VEX Motor Power Curves

[rkane]

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.

Torque Speed and Power - VEX 393 Measured.pdf (1.61 MB)

[Team80_Giraffes]

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

https://vexforum.com/t/motor-torque-speed-curves-rev2/21868/1

[jpearman]

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.

[rkane]

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.