1 Introduction
With
the increasing popularity of
frequency converters
in industrial production, frequency converters, as a speed regulating and energy-saving product, have been increasingly valued by industrial and mining enterprises and have become an indispensable tool in industrial production and life. Frequency converters are playing a powerful role in various industries related to the national economy and people's livelihood, such as petroleum, chemical industry, building materials, electricity, mining, plastics, metallurgy, and water conservancy. According to statistics, in the current and next few years, the sales rate of frequency converters will increase by 10% to 15%, that is, about 5 to 9 million kw per year, and frequency converters of various power levels, various performances, and various uses will be used by users. Therefore, how to let users use frequency converters well and truly give play to the advantages of frequency converters in convenient speed regulation, energy saving, and improved production technology is the first issue that frequency converter manufacturers should consider.
This article takes the use of Fengguang brand inverters among users as an example, and combines some problems encountered by the author during the user's installation and debugging process to briefly discuss the installation and debugging issues of the inverter for reference by the majority of inverter users when using it.
2. Installation and use conditions of the inverter
Just like other electrical equipment or devices, the inverter, as a power electronic device, also has strict usage conditions and application scenarios. Any installation and use that violates the product specifications will be illegal and will inevitably cause losses to equipment, people, money and property.
2.1 Inverter operating environment and precautions
2.1.1 Working environment required by the inverter
(1) The working environment temperature is -10℃～+40℃, and the change of the working environment should not exceed ±5℃/h.
(2) Relative humidity: The maximum relative humidity of the air shall not exceed 90%, the rate of change of relative humidity per hour shall not exceed 5%, and condensation shall not occur.
(3) There shall be no conductive or explosive dust, and no gas or steam that may corrode metal or destroy insulation at the operating location.
(4) Allowable vibration conditions at the inverter installation site:
The vibration frequency is 10~150hz, and the vibration acceleration is no more than 5m/s2. When the inverter may resonate due to the vibration of the installation base, vibration reduction measures should be taken to avoid the resonant frequency.
(5) AC input power
The voltage fluctuation does not exceed ±20%.
The frequency fluctuation does not exceed ±2%, and the frequency change does not exceed ±1% per second.
Unbalance of three-phase voltage: negative sequence component does not exceed 5% of positive sequence component;
Power supply harmonic component: The root mean square value of the voltage relative harmonic content does not exceed 10%.
(6) Altitude: not more than 1000m.
2.1.2 Notes
(1) Non-professionals are not allowed to open the cover or door for use or inspection;
(2) The inverter has been subjected to a withstand voltage test before leaving the factory. The user is not allowed to and does not need to conduct a withstand voltage test on the inverter again;
(3) Do not connect large capacitors to the motor to improve the power factor;
(4) The casing is reliably grounded;
(5) Do not change the three-phase input to two-phase input, otherwise there will be phase loss protection;
(6) When operating at low frequency, the effect of the motor’s own fan and lubrication effect must be considered; when operating at high frequency, the bearing capacity of the bearing must be considered.
These explain the most basic rules for inverter installation. You must fully understand and be familiar with these rules.
2.2 Preparation before installation
To install a frequency converter properly so that it can operate normally and meet technical and process requirements, in addition to meeting the above basic rules, you should also pay attention to the following points:
(1) Before installation, you must first be familiar with and master the production process and technical requirements, understand its load conditions, and understand the role and position of the inverter in the system. Do you want to save energy, improve the production process, or both? In some situations, there is no room for energy saving, but it is inappropriate to insist on energy saving by the inverter.
(2) From an electrical point of view, the load of the inverter is first the motor. Therefore, before installation, you must first have a clear understanding of the motors on site, including rated voltage, rated current, number of motor poles, rated power, etc. The installed inverter must match them. In some special occasions, such as heavy loads, altitudes exceeding 1000m (i.e. exceeding the standard altitude), coal mine hoist inverters, etc., the inverter must be one or even two power levels higher than the load motor. Generally, the inverter is not allowed to have a lower power level than the load motor, so as to avoid the inverter overloading and failing to carry the load or frequent overload protection, causing unnecessary trouble.
