Stepper motors are commonly used in linear motion applications for their precise positioning capabilities and good holding torque. Though the basic operating principle is the same for all stepper motors, there are differences in operation and performance between the three primary types — permanent magnet, variable reluctance, and hybrid.

Permanent magnet stepper motors

Permanent magnet stepper motors have a permanent magnet rotor that is axially magnetized — meaning it has alternating north and south poles parallel to the rotor shaft. These motors are also called can-stack or tin-can motors because their stators consist of two coils contained in soft iron housings (so-called cans) with teeth on the inside that interact with the rotor.

The most common type of permanent-magnet stepper motor is called a can-stack or tin-can motor for the way the stator includes two coils contained in soft iron cans.

Permanent-magnet steppers typically have a step angle of 3.6° (100 steps per revolution) although models are available with 1.8° step angles. These motors can operate in full-step, half-step, or microstepping modes for higher resolution. Permanent magnet stepper motors have higher torque capabilities than variable reluctance types we’re about to cover, but at limited speeds … so they’re best for applications that require low to moderate torque at low speeds.

Variable reluctance stepper motors

Variable reluctance stepper motors have the simplest construction out of all three stepper-motor types. They include a nonmagnetic toothed rotor made of soft iron as well as a wound electromagnetic stator. Because the rotor is not magnetized, there’s no attraction between the rotor and stator when the windings aren’t energized — so variable reluctance motors don’t produce detent torque.

Their dynamic torque production is relatively low, but variable reluctance stepper motors have less torque dropoff at higher motor speeds. That means for moderate to high-speed applications, they are often a better choice than permanent-magnet designs. One caveat: Variable reluctance stepper motors are notoriously noisy, so are unsuitable for applications for which quiet operation is a design objective.

Hybrid stepper motors

Many NEMA stepper linear actuators have at their core hybrid stepper motors. These electromechanical linear devices leverage the way in which hybrid stepper motors impart simple and efficient operation.

A combination of permanent magnet and variable reluctance designs, hybrid stepper motors have a permanent-magnet toothed rotor made from two sections or cups that are opposite in polarity — with teeth arrays that are radially offset from each other. The electromagnetic stator is also toothed. The rotor teeth help guide the magnetic flux to preferred locations in the air gap … and that in turn improves their holding, dynamic, and detent torque when compared to those of permanent magnet and variable reluctance stepper-motor types.

Hybrid stepper motors can also achieve higher resolution than other stepper motor types, with step angles as small as 0.72° (500 steps per revolution) in full-step mode — and with even higher resolution when half-stepping or microstepping is used. The hybrid design is arguably the most widely used stepper-motor type even though its more complex construction makes it the highest-cost option of the three stepper motor variations.

Double and triple-stack hybrid stepper motors

Of the three primary stepper motor designs — permanent magnet, variable reluctance, and hybrid — hybrid stepper motors are arguably the most popular in industrial applications. Hybrid stepper motors are constructed with a rotor made of two sections or cups with a permanent magnet between them. This causes the cups to be magnetized axially — with one cup polarized north and the other cup polarized south. The surfaces of the rotor cups have precisely ground teeth (typically 50 or 100 teeth per cup) and the cups are aligned with an offset of ½ tooth pitch between the two sets of teeth.

In a hybrid stepper motor, the stator poles are also toothed — and when pulses are delivered to the stator by the stepper drive, these poles are magnetized … causing the rotor to turn so that the teeth of the rotor and stator align N-S or S-N.

This hybrid design (with teeth on both the rotor and stator) allows the motor to optimize magnetic flux, and therefore produce higher torque than permanent magnet or variable reluctance designs. Hybrid stepper motors can also achieve step angles as small as 0.72° in full-step mode and operate at higher speeds than other designs.

Although proprietary designs and production methods let manufacturers optimize the torque output (as well as step accuracy and speed characteristics) of their hybrid stepper motors, torque production is still closely tied to the frame size of the motor.

Stepper motors generally adhere to the NEMA ICS 16-2001 standard for frame sizes, which specifies mounting dimensions such as flange size and bolt circle diameter. However, one dimension not covered by the NEMA standard is motor length. And this flexibility in motor length for a given frame size provides manufacturers with another option for increasing the torque production of a particular NEMA size stepper motor — by creating motors with longer stack lengths. For example, double- and triple-stack stepper motors are now common offerings from several manufacturers.

Double and triple-stack hybrid stepper motors simply have multiple rotors and stators, stacked end-to-end. With multiple rotor and stator sections, the motor can produce more torque without the need to increase the frame size. Only the length of the motor increases. (Note that a few manufacturers also produce quad-stack stepper motors.)

However, in double- and triple-stack (and quad-stack) stepper motor designs, torque falls off faster as speed increases than it does in single-stack designs. This is because the added rotor and stator sections also increase the motor’s inductance. And higher inductance means the electrical time constant of the motor — the amount of time it takes the current in the windings to reach 63% of its maximum value — is also increased. When a stepper motor operates at high speeds, a high electrical time constant means there isn’t enough time for the current (and, therefore, torque) to reach its maximum value at each motor step, resulting in a torque drop-off as speed increases.

Another way to increase the torque from a stepper motor without increasing the NEMA frame size is to use a gearbox with the motor. The addition of a gearbox increases the torque delivered from the motor to the load — and can also provide better inertia matching between the motor and the load. Plus when connected to a gearbox, the motor can operate at higher speeds … which helps reduce or avoid resonance and oscillations.

How hybrid motor version affects linear-actuator choice

Specifying a hybrid-stepper linear actuator includes steps generally applicable to any motor-driven screw-based actuator. First, the design engineer should define required force, travel, and stroke speed for the axis in question — along with its target life in stroke cycles. (Typical peak linear force ratings for NEMA stepper linear actuators range from 180 to 1,800 N.) Design values and supplier references yield rated force and predicted loss of power beyond a given stroke count. Required power in Watts or Joule/sec depends on stroke length multiplied by force divided by time allotted per stroke. In some instances, manufacturer-supplied performance charts can help engineers more quickly identify actuators meeting all the design criteria.

Note that NEMA stepper-based linear actuators from some manufacturers are specifically designed for power density — with motor features to yield up to 50% more force (and better precision) than comparable linear actuators from competitors. That’s especially useful in laboratory and medical equipment or designs that are intended to be portable.

In some cases, a given series of one particular linear actuator can also sport stepper motors of one frame size but various stack lengths to provide a variety of holding torque capabilities … and higher linear force for longer stacks. Some linear-actuator suppliers even offer stepper motor options having different winding arrangements to satisfy specialty linear-speed requirements.

Linear Motion Tips | linearmotiontips.com