1 Introduction
With the rapid updates and development of technologies such as motors, home electronics, computers, and communications, the demand for permanent magnet materials is increasing, and their performance is becoming more demanding. Currently, most permanent magnet materials use neodymium iron boron, ferrite, AlNiCo, and Samarium Cobalt, and possess characteristics such as high coercivity and stable performance. These materials are magnetized by a high-voltage, high-current pulse discharge from a magnetizing power supply to a solenoid. Production requires a highly efficient, stable, and precise magnetizing power supply, and in-machine testing of the magnetic flux of the permanent magnet material after magnetization. This paper introduces a highly efficient automatic magnetizer that integrates magnetization and measurement, using a PLC for system control and a touchscreen for parameter adjustment and work display.
2 Electromagnetic exchange
The magnetizer generates a strong magnetic field based on the pulse discharge of the capacitor's energy storage, which magnetizes ferromagnetic materials [1]. Before electromagnetic exchange, the energy stored in the capacitor...
In the formula, uc is the terminal voltage of the storage capacitor, and c is the capacitance of the storage capacitor. Changing the voltage or capacitance of the capacitor can adjust the amount of electric field energy stored by the capacitor. Currently, capacitors with a voltage range of 2kV to 3.5kVdc can store energy exceeding 100kJ.
When capacitor C is charged to the set voltage u0, the charging power supply is disconnected, and then the series circuit of lr is connected. The charge stored in capacitor C then rapidly discharges through lr in a pulse form, resulting in a very large pulse current peak. The terminal voltage uc of the capacitor discharge satisfies...
In the formula, l is the inductance of the solenoid in the charging head, and r is the sum of the resistance of the solenoid, the connecting wire of the discharge circuit, the contact resistance, and the internal resistance of the discharge device (ignoring the distributed capacitance and distributed inductance of the circuit).
For pulse magnetization, the magnetizing head is often selected
That is, under underdamped conditions, when the capacitor discharge current causes the peak value of the pulse magnetic field to reach 3 to 5 times the coercivity of the magnetized material in the magnetizing coil, saturable magnetization can be achieved.
There are three types of magnetic flux detection: magnetoelectric flux, electronic flux, and digital integrating flux [2]. Figure 1 shows an electronic flux circuit, which consists of a detection coil and an integrating circuit. When the magnetic flux linked in the detection coil changes by δφ, an electromotive force is induced in the coil, and the output voltage after integration is...
In the formula, n is the number of turns of the detection coil, r is the resistance, and c is the integrating capacitance.
3 Control System Design
Figure 2 shows a schematic diagram of the magnetizer system. The circuit consists of an adjustable DC high-voltage power supply, a discharge switch circuit, a PLC controller, a touch screen, magnetic flux detection, and a magnetizing head. Control requirements:
① Adjust the magnetizing current by adjusting the control angle of the thyristor;
②Automatic detection of magnetized products
Magnetic flux intensity;
③ Human-computer interaction, i.e. setting parameters and displaying operating status;
④ Control and operation of the PLC-based real-world system;
⑤ Overcurrent and overvoltage protection for power components;
⑥ It has input short-circuit protection, ensuring safe operation.
3.1 Magnetizing Circuit
The magnetization circuit consists of a main circuit and a trigger circuit. The circuit diagram of the magnetizer is shown in Figure 3. The main circuit mainly consists of AC voltage regulation and boost, rectification and energy storage, and discharge circuits. The input voltage of the boost transformer T is adjusted by regulating the phase shift angle (or conduction angle) of the bidirectional thyristors VT1 and VT2, and then a pulsating DC voltage is obtained through the bridge rectifier circuit, storing electrical energy in the capacitor bank CL. When the thyristor VT7 is turned on, it instantly generates a strong pulse current discharge to the magnetizing head, rapidly magnetizing the material. In the bidirectional thyristor synchronous phase-controlled trigger circuit, the output voltage Vout of the analog module FX0N-3A controls the conduction angle to adjust the voltage at the upper end of the energy storage capacitor [2, 3, 4].
3.2 System Control Circuit
Figure 4 shows the system control circuit. The Mitsubishi FX1N-24MR is selected as the system master controller. The analog signal FX0N-3A has two inputs and one output. The inputs detect the current signal and the magnetic flux signal, and the output controls the conduction angle of the bidirectional thyristor.
3.3 Control Program Design
The control program has both manual and automatic modes. The manual control program is used for debugging and maintenance, such as capacitor switching, capacitor charging checks, and magnetization checks.
The automatic control program includes a sequential control program, a capacitor grading charging subroutine, a magnetic detection subroutine, an HMI interface program, door closing and charging head connection, overvoltage and overcurrent control, etc. Since the entire operation follows a continuous flow, sequential control is used to chain these subroutines together, which simplifies programming and makes the code more readable using STL instructions.
The capacitor-stage charging subroutine takes into account the possibility of a large inrush current when the capacitor is charging in a zero-state condition, which could damage the bridge rectifier circuit and the bidirectional controllable capacitor. The energy storage capacitor is charged in two stages, starting with a current-limiting resistor r1, and then shorted by the contacts of km2 for full-voltage charging.
After the magnetized workpiece is pushed by the air valve to the coil for detecting magnetic flux, the integrating capacitor in Figure 1 should be short-circuited to discharge (zeroing the detection). Then, the magnetic workpiece is inserted into the coil, and the magnetic flux of the product can be checked, thereby identifying the performance requirements of this batch of products. At the same time, the conduction angle of the bidirectional thyristor can be stabilized to ensure the quality of the product.
The touchscreen uses a Mitsubishi F940GOTO, which is used for setting parameters and displaying operating status. It allows setting the number of magnetizing poles and magnetizing current, and displays magnetic flux and operating status. The HMI interface program configures the communication between the touchscreen and the PLC.
4. Conclusion
The magnetizer utilizes pulse discharge of stored capacitors, achieving a maximum instantaneous discharge current exceeding 30 kA. It generates an extremely high-intensity magnetic field within 10 ms without impacting the power grid. When paired with a suitable magnetizing coil, it can instantaneously generate a magnetic field exceeding 30,000 Oe (Oersted), providing even better magnetization results for highly coercive magnets such as neodymium iron boron magnets. The integrated magnetization and flux detection system is suitable for assembly line operations, offering high efficiency, reliability, and interference resistance. However, further research is needed to minimize the impact of power electronic devices on the surrounding environment during switching on and off.