(3) The electrical insulation of the motor must be tested before installation. Motors with poor insulation cannot be installed with inverters. Although the inverter is equipped with short-circuit protection, momentary grounding may also cause damage to some inverters.
(4) Before installation, you should carefully read the instruction manual of the inverter and be familiar with which parameters to set based on the on-site process and how to set the parameters.
(5) For some occasions, especially those that require automatic control and require auxiliary accessories, such as pressure gauges, sensors, pressure transmitters for water supply and some supporting facilities, such as PID regulators, temperature controllers, timers, etc., some also require remote control devices, and they must be mastered proficiently, so that they can be installed and debugged quickly.
(6) Wiring must be carried out strictly in accordance with the inverter's instruction manual, including the main line and control line. In some cases, the specifications can only be higher than those required by the manual but not lower. Where crimping of terminal lugs is required, they must be crimped strictly in accordance with the requirements, and the specifications and processes must meet the standards.
(7) In the case of complex modern industrial control, electromagnetic compatibility issues must also be considered, and the interference and anti-interference of the inverter must be considered. If necessary, electromagnetic filtering devices should be installed. In some cases, the motor may be far away from the inverter, so it is necessary to consider installing output reactors and filters.
(8) For potential loads, such as winches, hoists, and elevators in coal mine main shafts, due to the existence of regenerative power generation, it is necessary to consider installing a braking unit and a matching braking resistor to prevent the inverter from overvoltage protection or damage.
The above issues are what we need to understand and master before installing the inverter. If you are not familiar with these contents, the installation or debugging of the inverter may not be smooth or even unsuccessful, causing damage to the equipment or failure to use it normally. This is what we must remember.
3 Installation and commissioning of medium voltage inverter
The medium voltage inverter mentioned here refers to the inverter with voltage level from 750v to 2300v, which is divided into two voltage levels, 1140v and 2300v. The load of this type of inverter is generally submersible electric pumps used in oil fields, and 1140v is also used for oil pumping units. Let's briefly talk about the installation and debugging methods.
3.1 Installation steps
(1) Connect the control line
To confirm the working status of the original power frequency operation, we must first understand the working process of the submersible electric pump.
A submersible electric pump is an electric pump placed 1000m-3000m deep underground. Due to the long cable, the power supply equipment on the ground must be able to compensate for this part of the cable loss. Therefore, the original industrial frequency device must provide a power supply voltage 100-300v higher than the rated voltage of the motor.
Electric pumps and cables are high-temperature, high-voltage equipment, and insulation tests must be performed before installation. Using a 2500V megohmmeter to test, the insulation resistance should not be less than 50mΩ.
It is necessary to check the voltage and working current of the original industrial frequency power supply for reference during debugging.
Due to the different depths of the submersible pumps in the oil field, the voltage of the power supply of the submersible pumps is different. Therefore, for the inverters of the same voltage level, there are different voltage inputs, and the control voltage of the inverter is different. In order to solve this problem, the author installed a transformer inside the inverter, and the voltage output is the same for different voltage inputs. For this type of inverter, its output is only 220v, 110v, and 380v. 220v is used to power the control circuit, 110v is used to power the motor protector, and 380v is used to power the voltmeter. Its control wiring is shown in Figure 2.
Therefore, it is necessary to select the appropriate control power supply terminal according to the original power supply voltage, and connect the two wires that were left empty when leaving the factory or the two wires connected to the highest voltage level (tested at the highest voltage when leaving the factory) to the terminals of the original voltage level. Because there are two transformers, they must be reconnected. See Figure 3.
(2) Connect to the main line
According to the wiring specifications, connect the power line to the three-phase input terminal (or marked r, s, t) of the inverter, the motor line to the three-phase output terminal (or marked u, v, w) of the inverter, and the ground line to the terminal marked with "⊥" on the inverter. Tighten the corresponding bolts.
3.2 Debugging
(1) Pull down the isolating switch on the cabinet door and turn off the "frequency conversion start" button on the cabinet door to make it disconnected. After the power is supplied, use the high voltage range "2500V" of the MF-47 or 500 multimeter to measure the three-phase input voltage (pay attention to safety). It should be within the specified range and the three phases should be balanced. If it is not correct, the power supply should be turned off to check the power supply.
