# Overcurrent protection, limit for nominal current or stall?

Background: I'm currently working on a robotics design that will drill through soil.

I have selected a 48V motor who's nominal current is 3.17A and has a stall current of 42A. I will be driving the motor with a microcontroller and motor driver.

However I am having some trouble with my design.

The motor driver includes over current protection circuitry, and I need to select a resistor to set the max allowable current.

Would it be more optimal to use a resistor that caps the current at exactly 3.17A? Or would it be better to provide some margin, and make it 4A?

The over current circuitry has a feature where it resets itself after every PWM cycle and checks if the current is too large again. I believe this is "current chopping" so the motor should start up. (Startup time is not necessarily important for this.)

Here's a link to the datasheet for the motor in question, the 148877 https://www.maxongroup.com/medias/sys_master/root/8841119236126/EN-141.pdf

Any input would be appreciated. Thank you for reading

• 42A/3.17A = 13.25. That is a very high ratio between stall current and current at nominal operating power.I would be concerned that if you limit the current to 3.17A, you may not have enough starting torque to overcome whatever starting load you have. Motors are typically designed to operate with overcurrent for a short period of time while they get up to speed. Do you have a datasheet for the motor? Dec 16, 2020 at 15:46
• Thank you so much for your speedy reply @MathKeepsMeBusy . (maxongroup.com/medias/sys_master/root/8841119236126/EN-141.pdf) The motor in question is the 148877 Dec 16, 2020 at 15:54
• I'm a little confused. The motor appears to be a DC servo motor, but the drive appears to be 3 phase. How were you planning to connect these? I may be missing something, like the motor uses three phases, or you are only planning to use two phases of the driver, or something else? Dec 16, 2020 at 16:12
• Overcurrent protection may be set lower than max. You will get slow start, depends of load on motor (mechanical). If it is allowed in your project. Dec 16, 2020 at 16:13
• @MathKeepsMeBusy. My apologies! Please see this link for the data sheet. I linked the wrong sheet! ti.com/lit/ds/symlink/… Dec 16, 2020 at 16:20

## 1 Answer

The stall current actually doesn't represent any working range of the motor. It's just a current with locked rotor and nominal voltage, the only thing that limits the current is the winding resistance.

The valuable data for your motor is the nominal current. The motor current can also exceed this value for short time, but then it has to drop to let the motor cool down. The good protection would have to take into account the integral of i^2*t, not just a single peak value.

If you want to be on safe side then set the max. current to accordng to the nominal current, but you will lose the ability of high dynamics. The correct way would be a current limit approx. from 200% to 300% and an additional I^2*t limit, or a temperature sensor that cuts the power off at certain temp.

• Thank you so much for your input. This is definitely the correct approach. Dec 17, 2020 at 8:18