Stepper motor versus servo motor: which is right for your application?

In 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. In this article, Mike Keefe of KEB America explains the differences between stepper and servo motors and when it is appropriate to use each technology.

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°.

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.

DC input stepper drives are very compact and can even be mounted in DIN rails in an electrical enclosure.

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.

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.

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.

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 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.

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. 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.

KEB (UK) Ltd

5 Morris Close
Park Farm Industrial Estate
Wellingborough
NN8 6XF
UNITED KINGDOM

+44 (0)1933 402220

info@keb.co.uk

www.keb.co.uk

More products
Nabtesco delivers solutions for every drive application
To continue meeting increasing demand, Nabtesco is substantially expanding its production capacities with the construction of a new factory in Hamamatsu, 250 km southwest of the its headquarters in Tokyo – which will double the annual production capacity to two million precision gearboxes by 2026.
Festo showing ‘Progress in Motion’ at PPMA 2022
The Festo stand at PPMA 2022 will focus on key applications for the process and packaging sector. These include innovative process valve controls, servo-controlled flow wrapping and a new solution for controlling web tension with dancing rollers.
IFM Electronic makes IIoT easy at PPMA 2022
At this year’s PPMA Show, IFM Electronic is focusing on its moneo IIoT platform. This has been developed specifically to provide a convenient solution to the challenge of converting the huge amount of data produced by modern plant and machines into useful information that can be used to reduce downtime, optimise efficiency and cut costs.
Bürkert extends capabilities with Fine Controls partnership
The key distribution partner agreement between Bürkert and Fine Controls brings OEMs or end users in process control a distributor who can supply all the parts necessary to build a complete process system, as well as offering design and specification support.
Individual service for signal integrity
The demands on connection technology performance are constantly increasing. By providing customised support, Phoenix Contact helps customers select connection components to ensure the highest possible signal integrity.
Aveva MES rises in the leader ranks of the latest Gartner report
Aveva’s Manufacturing Execution Systems (MES) offering, which is deployed in the UK & Ireland by AVEVA Select partners SolutionsPT, has been recognised as a global leader in industrial software in the latest 2022 Gartner Magic Quadrant for Manufacturing Execution Systems.
Siemens selected by Microsoft for RAMP Programme
Siemens Digital Industries Software has been selected to participate in the Rapid Assured Microelectronics Prototypes (RAMP) Phase II initiative.
Thomson Industries exhibits linear actuators for the marine industry
Thomson Industries will be exhibiting at the 30th Shipbuilding, Machinery and Marine Technology (SMM) trade fair in Hamburg in September, showcasing its comprehensive line of electric linear actuators for marine applications.
Harwin and MSA Components sign German distribution deal
Harwin is bringing greater strength to its German supply chain through a new strategic partnership with MSA Components. The agreement allows MSA to offer all the latest products from Harwin’s extensive portfolio.
Mould making at Emka for injection moulding of plastics and metals
Emka reports that it is now one of the top three mould makers in Europe – producing 900 moulds a year for plastic injection, including polyamide/glass and Grivery, as well as zinc and aluminium die casting.

Login / Sign up