HOW A STEPPER MOTOR WORKS

The stepper motor works like an electrical machine and converts electrical energy into mechanical energy, which it releases via a shaft. With the help of a highly realistic 3D animation, we describe, among other things, the design features of a stepper motor, such as the offset toothed rotor. It allows a very high torque to be achieved as well as a constant speed to be maintained or a certain position to be approached very accurately and with no additional feedback.

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This animation explains the components that make up a stepper motor. First, we see the permanent magnet core of the rotor. Attached to this are the soft-magnetic, toothed dynamo sheets for mounting the shaft and the ball bearings.

Shown next is the stator, which is also made up of soft-magnetic plates that are insulated from one another. Seated in this is the coil body, which is made of plastic and wound with copper wire. These windings are connected to the connection cables of the motor.

In the final step, the rotor and stator are assembled and secured to the front and rear bearing shells. The corrugated washers provide the rotor with spring suspension in the axial direction and also serve to compensate for tolerances.

The individual components will be discussed again later in detail.

The die-cast aluminum end caps used on a standard motor perform an important function on a stepper motor: on the one hand, they serve to precisely align the motor shaft with the motor housing in order to achieve the most precise total radial runout possible. On the other hand, they are used to align the rotor with the stator, enabling an air gap of just 0.05 mm between the two parts.

A permanent magnet is seated in the core of the rotor and thereby forms the magnetic antipole to the electromagnet in the stator. The additional toothing in combination with the small air gap between rotor and stator allows a high position accuracy as well as a high torque to be achieved. Toothing is provided by means of soft metal plates, which are punched to form a rotor body.

Located in the stator is a plastic body with 8 pole shoes, which are wound onto the coils of the electromagnet. Used here is a temperature-resistant, high-performance plastic, allowing the motor to reach insulation class F (= 130 degrees Celsius).

The enameled copper wires of the stator are available in various thicknesses and vary in resistance and inductance. Depending on whether a slowly or quickly rotating motor is desired, different wires are used.

The motor leads are either soldered directly to the enameled copper wire of the windings or switched via a board that is integrated in the rear bearing shell. The motor windings can be wired in series or in parallel. The resistance and inductance and, thus, the motor behavior, are thereby changed. Motors wired in parallel are very well suited for dynamic operation.

Hybrid stepper motors are produced exclusively with ball bearings. They are the only movable element in the motor and determine its service life. Systems that have a proper mechanical design reach a service life > 20,000 hours provided that the maximum permissible axial and radial forces are not constantly exceeded. In harsh environmental conditions, specially protected ball bearings must be used.

The permanent magnet installed in the rotor is made of especially strong rare earth metals. In combination with the soft-metal rotor plates, it forms a magnetic unit and, simultaneously, the magnetic antipole to the electromagnet in the stator.

Like the rotor, the stator of the stepper motor consists of punched, soft-metal plates that are electrically separated from one another. It is equipped with eight pole shoes situated opposite one another and with teeth at the end. The geometric arrangement of rotor and stator teeth results in a rotating movement when power is supplied to the electromagnet in the stator.

The shaft is the part of the stepper motor that transfers the kinetic energy. It is manufactured with very high precision from electrically, non-magnetic stainless steel. For motors on which an encoder or brake is to be mounted, the shaft is extended and led out of the rear bearing seat. Hollow shafts can also be mounted.

With the help of the corrugated washers, the springs of the motor shaft are preloaded. This increases the life expectancy of the ball bearings and compensates for production-related tolerances. Using special bearing shells, it is possible to preload the motor shaft in a defined manner, e.g., to achieve a low axial play.