An electric motor (English: Electric machinery, commonly known as a "motor") is an electromagnetic device that converts or transmits electrical energy based on the law of electromagnetic induction. It is represented by the letter M (formerly D in the old standard) in circuit diagrams. Its main function is to generate driving torque, serving as a power source for electrical appliances or various machines.
1. Use of electric motors
Control motors used in industries such as industry, agriculture, and transportation mainly include servo motors, stepper motors, torque motors, switched reluctance motors, and brushless DC motors.
Among various types of electric motors, the most widely used is the AC asynchronous motor (also known as an induction motor). It is convenient to use, reliable in operation, inexpensive, and robust in structure, but its power factor is relatively low, and speed regulation is more difficult. Synchronous motors are commonly used for large-capacity, low-speed power machines because they not only have a high power factor, but their speed is also independent of the load size, determined only by the grid frequency, resulting in more stable operation. DC motors are often used in applications requiring a wide range of speed regulation, but they have commutators, complex structures, are expensive, difficult to maintain, and are unsuitable for harsh environments.
Since the 1970s, with the development of power electronics technology, the speed control technology of AC motors has gradually matured, and the price of equipment has decreased, leading to its application. The maximum output mechanical power that a motor can withstand under specified operating conditions (continuous, short-time, or intermittent periodic operation) without causing overheating is called its rated power. The specifications on the nameplate should be carefully observed during use.
When operating an electric motor, it is crucial to ensure that the characteristics of the load match the characteristics of the motor itself to prevent overspeeding or stalling. There are many methods for adjusting the speed of an electric motor, catering to the varying speed requirements of different production machinery. Generally, the output power of an electric motor changes with its rotational speed when its speed is adjusted. From an energy consumption perspective, speed adjustment can be broadly categorized into two types: one is to maintain a constant input power and adjust the output power by changing the energy consumption of the speed control device to regulate the motor's speed; the other is to control the motor's input power to regulate its speed.
2. Development of Electric Motors
Judging from the current development of electric motors, energy saving is the goal, intelligent control is the trend, and brushless DC motors are the hot topic.
Energy conservation is a key aspect of development.
With the global energy crisis becoming increasingly prominent, governments and businesses worldwide are striving to find ways to achieve sustainable energy development, with increasing energy supply and reducing energy consumption being common methods. Currently, my country also faces numerous energy constraints, making it particularly urgent to balance energy and economic development.
From the perspective of energy conservation and environmental protection, high-efficiency electric motors are the current international development trend. Europe, based on motor operating time, has established the CEMEP standard, which classifies efficiency into three levels: eff1 (highest), eff2, and eff3 (lowest), implemented in stages between 2003 and 2006. The IEC 60034-30 standard classifies motor efficiency into four levels: IE1 (corresponding to eff2), IE2 (corresponding to eff1), IE3, and IE4 (highest). my country has committed to implementing IE2 and higher standards from July 1, 2011. International requirements and the domestic energy shortage situation necessitate making energy-saving development of electric motors a top priority.
Intelligent control is the trend
With the development of communication technology, intelligent control has become a hot topic in the field of motors. The fully automatic washing machines and automatic curtains we use in our daily lives all convey intelligent signals. Motor control is also becoming simpler and more intelligent. PLC, FPGA, DSP and other technologies are increasingly being integrated into the motor industry chain.
Intelligent motor control offers far more functionality than traditional motor control. Intelligent motors achieve low-carbon operation and significantly reduce the probability of failure and downtime, making it an inevitable trend in motor control development. For example, intelligent power modules can be used, utilizing microcontrollers or DSPs to form a power interface with the motor, which can reduce motor size and simplify design.
This module offers advantages over previous discrete solutions in terms of lower parasitic inductance and higher reliability because all power devices within the module use chips from the same batch, ensuring consistent test performance. This intelligent power module can directly interface with the low-voltage TTL or CMOS output of a microcontroller and includes protection circuitry.
The module incorporates a thermistor for monitoring junction temperature, logic protection circuitry to prevent shoot-through between upper and lower bridge arms, dead-time control, and drive waveform shaping circuitry to minimize EMI. Within the module, each driver IC can be optimized to perform switching operations on power devices with minimal EMI and drive losses. This significantly contributes to energy conservation.
Brushless DC motors are a hot topic
After years of development and application, the industry is now focusing on brushless DC motors, especially micro-motors. Brushless DC motors use semiconductor switching devices to achieve electronic commutation, replacing traditional contact commutators and brushes with electronic switching devices, thus offering outstanding advantages such as high reliability, no commutation sparks, and low mechanical noise.
Energy-saving retrofits of motor systems utilize new technologies and materials to improve energy consumption, thereby increasing motor efficiency by reducing losses in electromagnetic, thermal, and mechanical energy. The DC inverter technology widely promoted in the home appliance industry is a prime example of energy-saving design. Experts claim that replacing all newly installed motors and their drive systems with high-efficiency, energy-saving motors annually could save over 100 billion kilowatt-hours of electricity, exceeding the total annual power generation of the Three Gorges Dam, while also reducing carbon dioxide emissions by nearly 100 million tons. "A small step for individual savings leads to a giant leap for collective savings," and the overall energy-saving effect of promoting high-efficiency, energy-saving motors is very promising.
