I set up a winch system to measure current at certain torque to reproduce the Power curve for the 393 VEX motor. But I’m not getting the same results at the motor specifications.
I have a 20-V Power Supply, which I believe is topped out at 2.5-Amps. The amp meter on the supply only goes up to 2-amps.
I have the power supply set to 7.2-V, and hooked up to a bread board. The power runs through a bumper switch, then through the motor.
I have the motor hooked to a 4-inch diameter traction wheel which is acting as my winch. I’m hanging weights from the winch and recording current as the winch lifts the weight. Based on the wheel’s radius of 2-inches being my “torque arm” (2-in = 0.0508-m), I can calculate Torque in Nm from the weight of my object in Newtons * 0.0508-m. When I let the winch spin freely, I’m getting the 0.37-A free current as specified on VEX website for the 393 motor. However, when I hook weights up and start lifting, I’m getting much more current draw than is specified from plotting VEX’s free current and stall current.
For example, at 7.1-Newtons, I get a torque of 7.1-N * 0.0508-m = 0.36-Nm. The ammeter on the Power supply reads approximately 1.6-Amps (it’s not a digital meter, just the old dial type). Based on the slope of VEX’s chart, you should only be drawing around 1.33-A for 0.36-Nm of torque. Some other measurements were as follows:
0.29-Nm at 1.2-A versus VEX chart shows it should only draw 1.14-Amps
0.44-Nm at 1.9-A versus VEX chart shows it should only draw 1.54-Amps
Any thoughts? I’d say maybe that their is some added current from the friction of my winch, but then why would the free current be spot on? Am I splitting hairs here? Are my measurements close enough the specifications for my crude winch and old power supply set up??