Electric motors play a crucial role in many motion control functions across a wide range of industries, including packaging, food and beverage, manufacturing, medical, and robotics. We can select from several motor types based on functionality, size, torque, precision, and speed requirements.
As we all know, electric motors are an important component of transmission and control systems. With the development of modern science and technology, the focus of electric motors in practical applications has begun to shift from simple transmission to complex control, especially the precise control of motor speed, position, and torque. However, electric motors have different designs and drive methods depending on the application, which may seem very complicated at first glance. Therefore, to help people understand the purpose of rotating electric motors, a basic classification has been made. Below, we will introduce the most representative, commonly used, and basic types of electric motors—control motors, power motors, and signal motors.
Control motor
Control motors are mainly used for precise speed and position control, serving as "actuators" in control systems. They can be categorized into several types, including servo motors, stepper motors, torque motors, switched reluctance motors, and brushless DC motors.
1. Servo motor
Servo motors are widely used in various control systems. They convert input voltage signals into mechanical outputs on the motor shaft, driving the controlled components to achieve control objectives. Generally, servo motors require that the motor speed be controlled by the applied voltage signal; the speed should change continuously with variations in the applied voltage signal; the torque should be controlled by the current output of the controller; and the motor should have a fast response, small size, and low control power. Servo motors are mainly used in various motion control systems, especially follow-up systems.
Servo motors are divided into DC and AC types. The earliest servo motors were ordinary DC motors, used only when high control precision was not required. Currently, with the rapid development of permanent magnet synchronous motor technology, most servo motors are either AC permanent magnet synchronous servo motors or DC brushless motors.
2. Stepper motor
A stepper motor is an actuator that converts electrical pulses into angular displacement. In simpler terms, when a stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle in a predetermined direction. We can control the amount of angular displacement by controlling the number of pulses, thus achieving precise positioning; we can also control the speed and acceleration of the motor by controlling the pulse frequency, thus achieving speed regulation. Currently, commonly used stepper motors include reactive stepper motors (VR), permanent magnet stepper motors (PM), hybrid stepper motors (HB), and single-phase stepper motors.
The main difference between stepper motors and ordinary motors lies in their pulse-driven nature, a characteristic that allows stepper motors to be integrated with modern digital control technology. However, stepper motors are inferior to traditional closed-loop controlled DC servo motors in terms of control precision, speed range, and low-speed performance; therefore, they are mainly used in applications where high precision is not required. Due to their simple structure, high reliability, and low cost, stepper motors are widely used in various fields of production practice, especially in CNC machine tool manufacturing. Because stepper motors do not require A/D conversion and can directly convert digital pulse signals into angular displacement, they have long been considered the most ideal actuator for CNC machine tools.
Besides their application in CNC machine tools, stepper motors can also be used in other machinery, such as as motors in automatic feeders, as motors in general-purpose floppy disk drives, and in printers and plotters.
In addition, stepper motors also have many defects; because stepper motors have an unloaded starting frequency, they can run normally at low speeds, but they cannot start if the speed exceeds a certain level, accompanied by a sharp whistling sound; the precision of microstepping drivers from different manufacturers may vary greatly, and the higher the microstepping number, the more difficult it is to control the precision; and stepper motors have significant vibration and noise when rotating at low speeds.
3. Torque motor
A torque motor is a flat, multi-pole permanent magnet DC motor. Its armature has a higher number of slots, commutator segments, and series conductors to reduce torque and speed ripple. There are two types of torque motors: DC torque motors and AC torque motors.
Among them, the DC torque motor has very small self-inductance reactance, so it has excellent responsiveness; its output torque is proportional to the input current and is independent of the rotor speed and position; it can be directly connected to the load and run at low speed without gear reduction in a near-stall state, so it can generate a high torque-to-inertia ratio on the load shaft and eliminate the systematic errors caused by the use of reduction gears.
AC torque motors can be divided into synchronous and asynchronous types. Currently, the most commonly used is the squirrel-cage asynchronous torque motor, which features low speed and high torque. Generally, AC torque motors are frequently used in the textile industry. Their working principle and structure are the same as those of single-phase asynchronous motors, but due to the higher resistance of the squirrel-cage rotor, their mechanical characteristics are softer.
4. Switched reluctance motor
Switched reluctance motors are a new type of speed-regulating motor. They are extremely simple and robust in structure, low in cost, and offer excellent speed regulation performance, making them a strong competitor to traditional control motors and possessing significant market potential. However, they currently also have issues such as torque ripple, high operating noise, and significant vibration, requiring time for optimization and improvement to adapt to practical market applications.
5. Brushless DC motor
Brushless DC motors (BLDCMs) were developed based on brushed DC motors, but their drive current is unequivocally alternating current (AC). Brushless DC motors can be further divided into brushless speed motors and brushless torque motors. Generally, brushless motors have two types of drive current: trapezoidal wave (usually a "square wave") and sine wave. Sometimes the former is called a DC brushless motor, and the latter an AC servo motor; more accurately, it is a type of AC servo motor.
To reduce rotational inertia, brushless DC motors typically employ a slender structure. They are significantly smaller and lighter than brushed DC motors, resulting in a 40%–50% reduction in rotational inertia. Due to challenges in manufacturing permanent magnet materials, the capacity of brushless DC motors is generally limited to below 100kW.
This type of motor has good linearity in mechanical and regulatory characteristics, a wide speed range, long life, convenient maintenance, low noise, and does not have a series of problems caused by brushes. Therefore, this type of motor has great application potential in control systems.
