# Measuring Drivetrain Efficiencies



## major (Apr 4, 2008)

JCC said:


> here's my question:
> 
> If using a brushed DC motor, would using the torque constant and the instantaneous current information be an accurate method to determine the instantaneous torque information?


 
Kind of, if you allow transients to settle out so you take the value at steady state. Otherwise the rotor polar moment of inertia will enter into the equation. And the Kt * Ia figure gives the torque at the air gap so does not include the rotational losses in the motor. A good approximation of the rotational losses can be measured based solely on RPM (at no load) and then applied to the load test torque calculation as a correction factor.



JCC said:


> This is assuming way too much current is not being dumped into the motor.


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## jwiger (Oct 18, 2014)

What kind of load does the dyno use? Even if it isn't calibrated to an industry standard you could still compare transmissions. Your measurements might be in "JCCmeters" but if it's repeatable it's still valid to say which transmission has more loss.


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## JCC (Mar 19, 2015)

Major,

How would you recommend going about that? I'm assuming that the rotational losses would vary with RPM, so I'll likely need a number of data points to get a good approximation.



major said:


>


I was simply referring to the idea that the motor would be run under typical conditions, rather than extreme conditions where excessive current could lead to demagnetization and performance loss.


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## Duncan (Dec 8, 2008)

Hi JCC

Torque is the easiest thing to measure,
Your dyno will be on bearings with a reaction arm and a load cell on the arm
To calibrate it bolt your own reaction arm onto the case and hang weights on it


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## GerhardRP (Nov 17, 2009)

JCC said:


> As I have spare parts laying around and access to a dynamometer, I would like to measure the efficiency of various drivetrains, such as a belt cvt, chain, gear, etc, when coupled with an electric motor.
> 
> Assuming the dynamometer is calibrated, I can get torque and speed information. However, without buying an expensive torque sensor for the motor shaft, I pretty much only have rpm, voltage, and current information.
> 
> ...


For a study of efficiency, I would compare power out /power in. With the motor, if you operate at something like its continuous rating, the efficiency will be about 90% of the electrical power in -- just current times voltage.
Is your dynamo-meter flexible enough that you could rig a "transmission" consisting of a straight drive shaft? Then you could measure the motor curves directly.
For a discussion of motor power and theory, you might look at an old thread of mine: http://www.diyelectriccar.com/forums/showthread.php/dc-motor-theory-and-model-39931.html Look at the first post and also number 114 which contains a spread sheet for calculating various parameters for a WarP 9 mmotor.


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## JCC (Mar 19, 2015)

jwiger said:


> What kind of load does the dyno use? Even if it isn't calibrated to an industry standard you could still compare transmissions. Your measurements might be in "JCCmeters" but if it's repeatable it's still valid to say which transmission has more loss.


It's a Dyno-mite dynamometer meant for go karts, atvs, and bikes, shown with an example below:









It's not the most ideal setup since it generally requires a chassis, and an out-of-car engine dynomometer would definitely be better. If I'm patient enough to wait at least a few months, I might have the chance to use a bench dynomometer used for small engines.

Comparing transmissions/drivetrains was definitely one of my ideas. One thought was that if I could use an identical throttle input from one test to the next, such as with a preprogrammed 5V microcontroller, I could more easily compare the graphs. This generally works for single speed transmissions with the same overall gear ratio, but I'm not quite sure how easily the shift range of a cvt could be compared to a chain. Since the cvt would be changing the gear ratio, I'm inclined to think that it would be hard to compare the graphs.


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## jwiger (Oct 18, 2014)

Good point on the CVT shifting. There may be a way to hold it at fixed ratio, and try a few ratios out to plot an average drivetrain loss.


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## piotrsko (Dec 9, 2007)

I would think that it does not matter what ratio a true cvt is using unless there are other devices added to the power stream to hold those ratios.


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