December 2021

Focus on: Drives, Motors & Controls Process, Controls & Plant Annual Buyers’ Guide 2022 www.pwemag.co.uk Plant & Works Engineering | 27 In short, stepper motors are ideal for a wide variety of industries and small power applications, ranging from 3D printers, adjustable conveyors and small metering pumps. What is a servo motor? While servo motors simply mean a motor controlled with feedback, in the world of automation it is accepted to specifically mean an AC permanent magnet synchronous motor. These motors have a stator similar to that of an induction motor, but their rotors are constructed with rare earth magnets. This eliminates the slip found in induction motors and allows the servo motor to run at the synchronous speed defined by the frequency and number of poles (rpm = 120*Frequency# of Poles). Advantages of servo motors AC servo motors use rare earth permanent magnets, so they offer best-in- class torque density. This is preferred for very dynamic motor applications that must accelerate and decelerate quickly. Servo motors include some sort of feedback, whether it is a resolver or encoder. The shaft feedback is critical because it is used in the servo controller to control rotor position and torque. Unlike a stepper motor, servo motors are capable of providing momentary peak torque across its speed range. A servo motor’s peak torque can be very important in certain applications and can result in an optimised motor selection. In addition, AC servo motors operate at much higher speeds compared to stepper motors. Critical motion control applications that involve functional safety and the handling of critical or expensive products (e.g. semiconductor machines) should use servo motors with feedback. The low cost benefit of a stepper motor/control simply does not justify the risk of injury or product failure. Common examples for their best use are in packaging machines, material handling and robotics. Limitations of servo motors The main disadvantage of servo motors compared to stepper motors is going to be the cost of the motor and the associated drive that controls it. This is particularly true for smaller power applications (e.g. 500W and below). If the application does not require the precision and high torque overload that a servo motor offers, then a stepper motor is likely to be the better choice. Servo motors are well suited to a range of industries and applications such as powering conveyor systems for packaging machines, metal cutting and forming machines, as well as robotics. *Mike Keefe is from KEB America Automation with Drive www.keb.co.uk KEB (UK) Ltd. 5 Morris Close Park Farm Industrial Estate Wellingborough NN8 6XF Tel: 01933 402220 E-Mail: info@keb.co.uk SMART AND DIGITAL AUTOMATION Packaging Machinery Theatre Systems Metal Processing Textile Machinery Process Engineering Lift & Escalators Intra- logistics Plastics Machinery e-mobility Automotive Wood Working Wind Power SPECIALISING IN SOLUTIONS