Today, with the rapid development of science and technology, with the continuous improvement of industrial automation, inverters have also been widely used. Choosing a suitable inverter is very important to ensure the normal and efficient work of the inverter, but in practice, it is necessary to pay attention to some misunderstandings.
Myth 1: Connect electromagnetic switches and electromagnetic contactors in the output circuit of the inverter
In practice, some occasions require the use of contactors for inverter switching: such as switching to the power frequency state when the frequency conversion is faulty, or the one-to-two method is selected, one motor is faulty, and the inverter turns to drag another motor. Therefore, many users will think that the installation of electromagnetic switches and electromagnetic contactors in the output circuit of the inverter is a standardized equipment and a safe way to disconnect the power supply, but in fact, there is a greater danger in this practice.
Disadvantages: When the inverter is still running, the contactor is disconnected first, and the load is suddenly stopped, and the surge current will make the overcurrent maintenance action, which will have a certain impact on the main circuit of the rectifier inverter. Seriously, it can even damage the IGBT of the inverter output module. Together, when the rational motor is loaded, the rational magnetic field energy cannot be released quickly, and high voltage will occur, which will damage the insulation of the motor and the connecting cable.
Coping strategy: connect the output side of the inverter directly with the motor cable, and the normal start-stop motor can be completed by triggering the inverter control terminal to achieve the role of soft start and soft stop. If it is necessary to use the contactor on the output side of the frequency conversion governor, it is necessary to carry out the necessary control interlock between the output of the frequency conversion governor and the action of the contactor, so as to ensure that the contactor can only operate when the frequency conversion governor has no output.
Myth 2: When the equipment is out of operation normally, disconnect the AC input power supply of the inverter
When the equipment is normally down, many users are accustomed to disconnecting the AC input power switch of the inverter, thinking that it is safer and can also save energy.
Disadvantage: This approach seems to be able to maintain the inverter from the impact of the power supply on the surface. In practice, the inverter is not charged for a long time, coupled with the influence of on-site environmental humidity, the internal circuit board will be slowly oxidized due to moisture, and gradually show a short circuit. This is the reason why after the inverter is out of power and out of operation for a period of time, soft problems will be frequently reported when the power is sent again.
Coping strategy: In addition to equipment maintenance, the inverter should be kept live for a long time. In addition, the upper and lower fans of the inverter control cabinet should be opened, the desiccant should be placed in the cabinet or the heater should be actively controlled by temperature and humidity to keep the ventilation and environment dry.
Myth 3: The inverter control cabinet installed in the open air or dust environment is sealed type
The inverter control cabinet used in some factories and mines, basements, and open-air installations will withstand strict testing in harsh environments such as high temperature, dust, and humidity. For this reason, many users choose a sealed inverter cabinet. Although this can play a role in rainproof and dustproof to a certain extent, it also brings the problem of poor heat dissipation of the inverter.
Disadvantages: The tight sealing of the control cabinet will cause the internal components to overheat due to the lack of ventilation and heat dissipation capacity, and the maintenance of thermal components will cause a fault to trip, and the equipment will be forced to stop operation.
Strategy: Install a breathable rain cover on the upper part of the inverter control cabinet with a dust filter and act as an exhaust port. The lower part is also slotted with an electric fan with a strainer to act as an air intake. It can form air circulation and filter dust in the environment together. Cooling air flow direction: from the bottom to the top. The distance between the transverse devices of the inverter should not be less than 5mm, and the temperature of the cooling air entering the inverter should not exceed +40 degrees Celsius. If the ambient temperature is above +40 degrees Celsius for a long time, consider installing the inverter in a small room with air conditioning.
In the control box, the inverter should generally be installed in the upper part of the box, and it is absolutely not allowed to put the heating element or the element that is easy to heat up close to the bottom device of the inverter.
Myth 4: In order to improve the voltage quality, the power factor compensation capacitor is connected in parallel at the output end of the inverter
Due to the constraints of power capacity, the voltage quality of some enterprises cannot be guaranteed, especially when large-scale electrical equipment is put into use, the bus voltage in the plant will decrease, and the load power factor will obviously decrease. In order to improve the voltage quality, users generally connect power factor compensation capacitors in parallel at the output end of the inverter, hoping to improve the power factor of the motor.