(2) After the power supply is normal, close the isolating switch and use a multimeter to check whether the 220V voltage of the control power supply (just measure the wiring box inside) is within the specified range. According to its ±20% fluctuation, it should be within the range of 220V±20V. If it exceeds this range, pull down the high-voltage isolating switch and adjust the connector of the control transformer until the requirements are met.
(3) Use a single-phase plug to take one wire and connect it to the "c" terminal of the control terminal block, and plug the other two phases into the junction box (on the old model, one wire is connected to "c" and the other wire is connected to "d"). Close the isolating switch, and the inverter should be able to supply control power. The panel will display "43.21" and after a few seconds, the "pro" light will turn on (new model), or only the "pro" light will turn on (old model). If there is no display, turn the plug over and reinsert it.
(4) After the panel display is normal, check whether the control function of the inverter is normal. Use a multimeter to detect the voltage across the delay thyristor (i.e. the voltage on the delay resistor), which should be around 1.00V. Turn the inverter's "on/off" switch to the "on" position, the inverter should be able to start, adjust the "frequency adjustment" knob, and the inverter frequency should rise from "2.00" to "50.00". Use a multimeter to measure the inverter's three-phase output terminals (at this time, it should be measured from one side of the inverter, because the cabinet is not connected to the main power and the inverter's main contactor is not energized). The three-phase voltage should be balanced with each other and the neutral line should also be balanced with each other. The voltage is roughly 15V for the three-phase voltage of 2300V level and 10V for the neutral line; 9V for the three-phase voltage of 1140V level and 5V for the neutral line. If it is unbalanced, check whether the cabinet has dropped or other problems during transportation until the voltage is completely balanced.
(5) Setting of operating parameters. This includes two steps: setting of inverter parameters and motor protection device parameters.
The inverter parameters are set. Since the inverter is usually designed for 2300v/125kw or 1140v/75kw when it leaves the factory, it should be reset according to the requirements of the on-site load, including the rated current and overload protection current. Other parameters generally do not need to be modified.
Parameter setting of motor protector. The parameters of motor protector include underload current, overload current, undervoltage setting, overvoltage setting, etc. Here is an explanation. Because the submersible electric pump is an electric pump placed deep in the oil layer under the oil well, it relies on the circulation of oil to dissipate heat during operation. Once the well is insufficiently supplied with fluid, the motor will run out of the oil layer and the motor will run dry, which will easily burn the electric pump. Therefore, underload protection should be set. When the operating current is lower than 80% of the rated current, the motor will stop running to protect the motor.
The parameter setting method can be carried out by referring to the corresponding manual. There are two main types, BK-3 type and BK-J1 type.
The method for BK-3 type is: when the display shows "P" (just after power on or pressing the "up" key four times), the parameter value can be modified. According to the required value, press the corresponding number key, then press the "up" or "down" key, and finally press the corresponding function key to complete the parameter value modification.
The method of bk—ji form is: in the ready screen (just powered on or pressing the "Shift" key four times), press the "Shift" key once, and then press the "Set Value" key once, and the screen will display the set value operation screen. There is a blinking cursor on the screen, and the corresponding data can be entered at the blinking cursor position. Press the "Move" key, the cursor can move up and down, and the arrow ("↑" or "↓") in the upper right corner of the screen indicates the current moving direction. Press the "Shift" and "Direction" keys to change the direction of the arrow. After entering, press the "Shift" key four times to save the modification results. As shown in Figures 4 and 5.
Some wells may have insufficient fluid supply and need to be operated at a lower frequency. Therefore, depending on the specific situation, the parameters can be reset according to the actual operating current after the pump is running.
(6) Load operation
After the parameters are set, you can unplug the plug on the plug box and remove the wires on the external point "c" (or points "c" and "d"), and you can run with load.
