December 2021

I n today’s industrial marketplace, it can be challenging and confusing when choosing the correct motor for an application. As well as all the different types of motor, the information provided on each is often either too basic or too technical to be helpful. A stepper motor is a type of brushless DC motor. They are designed to provide a certain number of defined steps per revolution. For example, a motor with 200 steps/revolution will offer a position resolution of 1.8°/pulse (360°/200 pulses). Each time the motor is pulsed by the stepper controller, it will move in the forward or reverse direction according to the number of pulses received. In the stepper example above, 100 pulses would cause the motor to rotate 180°. Advantages of stepper motors A stepper motor is able to offer open-loop positioning, which is a big advantage compared to open-loop induction motors, which are not able to provide positioning. Most stepper motors do not include any feedback which lowers the overall cost of a stepper and control solution. Their low cost is a primary reason for their widespread use. One advantage in industrial applications is that a stepper motor can be run with a DC powered stepper drive. They do not require larger inverters that input 230/460VAC or bulky fusing and cabling, which can be quite significant with multi-axis applications. Limitations of stepper motors It is possible for a stepper to ‘lose’ position. This can happen if the stepper motor does not provide sufficient torque to move the shaft and load. It can also happen at high speeds where the stepper motor goes too fast to lock in at each step. This could also happen if the inertia of the load carries it past the intended step when it is stopped. If the position accuracy of the stepper is critical, some motors will add feedback such as an incremental encoder. The feedback will identify if the actual shaft position deviates from the expected position, and the motor can correct itself as needed. The exception is when the load torque exceeds the rated torque of the stepper. In this case, the motor will stall and not be able to correct itself. Steppers do not retain any sort of known positioning when they are powered down. Therefore, they need to complete a homing routine every time they are powered up. You can see this when you turn on your printer at home. You will see the motors complete a routine that resets to a known ‘home’ position. Process, Controls & Plant Focus on: Drives, Motors & Controls 26 | Plant & Works Engineering www.pwemag.co.uk Annual Buyers’ Guide 2022 Stepper motor versus servo motor: which is right for your application? Mike Keefe* explains the differences between stepper and servo motors and when it is appropriate to use each technology.