Several important problems in field installation and debugging of frequency converter

Frequency converter is a kind of electric energy control device which uses the on-off function of power semiconductor devices to convert power frequency power into another frequency. The frequency converter we use now mainly adopts the AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion). Firstly, the power frequency AC power supply is converted into DC power supply through the rectifier, and then the DC power supply is converted into the AC power supply with controllable frequency and voltage to supply the motor. The circuit of frequency converter is generally composed of four parts: rectifier, intermediate DC link, inverter and control. The rectifier part is three-phase bridge uncontrolled rectifier, the inverter part is IGBT three-phase bridge inverter, and the output is PWM waveform, and the intermediate DC link is filter, DC energy storage and buffer reactive power.

Inverter selection:

The following points should be determined when selecting the frequency converter:

1. The purpose of frequency conversion, constant voltage control or constant current control, etc.

2. The load type of frequency converter, such as vane pump or positive displacement pump, should pay special attention to the performance curve of load, which determines the application method.

3. The matching between inverter and load;

1. Voltage matching; the rated voltage of frequency converter is consistent with the rated voltage of load.

2. Current matching; for ordinary centrifugal pump, the rated current of frequency converter is consistent with that of motor. For special loads such as deep water pump, it is necessary to refer to the motor performance parameters to determine the inverter current and overload capacity with the maximum current.

3. Torque matching; this situation may occur under constant torque load or with reducer.

4. When using frequency converter to drive high-speed motor, the output current value increases due to the small reactance of high-speed motor and the increase of high-order harmonic. Therefore, the capacity of frequency converter for high-speed motor is slightly larger than that of ordinary motor.

5. If the frequency converter needs long cable operation, measures should be taken to restrain the influence of long cable on the ground coupling capacitance, so as to avoid the insufficient output of frequency converter. Therefore, in this case, the capacity of frequency converter should be enlarged by one gear or output reactor should be installed at the output end of frequency converter.

6. For some special applications, such as high temperature, high altitude, this will cause the inverter capacity reduction, the inverter capacity should be enlarged by one gear.

Inverter control schematic design:

1. Firstly, confirm the installation environment of frequency converter;

1. Working temperature. The inverter is a high-power electronic component, which is easily affected by the working temperature. The product is generally required to be 0-55 ℃. However, in order to ensure the safety and reliability of the work, it should be considered to leave room when using, and it is better to control it below 40 ℃. In the control box, the frequency converter should be installed on the top of the box, and the installation requirements in the product manual should be strictly observed. It is absolutely not allowed to install heating elements or easily heating elements close to the bottom of the converter.

2. Ambient temperature. When the temperature is too high and the temperature changes greatly, the inner part of the inverter is prone to condensation, and its insulation performance will be greatly reduced, and even cause short circuit accident. If necessary, desiccant and heater must be added in the box. In the water treatment room, the water vapor is generally heavy. If the temperature changes greatly, this problem will be more prominent.

3. Corrosive gas. If the concentration of corrosive gas is high, it will not only corrode the leads and printed circuit boards of components, but also accelerate the aging of plastic devices and reduce the insulation performance.

4. Vibration and shock. When the control cabinet with frequency converter is subject to mechanical vibration and impact, it will cause poor electrical contact. Huaian thermal power has such a problem. At this time, in addition to improving the mechanical strength of the control cabinet and keeping away from the vibration and impact sources, seismic rubber pads should be used to fix the vibration components such as electromagnetic switches outside and inside the control cabinet. After running for a period of time, the equipment should be inspected and maintained.

5. Electromagnetic interference. Due to rectification and frequency conversion, a lot of interference electromagnetic waves are generated around the converter. These high-frequency electromagnetic waves have certain interference to the nearby instruments and instruments. Therefore, the instrument and electronic system in the cabinet should use metal shell to shield the interference of frequency converter to the instrument. All components shall be reliably grounded. In addition, shielded control cables shall be used for wiring between electrical components, instruments and instruments, and the shielding layer shall be grounded. If the electromagnetic interference is not handled properly, the whole system can not work, resulting in control unit failure or damage.

2. The distance between inverter and motor determines the cable and wiring method;

1. The distance between inverter and motor should be as short as possible. In this way, the capacitance to ground of the cable is reduced, and the emission source of interference is reduced.

2. Shielded cable is used for control cable, shielded cable is used for power cable, or conduit is used for shielding from inverter to motor.

3. The motor cable shall be independent of other cables and the minimum distance shall be 500mm. At the same time, it is necessary to avoid long distance parallel running of motor cable and other cables, so as to reduce the electromagnetic interference caused by the rapid change of inverter output voltage. If the control and power cables cross, they should be crossed at 90 degrees as far as possible. Analog signal line related to frequency converter shall be separated from main circuit, even in control cabinet.

4. It is better to select shielded twisted pair for analog signal line related to frequency converter, and shielded three core cable (its specification is larger than that of ordinary motor) or comply with the user manual of frequency converter.

3. Inverter control schematic diagram;

1. Main circuit: the role of the reactor is to prevent the high-order harmonics generated by the frequency converter from returning to the power grid through the input circuit of the power supply, thus affecting other power receiving equipment. It is necessary to determine whether to add a reactor according to the capacity of the frequency converter. The filter is installed at the output end of the frequency converter to reduce the high-order harmonic output of the frequency converter. When the distance between the inverter and the motor is relatively long, the reactor should be installed at the output end of the inverter, Filters should be installed. Although the frequency converter has various protection functions, the open phase protection is not perfect. The circuit breaker plays an overload in the main circuit, and the phase loss protection can be selected according to the capacity of the frequency converter. The overload protection of frequency converter can be used instead of thermal relay.

