One quick 393 motor question

So… what is the maximum safe stalling value for a 393 motor? We have experimented with stalling a 393 at motor value of 20, and we do not see any PTC issues. This year I put so much elastic assist on the lift that it would not stay down… So I thought applying a constant value to motors, like 25 or 30, might help keep the lift down, intake nicely placed and robot in size. So… how far can I go? Is stalling a motor with value 30 safely under PTC limit? What is the cut-off line?


This is very hard to tell, because there are a lot of variables in when the PTCs will trip. For example, one thing that will affect the maximum power before the PTC trips is if the PTC is warm already from earlier use or stalling. Any reasonably high value will trip the PTCs, it is just a question of time. I think that 30 should be fine, but you will have to try it out, as it may be affected by earlier motor usage or affect the PTCs tripping later in the match.

Also keep in mind that you can’t apply motor power to keep the lift in 18", as the robot will be off/disabled at the start of the match and when being sized.

As a side note, are you planning to lift lots of cubes, or why are so many elastics necessary? The best amount of elastics is so that is balances with the weight of the lift and around half of the weight you will lift.

Thanks Kevin. We will test it out with 20 ~ 30.
The lift is a reverse double six, and like any other reverse double lifts, the rubber bands undergo large amounts of shape change from top to bottom. The elastics is still under tuning, however even though the lift is over tensioned, the gears still skip a bit under pressure at the top where rubber bands get loose, and this is without cubes.
Because we have a PD controller and constantly sends values around 10 and 20 to the lift, we will be careful.

If your gears are skipping, I would highly recommend you take a look at how you can prevent that, as it can be or turn into an issue. Make sure the gears fully mesh and are well supported, and that the frame/supports wear the gears are isn’t bending or deforming under stress.

Also, as the elastics have more force at the bottom that at the top, you will have to watch out for the gears skipping at the bottom with the elastics.

This shows the theoretical value of current for a given control value on the 393 motor.

The PTC safe current is 0.9A, so the theoretical maximum value is somewhere around 33.

Theory does not always work when it comes to PTC devices, try 30 and see what happens. I usually try to stay much lower.

Thanks so much Mr. Pearman. This graph is awesome… why didn’t I find it before…

Because I just made it for you.

The simplest way to calculate this is to just use Ohms law, I = V/R. The resistance of the motor winding is about 1.5Ω, voltage is the battery voltage multiplied by control value/127. The more correct way is to use the equations that vamfun and I created for the smart motor library (which is what I did above, although I tweaked it slightly) and use a motor velocity of 0 with varying command values.

And this is why we love JPearman

Exactly. Thank you again Mr. Pearman. Also, I did not know motor resistance was about 1.5 ohm… Doesn’t motor resistance vary depending on the spinning situation of the motor?

For stall current, you don’t have to worry about the rotation of the motors.