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DC Motor Locked Rotor Torque Measurements

DC Motor Locked Rotor Torque Measurements

Following my long-winded spiel on Torque and Speed in DC Motors, I went ahead measured the locked rotor torque output of one of the M260 motors in our Sparkfun Inventor's Kit. Measurements were made at several operating points (currents) to show the torque to current relationship. I used a 20 mm arm connected to the motor shaft to press against a kitchen scale, and a power source to control input current. Here's the setup:


Here's a clip of the actual measurements being made: (YouTube).

Moment arm was 3D printed, available here: It's a tight fit. Also available in two halves to be glued together. It was printed at 0.04 mm layer height with a 0.40 mm nozzle in PLA. Input current and voltage were monitored with the meters, an EEVBlog µCurrent used to increase current measurement accuracy. Button on the left rigged to complete the circuit, off-camera power source used to control current and limit input voltage (max 800mA and 3V). Motor is held above scale in a clamp so that the moment arm presses down at about 90 degrees, making for an accurate torque calculation/measurement.

Here's the moment arm:


Here's the results for a single motor position:



Excel file (and pdf) with measurements and comparison to voltage and power available here:

At a constant speed - zero, in this case - the torque also correlates with voltage (and power). This is of little use with a motor, however, as it's speed will typically be allowed to change when driven at different operating points.

These measurements could have been made more accurate by rotating the motor and repeating them. Instantaneous locked rotor torque changes with stator angle due to the interaction of the permanent magnets and the ferromagnetic materials. (The magnets pull and push by different amounts when they are different distances away from the electromagnets, regardless of the electrical operating point.)

Locked rotor torque measurements can be taken as the maximum torque a DC motor can provide given a certain amount of current. It will decrease approximately linearly with speed (rpm) to zero at the maximum free-spinning speed for a certain input voltage.


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