2. Control circuit: manual switching of power frequency frequency conversion, so that the power frequency can be switched manually in case of frequency conversion failure. Since voltage cannot be applied at the output end, the fixed power frequency and frequency conversion should be interlocked.

4. Grounding of frequency converter;

Proper grounding of frequency converter is an important means to improve system stability and suppress noise. The smaller the grounding resistance of the grounding terminal of the frequency converter is, the better. The cross section of the grounding wire is not less than 4mm, and the length is not more than 5m. The grounding of frequency converter shall be separated from the grounding point of power equipment, and shall not be grounded together. One end of the shielding layer of the signal line is connected to the grounding end of the frequency converter, and the other end is floating in the air. The inverter is electrically connected with the control cabinet.

Inverter control cabinet design:

The frequency converter should be installed inside the control cabinet. The following problems should be paid attention to in the design of the control cabinet

1. Heat dissipation problem: the heating of frequency converter is caused by internal loss. In the frequency converter, the main circuit is the main loss, accounting for about 98%, and the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the frequency converter, it is necessary to heat the inverter. We usually use the fan to heat the inverter. If the fan can not work normally, the inverter should be stopped immediately. For the high-power inverter, the fan should be added on the control cabinet. The air duct of the control cabinet should be designed reasonably, and all air inlets should be equipped with fans Set up dust-proof net to exhaust air smoothly, avoid eddy current in cabinet and form dust accumulation at fixed position; select matching fan according to ventilation quantity in inverter instruction manual, and pay attention to vibration prevention during fan installation.

2. Electromagnetic interference problems:

1. Due to rectification and frequency conversion, a lot of interference electromagnetic waves are generated around the frequency converter. These high-frequency electromagnetic waves have certain interference on nearby instruments and instruments, and will produce high-order harmonics, which will enter the whole power supply network through the power supply circuit, thus affecting other instruments. If the power of the inverter is very large, accounting for more than 25% of the whole system, the anti-interference measures of the control power supply should be considered.

2. When there are high frequency impact loads in the system, such as welding machine and electroplating power supply, the inverter itself will be protected due to interference, so the power quality of the whole system should be considered.

3. Protection issues need to pay attention to the following points:

1. Waterproof and anti condensation: if the frequency converter is placed on the site, it is necessary to pay attention to the pipe flange or other leakage points above the inverter cabinet, and there should be no splashing water near the frequency converter. In short, the protection grade of the cabinet on site should be above IP43.

2. Dust prevention: all air inlets shall be equipped with dust screen to prevent the entry of flocculent debris. The dust screen shall be designed as detachable type to facilitate cleaning and maintenance. The mesh of the dust-proof net shall be determined according to the specific situation on site, and the joint of the dust-proof net and the control cabinet shall be handled strictly.

3. Anti corrosive gas: in the chemical industry, this situation is more common. At this time, the frequency conversion cabinet can be placed in the control room.

Wiring specification of frequency converter:

Signal line and power line must be separated from each other: when using analog signal for remote control of frequency converter, in order to reduce the interference of analog quantity from frequency converter and other equipment, please separate the signal line of control frequency converter from strong current circuit (main circuit and sequence control circuit). The distance should be more than 30cm. Even in the control cabinet, the same wiring specifications should be maintained. The longest control return line between the signal and the frequency converter shall not exceed 50m.

The signal line and power line must be placed in different metal pipes or metal hose respectively: if the signal line connecting PLC and frequency converter is not placed in the metal pipe, it is very easy to be interfered by frequency converter and external equipment; at the same time, since the frequency converter has no built-in reactor, the input and output power lines of the frequency converter will have strong interference to the outside, so the signal is placed The metal pipe or hose of line No. 1 should be extended to the control terminal of frequency converter to ensure the complete separation of signal line and power line.

1. The analog control signal line shall use double stranded shielded wire with the wire specification of 0.75mm2. During wiring, it must be noted that the cable stripping should be as short as possible (about 5-7mm). At the same time, the shielding layer after stripping should be wrapped with insulating tape to prevent interference caused by contact between shielding wire and other equipment.

2. In order to improve the simplicity and reliability of wiring, it is recommended to use crimping bar terminals on signal lines.

Operation of frequency converter and setting of relevant parameters:

The frequency converter has many setting parameters, and each parameter has a certain range of selection. In use, the frequency converter cannot work normally due to improper setting of individual parameters.

Control mode: speed control, torque control, PID control or other methods. After the control mode is adopted, static or dynamic identification is needed according to the control accuracy.

Minimum operating frequency: the minimum speed of motor operation. When the motor runs at low speed, its heat dissipation performance is very poor. If the motor runs at low speed for a long time, the motor will be burnt out. Moreover, at low speed, the current in the cable will increase, which will also lead to cable heating.

Maximum operating frequency: the maximum frequency of general frequency converter is 60Hz, some even 400Hz. High frequency will make the motor run at high speed. For ordinary motor, its bearing can't run at rated speed for a long time. Whether the rotor of motor can bear such centrifugal force.

Carrier frequency: the higher the carrier frequency is set, the greater the higher harmonic component is, which is closely related to the cable length, motor heating, cable heating, inverter heating and other factors.

Motor parameters: the frequency converter sets the power, current, voltage, speed and maximum frequency of the motor in the parameters, which can be directly obtained from the motor name plate.

Frequency hopping: resonance may occur at a certain frequency point, especially when the whole device is relatively high; when controlling the compressor, avoid the surge point of the compressor.