What is a Stepper Motor?

Understanding Stepper Motor Types and Operating Modals for Motion Control

Stepper motors are an important component of any motion control system. In general, stepper motors look similar to AC induction-type motors. They have both a stator AND a rotor. Stepper motors are a DC synchronous type motor. An induction motor's spin is difficult to control, but a stepper motor rotates with remarkable precision. Stepper motors are capable of producing full, instantaneous torque from a standstill. These motors are very useful in motion control applications that require precision, repeatability and power.

Types Of Stepper Motors

There are three types main stepper motors.

Permanent magnet stepper. PM steppers are rotors with permanent magnets. These permanent magnets interact with the stator's electromagnets to create rotation and torque. PM steppers have lower power requirements and produce more torque for each unit of input power.

Variable Reluctance Stepper. The VR stepper is not made with permanent magnets. Instead, they are made from plain iron. They resemble a gear and have protrusions, or "teeth", around the circumference. VR steppers are equipped with teeth, which allow for very high levels of angular resolution. However, this precision often comes at a cost of torque.

Hybrid Syncronous stepper. The best features from both PM and VR steppers are used in the HS stepper's rotors. An HS motor's rotor is equipped with a permanent magnetic core. However, the circumference of the HS motor is made from plain steel and has teeth. The hybrid synchronous motor is therefore capable of high angular resolution while also producing high torque.

Operating Modes

There are three modes of operation for stepper motors. As an example, let's take a rotor that has 200 teeth. The three main operating modes are:

Full Step Mode. The rotor takes 200 steps each 360deg. Two of the phases on a stator phase are always energized when the operation is complete. This allows for maximum torque, but the angular resolution is limited due to the number and size of the teeth on the rotor.

Half-Step Mode. For every 360deg turn of the motor shaft the rotor moves through 400 steps each measuring exactly 0.9deg. Half-step operation involves alternating between one or two phases being energized on the stator. This allows for twice as much angular resolution, which improves positioning accuracy. However, torque is affected.

Micro Step Mode. The rotor moves through 51,200 distinct steps each 360deg. Micro-step operation can allow phases on the stator for either partial energized (de-energized) or energized (energized). This mode is useful for applications where precise positioning is required, but torque rating can be reduced up to 30%.

Important Considerations

You should analyze the torque-speed curve when choosing a steppermotor for your application. This data should be obtained from the manufacturer. It shows the motor's torque at a particular speed. The application's requirements must be closely matched by the motor's torque-speed curve. Otherwise, performance of the system won't be as good as it should.