Science popularization, industrial application and development trend summary of various motors
Electric motor ( English: electric machine, commonly known as " motor" ) refers to an electromagnetic device that converts or transfers electric energy according to the law of electromagnetic induction. it is indicated by the letter m ( the old standard is d ) in the circuit. Its main function is to generate driving torque as a power source for electrical appliances or various machines.
1, the use of motor
Control motors in industry, agriculture, transportation and other fields mainly include servo motors, stepping motors, torque motors, switched reluctance motors and brushless DC motors.
The most widely used of all kinds of motors is AC asynchronous motor ( also called induction motor ). It is easy to use, reliable in operation, low in price and firm in structure, but it has a low power factor and is difficult to regulate speed. Synchronous motors are commonly used in power machines with large capacity and low rotation speed, because synchronous motors not only have high power factor, but also their rotation speed has nothing to do with the size of the load and only depends on the frequency of the power grid and work more stably. DC motors are often used in occasions requiring wide-range speed regulation, but they have commutator, complex structure, high price, difficult maintenance and are not suitable for harsh environments.
Since the 1970s, with the development of power electronics technology, AC motor speed regulation technology has gradually matured and the equipment price has been decreasing, thus it has been applied. The maximum output mechanical power that the motor can undertake under the specified working system ( continuous, short-term operation and intermittent periodic operation ) without causing overheating of the motor is called its rated power, and attention should be paid to the regulations on the nameplate when it is used.
When the motor is running, attention should be paid to matching the characteristics of its load with those of the motor to avoid runaway or stalling. There are many methods of motor speed regulation, which can meet the requirements of different production machinery speed changes. In general, the output power of the motor varies with the speed of the motor. From the point of view of energy consumption, speed regulation can be roughly divided into two types: one is to keep the input power unchanged and adjust the output power to adjust the rotation speed of the motor by changing the energy consumption of the speed regulation device; The second is to control the input power of the motor to adjust the rotational speed of the motor.
2. Development of Motor
According to the current development of motor, energy saving is the development goal, intelligent control is the development trend, and brushless DC motor is the development hotspot.
Energy conservation is the key to development
With the global energy crisis becoming more and more prominent, governments and enterprises all over the world are trying to find ways to achieve sustainable energy development. Increasing sources of income and reducing expenditure is the current common method. At present, our country also faces many problems in energy restriction, and it is particularly urgent to balance the relationship between energy and economic development.
From the perspective of energy conservation and environmental protection, high efficiency motors are the current international development trend. European CEMEP standards, based on the running time of motors, divided the efficiency into eff1 ( highest ), eff2 and eff3 ( lowest ) levels, which were implemented step by step from 2003 to 2006. IEC 60034-30 divides motor efficiency into four grades: IE1 ( corresponding to eff2 ), IE2 ( corresponding to eff1 ), IE3 and IE4 ( highest ). China is committed to implementing IE2 and above standards from July 1, 2011. Both the international requirements and the domestic energy shortage situation need to take the energy-saving development of motors as the first task.
Intelligent control is the trend
With the development of communication technology, intelligent control has also become a hot topic in the field of motors. The fully automatic washing machines and automatic curtains used in our life all transmit intelligent signals. Motor control tends to be simplified and intelligent. PLC, FPGA and DSP are increasingly integrated into the motor industry chain.
The function of intelligent motor control is much more than that of traditional motor control. Intelligent motor realizes low-carbon operation, and the failure probability and downtime are greatly reduced. It has become the inevitable trend of motor control development. For example, an intelligent power module can be used to form a power interface between the microcontroller or DSP and the motor, which can reduce the size of the motor and simplify the design.
Compared with previous discrete solutions, this module has the advantages of smaller parasitic inductance and higher reliability because all power devices in the module use the same batch of chips and have consistent test performance. This intelligent power module can interface directly with the microcontroller's low voltage TTL or CMOS output and has a protection circuit.
The module is internally provided with a thermistor for monitoring junction temperature, a logic protection circuit for preventing upper and lower bridge arms from passing through, dead time control, and a drive waveform shaping circuit for minimizing EMI, etc. In the module, each driving IC can be optimized to complete the switching action of the power device with minimum EMI and driving loss. It is a great promotion to energy conservation.
Brushless DC motor is a hot spot
After years of development and application, the industry is focusing on brushless DC motors, especially the development of micro-special motors. Brushless DC motor uses semiconductor switching device to realize electronic commutation, and replaces traditional contact commutator and brush with electronic switching device, which has outstanding advantages of high reliability, no commutation spark, low mechanical noise, etc.
Energy - saving reform was carried out on the motor system, new processes and materials were adopted to improve energy consumption, and the efficiency of the motor was improved by reducing the loss of electromagnetic energy, thermal energy and mechanical energy. At present, the widely publicized DC frequency conversion in the household appliance industry is a model of energy-saving design.
