The step motor, a self-positioning wonder

Step motors are “special” in that they are self-positioning and therefore do not require an encoder to operate. This immediately gives step motors a cost advantage over servo motors which require an encoder to operate in a position mode. Furthermore, step motors are sometimes constructed in such a way that they do not require any magnetic material in their rotor (the part of the motor that rotates) or the stator (the part that is connected to the motor frame). Instead, the “torque creating” part of a step motor can be constructed entirely of iron or similar materials which focuses the magnetic flux lines, and of copper wire which creates the electromagnetic force. Step motors also contain ball bearings and other mechanical items, but these items are common to all motors and do not help us distinguish one motor type from another.

In addition to being inexpensive to construct, step motors are also “brushless,” meaning there is no physical contact with the electrical portion of the rotor. This means that none of the problems that can occur with mechanical commutators, such as degradation due to wear or electrical arcing, occur with step motors. Finally, step motors produce a relatively high torque for a given package size, and also have a high holding (resting) torque.

Despite these advantages step motors have a few drawbacks. The most significant is that step motors create noise which is often audible, and induce vibrations which can disturb the load or affect parts of the system which are vibration sensitive. Vibration can be reduced using microstepping techniques or even mechanical dampers but these solutions seldom eliminate the problem completely.

Another significant limitation of step motors is that they have relatively low high-end speed. While step motors have been driven to operate at 10,000 RPM and beyond in some applications, for most systems, 5,000 RPM or less is the most that can be expected. In addition to a low top speed, the torque that is available from a step motor drops significantly at higher velocities.

It is mainly due to these limitations that step motors are generally not available in power ranges above several hundred watts. The most common “NEMA” motor sizes for step motors are 17, 23, and 34. Larger sizes are sometimes available but they are not as common.