Press the "Frequency Start" button on the cabinet door, and after the set delay time is up, the contactor inside the machine is closed, the inverter displays "43.21" and "pro" is on (new model) or only "pro" is on (old model), turn the "start and stop" switch on the inverter to the "start" position, and the inverter can gradually increase from the lowest frequency. Adjust the "frequency adjustment" knob to increase the frequency to 30hz first, and use a calorimeter to measure the three-phase output current, which should be basically balanced, and the imbalance should not exceed 20%. Then increase to the frequency required by the user. Use a calorimeter to measure the input and output currents, and the three phases should be basically balanced.
If the frequency does not rise during the adjustment process, that is, when the frequency adjustment knob is adjusted, the frequency does not rise, the output current continues to increase, and it is in the speed limit protection state until the overcurrent protection is activated, this may be caused by insufficient low-frequency compensation. You can adjust the compensation potentiometer on the main control board (old model) or set the inverter parameter "low-frequency compensation" (new model) to solve it. When adjusting, pay attention to the output current as long as it can rise, and do not adjust too much to avoid over-compensation. During the operation, the current is too large, causing the motor and inverter to over-current and malfunction.
After normal operation, the parameters of the motor protector can be readjusted according to the operating current to achieve a state of reliable protection and normal operation.
The forward and reverse rotation of the motor must be confirmed. According to experience, in general, the forward current is larger than the reverse current, and the pressure indication at the wellhead is higher. In the reverse rotation, the pressure gauge pointer generally does not move. Some wells are slow to produce oil, and it takes some time to confirm.
Since this frequency conversion cabinet is also equipped with industrial frequency backup, it can be temporarily put into industrial frequency when there is a problem with the frequency conversion, so as not to delay the production. Therefore, after the frequency conversion is normal, the industrial frequency operation should be tested, and its forward and reverse rotation should also be debugged normally. When adjusting the forward and reverse rotation, the frequency conversion adjusts the output and the industrial frequency adjusts the input, so that both the industrial and frequency conversion can operate normally.
After adjustment, use frequency conversion to run normally, start the motor protector, and after the protector displays normal, turn on the "frequency conversion start" knob (the motor protector is turned on for protection, it will not work if it is not turned on), and the indications of various instruments on the cabinet are normal, and the debugging is completed.
4 Installation and debugging of low voltage inverter
Since low voltage inverters carry a wide variety of loads, the types vary greatly, so the installation and commissioning are also quite different.
From the perspective of using frequency converters, low-voltage loads can be roughly divided into two types: those with inertia and those without inertia. The most notable feature of inertia loads is that they generate electricity during rapid shutdown. How to deal with this energy will be the key to whether the frequency converter can operate normally.
4.1 Installation
When the inverter is installed for the first time or used after long-term storage, it should be thoroughly inspected. The method is as follows:
(1) Visual inspection: check for damage, rust on metal parts, and frost and condensation. If there is frost and condensation, dry it for 4 hours (60℃) or place it at room temperature with ventilation for 24 hours.
(2) For small power inverters, gently turn over the chassis and pay attention to whether there are any abnormal sounds inside the chassis. If there are any abnormal sounds, you should open the chassis and find and remove the foreign objects inside.
(3) For inverters with relatively large power, the casing should be opened to check whether there are any dropped wires or loose bolts due to shaking during transportation or storage. If so, they should be re-welded or tightened.
A few points to note here:
The installation of the inverter should comply with the working environment required above.
The inverter can be wall-mounted or cabinet-mounted. It must be installed firmly to ensure safety during operation. For ventilation and heat dissipation, the inverter must be installed vertically, as shown in Figures 6 and 7.
For cabinet structures, sufficient space should be left around for easy operation and heat dissipation by staff: the front spacing should be no less than 1.5m, and the back and sides should be no less than 1m.
For wall-mounted inverters, sufficient heat dissipation space should be left around the inverter. The upper part of the inverter should be at least 1m away from the top of the room, and the lower part should be at least 1m away from the ground to ensure smooth ventilation of the inverter and reliable operation.
Some rooms are tightly sealed, so you need to cooperate with the user to install exhaust fans or air conditioners. In some places, the environment is dirty and humid, so you need to take isolation measures to prevent dust and moisture.
