measuring motor efficiency 3

[polaris12]

Hi,
I'd like to test thyristor based motor controller (measuring voltage zero crossing to regulate the current flow) on the on 3 phase induction motor. What would be appropriate equipment and what electrical and mechanical parameters I have to measure to obtein the most reliable assess of efficiency and possible savings. Please, recommend some non-costly meter or device. The motor 37kW is in the field and I would like to find some less intrusive but still reliable method.

 

[jOmega]

If you are looking to do this on the cheap... suggest you forget it.

There is no simplistic method for making such a determination. You will have to spend the $$$$$$$$$ to obtain the necessary equipment.

First you need to measure the power into the motor at the fundamental frequency... that is, the Watts input.

Next you need to measure the mechanical power out of the motor and that is done with a dynamometer... with a torque sensor to measure the shaft torque and speed...that runs in the neighborhood of >$10+ K .... not incuding the instrumentation etc....

 

[electricpete]

IEEE112 has some great info.

 

[UKpete]

On the electrical side, the voltage and current waveforms are not sinusiodal so you must use a TRUE RMS instrument with good frequency response (say, up to 5 or 10kHz) so that the harmonic power is included in the measurement. A good budget power analyzer that I have used is the Voltech PM300, see:

http://www.voltech.com/products/pwr_anl/index.htm

- but even this will cost perhaps US$2000. You will also need three CT's (current transducers) of adequate current rating and frequency response to use with the analyzer, these won't cost that much, see:

http://www.lem.com/

More difficult is the torque measurement. JOmega is right, I don't think this will be cheap or easy to apply in the field.

 

[polaris12]

Thank you for your helpful answers! Maybe with measured Input Power (some Power Analyzer) and Slip Method for Output Power we may expect ratio: Efficiency with motor controller on to Efficiency with motor contr. off aproximately correspond to the real ratio of these two values (presuming that errors will affect results the same way)? Unfortunately, we must admit low accuracy, so what would be the simple, non-intrusive method in that case?

 

[ScottyUK]

I agree with UKpete's recommendation of Voltech power analysers. We have a PM3000A which is exceptionally versatile, but they ain't cheap. Suggest you hire an analyser from your local instrument rental company.

Can't help with torque measurement, other than to suggest that one of the universities might have a dynamometer rig which you could rent time on.

 

[rewindr]

Binsfeld Engineering makes a shaft mounted battery operated strain gage torque measuring device I have used successfully to measure output torque of a round shaft. The equipment cost 3-4K.

A watt meter can be used to measure input although we have used more sophistated instrumentation.

Results appear to be accurate to around 99%.

Obviously if you know the electrical input and the mechanical output the efficiency is calculable.

In addition to efficiency studies, I have used this equipment to determine the output of a diesel engine in order to size a replacement electric motor. Also if your equipment is fast enough with a high sampling rate you can see a torque sine wave or determine locked rotor torque useful for estimating inertia.

You can isolate electrical vs. mechanical issues, like where the gearbox guy says it's not his fault and the motor guy says the motor is OK.

When one knows the torque the answer becomes obvious.

 

[electricpete]

That looks like an interesting device.

Two quick questions:
1 - Do you know if they publish the accuracy?
2 - How is imbalance addressed? Is there a counterweight on other side of the shat?

 

[electricpete]

3 - Is there additional cost for the receiver unit that accepts data from the shaft-mounted transmitter?

 

[UKpete]

may I add another - what shaft speeds does it work up to?

 

[polaris12]

By the way, what's your opinion about TRIAC constant speed motor controllers (can we expect significant savings - are they cost effective, how they affect other equipment, what's with harmonics etc)? What different type are avalable? And, could you recommend some if your opinion is positive? That will help a lot, thank you in advance!

 

[plc123]

Hi,
there should be formula from motor manufacturer
that shows heat exhange dependance ( motor T, air flow speed, ambient Temp.).
All effectiveness questions coming to
ratio: what portion of cosumed active energy coming to heat.
If you have active energy reading and calculated
heat dissipation, then efficiency is available.

Thanks,
Vladimir.

 

[electricpete]

Here was one thread on Power Planner which is a NOLA type device that attempts to save power by reducing voltage seen by the motor under low-load conditions:
thread237-56540

Note the links to MarkE's page... he has some very good info on it.

We have discussed it appears sometimes the claims of these devices are hyped up beyond realistically-achievable gains. The most benefit is possible for smaller motors which are typically inefficient. And the gains only occur when the motors are lightly loaded.

 

[polaris12]

Electricpete, it was useful to read the thread you mentionned - thank you! So, as a conclusion, if we summarize and make (short) list about possible reasonable application, what they would be? Are injection-moulding mashines on the list, and how's with elevators and escalators for example?

 

[polaris12]

Forgot, plc123, could you, please explain : If you have active energy reading and calculated
heat dissipation, then efficiency is available.
How to calculate the savings?

 

[plc123]

Hi polaris12,
Active energy readings available from on board watt meter (useful energy plus losses), losses as heat dissipation available using thermal dissipation model from manufacturer:
heat loss in watts as function from ambient T, rpm .
Thanks,
Vladimir.

 

[jbartos]

Questions to the original posting marked ///\\
I'd like to test thyristor based motor controller (measuring voltage zero crossing to regulate the current flow) on the on 3 phase induction motor. What would be appropriate equipment and what electrical and mechanical parameters I have to measure to obtein the most reliable assess of efficiency and possible savings.
///Please, are you interested in the motor controller efficiency measurement or as the title reads "measuring motor efficiency" implying the interest in the motor efficiency, or both? Considering the motor efficiencies, these are usually available from the motor nameplate and from the motor manufacturers.\\ Please, recommend some non-costly meter or device.
///Please, clarify meter or device for what purpose.\\ The motor 37kW is in the field and I would like to find some less intrusive but still reliable method.
///Please, would you clarify for what that method is supposed to be?\\///

 

[rewindr]

Binsfeld publishes accuracy but check with them. The device is self balancing as is consists of a battery and sending unit located on opposite sides of shaft. It includes a reciever that can be scaled in a fashion to make the conversion intuitive, such as 100 in-lbs / volt. There is no upper limit on the shaft diameter. Lower limit is 1 inch or so.

 

[polaris12]

For JBartos: Yes,we are interested in both, motor efficiency and it's measuring. The motor is part of injection-moulding machine with 1 sec cycle and we would like to know wether any savings are possible while low period of pressure.We need some kind of "power quality" meter and being without experience in this field,we are grateful for every comment helping to decide wich meter would be appropriate and cost-efficient.The thread 237-56540, recommended here (thank you, electricpete)clairified something, but injection-moulding machine was not menntioned directly so we are still not shore - may it be the candidate for saving or not.

 

[jbartos]

Suggestion to the previous posting:
If the voltage and current waves are undistorted, then
Wattmeter reading on the drive input Wdrive,input might be measured, and Wdrive,output might be measured. Then, the drive efficiency would be EFFdrive=Wdrive,output/Wdrive,input.
If the waveforms are distorted by harmonics, which is probably the case, then the input current and voltage and output current and voltage need to be rectified and filtered to be then measured by dc ammeter and voltmeter.
The motor Wmotor,input=Wdrive,output
and the Wmotor,output can be obtained via a dynamometer measurement of HP.
Then, EFFmotor=Wmotor,output/Wmotor,input