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Eric Rice • National Marketing Director • Applied Motion Products
Step motors offer the automation industry a cost-effective and simple method to digitally
control motion in a wide range of applications — including packaging equipment, 3D printers,
material handling and sorting lines, benchtop CNC machines, and more. They serve as critical
components of many rotary and linear positioning axes.The cost-performance benefits of step motors lie in their simplicity and their ability to
position accurately in open-loop control schemes, without any feedback from the motor to the
controller. Getting the optimal performance benefits of an open-loop stepper system requires
understanding how to specify and install a step motor into an application. Following are six
common mistakes that step motor users, both novice and experienced, can easily avoid.1
“The torque specification of the step motor is higher than what I’m
seeing in practice.”After calculating the torque required to move the load in an application, a user selects
a step motor based on (1) the holding torque specification of the step motor or (2) the speed-
torque curve. Once mounted and coupled to the load, the motor doesn’t produce the amount of
expected torque.
The first mistake is using the holding torque as a measure of performance to specify the step
motor. Holding torque defines the torque a motor produces when maintaining a position and
not moving. It is generally a poor indicator of the torque the motor produces when moving.
When a step motor starts moving, the produced torque falls precipitously from the holding
torque value, even after just a few rpms. As speed increases, the torque falls further. For this
reason, don’t select a step motor based on holding torque alone. Instead, refer to published
speed-torque curves.
The second mistake is failing to understand the nature of speed-torque curves. A speed-
torque curve represents the torque at which the step motor stalls. When a step motor stalls, the
rotor loses synchronization with the stator and the shaft stops turning.
To ensure the step motor continues to turn and provides enough torque to move the load,
evaluate the speed-torque curves by estimating a margin of safety. A simple way to do this is102 August 2019 http://www.designworldonline.com DESIGN WORLD
DESIGN WORLD http://www.designworldonline.com August 2019 103
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