When two or more inverters are installed in a control cabinet, they should be installed side by side (horizontally arranged) as much as possible. If they must be arranged vertically, a shelf should be added between the two inverters to prevent the hot air from the lower inverter from entering the upper inverter. See Figure 8.
4.1.1 Main circuit wiring
4.1.1.1 Basic wiring
The installation of the main line is relatively simple. Connect the power line to the input terminal (or the terminal marked with r, s, t) of the inverter, connect the motor line to the output terminal (or the terminal marked with u, v, w) of the inverter, and reliably ground the ground terminal of the inverter through the ground wire. See Figure 9.
Notice:
(1) The input and output terminals of the inverter must not be connected incorrectly. If the power supply line is connected to the output terminal by mistake, no matter which inverter tube is turned on, it will cause a short circuit between the two phases and quickly burn out the inverter tube. See Figure 10.
(2) The circuit breaker in front is usually required to be installed, mainly for disconnecting the circuit. When the inverter fails, especially when the rectifier circuit or the main circuit is damaged, the large current can trip the circuit breaker in time and disconnect it from other circuits of the grid to avoid affecting other circuits.
(3) The crimping of the wire ends should be reliable. Generally, a wire nose with equivalent capacity should be used for crimping. It must be pressed tightly to avoid overheating and burning of the wires or terminals due to long-term operation of large current.
4.1.1.2 Selection of wire diameter
Generally, the wire diameter should be selected according to the wiring requirements of the motor. In special occasions, a larger specification should be selected, especially when the motor is far away from the inverter. The selection should be based on the principle of larger rather than smaller.
4.1.1.3 Grounding
Each inverter has a dedicated grounding terminal "e" or "⊥", and the user should reliably connect this terminal to the ground.
When the inverter and other equipment, or multiple inverters are grounded together, each device must be connected to the ground wire separately. It is not allowed to connect the ground terminal of one device to the ground terminal of another device before grounding. As shown in Figure 11.
4.1.2 Wiring of control circuit
After the main wiring of the inverter is completed, the inverter can be operated. However, in general, due to the convenience of control and monitoring, the operation and display parts of the inverter need to be led to a convenient place. In some cases, the on-site environment is poor, and the inverter is not suitable for installation. Instead, it is installed in the power distribution room with a better environment, and the control part is led to the site. In some cases, such as the elevator inverter, the control part needs to be connected to the original system, and the control line also needs to be led out.
The control lines are divided into analog and digital.
4.1.2.1 Analog
It mainly includes: given signal line and feedback signal line on the input side; frequency signal line and current signal line on the output side.
The anti-interference ability of analog signal lines is poor, so shielded lines must be used. The end of the shielding layer close to the inverter should be connected to the common end of the control circuit, but not to the ground terminal (e) of the inverter or the earth. The other end of the shielding layer should be left hanging, as shown in Figure 12.
The wiring principles should be followed:
(1) Keep it at least 100mm away from the main circuit.
(2) Avoid crossing the main circuit as much as possible. If crossing is necessary, it should be done vertically.
4.1.2.2 Switch quantity.
Control lines such as start-stop, inching, and multi-speed control are all switch control lines.
Generally speaking, the wiring principles of analog control lines are also applicable to switch control lines. However, switch control lines have strong anti-interference capabilities, so when the distance is not very far, unshielded lines are allowed, but the two lines of the same signal must be twisted together.
4.2 Debugging
There is no fixed pattern for debugging the frequency converter, which can generally be divided into several steps: "first no load, then light load, and then heavy load".
4.2.1 No-load inspection and parameter preset
The wind and solar inverter can be checked without connecting to the main power supply, but only to the control power supply. This is not possible with other inverters.
For small-power inverters, you can remove the short-circuit sheet on the main terminal block and supply 380V power to the three-phase input. Large-power inverters generally have control voltage input terminals, from which you can use a one- or two-core cable to connect a 380V single-phase power supply to check the inverter (Note: before connecting, remove the two wires connected to the three-phase input at both terminals). Refer to the manual to familiarize yourself with the use of each keyboard and how to set parameters.
