Frequency converter commissioning and maintenance

1 Introduction

Since the 21st century, with the continuous maturity and improvement of domestic variable frequency speed regulation technology, frequency converters have replaced traditional DC speed regulation and hydraulic coupling, hydraulic resistance and electromagnetic speed regulation equipment due to their wide speed regulation range, high precision, high stability, strong motor speed regulation characteristics, and significant energy-saving effects. They are widely used in all walks of life in the national economy. Among them, the installation, commissioning, maintenance and fault handling of frequency converters have troubled many frequency converter agents, electrical engineers, and direct users. In order to facilitate readers' understanding and operability, combined with my work practice, taking a certain brand of frequency converter as an example, I will share with my peers the accumulated experience in the installation and commissioning, fault analysis and handling, maintenance and maintenance of frequency converters and their motor systems.

2. Debugging and precautions of inverter

2.1 Precautions before debugging

(1) Master and be familiar with the panel operation keys

The inverter has an operation panel. The functions are similar even though the brands are different. For example, the inverter operation panel consists of a four-digit LED digital tube monitor, a light-emitting diode indicator light, and operation buttons. See Figure 1.

The operation buttons include: run button (run), stop/reset button (stop/reset), programming button (prg), exit button (esc), multi-function button (m), shift button (》》), forward/reverse button (fwd/rev), and a potentiometer knob. Before starting the commissioning, the on-site personnel should first master and be familiar with the functions of each function button on the inverter operation panel in conjunction with the operation manual.

(2) Check before powering on

Before debugging the inverter, you must first carefully read the product technical manual, especially to see if there are any new contents and precautions;

Check the input and output terminals against the technical manual to see if they meet the requirements;

Check whether the wiring is correct and tight, and never connect them wrongly or reversely;

Check whether the shielding part of the shielded cable is connected correctly as specified in the technical manual.

(3) Power-on inspection and debugging

On the basis of the inverter power-off inspection being correct, the contents and steps of the inverter power-on inspection and debugging are established. The basic steps to be taken are:

No-load test with power supply;

Running with motor at no load;

Test run with load;

Online coordination with the host computer, etc.

2.2 Connect power supply and run without load

After connecting to the three-phase AC power supply, first press the jog key (m) for trial operation, then press the run key (run) to run the inverter to 50hz, use a multimeter to measure the three-phase output of the inverter (u/t1, v/t2, w/t3), the phase voltage should be balanced (370v~420v); the DC bus voltage should be (500v~600v). Then press the stop key (stop/reset), and when the frequency drops to 0hz, connect the motor line.

2.3 Test run with load

(1) When setting the number of poles, rated power, rated speed, and rated current of the motor, the operating current of the inverter must be considered comprehensively;

(2) Select the execution mode of the parameter self-tuning function

Static parameter self-tuning: the parameters are self-tuned when the motor cannot be disconnected from the load;

Rotation parameter self-tuning, parameter self-tuning can be performed when the motor can be disconnected from the load;

Note: When starting parameter auto-tuning, please ensure that the motor is in a stationary state. If overcurrent or overvoltage faults occur during the auto-tuning process, the acceleration and deceleration time can be appropriately extended.

(3) Set the upper limit output frequency, lower limit output frequency, base frequency, and torque characteristics of the inverter;

(4) Set the inverter to its built-in keyboard operation mode, press the manual key, run key, and stop key, and observe whether the motor reverses and whether it can start and stop normally;

(5) Be familiar with the protection code when the inverter fails, observe the factory value of the thermal protection relay, observe the setting value of the overload protection, and modify it when necessary.

2.4 System Debugging

(1) Manually operate the run and stop buttons on the inverter panel, observe the motor running and stopping process and the inverter display window to see if there are any abnormal phenomena. If there are any abnormal phenomena, change the preset parameters accordingly and then run again;

(2) If the inverter overcurrent protection is activated during the process of starting or stopping the motor, the acceleration and deceleration time should be reset. The acceleration of the motor during acceleration and deceleration depends on the acceleration torque, while the frequency change rate of the inverter during the start and brake process is set by the user. If the motor's rotational inertia or motor load changes, when the speed is increased or decelerated according to the pre-set frequency change rate, the acceleration torque may be insufficient, causing the motor to stall, that is, the motor speed is not coordinated with the inverter output frequency, causing overcurrent or overvoltage. Therefore, it is necessary to reasonably set the acceleration and deceleration time according to the motor's rotational inertia and load so that the frequency change rate of the inverter can be coordinated with the motor speed change rate.

