Stepper motors are used in industrial automation and robotics because they can move in increments. These motors are used in IC manufacturing cells. They play an indispensable role in the design of CNC (Computer Numerical Control) machines and XY plotters.
Stepper motor working principle
A stepper motor is a DC motor that has permanent magnets arranged in the rotor, and two, three, or four sets of coils, called stages, placed around the stator. The windings are connected to an external logic driver that sequentially supplies voltage pulses to the windings. The motor responds to these pulses and performs start, stop, and reverse operations on command.
Both the rotor and stator have a defined number of teeth to accommodate the designed step angle. The step angle is defined as the angular displacement of the rotor in response to each pulse. The rotor position depends on the step angle and the number of pulses. The rotational speed depends on the pulse rate (not the power supply voltage) being precisely controlled; thus making the stepper motor an ideal driver for operations involving precise positioning. Unlike control and servo motors, no feedback control winding is needed to close the loop and monitor the rotor's position and speed.
Stepper motor systems must accelerate and decelerate at a rate that allows the motor to overcome the inertia of the drive system. Therefore, the rotor has a small diameter and a relatively long length. If a stepper motor is dynamically overloaded, it will slip phases. These motors are suitable for applications where the load is within the motor's capacity range.
Basic classification of stepper motors
Based on their structural type, stepper motors can be divided into three types: variable reluctance (VR) stepper motors, permanent magnet (PM) stepper motors, and hybrid stepper motors.
A variable reluctance (VR) stepper motor is a type of motor whose rotor is made of ferromagnetic material. Therefore, when the stator is excited, it becomes an electromagnet, and the rotor experiences a pulling force in a specified direction. Ferromagnetic materials always attempt to adjust themselves along the path of least magnetic reluctance. By exciting the coils, a magnetic field is generated, changing the air gap reluctance. Hence the name variable reluctance stepper motor. In this type of motor, the direction of the motor is independent of the direction of the current in the windings.
In a permanent magnet (PM) stepper motor, the rotor is permanently magnetized. Therefore, the motor's motion is due to the attractive and repulsive forces between the stator and rotor magnetic poles. In this type of motor, the direction of the motor depends directly on the direction of the current in the windings, as the magnetic poles reverse, thus changing the direction of the current flowing through the rotor.
Hybrid stepper motors, as the name suggests, are motors designed to provide higher efficiency by combining the advantages of permanent magnet stepper motors and variable reluctance stepper motors. VR and PM stepper motors are the most common types. The only difference is that in a variable reluctance stepper motor, the rotor is made of ferromagnetic material, while in the case of a permanent magnet stepper motor, the rotor is permanently magnetized.
This article introduces the working principle of a stepper motor. By reading the full text, you will understand that a stepper motor is a DC motor with permanent magnets arranged in the rotor, and two, three, or four sets of coils, called stages, placed around the stator. The windings are connected to an external logic driver, which sequentially provides voltage pulses to the windings. The motor responds to these pulses and performs start, stop, and reverse operations upon command.
