Current output on the 18V25A motor driver board

Support Motor controllers/drivers and motors
1

Hi! I’m using the Pololu G2 High-Power Motor Driver 18v25 to control a motor that is rated for 18V, the system is powered by an 18.5V lithium ion battery. When I load the system the motor grinds to a halt but when I put an ammeter in series with the motor driver board and the motor it is only outputting 6A rather than the 25 that I expected. Is there a reason for this? My current PWM is set to 255.

2

Hello.

It sounds like you are stalling your motor. I am not sure what motor you are using, but please keep in mind that stalling or overloading motors usually decreases their lifetimes significantly and could even result in immediate damage. A general recommendation for brushed DC motor operation is 25% or less of the stall current (for its rated voltage).

As for why you are seeing a 6A current measurement instead of 25A, when you send a PWM signal to the driver you are controlling the voltage (not the current) across the motor outputs. As long as your battery is capable of supplying enough power, the motor’s current draw will depend on the magnitude of that voltage and the load on the motor. Essentially, your battery and driver do not push current; your motor pulls/draws the current it needs.

- Patrick

3

Thank you for this information! The stall current for my motor is ~65 A. I just want to be sure, low current from the driver board is a loading issue and not the result of some sort of software issue on my end?

4

That is a pretty powerful motor, so you will need to be careful about how you control it with the driver. While the 18v25 driver can handle large current spikes for short durations, it does not have an over-temperature shut-off, so if your motor to tries to draw too much power, it could damage the driver. If you have not already, I recommend carefully reviewing our product page documentation, especially the information under the “Current sensing and limiting” and “Real-world power dissipation considerations” headers.

Can you measure the following voltages when your run your program driving the motor?

  • VIN
  • PWM
  • OUTA
  • OUTB

For now, to try and keep things safer, I recommend only testing your motor with no load attached.

- Patrick