The method to check whether this setting is reasonable is to first select the acceleration and deceleration time according to experience. If overcurrent occurs during the starting process, the acceleration time can be appropriately extended; if overcurrent occurs during the braking process, the deceleration time can be appropriately extended. On the other hand, the acceleration and deceleration time should not be set too long, which will affect production efficiency, especially in the case of frequent starting and braking.

(3) If the inverter still protects within the limited time, the start/stop operation curve should be changed from a straight line to an S-shaped, U-shaped line or an inverse S-shaped, inverse U-shaped line. When the motor load inertia is large, a longer start/stop time should be used, and the operation curve type should be set according to its load characteristics.

(4) If the inverter still has an operating fault, you should try to increase the current limit protection value, but you cannot cancel the protection. You should leave at least 5%-10% protection margin. This function is especially suitable for occasions where the speed or load changes sharply.

(5) If the inverter fails to drive the motor to reach the preset speed during startup, there may be two situations:

The electromechanical resonance of the system can be judged from the sound of the motor running. The resonance point can be avoided by setting the frequency jump value. Generally, the inverter can set three jump points. When the V/F controlled inverter drives the asynchronous motor, the current and speed of the motor will oscillate in certain frequency bands. In severe cases, the system cannot run, and even overcurrent protection occurs during acceleration, which prevents the motor from starting normally. This is more serious when the motor is lightly loaded or has a small moment of inertia. Ordinary inverters are equipped with a frequency jump function. Users can set the jump point and jump width on the V/F curve according to the frequency point where the system oscillates. When the motor accelerates, these frequency bands can be automatically skipped to ensure the normal operation of the system.

The torque output capacity of the motor is not enough. The factory parameter settings of inverters of different brands are different. Under the same conditions, the load capacity is different. The load capacity of the motor may also be different due to different inverter control methods; or due to different system output efficiency, the load capacity may be different. In this case, the torque boost value can be increased. If it cannot be achieved, the manual torque boost function can be used. Do not set it too large, as the temperature rise of the motor will increase. If it still does not work, a new control method should be used, such as the method of using a constant v/f ratio. When the startup does not meet the requirements, the speed sensorless vector control method can be used instead, which has a greater torque output capacity. For fan and pump loads, the torque curve value should be reduced.

2.5 System debugging of inverter and host computer

In automation systems, the application of serial communication between inverters and host computers is becoming more and more widespread. By connecting with remote control systems, the following can be achieved:

(1) Adjustment of inverter control parameters;

(2) Control and monitoring of frequency converters;

(3) Fault management of the inverter and restart after fault.

Therefore, when selecting a frequency converter, many users have put forward more stringent requirements on the communication function of the frequency converter, requiring the frequency converter to achieve fast and accurate data exchange with the host control system, PLC controller, text display human-machine interface and touch screen human-machine interface and other devices to ensure the integrity of the control system function.

2.6 Notes on system debugging

(1) After the manual basic setting is completed, if there is a host computer in the system, connect the control line of the inverter directly to the control line of the host computer, and consider changing the operation mode of the inverter to the host computer operation command given. According to the needs of the host computer system, adjust the range of the inverter receiving frequency signal terminal to 0-5v or 0-10v, and the response speed of the inverter to the analog frequency signal sampling. If another monitoring head is required, the monitoring amount of the analog output should be selected, and the range of the inverter output monitoring amount terminal should be adjusted.

(2) Problems that may be encountered during online debugging between the inverter and the host computer

After the host computer gives a control signal, the inverter does not execute or receive instructions;

After the host computer gives the control signal, the inverter can execute the instruction but with errors or inaccuracies.

Reason: Some host computers (such as PLC) generally output 24V DC signals, while the inverter's main control board terminals only receive passive signals. If you directly put wires from the PLC terminals to the inverter's main control board terminals, the inverter will not work. At this time, you should consider adding an external 24V DC relay to output a switch signal to the inverter's main control board terminals, which can also improve anti-interference capabilities. At the same time, check whether the inverter's support protocol and interface mode are correct.

The above is the basic debugging process of the inverter-AC motor v/f control mode. Whether the system can run safely and reliably, the entire installation and debugging process of the inverter and the load is very important. Here we should especially remind you that you should first read the product technical manual carefully, check the hard structure of the inverter one by one according to the manual, master its characteristics, and then debug step by step according to the above recommended steps.

