AC variable frequency speed regulation technology is an important development direction of modern power transmission technology. With the application of power electronics technology, microelectronics technology and modern control theory in AC speed regulation system, variable frequency AC speed regulation has gradually replaced the past slip speed regulation, pole changing speed regulation, DC speed regulation and other speed regulation systems, and is more and more widely used in many fields of industrial production and daily life. However, due to some factors in the use environment, service life and human operation, the service life of the frequency converter is greatly reduced, and there are also various faults in use.

1 。 Determine the fault based on the static test results of the frequency converter

First of all, you can do a static test on the frequency converter. Generally, the general-purpose frequency converter includes the following parts: (1) rectifier circuit; (2) DC intermediate circuit; (3) Inverter circuit; (4) Control circuit.

Static test is mainly a test of high-power transistors (power modules) in rectifier circuit, DC intermediate circuit and inverter circuit. The tool is mainly multimeter. The rectifier circuit is mainly a forward and reverse test of the rectifier diode to judge its quality. Of course, we can also use a voltage withstand meter to test it. The DC intermediate circuit mainly measures the capacity and withstand voltage of the filter capacitor. We can also observe whether the safety valve on the capacitor bursts and whether there is liquid leakage to judge whether it is good or bad. The judgment of power module is mainly to judge the freewheeling diode in the power module. For IGBT module, we also need to judge whether it can be turned on and off when there is trigger voltage.

2. Judge the fault point through the display of the frequency converter

(1) OC. Overcurrent fault is probably the most common fault in the frequency converter. First of all, eliminate the faults caused by parameter problems. For example, current limitation and too short acceleration time may lead to overcurrent. Then we must judge whether there is something wrong with the current detection circuit. Take fvr075g7s-4ex as an example: we sometimes see that fvr075g7s-4ex also has current display on the panel when the motor is not connected. Where does the current come from? At this time, it is necessary to test its three Hall sensors. In order to determine which phase sensor is damaged, we can turn on the machine every time we remove one phase sensor to see whether there will be overcurrent display. After such a test, the OC fault can be basically eliminated.

(2) OV. The overvoltage fault should first be eliminated due to parameter problems. For example, the deceleration time is too short, and the overvoltage caused by regenerative load. Then we can see whether there is a problem with the voltage at the input side. Finally, we can see whether the voltage detection circuit has a fault. Generally, the voltage sampling point of the voltage detection circuit is the voltage of the intermediate DC circuit. We take Sanken svf303 as an example. It is sampled by the DC circuit (about 530v DC) and then isolated by the optocoupler after reducing the voltage through a resistance with a large resistance. When the voltage exceeds a certain value, it displays "5" overvoltage (this machine is displayed by the digital tube). We can see whether the resistance is oxidized and whether the optocoupler has a short circuit, etc.

(3) UV. Under voltage, we can first check whether there is a problem with the voltage at the input side, and then look at the voltage detection circuit. The fault diagnosis is the same as overvoltage.

(4) FU. The fast fuse fault detection function has been introduced in most of the current frequency converters. (especially high-power frequency converter) take lg030ih-4 frequency converter as an example. It is mainly used to sample and detect the voltage in front of and behind the fast fuse. When the fast fuse is damaged, there will inevitably be no voltage at one end of the fast fuse. At this time, the isolation optocoupler acts and a Fu alarm appears. Replacing fast melt should solve the problem. Special attention should be paid to whether there is a problem in the main circuit before replacing the fast melt.

(5) OH. Overheating is mainly caused by poor internal heat dissipation of the frequency converter. We can check the cooling fan and ventilation channel.

(6) Sc. short circuit fault we can check whether there is a short circuit inside the frequency converter. Check the internal circuit. There may not be a short circuit. At this time, we can check that the power module may have a fault. Under the condition that the driving circuit is normal, replacing the power module should be able to repair the machine.

There are many kinds of frequency converter faults. The maintenance workers in the first steel-making workshop came into contact late and knew little about the basic knowledge of frequency converter. We can only constantly summarize in practice and find out a set of methods to deal with frequency converter faults quickly and effectively.

