Measuring torque acting on motor shaft?

[robertjordan]

I have an electric motor that I am using to turn an auger to dispense pet food. I want to measure the average torque required to turn this auger at a given rpm.

I know Power_in = Voltage into motor * Current into motor
and Power_out = Torque exerted by shaft * RPM of auger
and Power_out = Efficiency of motor * Power_in

But how can I find the efficiency of the motor at different voltage inputs? I don't know anything about this motor (I don't even know if it's an AC or DC motor) beyond that it works when connected to a battery and that it changes directions when I swap the leads. How can I find average torque exerted on the shaft by the pet food? I can measure RPM, VoltageIn, and CurrentIn...

 

[insightful]

But how can I find the efficiency of the motor at different voltage inputs?

Yours is definitely DC. I'd peruse these motors and find one that's close. From 60 to 75% efficient is common:

http://www.jameco.com/webapp/wcs/st...e~category_root^prodPage~15^page~SEARCH%2BNAV

 

[CWatters]

It might be possible to measure the torque directly, for example by measuring the torque the motor applies to it's mount. The other possibility is to insert a sensor between motor and auger.

https://www.transducertechniques.com/sts-torque-sensor.aspx

 

[David Lewis]

I wouldn't worry about efficiency yet because it varies depending on the motor's operating point. The torque constant can be deduced from speed versus voltage curves. Speed and voltage are easy to measure. From what I've seen, measuring torque directly of a small DC motor requires comparatively elaborate and sensitive instrumentation.

Anyway, let's assume, for example, you want your motor to operate at maximum efficiency. In that case you then want the torque absorbed by the auger to equal the torque produced by the motor at the motor's maximum efficiency speed.

Technical correction: In the power out formula, torque is a physical quantity, whereas RPM is a unit of measure. I think you'd like all the variables to be physical quantities in this type of equation.