Currently having issues with a VEX 393 motor stalling out. Im building a robot on the larger side and have the motor drving its wheels so there is a decent amount of torque to move it. However im having issues with after it starts to move, the motor will randomly cut out and stop. It is being powered by an Arduino breadboard wired to 18V in the “Vin” terminal with an L239D controlling everything.
The main question I have is can if I use 9V versus 18V will it change the stall torque of the motor? and how? and once the RPM of the motor increases does the stall torque change? and how?
here’s some of @jpearman 's earlier work on 393 motor.
And btw, there should be a PTC inside the motor that will limit the amount of current that you can pass to the motor (it will tripped once it goes past certain temperature for a short amount of time).
the motor is powered out of the output of the chip which i believe has an output range of 4.5-36V and i’ve had no issues testing without a load so i am assuming that the chip is suppling the max voltage the motor can take. however i am not super familiar with the motor chip so i am unsure. the entire thing is for a school project so i am just learning everything as i go…
i would not be surprised in the slightest if the motors are damaged/barley working. i have noticed chronic use of 9V batteries directly to the motors and we were never informed of their voltage limit so the whole idea slipped my mind.
and i had been doing testing so there is potential the motor was getting hot and set it off.
This sounds like the motor protection is disabling the motor, the so called PTC.
When driving a 393 from a VEX brain such as the cortex, the PTC in the motor would last perhaps 5 seconds when stalled. If the motor is receiving 18V, when the motor is stalled that’s going to equate to more current, the PTC will heat up more quickly and the motor may be disabled after only 2 or 3 seconds. Even when the motor is not stalled, higher voltage will cause the PTC to trip sooner.
Lots of useful info on PTC here I posted in a previous life.
The PTC is probably going to trip almost instantly once resistance is applied to the motor at those voltages, but the extra voltage should give a bump in current (same resistance + more voltage = higher current), so I think that yes the torque of the motor would increase at stall.
Just doing some quick math here:
The 393 motor is listed as having a stall current of 4.8A. This value was measured at 7.2V here:
We can then infer the effective resistance of the 393 motor when stalled using Ohm’s law (V= IR) rearranged in our case to be R = V/I, which works out to 1.5 Ohms.
Rearranging the equation again to I = V/R, we can calculate how much current would be pushed through a stalled 393 motor at 18v to be a whopping 12 Amps. Given that @jpearman measured the PTC to trip at 3A in about ~3 seconds, I wouldn’t expect it to last long at these kinds of currents. However, lets have some fun and say we’re really interested in how much torque we can get the 393 to produce in the time it takes for the PTC to trip.
Given the relationship between current and torque is basically linear, it’s easy to calculate how much torque the motor would be producing in our 18v/12A stall situation: 37.5 in/lbs
