second, synchronous motor and asynchronous motor
the working principle of asynchronous motor (induction motor) is to generate induced current and electromagnetic torque in the rotor through the rotating magnetic field of the stator, and there is no direct magnetic field in the rotor. Therefore, the rotor speed must be less than the synchronous speed (without this difference, that is, the slip ratio, there is no rotor induced current), so it is called asynchronous motor. The rotor of a synchronous motor itself generates a magnetic field in a fixed direction (generated by a permanent magnet or DC current), and the rotating magnetic field of the stator "drags" the rotor magnetic field (rotor) to rotate, so the rotating speed of the rotor must be equal to the synchronous speed, so it is called a synchronous motor. The rotating speed n of permanent magnet synchronous motor is always constant at n=6f/p, where f is the set frequency and p is the motor pole number.
because there is no need to absorb reactive current from the power grid and there is no copper loss or iron loss on the rotor, the power factor of synchronous motor can be kept close to 1 in a wide load range, the machine efficiency is improved by about 8% compared with that of asynchronous motor with the same capacity, and the power index (η xcos φ) is improved by about 18%. The power density of pm synchronous motor is about 5% higher than that of asynchronous motors with the same capacity.
3. Comparison of energy saving
1. Comparison of operating efficiency of permanent magnet synchronous motor and asynchronous motor under different loads:
As can be seen from the figure, the operating efficiency of permanent magnet synchronous motor is obviously higher than that of asynchronous motor.
2. Actual test
1). Test conditions:
The test sample (disc-shaped sheet) was injected with the same mold on two injection molding machines, and the raw material was ordinary PP. The two injection molding machines are HTF86X1/J2 (using synchronous servo motor and quantitative gear pump) and HTF8/J1 (using asynchronous motor and variable displacement pump).
2). Test power curve of permanent magnet synchronous servo motor (see the figure below)
The test time of both injection molding machines is one hour, and the injection cycle is 19.s, in which the cooling time is 3s, and the pause time of mold support is 2 seconds. The actual power consumption of HTF86X1/J2 and HTF8/J1 is 2.12 kWh and 2.75 kWh, respectively. According to the calculation, HTF86X1/J2 saves .63 kWh per hour compared with HTF8/J1, which can save about 22.9% electricity.
3) Analysis of energy-saving effect:
Under the same conditions, using frequency converter to control synchronous motor can save more than 2% electricity than controlling asynchronous motor. The energy saving effect of inverter control compared with variable pump control is mainly determined by the ratio of idle time to machine running time. With the development of frequency conversion control technology, the improvement of control methods and the improvement of control accuracy, its energy saving will become more prominent. According to the above data, the energy-saving effect is very obvious when the idle time only accounts for a quarter of the total cycle.
IV. Motor control
Simply using frequency converter and synchronous motor can not achieve good control effect. Therefore, the Science and Technology Department of Haitian Company developed a frequency converter control module with closed-loop control function and applied for a patent.
figure 3 is the general principle of this technology. The flow signal and pressure signal in the figure are input signals given by the main controller. The pressure sensor feeds back the actual value of the system pressure to the control algorithm module in real time, and outputs the driving signal to the frequency converter after calculation. When the pressure sensor signal and the pressure signal are compensated by superposition, they are converted into P and Q with the flow signal. In operation, the system pressure of the injection molding machine is not higher than the given pressure, and the flow rate is not greater than the given flow rate. When the pressure feedback signal is less than the given pressure signal, the output flow is equal to the given flow of the flow signal; When the pressure feedback signal is close to the given pressure signal and the output flow is less than the given flow, the pressure closed-loop control is carried out.
under the control of this algorithm, the actual pressure can follow the given pressure well. At the same time, the speed and pressure are used to control the running process of the machine at the same time, which makes the control more comprehensive and greatly improves the response speed and control accuracy of the whole machine.
V. Summary
Although the control technology of asynchronous motor is becoming more and more perfect, its own power factor, efficiency and heating problems can never be overcome. Synchronous motor technology is becoming more and more mature, and its manufacturing cost is getting closer to that of asynchronous motor, which is replacing asynchronous motor in more and more occasions. With the increasing shortage of power resources, the government and enterprises are paying more and more attention to the development and promotion of environmental protection and energy-saving products, and the application of synchronous motors in injection molding machines will become a trend.
AC asynchronous motor is widely used in various fields of industrial production because of its simple structure, low price and adaptability to various industrial conditions. The appearance of frequency converter has broken the situation that asynchronous motor can not perform stepless speed regulation and improved its application effect. However, variable frequency speed regulation is still limited by the shortcomings of asynchronous motor, such as low efficiency, small torque at low speed, large harmonic loss, large speed change rate and poor dynamic performance. Moreover, the problems of low efficiency, small power factor and low-speed heating of asynchronous motor still cannot be improved.