Routine maintenance of injection molding machine frequency converter

    1. Routine maintenance and repair of frequency converter

    First of all, maintenance personnel must be familiar with the basic working principles and functional characteristics of the frequency converter, and have basic knowledge of electrician operation. Before inspecting and maintaining the inverter, the main power supply of the equipment must be cut off, and the main circuit voltage of the inverter must drop to a safe value (about 30 V) before proceeding. Before powering on the inverter, you should check the temperature and humidity of the surrounding environment. Excessive temperature will cause the inverter to overheat and alarm, and in severe cases, it will directly cause damage to the power components of the inverter; too humid air will cause internal flashover of the inverter. When the inverter is running, you should pay attention to whether the cooling system is normal, whether there are any abnormal noises from the inverter and motor, whether the display panel of the inverter displays normally, whether the output U, V, W three-phase voltage and current are balanced, etc. During routine maintenance and repair of the inverter, attention should be paid to checking the following points:

1) Whether the incoming line voltage and current of the frequency converter are normal;

2) Whether the temperature and humidity of the environment where the frequency converter is located are normal;

3) Is the temperature of the radiator normal?

4) Whether the sound of the inverter cooling fan is normal;

5) Check whether the high-power resistor on the circuit board of the frequency converter changes color, and whether the capacitor leaks or bulges; 

6) Whether the wiring and connectors in the frequency converter are loose.

    2. Common faults and maintenance strategies of frequency converters

2.1

    Damage to the frequency converter rectifier module Damage to the frequency converter rectifier module is one of the common faults of the frequency converter. The early production of frequency converter rectifier modules all used diodes. At present, most rectifier modules use thyristors. The rectifier modules of medium and high-power ordinary frequency converters are generally three-phase full-wave rectifiers. The rectifier devices are easy to overheat and breakdown. When they are damaged, the fast fuse will blow and the whole machine will shut down. When replacing the rectifier module, it is required to evenly apply a layer of silicone grease with good heat transfer performance on the surface in contact with the heat sink, and then tighten the mounting screws. If the rectifier module of the same model is not available, it can be replaced by another type of rectifier module of the same capacity. The damage of the rectifier module is often closely related to the external power supply of the machine. Therefore, when the rectifier module fails, it cannot be blindly powered on and the peripheral equipment should be checked first.

2.2 

    Frequency converter charging circuit failure General-purpose frequency converters are generally voltage-type frequency converters, which adopt AC-DC-AC working mode. Due to the large smoothing capacitance on the DC side, the charging current is very large when the frequency converter is connected to the power supply. It may cause the power switch to trip. For this purpose, a starting resistor is set in the charging circuit to limit the charging current. After charging is completed, the control circuit short-circuit the resistor through the contacts of the contactor or the thyristor. Charging circuit faults generally appear as the starting resistor is burned out, and the inverter alarm shows that the DC bus voltage is faulty. When the AC input power of the frequency converter is frequently switched on and off, or the contacts of the short-circuit contactor are in poor contact or the conduction resistance of the thyristor becomes large, the starting resistor will be burned out. In this case, you can purchase a replacement resistor of the same specification. At the same time, the cause of the burned resistor must be found. If the fault is caused by frequent switching on and off of the input power supply, this phenomenon must be eliminated. If the fault is caused by short-circuited contactor contacts or short-circuited thyristors, these components must be replaced. Then put the frequency converter into use.

2.3

    The frequency converter displays overcurrent

   Overcurrent occurs in the system during operation. The reasons are generally from the following aspects:

1) When the motor encounters an impact load or the transmission mechanism becomes "stuck", the motor current suddenly increases;

2) The output side of the inverter is short-circuited, such as the connecting wires between the output end and the motor are short-circuited, or there is a short-circuit inside the motor, etc.;

3) The frequency converter itself is not working properly. For example, the upper and lower devices of the same bridge arm in the inverter bridge are "through", causing the positive and negative poles of the DC voltage to be in a short-circuit state.

4) The inertia of the load is large, and the speed-up time is set too short. The speed of the motor rotor cannot keep up due to the large inertia of the load. As a result, the speed-up current is too large.

5) When the inertia of the load is large, /F5+ and the deceleration time is set too short, the motor rotor still maintains a high speed due to the large inertia of the load. As a result, the speed at which the rotor winding cuts the magnetic lines of force is too high and overcurrent occurs. .

    For the above fault phenomena, mainly check the following aspects:

1) Is the working machine stuck?

2) Use a megohmmeter to check the short circuit point on the load side;

3) Whether the inverter power module is damaged;

4) Whether the starting torque of the motor is too small, making the drag system unable to rotate;

5) Whether the speed-up time setting is too short;

6) Whether the deceleration time setting is too short;

7) Whether the torque compensation (v/f ratio) setting is too large, causing excessive no-load current at low frequency;

8) Whether the electronic thermal relay is improperly adjusted and the operating current is set too small, causing the inverter to malfunction.

If these problems are not caused by these problems, you can disconnect the current transformer on the output side and the Hall current sensor on the DC side, and run after reset to see if overcurrent still occurs, because the Hall sensor that detects current is affected by environmental factors such as temperature and humidity. Due to the influence, the operating point is prone to drift, resulting in overcurrent. If it still occurs, it is very likely that the 1PM module is faulty, because the 1PM module contains overvoltage, overcurrent, undervoltage, overload, overheating, phase loss, short circuit and other protection functions. Replacing the module of the same model should solve the problem.

2.4

    Overvoltage and undervoltage protection actions of the frequency converter Overvoltage and undervoltage protection actions of the frequency converter are mostly caused by fluctuations in the grid voltage. In the power supply circuit of the inverter, if there is a direct start or stop of a large-load motor, it will cause the grid voltage to fluctuate in a large range instantaneously, causing the overvoltage and undervoltage protection of the inverter to operate and unable to work normally. This situation generally does not last long, and the power grid can operate normally after the voltage fluctuation of the power grid passes. This situation can only be avoided by increasing the capacity of the power supply transformer and improving the quality of the power grid. In addition, the overvoltage fault of the frequency converter may also be caused by the frequency converter driving a large inertia load, because in this case, the deceleration stop of the frequency converter belongs to regenerative braking. During the stop process, the output frequency of the frequency converter decreases linearly. The frequency of the load motor is higher than the output frequency of the frequency converter. The load motor is in the power generation state, and the mechanical energy is converted into electrical energy and absorbed by the smoothing capacitor on the DC side of the frequency converter. When this energy is large enough, the voltage on the DC side of the frequency converter It will exceed the overvoltage protection setting value of the DC bus and trip. For this kind of fault, one is to set the deceleration time parameter longer, or increase the braking resistor, or add a braking unit; the other is to set the stop mode of the frequency converter to free stop. Another situation is that the rectifier part of the frequency converter is damaged or the detection circuit is damaged, causing a fault alarm. Voltage detection is generally done by sampling the DC bus voltage, and then comparing it with the overvoltage protection setting value, and then transmitting the difference to the microcontroller. device. This alarm will occur if there is a problem with any component in the rectifier bridge, filter capacitor, sampling circuit or comparison circuit.