Electric Motor Torque Measurement

[toastedhead]

Ok, is there a better way to measure torque that an electric motor is putting out than using a $5000 dollar load cell?

What we have is a test bench with a 150hp electric motor directly coupled to our rotary lobe blower. Im not entirely convinced that the output being displayed on the motor controller is true kw so Im thinking my conversion to HP and then to torque isnt accurate or real world.

Is it as simple as using a multimeter to measure to double check?

 

[drawoh]

toastedhead,

Can you disconnect your blower and add a brake?

Your brake would be a disc attached to your shaft, with a lever connected to a spring scale. You know the radius to the spring scale. You read the force off the spring scale. You will have to think through the linkage a bit.

JHG

 

[BobM3]

I'd contact the manufacturer of the drive first and get their input on how accurate the "motor torque output" reading is. You may also want to ask that question directly in the Electrical/Electronics Engineers Electric Motors... forum.

 

[electricpete]

I assume you have a 3-phase induction motor in steady state powered from non-vfd source.

You can estimate POWER INPUT as sqrt(3)*Vrms*Irms*cos(theta) where cos(theta) is the power factor. This assumes sinusoidal waveforms (if not sinusoidal than you have to get the fundamental portion). With a typical multimeter you won't get the angle. You could with a fluke scope-meter. Also there are instruments built to measure power (all measurements need to be made by someone who knows what they're doing with regard to safety and also polarity of connections).

Once you have power input, you need to estimate losses in the motor and subtract them from power input to deterine motor power output. Could guesstimate those losses based on nameplate info and loading level.

Then you need to know speed so you can compute torque from Torque = Power/(2*pi*speed) with appropriate unit conversions needed. You can guesstimate speed from current and nameplate info – slip is roughly proportional to load – varies between 0 slip at no-load and nameplate slip (sync speed – nameplate speed) at full load. There's a lot more that could be said (load relationship vs current, voltage correction, etc) but since slip is typically 1% or less for large motors, the estimate of slip is not critical for purposes of estimating speed to convert power to torque.

If you don't want to measure input power, an alternative is to estimate loading from slip itself in which case you'll want a much more accurate measurement of speed (than if you are just using slip to compute speed for purposes of converting power to torque). Again this would be based on principle of slip varying linearly with load. In this case your estimate of torque WILL be heavily dependent on your estimate of slip and you'll want as accurate a measurement of speed as you can get, and you may want to correct for voltage variation. Also note that motor nameplate speed is often rounded to the nearest 5 rpm.

All in all a lot more than just hooking up a multimeter. Let us know if you want more details on a specific approach.


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[toastedhead]

Actually, this motor is hooked to a variable freq drive which allows us to test the blower at various RPM. We know the RPM by the freq displayed and was varified by a Browne and Sharpe tach.

Is the display on the VFD accurate? For instance, if the display shows 24 amps current draw, can I then calculate the horsepower required to keep my unit running is 24 (if the motor eff is 94%, 3 phase, 460V?

Something isnt making sense to me though. When I take the blower off of the stand, and run the motor up to speed, it still registers 24 amps.

So this readout is meassuring output horsepower. Not horsepower based on feedback correct?

 

[electricpete]

When I take the blower off of the stand, and run the motor up to speed, it still registers 24 amps

Motors still draw current at no load. Your full load amps is probably around 240A. 10% of FLA is probably No load amps.

Is the display on the VFD accurate? For instance, if the display shows 24 amps current draw, can I then calculate the horsepower required to keep my unit running is 24 (if the motor eff is 94%, 3 phase, 460V?

No, power is not linear with current at low power, only above 75% load or so. Also you need to consider again whether voltage is still nameplate.

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[MintJulep]

How is the motor mounted?

How accurately do you need to measure the torque?

How are you calculating HP from the (assumed) amp reading from the VFD? Are you assuming that the voltage is a constant 460? Probably it's not.

 

[electricpete]

Regarding voltage variation we both alluded to. Typically voltage reduces in proportion to speed below base speed.

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[EdDanzer]

You could measure reaction torque in the base rather than measure shaft torque to reduce the cost of equipment.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[waross]

A couple of points about drives, current and torque.
Drives use a pulse width modulated waveform. The drive gurus generally recommend using the values that the drive reports. Not all meters will accurately measure the effective output waveform of a drive. The drive will.
If the drive reports kW out, apply a factor for motor efficiency and convert that to HP. The motor efficiency may drop a few percent when powered by a drive rather directly than from the grid.
24 Amps on a 150 HP motor at any common voltage is probably almost all magnetizing current which is not load dependent. Furthermore, the magnetizing current is 90 degrees out of phase with the load current further complicating estimates based on current at low loads.
For a sanity check, you may measure the inpt Voltage and current to the drive and calculate the KVA. I would estimate the combined losses of the drive and motor at about 15%, so use Volts x Amps x 1.73 x 85% / 1000 / 750 for an estimate of motor HP. The accuracy of this method is load dependent. It is a fairly good sanity check for loads between 75% and 100%. Below 50% loading the results with this method may be more confusing than helpful.
First use the values that the drive reports and use this method for a sanity check on loads above 75%.

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

 

[waross]

You may be missing some valuable responses posting motor questions in this forum.
Consider posting a link in the "Electric motors & motor controls engineering" forum to this thread.
Something like:

Heading:
Electric Motor Torque Measurement

Body:
Please see the thread at Electric Motor Torque Measurement
thread404-254045

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

 

[electricpete]

Error - infinite loop.

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[waross]

Hi Pete, I was suggesting that that link be posted in the Motor forum, not used here.
Sorry if I caught you in a "do" loop. grin

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

 

[electricpete]

Aha. Different error. When skimming the thread, my eye was drawn towards the link and the line above it (please se...). I was surprised when the click took me back where I started. My bad.

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