Some experts say that if only the newly-added motor and its dragging system are replaced by high-efficiency and energy-saving motors, more than 100 billion kilowatt hours of electricity can be saved each year, which will exceed the total amount of electricity generated by the Three Gorges Power Station throughout the year, while reducing carbon dioxide emissions by nearly 100 million tons. " One small step for individual saving and one big step for collective saving", the overall energy-saving effect of promoting high-efficiency and energy-saving motors is still very optimistic.
Although the state is actively promoting, there are still five major problems in energy saving of the motor system: basic work such as technical standards needs to be strengthened; High - efficiency general-purpose and special-purpose equipment to be developed; Reasonable matching of motor system and operation efficiency need to be improved. The construction of the third-party energy-saving service team needs to be strengthened. Incentive policies and institutional mechanisms also need to be improved.
3. Detailed explanation of various motors
Servo motors are widely used in various control systems, which can convert the input voltage signal into mechanical output on the motor shaft and drag the controlled element to achieve the control purpose.
Servo motors are divided into direct current and alternating current. The earliest servo motor was a general DC motor. Under the condition of low control precision, the general DC motor was used as the servo motor. The current DC servo motor is structurally a low-power DC motor, and its excitation is mostly controlled by armature and magnetic field, but usually controlled by armature.
According to the classification of rotating motors, DC servo motors can meet the requirements of the control system in terms of mechanical characteristics. However, due to the existence of commutator, there are many shortcomings: sparks are easily generated between commutator and brush, which interfere with the operation of the driver and cannot be applied to occasions with combustible gas. There is friction between brush and commutator, resulting in large dead zone. Complex structure and difficult maintenance.
AC servo motor is essentially a two-phase asynchronous motor, and its control methods mainly include amplitude control, phase control and amplitude-phase control.
In general, servo motors require that the rotational speed of the motor be controlled by the applied voltage signal. The rotational speed can continuously change with the change of the applied voltage signal; The response of the motor should be fast, small in size and small in control power.
stepmotor stepper motor
The so-called stepping motor is an actuator that converts electrical pulses into angular displacement. More generally, when the stepping driver receives a pulse signal, it drives the stepping motor to rotate a fixed angle in the set direction.
We can control the angular displacement of the motor by controlling the number of pulses so as to achieve the goal of accurate positioning. At the same time, the speed and acceleration of the motor can be controlled by controlling the pulse frequency so as to achieve the purpose of speed regulation.
At present, the commonly used stepping motors include reactive stepping motors ( VR ), permanent magnet stepping motors ( PM ), hybrid stepping motors ( HB ) and single-phase stepping motors.
The main difference between stepping motor and common motor lies in the form of its pulse drive. It is precisely this characteristic that stepping motor can be combined with modern digital control technology. However, the stepping motor is not as good as the traditional closed-loop DC servo motor in control accuracy, speed variation range and low speed performance. Therefore, it is mainly used in occasions where the accuracy requirement is not particularly high.
Because of its simple structure, high reliability and low cost, stepping motors are widely used in various fields of production practice. Especially in the field of NC machine tool manufacturing, stepping motor is always considered to be the most ideal execution element of NC machine tool because it does not need A/D conversion and can directly convert digital pulse signals into angular displacement.
In addition to the application in numerical control machine tools, stepping motors can also be used in other machines, such as motors in automatic feeder, motors in universal floppy disk drives, and also in printers and plotters.
In addition, stepping motors also have many defects. Due to the no-load starting frequency of the stepping motor, the stepping motor can run normally at low speed, but cannot start if it is higher than a certain speed, accompanied by a sharp whistling sound. The accuracy of subdivision drivers from different manufacturers may vary greatly, and the greater the subdivision number, the more difficult it is to control. Moreover, the stepping motor has large vibration and noise when rotating at low speed.
The so-called torque motor is a flat multi-pole permanent magnet DC motor. The armature has more slots, commutator segments and series conductors to reduce torque ripple and speed ripple. Torque motors include DC torque motors and AC torque motors.
Among them, the self-inductance reactance of DC torque motor is very small, so its response is very good. The output torque is proportional to the input current and independent of the speed and position of the rotor. It can be directly connected to the load in a low speed operation near a locked-rotor state without gear reduction, so that a high torque to inertia ratio can be generated on the shaft of the load, and system errors due to the use of reduction gears can be eliminated.
AC torque motors can be divided into synchronous and asynchronous motors. At present, squirrel-cage asynchronous torque motors are commonly used, which have the characteristics of low rotation speed and large torque. In general, AC torque motors are often used in the textile industry, and their working principle and structure are the same as those of single-phase asynchronous motors, but their mechanical characteristics are relatively soft due to the large resistance of squirrel-cage rotors.
Switched reluctance motor
Switched reluctance motor ( SRM ) is a new type of speed regulation motor, which is extremely simple and firm in structure, low in cost and excellent in speed regulation performance.