Basic gearmotor question

Hello. I recently purchased a 122:1 Metal Gearmotor 37Dx57L mm as i was looking for a motor that could run at low speed with high torque. From your catalog this seemed like the best option. I hooked this motor up last night i was shocked at how little resistance stalled the motor while running at low speeds. When run at higher speeds the torque is more reasonable. This is very unfortunate because as part of my student design competition this motor will not do the trick and the deadline is approaching fast. This is unless there is something i am missing here? My understanding is that at lower speeds the motor should have the same, if not more torque!

I am using a 9V battery source with a TB6612FNG Dual Motor Driver Carrier It seems this driver only sends out 4.5V when you hook up one motor as it is a dual motor driver. Is this correct? Is there any way to get the entire 9V out of the motor driver while only one motor is hooked up? do i need a different driver?

This motor is a tank. I suspect it can deliver the torque. I just need some insight on how to accomplish that.

thanks for your help


I think you’re confused because you’re mixing up a lot of variables. If you only increase the gear ratio, you will get less speed and more torque. If you only increase the load (e.g. try to hold the shaft), you will get less speed and more torque. However, it sounds like you’re lowering the power into the motor, which will give you less power out, both in terms of speed and torque. A high gear ratio is an easy way to get a lot of torque at low speed; if that is not an option, your only other option is to do some closed-loop control, where you apply plenty of power but then reduce the power once the speed is faster than you want it to be.

Regarding your electronics, your battery and motor driver are both way too small for this motor, which is probably why you’re only seeing 4.5V on the motor. The two channels are independent; if you run at a low enough voltage, using the two in parallel could be close to reasonable. In general, though, you should be using a driver such as the VNH2SP30 or MC33926 and a battery with at least a few amp-hours of capacity.

- Jan

Thanks for the quick reply.

My understanding was…
Torque = (voltage * current ) / (angular speed)
But clearly the relation was not in the same proportions as I anticipated. At low voltage the torque is minute compared to that of high voltages.

As for your recommendations…

[ul]Unfortunately, I have no extra gears to increase the ratio. Mounting would also be an issue here i suspect.

Buying another driver would allow me to increase the voltage to the motor however that would also increase the speed which is not what i want. Increasing the load at a high voltage would be a good solution to bring down the speed although I cannot think of any straight forward mechanics to achieve this at the moment.

Finally, i have no way to do feedback for a closed loop control. [/ul]

I guess what im looking for is a wizardly solution to this problem. If it means recommending some different pololu products that would be OK, however, it would be nice to work with what I have already. I need slow rotation (~.25rot/s max) and a reasonable torque (6kg-cm).

Thanks again for the help

Your equation is valid in the sense that torque times speed is the mechanical power out of a motor and voltage times current is the electrical power into the motor; however, you should be aware that does not take any motor efficiency into consideration. You can look at simple torque = 0 (no load) and speed = 0 (motor stalled) to see two cases where the power out is zero, but the voltage and current are both nonzero. However, even the equation as you have it clearly has torque going down with voltage.

Given has slowly you want to go, a high gear ratio is the easiest way to go. The only units we carry with higher gear ratios are some of the Tamiya plastic gearboxes.

- Jan