VEX Motor Power Curves

I recently tried to measure current draw for certain torque values for the VEX 393, and was hoping you could help with a question or two? We connected the 393 to a winch set up and lifted a series of increasing weights. The motor was hooked to a DC power supply and the voltage was set to 7.2-Volts. As each weight was raised by the winch, we were able to record the RPM’s and the current draw. My first question is whether we made a mistake by setting the voltage to remain at 7.2-Volts? It seems like other experiments I’ve seen had the voltage varying as torque changed?

Also, we could not get past about 0.8-Nm of torque. This drew 2.4-amps and we only had the winch spinning at 19-RPM. It just seems like the motor is not as powerful as we expected. We took many measurements from 0.0-Nm to 0.8-Nm of torque. When we plotted the trendlines for torque vs. current & torque vs. speed, we are not getting the same trends/slopes as the published values.
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Torque Speed and Power - VEX 393 Measured.pdf (1.61 MB)

That curve you made looks like a 269 power speed curve. Maybe James can give you more insights.

0.8 Nm is about 7 in-lb

It’s hard to say. There will be some voltage loss through all those breadboard connections and thin wire. There will be losses through the chain/gears and also friction losses. How were you measuring current, perhaps just the PSU? What did it show when the motor was stalled? First thing I would do is check voltage at the motor input and see what that was.

Thank you so much for the response! We will put an ammeter across the motor to measure voltage while it’s working. As you mentioned, we have a power supply that reads current as the winch is working. The current approached approximately 2.5-amps as the motor came to a stall at approximately 0.8-Nm of torque.