Permanent magnet synchronous motor has good dynamic response and large output torque, and its performance and efficiency can not be surpassed by the combination of asynchronous motor and frequency converter. In foreign countries, especially Japan, synchronous motors are not only used in all-electric injection molding machines, but also widely used in ordinary injection molding machines. At present, many companies, such as Japan Petroleum Research Institute, Daikin and BUCHER of Germany, have increased their research and development of electro-hydraulic hybrid power systems. China is rich in rare earth resources, which greatly reduces the production cost of motors. At present, the prices of permanent magnet synchronous motors and asynchronous motors are very close. With the demand of domestic plastic market for product quality and accuracy, the control accuracy of injection molding machine has been improved accordingly, and synchronous motor will be more and more used in injection molding machine.
Second, synchronous motors and asynchronous motors
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 stator, and the rotor does not directly generate magnetic field. Therefore, the rotor speed must be less than the synchronous speed (without this difference, that is, slip, there will be 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 rotor speed 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=60f/p, where f is the set frequency and p is the number of motor poles.
Because there is no need to absorb reactive current from the power grid and there is no copper loss and 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 permanent magnet synchronous motor is about 5% higher than that of asynchronous motor with the same capacity.
Third, the comparison of energy saving
1. Comparison of operating efficiency between permanent magnet synchronous motor and asynchronous motor under different loads;
As can be seen from the figure, the operation efficiency of permanent magnet synchronous motor is obviously higher than that of asynchronous motor.
2, the actual test
1). Test conditions:
The test sample (disc-shaped sheet) is injected with the same mold on two injection molding machines, the raw material is ordinary PP, and the two injection molding machines are HTF86X 1/J2 (using synchronous servo motor and quantitative gear pump) and HTF80/J 1 (using asynchronous motor and variable pump) respectively.
2) Test the power curve of the permanent magnet synchronous servo motor (see the figure below).
The test time of two injection molding machines is 1 hour, and the injection cycle is 19.0s, in which the cooling time is 3s and the die support pause time is 2s. The actual power consumption of HTF86X 1/J2 is 2. 12 kwh, and that of HTF80/J 1 is 2.75 kwh. According to the calculation, compared with HTF80/J 1 J2, HTF 86x1saves electricity by 0.63 degrees, which can save electricity by about 22.9%.
3) Energy saving effect analysis:
Under the same conditions, controlling synchronous motor with frequency converter saves more than 20% electricity than controlling asynchronous motor. The energy-saving effect of frequency conversion 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 mode and the improvement of control accuracy, its energy saving will be 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.
Fourth, motor control.
Only using frequency converter and synchronous motor can not achieve good control effect. Therefore, the Science and Technology Department of Haitian Company has developed a frequency converter control module with closed-loop control function and applied for a patent.
Fig. 3 is the general principle of this technology. The flow signal and pressure signal in the figure are the 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 together with the flow signal. In operation, the system pressure of 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.