After getting familiar with it, turn on the inverter, increase the frequency to 50hz, and use a multimeter (preferably a pointer type) to measure the three-phase output. The voltage should be completely balanced.
After the inspection, turn off the power and put the removed short-circuit piece back to its original position for low-power equipment (note that the two terminals of the short-circuit piece on the main circuit should be discharged before installation). For high-power equipment, remove the two external wires of the control terminal and restore the original wiring.
Do not connect the three-phase output of the inverter to the motor line first, and connect the 380V power supply to the three-phase input of the inverter to observe the no-load operation of the inverter. Other types of inverters can also be operated in this way.
(1) Become familiar with the keyboard, that is, understand the functions of each key on the keyboard, perform trial operations, and observe the changes in the display.
(2) Perform basic operations such as "start" and "stop" according to the instructions, observe whether the inverter is working normally, and further familiarize yourself with the operation of the keyboard.
(3) Preset parameters. Preset the main parameters according to the method described in the manual. Check whether the inverter's execution is consistent with the presets for several items that are easy to observe, such as the speed increase and speed decrease time, the inching frequency, and the frequency of each gear when there are multiple speed gears.
(4) Connect the external input control line and check the execution of each external control function item by item.
(5) Use a multimeter (preferably a pointer type) to check whether the three-phase output voltage is completely balanced.
4.2.2 Load operation
The above tests prove that the inverter is normal, that is, it can run with load. The load operation of the inverter includes light load test operation and heavy load operation, that is, normal operation. If conditions permit, you can also run the inverter with an empty motor first, but in general, it is directly run with load.
Before the trial run, the insulation of the motor should be checked. For low voltage 380-660V motors, the insulation resistance should not be less than 50 megohms when tested with a 1500V megohmmeter. The insulation resistance of water pump loads may be lower, but it should not be less than 2 megohms.
You also need to understand the operation of the load to have a clear idea. There are many types of low-voltage loads, such as fans, water pumps, mixers, wire drawing machines, plastic machinery, elevators, air compressors, belt conveyors, chemical fiber machines, machine tools, and many more. Different loads have very different working conditions, so they should be treated differently according to different situations. Some special machines must use a certain type of special inverter. For example, the elevator is a potential load and has the problem of processing regenerative electric energy. A special inverter for the elevator must be used. There are also chemical fibers and machine tools, and some of them also need to use special inverters. This type of inverter has special hardware, especially software, for special loads, so as to ensure the reliable operation of the inverter. Centrifugal loads, such as centrifugal fans and centrifuges, have relatively large operating inertia, and their speed-up and speed-down time are relatively long. If the setting is too short, the inverter will have overcurrent when speeding up, overvoltage when speeding down, and may even be damaged. Therefore, if rapid shutdown is not required, the time can be appropriately extended, generally equivalent to the free shutdown time of the equipment (according to experience, generally 300s). If rapid shutdown is required, a braking circuit should be added.
After clarifying the above issues, you can start the load operation. Connect the inverter output to the motor line. Supply power.
(1) Jog or test run at low frequency. Observe the forward and reverse direction of the motor. If it is reverse, adjust it using the forward and reverse terminals of the inverter, or adjust the output wiring of the inverter after power failure.
Some machines may not allow reversal. In this case, the coupling between the motor and the machine should be disassembled first, the motor should be idle first, and the direction should be adjusted before connecting the coupling.
For loads such as submersible pumps, the direction of rotation cannot be seen on the well. Based on experience, when the current is small during forward rotation, the pressure gauge indicates an increase, and the amount of water supplied is large. When the pressure gauge indicates a low level during reverse rotation, the amount of water supplied is small, and the current is large. That is, judgment can be made based on experience.
(2) Start-up test. Start the working frequency from 0 Hz and slowly increase it to see if the traction system can start. At what frequency should the system start? If it is difficult to start, try to increase the starting torque. Specific methods include: increasing the starting frequency, increasing the u/f ratio, and using vector control.
