Accuracy of Motor Current as Torque Feedback?

Does anyone have any concrete numbers as to how much accuracy there is in trying to measure torque by measuring motor current? I've heard some
off hand figures of 10%, but nothing that was substantiated with any kind of legitimate research, or even casual observation.
Does anyone have any references that compared current vs a true torque sensor?
Thanks,
Chris
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" snipped-for-privacy@gmail.com" wrote:

No figures offhand, but I think the relationship will be between motor power (electrical input) vs torque. This means having to measure the input voltage and phase angle relationship with the input current as well.
What kind of motor do you have in mind? Is it part of a variable speed system (this will complicate things).
Other things that might have to be accounted for: variations in winding resistance due to load and/or ambient temperature changes. Changes in mechanical friction or windage loss due to environmental factors.

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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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----------------------------

--------- Depends on several factors- kind of motor, pf, slip and loading. For an induction motor at light load load, the magnetising current is highly inductive and possibly 30-50% of the full load current so such an estimate is poor. What you get is simply a ball park estimate even at full load. Lets face it, you could take the conventional open and short circuit tests on a motor and determine the equivalent circuit- then calculate torque at any given slip- and, if lucky, ending up in the +/- 5% to 10% range because the model is still simplified compared to the real thing.
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Don Kelly @shawcross.ca
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On 10/26/05 6:34 PM, in article V8W7f.305229$oW2.105454@pd7tw1no, "Don

If he is talking about a dc shunt motor or, even better a motor using a permanent magnet to provide the field, the current should be a reasonably proportional to the torque..
Bill
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estimate
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My guess is that this refers to a newish thing in ac speed drives and soft starters, sensorless flux control. A feedback element is removed, saving money, and the motor current is used in a pinch to calculate flux.
To the OP, I believe that you might find some Square D or Eaton (aka Cutler Hammer) or ABB literature that makes some comments about accuracy, and about lack thereof. If you really need to know, try an 800 number. I have an ABB ACS550 technical catalogue here, it says
Speed control Static accuracy: 20% of motor nominal slip Dynamic accuracy: <1% s with 100% torque step
Torque control Torque step rise time: <10ms with nominal torque Non-linearity: +-5% with nominal torque
If that helps you any. I think they don't give incredibly accurate speed control, and some time ago I think I read that they have worse accuracy at slower speeds.
Later,
j
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True, the results for a DC, separately excited motor will be better. Better still for a PM motor but maybe not as good for a shunt motor. Still a ball park estimate. One problem is that it assumes total flux per pole is constant- not necessarily true.
Don Kelly @shawcross.ca remove the X to answer ----------------------------

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Chris, I don't see how it could even meet the 10% figure in the real world, unless the entire system was tested and calibrated under real-world working conditions. But even at that, as soon as things start wearing, the calibrations are gone.
IMHO, there are just too many variables. Mechanical parts, winding differences, temperatures, bearing pre-loads, even what type of, and how much grease they put in the bearings could influence it.
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Anthony

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