Stepper Motors

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Selecting a stepper motor requires consideration of power, torque, physical constraints and several other factors.


How to select a stepper motor for your application


Hi and welcome to ENGINEERING.com’s motor series. I’m Alysa King. In this video we’ll show you how to select a stepper motor for your application. But first, let’s quickly review what a stepper motor is.

Stepper motors have a central cogged rotor that spins within the stators. The stators are placed around the rotor just like they would be in a traditional electric motor. But instead of having a freely rotating rotor, the rotor has cogs that align with cogs in the stators. In this graphical example there are 4 stators. As the current is applied to each stator in turn, the rotor shifts precisely the distance between the stators and then locks into the stators.

In a one-phase stepper motor, the current is applied to each stator in turn, thereby moving the rotor 90 degrees.

A more common method of stepping is a two-phase approach, which energized both phases, but only switches one polarity at a time. This forces the rotor to align itself between the "average" north and the "average" south magnetic poles. While a two-phase stator draws twice as much power, it delivers 41% more torque.

Either one or two phase stepper motors can move in discrete and highly precise steps.

When compared to a servomotor, stepper motors are typically less expensive because they don’t have all of the control components of a servomotor.

Stepper motors usually work in an open loop configuration without any feedback.

Certain advantages of stepper motors make them ideal for specific applications. For instance, in addition to being lower cost, stepper motors are excellent at maintaining torque at low rpms. They tend to be very low maintenance and rugged enough to operate in any environment.

Unfortunately, the strengths of stepper motors can also turn into disadvantages in certain applications. They consume current regardless of the load, and the torque decreases as you increase speed. And since they don’t have a feedback loop, they can slip.

Stepper motors are used in computer disk drives, printers, plotters, and many more consumer electronic applications. Because of their precision and price point, they are also commonly used in factory automation such as pick and place machines and robotics. And in optics applications you will see them in linear actuators, rotation stages and mirror mounts. They are also common in the packaging industry.

To specify a stepper motor you’ll have to follow a few key steps:

1. Calculate the power that will be available to the motor

2. Calculate the output power that your application demands, then multiply that by 1.5 to 2 times

3. Determine the physical space constraints so that you can size the motor

4. Select a motor for the highest torque and speed point in the application

5. Use a 30% safety factor from above the published torque speed curve

That’s it for ENGINEERING.com’s outline on selecting a stepper motor for your application. For more details, watch some of the related videos in this section.