3. Maintenance and repair of inverter

Due to the changes in the inverter's operating environment, such as the influence of temperature, humidity, smoke, etc., as well as the aging of the inverter's internal components, various inverter failures may occur. Therefore, the inverter must be inspected daily during storage and use; in order to reduce the failure rate and extend the inverter's service life, the inverter needs to be regularly maintained.

3.1 Daily Maintenance

When the system is turned on normally, pay special attention to any abnormal conditions, as follows:

(1) Whether the motor has abnormal sound and vibration;

(2) There is no abnormal heating of the inverter and motor, and no burning smell;

(3) Whether the working environment temperature is good;

(4) Is the load current meter the same as usual?

(5) The input current and input voltage of the inverter are within the allowable range for normal operation.

(6) The output current and output voltage of the inverter are within the rated value range (short-term current overload is allowed).

(7) Check whether the inverter's cooling fan is operating normally and whether there is any dirt or lint blocking the air duct.

3.2 Regular maintenance

The inverter needs regular inspection and maintenance. When inspecting the inverter, be sure to cut off the power supply and wait until the monitor has no display and the main circuit power indicator goes out before inspection. Based on experience, the items, contents and countermeasures that need to be inspected regularly are shown in Table 1 below.

3.3 Insulation test

The inverter has been subjected to insulation test before leaving the factory. Generally, it is best not to conduct insulation test again. If the test is necessary, it should be carried out strictly according to the following steps, otherwise the inverter may be damaged.

Disconnect all the connection wires of the control board circuit to prevent the test voltage from being connected to the control circuit and damaging the control board; first remove the wiring of the inverter main circuit, and then short-circuit all the terminals of the main circuit, as shown in Figure 2 below, and use a DC500V megohmmeter to measure the insulation resistance between them and the ground terminal E. The megohmmeter indication value ≥20mω is normal.

3.4 Fault analysis and measures of frequency converter

The common faults of the inverter are listed in Table 2 for reference by on-site technicians.

4 Simple judgment method for inverter main circuit fault

The main circuit of the inverter is shown in Figure 3. In daily maintenance, technicians can use a digital multimeter to simply determine whether the rectifier bridge, IGBT, IPM devices, etc. of the main circuit are damaged. For personal safety, the inverter must be powered off, and after waiting for 3 to 5 minutes, the inverter three-phase AC input terminals (R/L1, S/L2, T/L3) and the inverter three-phase AC output terminals (U/T1, V/T2, W/T3) can be removed before operation.

The following is a static judgment of the inverter main circuit. The actual judgment is based on the test with a motor, but it can provide a reference for simple judgment on site.

First, set the digital multimeter to the "diode" position, and then use the red and black test leads of the digital multimeter to detect the inverter main circuit according to the following steps (see Figure 4):

(1) The black probe of the digital multimeter touches the positive pole (p+) of the DC bus, and the red probe touches the inverter output terminals (u/t1, v/t2, w/t3) in turn, and records the displayed values ​​on the multimeter; then touch the red probe to (n), and the black probe to (u/t1, v/t2, w/t3) in turn, and record the displayed values ​​on the multimeter; if the six displayed values ​​are basically balanced, it indicates that there is no problem with the inverter IGBT inverter module, otherwise the IGBT inverter module at the corresponding position is damaged. The inspection phenomenon is: no output or "ocu1" fault is reported. As shown in Figure 4 (a).

(2) Use the black probe of the digital multimeter to touch the positive pole p (+) of the DC bus, and the red probe to touch the inverter input terminals (r/l1, s/l2, t/l3) in turn, and record the displayed value on the multimeter; then use the red probe of the digital multimeter to touch (n), and the black probe to touch the inverter input terminals (r/l1, s/l2, t/l3) in turn, and record the displayed value on the multimeter; if the six displayed values ​​are basically balanced, it means that there is no problem with the inverter diode rectification or soft-start resistor. On the contrary, if the rectifier module or soft-start resistor at the corresponding position is damaged, the inspection phenomenon is: no display or undervoltage fault, as shown in Figure 4 (b).

5 Conclusion

Based on the author's work experience, this article takes a brand of inverter as an example to introduce the basic matters such as installation, debugging, troubleshooting, care and maintenance of the inverter. It is hoped that it can provide reference and reference for colleagues in the industry in the technical transformation of electrical transmission equipment and the promotion of the popularization and application of high-tech products, so as to reduce major economic losses caused by low-level errors.