What is a stepper motor?

A stepper motor is an electromechanical device converting electrical energy into mechanical power. At the same time, a stepper motor is a brushless, synchronous motor that can divide a full rotation into a huge number of steps. When a stepper motor is applied with electrical command pulses in the proper sequence, the shaft or spindle of the stepper motor rotates in discrete steps, allowing precise control of the motor's position without any feedback mechanism, as long as the motor is sized for the application.

1. Construction of stepper motors

Stepper motors generally consist of front and rear end caps, bearings, center shaft, rotor core, stator core, stator assembly, corrugated washers, screws, and other parts. The two main parts of a stepper motor are the stator and the rotor. They are both composed of magnetic materials with six and four magnetic poles on them respectively. The six magnetic poles of the stator have control windings on them, and the two opposite poles form a phase.

2. Types of Stepper Motors

There are three basic types of stepper motors, reactive rotor: variable reluctance stepper motor(VR), active rotor: permanent magnet stepper motor(PM),  combination of VR and PM: Hybrid stepper motor (HY)

Variable reluctance stepper motors

Variable reluctance stepper motors are the simplest type of stepper and are made up of a soft iron multi-tooth rotor and a wound stator. Both poles are magnetized when DC current is applied to the stator winding.

When the rotor teeth are attracted to the energized stator poles, rotation occurs. As the magnets of variable reluctance stepper motors are smaller and lighter than those of permanent magnet stepper motors, they are faster. The smaller the area between the rotor and stator gears of a variable-resistance stepper motor, the smaller the loss of magnetic force.

For this type of stepper motor, although the structure is simple and easy to control, the resolution is low and the torque is small.

Permanent magnet stepper motors

Permanent magnets are incorporated into the motor structure of the permanent magnet stepper. The rotor position change will be caused by a change in the current direction in the coils, changing the magnetic poles. The rotor will rotate 90° as the direction of the current is properly changed. This single step of the motor, although useful in some applications, would be very large and inaccurate. Therefore, the really existing permanent magnet motors have more rotor poles and several magnets mounted on the rotor to increase the number of steps and positioning precision.

The magnetized rotor poles provide greater magnetic flux intensity and, as a result, permanent magnet motors exhibit better torque characteristics than variable reluctance stepper motors. The simple construction results in a moderately priced motor with fairly low resolution.

Hybrid stepper motors

Hybrid stepper motors are one of the most widely used types of stepper motors in the industry. Hybrid stepper motors combine the best features of variable reluctance and permanent magnet stepper motors, so they are more expensive. Hybrid stepper motors offer better performance in terms of step resolution, torque and speed.

The rotor of a hybrid stepper motor consists of permanent magnets, but unlike the models discussed above, the magnets are not mounted radially but magnetized axially. Normally, the rotor consists of two opposing magnetized rings placed on the motor shaft. It has slotted grooves in each ring to form rotor teeth.

The number of full steps per revolution can be calculated from the following formula:

• SPR = NR x Ø
• Where: SPR = number of steps per revolution
• NR = total number of rotor teeth (total for both yokes)
• Ø = number of motor phases
• or: NR = SPR/Ø

Comparisons	Variable reluctance stepper motor(VR)	Permanent magnet stepper motor (PM)	Hybrid stepper motor(HB)
Cost	Medium	Relatively cheap	Relatively expensive
Design Structure	Simple	Medium	Relatively complex
Resolution	Step angle: 1.8°, 0.9° or less	Step angle: 3°~30°	Step angle: 1.8°, 0.9° or less
Torque speed curve	No significant torque drop at high speed	Large torque at low speed, significant torque drop at high speed	Large torque at low speed, significant torque drop at high speed
Noise	More noise	Less noise	More noise (better with microstep subdivision)
Heat generation	High temperature rise (need heat sink)	Low temperature rise	Low temperature rise
Microstep subdivision	Usually operate in full step only	Ferrite or neodymium-iron-boron magnets	Neodymium-iron-boron magnets
Rotor material	Silicon steel sheet (no permanent magnets)	Full step, half step, micro step	Full step, half step, micro step
Rotor surface	Multi-tooth structure on the rotor outer surface	Smooth and toothless	Multi-tooth structure on the rotor outer surface
Magnetic torque	Generates only attractive torque	Both attractive and repulsive torque during operation	Both attractive and repulsive torque during operation

3. Types of Wiring

Another classification of stepper motors is based on the type of winding of the two-phase motor. Stepper motors are classified into unipolar and bipolar motors based on this classification.

The main difference is that unipolar motors operate with one current (voltage) polarity, while bipolar motors operate with two polarities, which means that the direction of current flow in the coils is variable. A further difference is that the motor coils must be connected in order to transfer power from the end of one coil to the beginning of the other. This connection method permits one polarity of current (voltage) to be used. A bipolar motor has a higher torque than a unipolar motor, but it is also more complicated to control it.

Bipolar stepper motor	Unipolar stepper motor
Higher torque	Lower torque
Higher anchorage due to windings	Lower anchorage
Smaller	Larger
Cheaper	More expensive
More complicated control, which requires a board that includes power and slew control stages	Easier control by requiring only one power circuit to be completed

4. Advantages of stepper motors

• Positioning - Due to stepper motors moving in precise repeatable steps, they perform well in applications that require precise positioning.
• Speed control - Precise motion increments provide excellent control of speed.
• Low-speed torque - Stepper motors have high torque at low speeds.

5. Disadvantages of stepper motors

• Low efficiency - Compared to DC motors, they use more current. Therefore they tend to run hot.
• Limited high speed torque - The torque of the motor drops quickly as speed decreases
• No feedback - Feedback is not used to specify possible missed steps
• High levels of vibration and noise - At very high speeds, operation of this motor is not easy.
• Possible loss of synchronization - Caused by factors such as unexpected changes in load
• Small torque toward inertia ratio
• Require a drive circuit