I. Basic knowledge of main principles of frequency converter.

After the three-phase 380V grid voltage is input from the L1, L2 and L3 input terminals of the frequency converter, it must first be rectified by the rectifier bridge of the frequency converter, and then filtered by the capacitor to output a DC voltage of about 530v (this 530v is the most frequently tested pilot that we often use to judge the quality of the rectifier part of the frequency converter, of course, the rectifier bridge is initially subject to the power-off test) and then through the inverter circuit, By controlling the on-off of the inverter circuit to output the voltage of the appropriate frequency we want (the most important thing for the inverter to convert frequency is to control the off of the inverter circuit to control the output frequency). There are countless kinds of inverter faults. Fortunately, now the inverter tends to be intelligent. General faults can be detected by itself, and its code is displayed on the control panel. Users can preliminarily determine the cause of the fault by checking the user manual. But sometimes, when the frequency converter is running, starting or loading, the indicator light suddenly does not light, the fan does not turn, and there is no output. At this time, we beginners don't know what to do. In fact, it's very simple. We just need to cut off the power supply of the frequency converter. Power off and test its rectifier and inverter parts. In most cases, you can know the fault. There is one thing to pay attention to here. You can't measure it immediately after power failure, because there is a large capacitor in the frequency converter with a high voltage of hundreds of volts. You must wait for more than ten minutes to measure it again, which should be paid attention to. Test of rectifier bridge and inverter circuit before power on of frequency converter. The specific measurement methods are as follows:

Find the "+" and "-" at the DC output end of the frequency converter, and then adjust the multimeter to the measuring diode gear. The black probe is connected to "+" and the red probe is connected to the input terminals L1, L2 and L3 of the frequency converter respectively. If the upper half of the rectifier bridge is intact, the multimeter should display a voltage drop of 0.3... And if it is damaged, the multimeter should display "1" over range. On the contrary, connect the red probe to "-" and the black probe to L1, L2 and L3 ends respectively to obtain the same results as above. If "1" appears, it indicates that the rectifier bridge is damaged. Then test its inverter circuit. The method is as follows: adjust the multimeter to resistance × In gear 10, connect the black probe to "+" and the red probe to the output terminals u, V and W of the frequency converter. The resistance value should be tens of ohms, and the reverse direction should be infinite. On the contrary, if you connect the red probe to "-" and repeat the above process, you should get the same result. In this way, after measurement, when judging that the rectifier and inverter parts of the frequency converter are in good condition, power on and measure the DC output end to see if there is about 530v high voltage. Note that sometimes the multimeter shows dozens of volts. Everyone thinks that the rectifier circuit is working, but in fact it does not work. It will output about 530v high voltage when it works normally, and the dozens of volts of voltage is induced from the inside of the frequency converter. If there is no high voltage around 530v, there is often a problem with the power version. In some frequency converters, a small patch resistor of the power version is burned, resulting in the power board not working, so that the frequency converter has no display and no output, the fan does not turn, and the indicator light does not light up. In this way, we can preliminarily judge which part of the frequency converter has failed, and then we can focus on testing the suspected fault part when disassembling and repairing.

II. Fundamentals of inverter technology

（1） Basic terms

1. Electronic line shaping --- els, many industrial production lines are composed of multiple machines, and the axes have a kinematic relationship. In the past, mechanical mechanisms were used to connect the shafts. If the shafts were connected electronically, each state has its own drive motor, which is called "electronic line shaping" (ELS). 2. Auto tuning, a technology commonly used in magnetic beam vector converter, can automatically monitor (find out) motor parameters, such as slip frequency / field current / torque current / stator impedance / rotor impedance / stator reactance / rotor reactance, etc. Only with these parameters can [special data estimation] and [slip (slip) compensation] be made. Because of this technology, good operation accuracy can still be obtained without encoder.

3. No encoder operation. In terms of speed control, compared with the open loop of the old variable frequency converter, the magnetic beam vector converter achieves a closed loop by the speed observation and calculation function. Good speed accuracy can be achieved without installing encoder on the motor side. Running without encoder has the following advantages: 1) precise wiring; 2) , there is no need to worry about the impact of RF noise on the encoder low-voltage signal; 3) There is no need to worry about the high failure rate of the encoder in the situation of multi vibration.

4. Vector control of frequency converter in AC motor, the magnetic field of rotor is generated by the induced current of stator winding. The stator current consists of two parts. One part affects the magnetic field, and the other part affects the motor output torque. To use AC motor, when speed and torque control are needed, the current that affects the torque must be separated for control, and the magnetic beam vector control can separate these two parts for independent control. (physical quantities with size and direction are called vectors)

5. Field weakingfield weaking circuit can be used to weaken the field current of the motor, change the balance relationship with the magnetic field, and make the motor run above the basic speed

6. When the torque required for constant torque application does not change due to speed, it is often used in [constant torque application]. Loads such as conveyor belts. [constant torque application] usually requires a large starting torque. [constant torque application] it is easy to have the problem of motor heating when running at low speed. The solutions are as follows: (1) increase the motor power; (2) Use the special motor for frequency converter equipped with constant speed cooling (that is, the cooling mode of the motor is forced air cooling).