Some loads, such as submersible pumps, have long motor leads and may have long-line effects. That is, due to the large harmonic components of the inverter output, the voltage at the motor end may increase. Therefore, pay attention to the changes in the motor current during adjustment. When the motor current is found to continue to increase, the machine should be shut down in time and the installation of an output reactor should be considered.
(3) Start test. Increase the given signal to the maximum and observe:
Changes in starting current;
Whether the entire traction system runs smoothly in the speed-up system.
If the circuit breaker trips due to excessive starting current, the speed-up time should be appropriately extended. If the starting current is too high in a certain speed range, try to solve it by changing the starting method (S-shaped, semi-S-shaped, etc.).
(4) Shutdown test: Adjust the operating frequency to the highest operating frequency, press the stop button, and observe the shutdown process of the traction system.
Check whether the machine trips due to overvoltage or overcurrent during the shutdown process. If so, the deceleration time should be appropriately extended.
When the output frequency is 0 Hz, check whether the traction system has creeping phenomenon. If so, DC braking should be added appropriately.
(5) Load test of the drag system
The main contents of the load test are:
If fmax>fn, the load capacity test at the highest frequency should be carried out, that is, whether it can be carried under normal load.
At the lowest operating frequency of the load, the heating condition of the motor should be examined. Make the drive system work at the lowest speed required by the load, apply the maximum load at this speed, perform a low-speed operation test according to the continuous operation time required by the load, and observe the heating condition of the motor.
The overload test can be carried out according to the possible overload conditions and duration of the load to observe whether the traction system can continue to work.
After adjustment, the inverter is officially put into load operation and should generally be observed for more than two hours to ensure reliable operation.
The above are the most basic steps of inverter debugging. In the process of inverter debugging, you may encounter various situations, such as inverter interference and anti-interference, power factor compensation, closed-loop operation, etc., which must be gradually mastered through specific practice.
4.2.3 Closed-loop operation
Here we will talk about closed-loop operation. It is the most important function of the frequency converter. In many cases, the advantages of automatic control can only be reflected through closed-loop operation.
(1) Principle of closed-loop system
Closed-loop operation is to select a physical quantity of the traction system (such as temperature, pressure, tension, liquid level, etc.), detect it at a certain point (this point should play a key role or be universal to the operation of the entire system) with a corresponding sensor or transmitter (such as thermocouple, remote pressure gauge, temperature transmitter, pressure transmitter, tension sensor, etc.), and then send it to the PID regulator to perform proportional, integral and differential operations with the expected value of the system (which can be set on the PID), and then send it to the frequency input terminal of the inverter to adjust the frequency of the inverter, and then adjust the speed of the motor, so that the entire traction system is in a state of automatic adjustment and stable operation.
The detection value of the system is called the feedback signal, and the expected value is called the given signal, or the target signal. The adjustment process of the system is the process of repeatedly comparing and calculating these two signals to make them as close as possible. Here we take constant pressure water supply as an example. As shown in Figure 13 below.
The pump motor in the figure is powered by the inverter vvvf. sp is a pressure sensor that detects the pressure on the pipeline. It can also be a pressure transmitter, a remote pressure gauge, etc., and the inverter supplies it with +24v or +5v power. After detecting the pipeline pressure, it converts it into a 4-20ma current or a 0-5v voltage signal and sends it back to the inverter.
After the inverter is set to pid effective, the inverter has two analog signal input terminals:
Target signal input terminal. That is, the given terminal vrf. It is a value corresponding to the pressure control target. It is set by the potentiometer on the inverter. It can also be set directly by the keyboard. When a dedicated pid controller is used, it is set by the dedicated sv setting window. In addition to being related to the required pressure control target, it is also related to the range of the pressure transmitter sp. When setting, it must be equivalent to the relevant range.
Feedback signal input terminal. That is, the auxiliary given terminal vpf. It receives the signal fed back from the pressure sensor sp.
(2) Control process
Assume: xt is the target signal, whose size corresponds to the required pipeline pressure. xf is the feedback signal of the pressure transmitter. Then the size of the inverter output frequency fx is determined by the composite signal (xt-xf).
