Understanding Stepper Motor Types & Operating Modes for Motion Control

Stepper motors are often an extremely important component in a motion control system. In a broad sense, stepper motors are similar to legacy AC induction-type motors in that they have both a stator and a rotor; however, that’s where the similarities end. Stepper motors are a type of DC synchronous motor. Whereas the rotation of an induction motor is largely uncontrollable, the rotation of a stepper motor can be controlled with a remarkable degree of precision. Stepper motors can produce full, instantaneous torque - even from a standstill. This makes them very useful for motion control applications, where accuracy, repeatability, and power are paramount.

Types Of Stepper Motors

There are three main types of stepper motors:

1. Permanent Magnet Stepper. PM steppers have rotors that are constructed with permanent magnets, which interact with the electromagnets of the stator to create rotation and torque. PM steppers usually have comparatively low power requirements and can produce more torque per unit of input power.
2. Variable Reluctance Stepper. VR stepper rotors are not built with permanent magnets. Rather, they are constructed with plain iron and resemble a gear, with protrusions or “teeth” around the circumference of the rotor. The teeth lead to VR steppers that have a very high degree of angular resolution; however, this accuracy usually comes at the expense of torque.
3. Hybrid Syncronous Stepper. HS stepper rotors use the best features of both PM and VR steppers. The rotor in an HS motor has a permanent magnet core, while the circumference is built from plain iron and has teeth. A hybrid synchronous motor, therefore, has both high angular resolution and high torque.

Operating Modes

Stepper motors have three main operating modes. Using a rotor with 200 teeth as a basic example, the operating modes are:

1. Full Step Mode. For each 360° rotation of the motor shaft, the rotor proceeds through 200 distinct steps, each exactly 1.8°. During full step operation, two of the phases on the stator are always energized. This provides maximum torque, but angular resolution is limited by the number of teeth on the rotor.
2. Half Step Mode. For each 360° rotation of the motor shaft, the rotor proceeds through 400 distinct steps, each exactly 0.9°. During half step operation, there is an alternation between having one or two phases on the stator energized. This provides twice the level of angular resolution for increased positioning accuracy but comes at the expense of torque.
3. Micro Step Mode. For each 360° rotation of the motor shaft, the rotor proceeds through 51,200 distinct steps, each exactly 0.007°. During micro-step operation, phases on the stator can be either energized, de-energized or partially energized. This mode is used in applications where highly accurate positioning is needed, although torque rating can be reduced by as much as 30%.

Important Considerations

When choosing a stepper motor for your application, it is important to analyze the motor’s torque-speed curve. This data, which should be available from the manufacturer, is a graphical representation of the motor’s torque at a given speed. The torque-speed curve of a motor must closely match the requirements of the application, otherwise, the performance of the system will not be as expected.

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