Disadvantage: Connecting the power factor compensation capacitor and the surge absorber to the motor cable (between the transmission unit and the motor) will not only reduce the control accuracy of the motor, but also form a transient voltage on the output side of the transmission unit, causing permanent damage to the ACS800 transmission unit. If a power factor compensation capacitor is connected in parallel on the three-phase input line of the ACS800, it is necessary to ensure that the capacitor and the ACS800 are not charged together to avoid damage to the inverter due to surge voltage. The current flowing into the inverter is used to improve the power factor, but the charging current cannot be started because the charging current forms an inverter overcurrent (OCT).
Strategy: Remove the capacitor and run it, and as for improving the power factor, it is useful to connect an AC reactor to the input side of the inverter.
Myth 5: The circuit breaker is used as the thermal overload and short-circuit maintenance of the inverter, which is better than the fuse
The circuit breaker has a relatively complete maintenance function, has been widely used in power distribution equipment, and has a tendency to replace the traditional fuse. At present, the complete sets of frequency conversion speed control equipment produced by many manufacturers are basically equipped with circuit breakers (air switches), in fact, there are also some safety risks.
Disadvantages: When the power cable is short-circuited, the circuit breaker maintenance action trips due to the inherent action time of the circuit breaker itself, and the short-circuit current will be introduced into the inverter during this period, resulting in component damage.
Strategy: As long as the cable is selected according to the rated current, the inverter drive unit can maintain itself, the input and the motor cable to avoid thermal overload and do not need additional thermal overload maintenance equipment. Equipping a fuse will allow the input cable to be maintained in the event of a short circuit, reducing device damage and avoiding damage to connected equipment in the event of a short circuit inside the transmission.
Check that the fuse action time of the equipment should be lower than 0. 5 seconds. The moment of operation depends on the type of fuse (gG or aR), the impedance of the power supply network, the cross-sectional area of the power cable, the data and the length. When using a gG fuse that exceeds 0. At 5 seconds of action, fast melting (aR) reduces the time of action to an acceptable level in most cases. It is necessary for the fuse to be of the non-delay type.
Circuit breakers do not provide maintenance to the transmission equipment quickly enough because they react slower than fuses. Therefore, when quick maintenance is required, fuses should be used instead of circuit breakers.
Myth 6: The inverter selection only needs to consider the load power
When purchasing inverters, many users generally only match the inverter capacity based on the power of the drive motor. In fact, the load driven by the motor is different, and the requirements for the inverter are also different.
Disadvantages: Because of the difference in the load characteristics of the motor, if the inductive elements are not fully considered, the inverter may be damaged due to improper use, and the necessary braking unit and filter may be not equipped, which may cause safety hazards.
Coping strategy: According to the characteristics and types of loads, the capacity and equipment of the inverter should be reasonably selected.
1. The fan and water pump are the most general loads: the requirements for the inverter are brief, and the capacity of the inverter is only equal to the capacity of the motor (the capacity of the air compressor, deep water pump, sediment pump, and fast-changing music fountain needs to be increased).
2. Crane load: This type of load is characterized by a large impact when starting, so the inverter is required to have a certain margin. Together, when the elbow is placed under the weight, there will be energy feedback, so it is necessary to use the braking unit or choose the common bus method.
3. Uneven load: some loads are sometimes light and sometimes heavy, and the capacity of the inverter should be selected according to the situation of heavy load, such as rolling mill machinery, crushing machinery, mixer, etc.
4. Large inertia load: such as centrifuge, punch, and rotary kiln of cement plant, this kind of load has a large inertia, so it may vibrate when it starts, and there is energy feedback when the motor decelerates...... A slightly larger inverter should be used to speed up the operation and avoid vibration. Cooperate with the braking unit to eliminate the regenerative energy.
After cooperating with other intelligent equipment (PLC, DCS system), the inverter can complete multiple control strategies and closed-loop conditioning, and it also has a relatively complete maintenance function. However, in practice, there are many misunderstandings. Facing up to the opposite location, avoiding danger, and rational use is the key to improving the power and service life of the inverter, and it can also reduce unnecessary losses.
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