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How to Measure Torque Required for motor to spin a rotating shaft?

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I've harvested a motor from a cordless drill and connected it to a belt which turns a rotating shaft.  The motor pulley and the pulley on the other side of the belt are roughly the same size, which a fairly small radius (5 mm maybe?).

The issue I'm running into, which I don't fully understand, is that the motor is drawing 15 Amps while spinning the shaft at a constant 600 RPMs.  My power supply won't go higher than 15 Amps, so I'm unable to increase the speed.  This amount of current draw seems very high given that the shaft is rotating at constant speed.

Note that I measured the no load current of the motor to be about 2 Amps.  Without load, I have no problem increasing the voltage to the motor, and thus increasing the speed.

This would lead me to believe that the torque required to maintain constant speed of the rotating shaft is causing the high current draw, but the shaft is only connected to ball bearings and seems fairly easy to turn when I turn it by hand.  In addition, it's situated vertically, so friction of the ball bearings should be the only force slowing it down.

So, I need to figure out if the shaft really does take a lot of torque to spin or if the motor, for whatever reason, can't handle very much torque (which would be surprising since it came from a cordless drill).

1.  How would I measure the torque required to keep the shaft spinning at constant speed through the belt?  


2.  What's the best way to measure the torque/current relationship for the motor?  How would I apply different amounts of torque to the motor in order to take the necessary data points?

Thanks for the help!

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  • 5 months later...

With any motor, the power is from 

P = IV 

The power (in Watts) used is proportional to the voltage (in Volts)  AND the Current (in Amps).....If you up your voltage, the ampage should drop significalty...as you've experimentally discovered, if the voltage is too low, the ability to supply a sufficient current (ampage) becomes the limiting factor.

New (DC) motors are rated for a no load speed at a specific VOLTAGE...the current increases subject to the load to match the power requirements...and CRITICALLY the wire requirements are rated on CURRENT. If you run your motor at too low a voltage, your wires will overheat - both externally and internally - the varnish will evaporate from the winding wires inside the motor and the motor will either burnout through a resulting internal shortcircuit or catch fire!

Hope this helps

I shan't - at the moment - expand further as your experimentally discovering lots on your own which is by far the best way to learn....but I'll be happy to look again for additional questions.

Good luck!

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