# Robotics DC motor help

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1. Nov 25, 2014

I'm currently trying to test the motors I bought for a robotics project to see if they'll work for our application and they seem to be much weaker than what should be expected based on the specs. During test, the battery is hooked directly to the motor and I'm measuring the current draw and the torque supplied to the wheel. The battery is a 7.2V 5Ah LiPo, so it should be able to much more current than what is needed for one of these motors.

The motors have the following specs at 6V (though it can be safely ran between 3 to 9V):
Free run current: 70 mA
Stall current: 1.6 A
Stall Torque: 125 oz*in

I'm measuring the torque on the motor by attaching a mass to a string and having the string wrap around the wheel and using the equation T=F*r=mg*r, where r is the radius of the wheel.

When testing the motors, the gears begin to slip with a 600g mass attached to the string. My calculations here are:

F=(.6 kg)*(9.81 kg/s^2)=5.9 N=17.9 oz

So,
T=(17.9 oz)*(2.16 in)=39 oz*in which is well below the maximum rated torque of the motor.

With the 600g mass the current was about 600 mA, which also suggests that I'm far from the maximum motor torque.

These results seem to imply that the motors are not performing as specified, but I wanted to make sure that I'm not doing something wrong while testing or with my calculations.

As a side note, these motors do have a 1000:1 gear ratio, so it makes sense that the gears would be the weak link, but only being able to get 30% of the specified torque seems extremely low.

Any ideas would be greatly appreciated.

2. Nov 25, 2014

### phinds

Doesn't sound to me like a battery problem but I do note that you have said nothing about what the battery is capable of producing in sustainable current. That is, it COULD be (seems unlikely) that the 5Ah rating is only good as long as you keep the battery to 10 ma and it has a peak current output of 20 ma (I'm using ridiculously low numbers here just to make the point).