=L2 or something similar to complete the problem? Any comments on what assumption might give the last required scalar equation?

Unbalance introduces one additional complexity which can be handled. .... we would assume calculate the torque created from the positive sequence components and subtract out reverse torque created by any negative sequence components.

 

[electricpete]

I thought a little more about it... the equivalent circuit above models I^2*R losses core losses, but not stray losses and friction/windage.

Stray losses it seems are the most difficult to deal with. You can certainly establish total losses including stray losses with a dyno test. But other IEEE 112 methods allow stray losses to be estimated.

Let us focus on friction and windage: from the motor electrical and speed standpoint, there is NO way to distinguish friction and windage from actual load. If I put in a lousy grease which increase friction, the response will be identical to an increase in load.

The simple un-deniable fact is that the pdma and Reliance box cannot distinguish true mechanical load from motor friction and windage. They must rely in some manner on the nameplate data, which may be an inaccurate condition of truth at the time of motor manufacture, and even more so at some point after years of service. After all, if we believe the motor nameplate data then why do we need the testing?

 

[electricpete]

To the original question: IEEE 112 discusses induction motor efficiency testing.

What is the size of the motor? Smaller motors and drives are more frequently tested with dyno.

Can the motor/drive be pulled out or do you have to do it in place?

There was a post very recently that discussed some kind of torque measuring devices suitable for in-place use.

 

[electricpete]

see rewindr's comments 8/18/03 at thread237-67726 regarding shaft-mounted strain gage which may be suitable for use in-place.

That appears to gives you a means to measure the output (torque) of the motor, which is the difficult quantity to get at.

The input power should be fairly easy to measure using suitable equipment. If you meausre input power to the drive you capture losses in the drive, cables, motor.

 

[electricpete]

My realier message today to jb incorrectly used speed where should have been horsepower: Should have been "...You will notice that nothing on the sample report relates to an actual measured torque or horsepower..."

 

[shenyee]

Thank you for all of you that reply.