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How to Select a DC Motor

Selecting a DC Motor

DC motors possess linear relationships that allow for very predictable operation. For instance, if enough voltage is applied across the terminals of a DC motor, the output shaft will spin at a rate proportional to that applied voltage. You can take the ratio of the applied voltage over the rated voltage and multiply that number by the no load speed and get the running speed. Also, if you decide to measure and plot the current and torque, you will have a simple straight line indicating yet another directly proportional relationship.

When torque demands increase, so does the current. Plotting the torque and speed together, you will find that only two points of data are needed. Those are the no load speed and the stall torque.

The entire motion control world, including manufacturers and designers, depend greatly on the premise that all these linear relationships will hold true. And they do hold true, because the laws of physics do not change! However despite their simplicity, selecting a DC motor for an application can still be a daunting task. There are many other variables that must be taken into account including dimensions, load, duty cycle, environment, feedback considerations, etc. Perhaps decoding some of the mysteries of motor operation will shed some light on the selection process.

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