Although the country is actively promoting energy conservation in motor systems, there are still five major challenges: basic work such as technical standards needs to be strengthened; efficient general-purpose and special-purpose equipment needs to be developed; the rational matching and operating efficiency of motor systems need to be improved; the construction of third-party energy-saving service teams needs to be strengthened; and incentive policies and institutional mechanisms need to be improved.
3. Detailed Explanation of Various Motors: Servo Motors
Servo motors are widely used in various control systems. They can convert input voltage signals into mechanical output on the motor shaft, driving the controlled components to achieve the control objective.
Servo motors are divided into DC and AC types. The earliest servo motors were ordinary DC motors, which were used when high control precision was not required. Structurally, current DC servo motors are essentially low-power DC motors, and their excitation often employs armature control and field control, but armature control is more common.
Rotary motors can be classified as follows: DC servo motors can well meet the requirements of control systems in terms of mechanical characteristics, but due to the presence of the commutator, they have many shortcomings: sparks are easily generated between the commutator and the brushes, which interferes with the operation of the driver and cannot be used in situations with flammable gases; friction exists between the brushes and the commutator, which will generate a large dead zone; the structure is complex and maintenance is relatively difficult.
An AC servo motor is essentially a two-phase asynchronous motor, and its control methods mainly include three types: amplitude control, phase control, and amplitude-phase control.
Generally, servo motors require that the motor speed be controlled by the applied voltage signal; the speed should be able to change continuously with the changes in the applied voltage signal; the motor should have a fast response, small size, and low control power.
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 angular displacement of the motor by controlling the number of pulses, thereby achieving precise positioning; at the same time, we can control the speed and acceleration of the motor by controlling the pulse frequency, thereby achieving speed regulation.
Currently, commonly used stepper motors include variable reluctance stepper motors (VR), permanent magnet stepper motors (PM), hybrid stepper motors (HB), and single-phase stepper motors.
The main difference between a stepper motor and a regular motor lies in its pulse-driven nature. This characteristic 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.
Because stepper motors are characterized by simple structure, high reliability, and low cost, they are widely used in various fields of production practice. Especially in the field of CNC machine tool manufacturing, stepper motors have always been considered the most ideal CNC machine tool actuators because they do not require A/D conversion and can directly convert digital pulse signals into angular displacement.
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 drawbacks; 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; furthermore, stepper motors have significant vibration and noise when rotating at low speeds.
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.
Switched reluctance motor
Switched reluctance motors are a new type of speed-regulating motor with an extremely simple and robust structure, low cost, and excellent speed regulation performance.
Brushless DC motor
Brushless DC motors (BLDCMs) were developed based on brushed DC motors, but their drive current is pure alternating current. Brushless DC motors can be further divided into brushless speed motors and brushless torque motors. Generally, there are two types of drive current for brushless motors: trapezoidal wave (usually a "square wave") and sine wave.
Sometimes the former is called a brushless DC motor, and the latter an AC servo motor; more precisely, it's a type of AC servo motor. To reduce rotational inertia, brushless DC motors typically employ a slender structure. Brushless DC motors are much smaller and lighter than brushed DC motors, resulting in a 40%–50% reduction in rotational inertia. Due to the challenges in manufacturing permanent magnet materials, the capacity of brushless DC motors is generally 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.
Variable frequency motor - Construction principle
Speed regulation and control of electric motors is one of the fundamental technologies for various industrial and agricultural machinery, as well as office and household electrical equipment. With the remarkable development of power electronics and microelectronics technologies, the AC speed regulation method using "dedicated variable frequency induction motor + frequency converter" is leading a revolutionary shift in the field of speed regulation, replacing traditional methods, due to its superior performance and economy.
Its benefits to various industries include: greatly improving the degree of mechanical automation and production efficiency, saving energy, improving product qualification rate and product quality, correspondingly increasing power system capacity, miniaturizing equipment, and increasing comfort. It is currently replacing traditional mechanical speed regulation and DC speed regulation solutions at a rapid pace.
Due to the special nature of variable frequency power supplies and the system's requirements for high-speed or low-speed operation and dynamic speed response, stringent requirements are placed on the electric motor, which is the main power source, bringing new challenges to the electric motor in terms of electromagnetics, structure, and insulation.
Applications of variable frequency motors
Variable frequency speed control has become the mainstream speed control solution and can be widely used in continuously variable transmissions across various industries. In particular, with the increasingly widespread application of frequency converters in the field of industrial control, the use of variable frequency motors is also becoming more and more widespread. It can be said that due to the superior performance of variable frequency motors compared to ordinary motors in terms of frequency control, we can hardly find variable frequency motors wherever frequency converters are used.
linear motor
Synchronous motors are used in various motion control systems, especially servo systems. Due to their advantages in reliability and maintenance, power factor, motor size and moment of inertia, control accuracy, and field weakening ratio, synchronous motors have become increasingly popular in large-capacity motors worldwide. For example, in industrial applications, high-power air compressors, water pumps, high-power hoists in the coal and non-ferrous metals industries, and large-capacity rolling mills in steel plants all utilize synchronous motors.
In recent years, the application of linear motors in machine tool feed servo systems has gained attention in the global machine tool industry, sparking a "linear motor craze" in industrialized regions of Western Europe. The biggest difference between using a linear motor for direct drive in machine tool feed systems and the traditional rotary motor transmission is the elimination of the mechanical transmission link between the motor and the worktable (slide).
