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What Are Stepper Motors?

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  • Written By: Emma G.
  • Edited By: Melissa Wiley
  • Last Modified Date: 28 November 2014
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Stepper motors are motors that rotate a set number of degrees in response to an electronic pulse. This method allows the rotation speed and magnitude to be minutely controlled without using a feedback system. Stepper motors are often used in disk drives, machine tools, and robotics. They are valued for their high torque at low speeds and because they tend to be extremely reliable in all environments.

A stepper motor is basically a circle of electromagnets arranged around a single permanent magnet, called a rotor. A pulse of electricity is sent to each electromagnet in turn, charging it. This causes the magnet to spin toward that electromagnet. That electromagnet is discharged, and then a charge is sent to the next one. Charging and discharging the electromagnets causes the rotor to spin 360 degrees on its axis. If the electronic pulses are fast enough, it results in continuous rotation of the rotor.

One movement of the rotor is called a step. Each step is the same size, with a variation of no more than 3 to 5 degrees. As each pulse is a discrete event, variations do not add up over time and cause a major loss of accuracy. This means stepper motors can never be more than 5 degrees away from perfect accuracy.

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The speed of the motor's rotation can be changed by varying the frequency of the pulse. If the pulses stop, as when no power is being sent to the motor, the rotor will align with the closest magnet and hold there. The attraction between the magnet and the metal is enough to keep the shaft from turning.

Stepper motors generally have three step modes. Full step mode causes the rotor to move one full step for each pulse. Many standard stepper motors are 200 step, meaning each pulse causes the rotor to move 1/200 of the 360-degree circle. A standard stepper motor set on full step mode will therefore rotate about 1.8 degrees for each pulse.

Half step mode allows the operator to double the number of steps per full rotation. A standard 200-step motor set on half step mode will effectively have 400 steps, because the rotor moves a half step for every pulse. Microstep mode divides the rotation into even smaller steps. Every division of a pulse allows for greater precision, but it also causes the motor to lose some of its torque.

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