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Exceeding Rated Motor Voltage on HPCB Micro Metal Gearmotor

Hi there,

My setup includes the following:

  • 1 Raspberry Pi Zero W

  • 1 Arduino Pro Micro

  • 2 Pololu Micro Metal Gearmotor #3072

  • 1 Pololu DRV8833 Dual Motor Driver Carrier #2130

  • 2 Pololu Magnetic Encoders #4761

I’ve noticed that the 6V version of the 30:1 HPCB Micro Metal Gearmotor #3072 performs significantly better at lower voltages than the 12V version of the 30:1 HPCB Micro Metal Gearmotor #3049 and even performs better at its rated voltage of 6V than the 12V motor at its rated voltage. What is going on internally to allow this to happen as both motors are advertised in the same way as HPCB motors?

My current setup requires the motors to output 1000 RPM at approximately 0.35 kg*cm for 0.5 seconds based on some conservative estimations. Looking at the performance curve of the 6V 30:1 Micro Metal Gearmotor #3072 in the Micro Metal Gearmotor datasheet, these requirements are clearly not possible at 6V. To overcome this issue, I intend to run the motors using 3 LiPo batteries at a voltage of 11.1 V. Since I’m only running the motors at this excessive voltage for 0.5 seconds, I’m thinking the motors won’t overheat. Will the motors be damaged under these conditions? Should I use another motor?

I also see that at 11.1 V, I will be above the rated voltage of the Motor Driver Carrier #2130 of 10.8 V. Will the motor driver be damaged?

Any other suggestions for how I can meet the above speed and torque requirements are welcome.

Hello.

Both the 6V and 12V high-power carbon brush (HPCB) micro metal gearmotors use carbon brushes, but the motors are not wound the same way. It not surprising that the 12V motors do not perform as well as the 6V motors at 6V because these motors are designed to have similar speed/torque performance at their respective rated voltages (the 12V motors are just able to do so with about half the current draw). However, as shown in our micro metal gearmotor datasheet, even at their respective rated voltages their performance is not exactly the same.

I do not expect any of our micro metal gearmotors to be able to reliably apply 0.35kg•cm at 1000rpm. Even if you power a 6V 30:1 HPCB gearmotor (product #3072) with a 3S LiPo, based on our datasheet, I would only expect your motor to theoretically run at about 600rpm with that much load, and doing so would likely result in thermal damage. Ultimately, I recommend using a larger motor that can deliver your required speed and torque, ideally at its rated voltage without drawing more than 20-25% of its stall current. Here is a link to our full selection of metal gearmotors. Alternatively, you might consider if you can distribute your load among multiple motors.

As opposed to the nominal voltage for motors, which can usually be exceeded at the expense of operating lifetime (if you can still keep the current draw low), the maximum voltage ratings for electronics like motor drivers are hard limits that should never be exceeded. Powering the DRV8833 (product #2130) with a 3S LiPo (which can be around 12.6V when fully charged) will damage it. Instead, you might consider using two of our MAX14870 carriers, or since you are using a Raspberry Pi, you might consider our Dual MAX14870 Motor Driver for Raspberry Pi.

- Patrick

Hi Patrick,

Thanks for the information. I had viewed the micro metal gearmotor datasheet you linked after reading some earlier posts on this forum. I’m hopeful I can still achieve my torque and speed requirements by bumping up the voltage after looking at those performance curves.

Thank you for also providing me with that resource for a higher rated motor driver carrier.

I’ve used 3 LiPos to power the DRV8833 (#2130) before and haven’t encountered any issues. As for the motor. I think you may have missed that I will only be powering the motor for 0.5 seconds. Hopefully over such a short time period there won’t be thermal damage to the motor.

I’m going to go ahead and test it out. I will report the results in this thread later.

We did consider that you only plan on powering the motor for 0.5s, but heat can build up quickly, especially when using motors above their rated voltage. The total cycle time will also matter since it will takes motors significantly longer to cool down than to heat up. Even if your motor does not burn out immediately, I expect running it like you described to reduce its operating lifetime.

Also, regarding your driver, please keep in mind that just because you have not had problems using the DRV8833 out of spec before does not mean you should expect it to continue to operate that way in the future.

- Patrick