Stepper motors rotate in discrete steps. These are DC motors with multiple coils arranged in groups or phases. Energizing each phase sequentially enables the shaft of the motor to rotate in single steps. It is possible to achieve very fine positioning and speed control with a computer controlling the stepping. This allows use of stepper motors for several industrial applications involving precision motion control. As stepper motors come in various sizes, styles, and electrical characteristics, it is important to know the parameters that allow selecting the right motor for the job.
Stepper motors are good for three things—positioning, speed control, and generating low-speed torque. As they move in repeatable and precise steps, stepper motors are appropriate for applications requiring meticulous positioning such as in 3-D printers, XY plotters, CNC machines, and camera platforms. With their precise incremental movement, stepper motors allow excellent control of their rotational speed suitable for robotics and process automation. Where regular DC motors generate very little torque at low speeds, stepper motors are the opposite, generating their maximum torque at low speeds. This makes then the right choice for applications requiring high precision at low speeds.
It is also necessary to know the limitations of stepper motors—low efficiency, limited high-speed torque, and no feedback. Stepper motors are notoriously low efficiency devices, as their current consumption is independent of the load they are driving. Moreover, when it is stationary and not doing work, a stepper motor draws the maximum current. The low efficiency of these motors manifests itself in the high amount of heat they generate. Contrary to that of other motors, stepper motors exhibit lower torque at high speeds than they do at low speeds. Even for steppers optimized for better high-speed operation, achieving that requires them to be paired with appropriated drivers. Servomotors achieve their positions aided by integral feedback. However, steppers have no such provision, achieving high precision when running open loop. Limit switches or home detectors are necessary for safety and for achieving a reference position.
Selecting a stepper motor for a specific task requires considering three major characteristics—motor size, step count, and gearing. The general concept is larger motors will deliver higher power. Manufacturers specify motor power in torque ratings, and NEMA numbers to specify their frame sizes. To decide whether the motor has the strength to meet your requirement, look at its torque ratings. While NEMA 57 is a monster size, 3-D printers and CNC mills usually use a NEMA 17 size motor. The NEMA numbers also specify standardized faceplate dimensions for mounting the motor.
The step count defines the positioning resolution. A motor can have a specific number of steps per revolution, which usually ranges from 4-400. For instance, step counts commonly available are 24, 48, and 200. Resolution of a stepper motor is specified in degrees per step. For instance, a motor rotating 1.8 degrees per step is actually rotating at 200 steps per revolution. A higher resolution motor usually sacrifices speed and torque. Therefore, motors with high step counts have lower RPMs and lower torques than do similar sized but low-step-count motors running at similar speeds.