7. Variable torque applications are often seen in centrifugal loads, such as pumps / fans / fans, etc. The purpose of using frequency converters is generally to save energy. For example, when the fan operates at 50% speed, the required torque is less than that required for full speed operation. The variable torque converter can only give the torque required by the motor to achieve energy-saving effect. Transient peak loads in secondary applications usually do not require additional energy to the motor. Therefore, the overload capacity of variable torque converter can be applied to most applications.

*The overload (current) capacity of the constant torque converter must be 150%/1minute of the rated value, while the overload (current) capacity of the variable torque converter only needs 120%/1minute of the rated value Because the rated current is rarely exceeded in the use of Centrifugal Machinery. In addition, the starting torque required for variable torque applications is also smaller than that for constant torque applications.

8. Special motor for frequency converter

The main features of the so-called [inverter duty motor] are as follows: 1) separate external force ventilation (external force air cooling); 2) , 10hz-60hz is constant torque output; 3) , high starting torque; 4) , low noise; 5) The motor is equipped with an encoder. * but not all motors called special motors for frequency converters have the above characteristics.

9. About speed regulation:

1) Speed regulation: adjust the running speed of the equipment according to the working conditions to achieve the purposes of energy saving, consumption reduction, wear reduction, production on demand, etc. 2) DC controller/motor: the DC controller adjusts the DC motor to adjust the speed. 3) AC inverter/motor: the frequency converter outputs three-phase AC current with variable frequency to control the speed of AC motor. 4) AC vector inverter: through complex calculation and transformation, the AC inverter controls the AC motor according to the control mode of DC motor, so as to achieve accurate speed control, torque control, improving output torque and other characteristics. 5) Servo control system: speed feedback or position feedback elements are introduced into the motion system to achieve extremely precise speed control, positioning control and high dynamic response through the role of negative feedback.

10. Several common industrial components:

1) Tacho generator: a speed measuring element, which is divided into AC and DC. 2) Resolver: an economical and accurate measuring element for speed and angular displacement.

3) Photoelectric encoder: a precise angular displacement and speed measuring element, which is suitable for being used as a feedback element in position control system.

4) PLC: Industrial Computing and control device, which realizes logic, timing, calculation and other control functions. It is generally used as the upper host of the whole automatic control system.

5) HMI (human machine interface): human machine interface.

6) Field bus system: a serial communication bus system applied to industrial control field, which greatly reduces the wiring cost and improves the anti-interference ability of control.

7) Distributed control: different from the traditional centralized control, it emphasizes the intelligence of each node device. Generally, the field bus system connects each sub device. It greatly improves the flexibility and reliability of the system application and reduces the computing burden of the upper computer.

11. Three terms about motor: 1) protection code: (ip**) inspect the ability of a device to prevent the entry of foreign matters and waterproof, making it one of the IEC standards. The two numbers represent the ability to prevent foreign matters and waterproof respectively. The higher the number, the smaller objects can be prevented from entering and the stronger the impact of water flow. Generally, the equipment with a protection grade of IP54 (dust-proof, anti splashing water drop) or above can be directly applied in the open air. 2) Insulation grade: it is one of IEC standards to investigate the ultimate temperature rise capacity of an electrical equipment (generally for motors) under the premise of ensuring good insulation characteristics. Generally, there are class B (85 degrees), class F (105 degrees) and class H (125 degrees).

3) Working system.

III. Introduction to frequency converter fault maintenance knowledge

1. What is a frequency converter?

Frequency converter is an electric energy control device that uses the on-off function of power semiconductor devices to convert the power frequency power supply to another frequency.

2. What is the difference between PWM and PAM?

PWM is the abbreviation of pulse width modulation in English. It is a way of adjusting the output and waveform by changing the pulse width of the pulse train according to a certain law. PAM is the abbreviation of pulse amplitude modulation in English. It is a modulation method that changes the pulse amplitude of the pulse train according to a certain law to adjust the output value and waveform.

3. What is the difference between voltage mode and current mode?

The main circuit of the frequency converter can be