Stepper Motor Basics: A stepper motor rotates a shaft by steps (i.e., fixed angular movements). Its internal structure eliminates the need for a sensor; the exact angular position of the shaft can be determined simply by counting the number of steps. This makes it suitable for a wide range of applications. Like all motors, stepper motors operate on the same principle: a fixed part (stator) and a moving part (rotor). The stator has gear-like protrusions wrapped with coils, while the rotor consists of either permanent magnets or a variable reluctance core. We'll delve deeper into different rotor structures later. Figure 1 shows a cross-section of a motor with a variable reluctance core as its rotor.
The basic operating principle of a stepper motor is this: energizing one or more stator phases generates a magnetic field through the coils, which the rotor aligns with. Sequentially applying voltage to each phase causes the rotor to rotate a specific angle, ultimately reaching the desired position. Figure 2 illustrates this principle. First, coil A is energized, generating a magnetic field that aligns the rotor. When coil B is energized, the rotor rotates 60° clockwise to align with the new magnetic field. The same process occurs when coil C is energized. The color of the stator teeth in the figure below indicates the direction of the magnetic field generated by the stator windings.
There are basically three types of rotors in stepper motors: Permanent magnet rotors: These rotors are permanent magnets aligned with the magnetic field generated by the stator circuit. This type of rotor provides good torque and braking torque. This means that the motor resists (even if not strongly) changes in position regardless of whether the coils are energized. However, compared to other rotor types, this type of rotor has the disadvantages of lower speed and resolution. Figure 3 shows a cross-section of a permanent magnet stepper motor.
Variable reluctance rotors: The rotor is made of an iron core with a special shape that aligns with the magnetic field (see Figures 1 and 2). This rotor more easily achieves high speed and resolution, but it generally produces lower torque and lacks detent torque. Hybrid rotors: This rotor has a unique structure that is a hybrid of a permanent magnet and a variable reluctance rotor. The rotor has two axially magnetized caps with alternating small teeth. This configuration combines the advantages of both permanent magnet and variable reluctance rotors, particularly high resolution, high speed, and high torque. However, higher performance demands come with a more complex structure and higher cost. Figure 3 shows a simplified schematic of this motor structure. When coil A is energized, a small tooth on the rotor's N cap aligns with a stator tooth magnetized S. Simultaneously, due to the rotor's structure, the rotor's S cap aligns with a stator tooth magnetized N. Although the operating principle of a stepper motor is the same, the actual motor structure is more complex and has a larger number of teeth than shown in the figure. This large number of teeth enables extremely small step angles, as small as 0.9°.
The stator is the part of the motor responsible for generating the magnetic field with which the rotor is aligned. The key characteristics of the stator circuit relate to its number of phases, pole pairs, and wire configuration. The number of phases refers to the number of individual coils, while the number of pole pairs indicates the primary tooth pairs occupied by each phase. Two-phase stepper motors are the most common, while three-phase and five-phase motors are less common (see Figures 5 and 6).
As mentioned above, the motor coils need to be energized in a specific sequence to generate a magnetic field that the rotor will align with. The following devices (starting with those closest to the motor) can provide the necessary voltage to the coils for proper motor operation: Transistor bridge: This device physically controls the electrical connections of the motor coils. A transistor can be thought of as an electrically controlled circuit breaker; when closed, the coils are connected to a power source, allowing current to flow through them. A transistor bridge is required for each motor phase. Pre-driver: This device controls transistor activation and is controlled by an MCU to provide the required voltage and current. The MCU is a microcontroller unit (MCU), typically programmed by the motor user. It generates specific signals to the pre-driver to achieve the desired motor behavior. Figure 7 shows a simplified schematic of a stepper motor control scheme. The pre-driver and transistor bridge can be contained in a single device, the driver.