If the pipeline pressure p exceeds the target value, then xf》xt→(xt—xf)《0→inverter output frequency fx↓→motor speed nx↓→pipeline pressure p↓→until it matches the required target pressure (xt≈xf).
On the contrary, if the pipeline pressure p is lower than the target value, then xf《xt→(xt—xf)》0→inverter output frequency fx↑→motor speed nx↑→pipeline pressure p↑→until it matches the required target pressure (xt≈xf).
There is a contradiction in the above process: on the one hand, we require that the actual pressure of the pipeline (whose size is proportional to xf) should be infinitely close to the target pressure (whose size is proportional to xt), that is, we require (xt-xf)→0; on the other hand, the output frequency fx of the frequency converter is determined by the subtraction of xt and xf. It can be imagined that if (xt-xf) is directly used as the given signal xg, the system will not work.
How to solve the above problems leads to the use of pid.
(1) The method to solve the above problem is to amplify (xt-xf) and then use it as the frequency given signal, that is:
xg=kp(xt-xf)
In the formula, kp is the proportional gain (i.e. the amplification factor).
The above relationship is shown in Figure 14. Since xg is the result of (xt--xf) being enlarged proportionally, this link is called the proportional link. Obviously, the larger kp is, the:
(xt-xf)=xg/kp
The smaller it is, the closer xf is to xt. Here, xf can only be infinitely close to xt, but cannot be equal to xt. That is, there will always be a difference between xf and xt, usually called the static error, represented by ε, and the static error should be as small as possible.
The introduction of the proportional gain link has brought about a new contradiction: in order to reduce the static error, the proportional gain should be increased as much as possible, but because the system has inertia, if kp is too large, it will easily cause the controlled quantity (pressure) to fluctuate and form oscillation, as shown in Figure 15.
(2) Integration (i) The purpose of introducing the integration step is
Make the change of the given signal xg proportional to the integral of the product kp(xt-xf) over time. That is, although kp(xt-xf) increases (or decreases) a lot at once, xg can only increase (or decrease) gradually within the "integration time", thereby slowing down the change speed of xg and preventing oscillation. The longer the integration time, the slower the change of xg.
As long as the deviation is not eliminated, (xt-xf≠0), the integration will not stop, thus effectively eliminating the static error, as shown in Figure 15.
However, if the integral time is too long, the controlled quantity (pressure) will be difficult to recover quickly when the controlled quantity (pressure) changes sharply.
(3) Differentiation (d) The role of the differentiation link is to give a larger adjustment action in advance according to the change trend of the deviation, thereby shortening the adjustment time and overcoming the disadvantage of delayed recovery due to too long integration time. As shown in Figure 15.
The inverter is usually set with a simple PI regulator inside. This can meet the requirements for simpler closed-loop control, such as water supply and simple wind pressure control. However, for more complex control occasions, such as air compressors, air conditioners, centrifugal fan closed loops, temperature, liquid level, etc., a dedicated PID regulator is generally required to obtain better control. When debugging PID, refer to the corresponding manual and carefully adjust each parameter in order to achieve the best operating effect and enable the system to operate automatically and stably.
5 Conclusion
Based on my many years of experience in the installation and commissioning of frequency converters, this article briefly discusses the most basic installation and commissioning methods of frequency converters, as well as the methods for dealing with some simple problems during the installation and commissioning process. The purpose is to summarize my own experience, communicate with colleagues in the industry, and jointly explore the use of frequency converters, in order to better play the powerful role of frequency converters and better serve the vast number of industrial and mining enterprise users. At the same time, it also provides a less mature manual-like tool for frequency converter users. Of course, with the complexity of control systems, more and more advanced control methods are being continuously applied to industrial control systems, such as PLC, DCS control, industrial control computers, etc. Due to limited space, I cannot state them one by one here, and I hope readers will forgive me. Due to my limited ability, the shortcomings and errors in the article are inevitable, and I hope that the vast number of colleagues will not hesitate to give advice. Now with the development of my country's national economy, energy conservation and consumption reduction have become a problem to be solved. I believe that the industry we are engaged in will become the most direct means for the country to save energy and reduce consumption, and make our own efforts for the development of the national economy.
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