Misunderstanding 1: Connect the electromagnetic switch and electromagnetic contactor in the output circuit of the inverter
In practical applications, some occasions need to use the contactor to switch the inverter: if the frequency conversion fault is switched to the power frequency state, or when one to two is used, one motor fails, the inverter turns to drag another Motor and other conditions. Therefore, many users will think that installing electromagnetic switches and electromagnetic contactors in the output circuit of the inverter is a standard configuration, which is a way to safely disconnect the power supply. In fact, this practice has a great hidden danger.
Disadvantages: When the inverter is still running, the contactor is disconnected first, and the load is suddenly interrupted. The surge current will cause the overcurrent protection action, which will have a certain impact on the rectifier inverter main circuit. Serious, it may even cause damage to the inverter output module IGBT. At the same time, when the load is inductive motor, the inductive magnetic field energy cannot be released quickly, which will generate high voltage and damage the insulation of the motor and the connecting cable.
Coping strategy: Connect the output side of the inverter directly to the motor cable. The normal start-stop motor can be realized by triggering the inverter control terminal to achieve the soft start and soft stop effect. If it is necessary to use the contactor on the output side of the inverter, it is necessary to control the interlock between the output of the inverter and the action of the contactor to ensure that the contactor can only act when there is no output from the inverter. .
Misunderstanding 2: Disconnect the AC input power of the inverter when the equipment is out of service normally.
When the equipment is out of service normally, many users are accustomed to disconnecting the AC input power switch of the inverter, which is considered to be safer and energy-saving.
Disadvantages: This approach seems to protect the inverter from power failures. In fact, the inverter does not charge for a long time, and the humidity of the on-site environment will cause the internal circuit board to be damp and slowly oxidize and gradually appear a short circuit. This is the reason why the soft fault is frequently reported when the inverter is powered off again after a period of power failure.
Coping strategy: In addition to equipment maintenance, the inverter should be in a live state for a long time. In addition, the upper and lower fans of the inverter control cabinet should be turned on, the desiccant should be placed in the cabinet or the automatic temperature and humidity control heater should be installed to keep the ventilation and the environment dry.
Misunderstanding 3: Inverter control cabinet installed in open air or dust environment adopts sealed type
In some factories, mines, basements, open-air installation and use of inverter control cabinets, will withstand the harsh test of harsh environments such as high temperature, dust, humidity. To this end, many users will choose a sealed type of frequency conversion cabinet. Although this can play the role of 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 inverter to overheat due to insufficient ventilation and heat dissipation capability, and the thermal element protection action will cause the fault to trip and the equipment will be forced to stop.
Coping strategy: Install a breathable rain cover on the top of the inverter control cabinet with a dust filter and as an exhaust port. The lower part is also slotted to install a fan with a strainer as an air inlet. Air circulation can be formed while filtering dust in the environment. Cooling air flow direction: from bottom to top. The lateral installation distance between the inverters should be no less than 5mm, and the cooling air temperature 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 frequency converter in an air-conditioned chamber.
In the control box, the inverter should generally be installed in the upper part of the cabinet. It is absolutely not allowed to install the heating element or the heat-prone component close to the bottom of the inverter.
Misunderstanding 4: In order to improve the voltage quality, parallel power factor compensation capacitors at the output of the inverter
Due to the limitation of power capacity, some enterprises cannot guarantee the voltage quality. Especially when large-scale power equipment is put into use, the bus voltage in the station will decrease and the load power factor will obviously decrease. In order to improve the voltage quality, the user usually parallels the power factor compensation capacitor at the output of the inverter, hoping to improve the motor power factor.
Disadvantages: Connecting a power factor compensation capacitor and a surge absorber to the motor cable (between the drive unit and the motor), their effects not only reduce the control accuracy of the motor, but also form a transient voltage on the output side of the drive unit. Causes permanent damage to the ACS800 drive unit. If a power factor compensation capacitor is connected in parallel on the three-phase input line of the ACS800, it must be ensured that the capacitor and the ACS800 are not charged at the same time to prevent the surge voltage from damaging the inverter. The current in the inverter flows into the capacitor for improving the power factor, and the inverter overcurrent (OCT) is caused by the charging current, so it cannot be started.
Coping strategy: After the capacitor is removed and operated, as for improving the power factor, it is effective to connect the AC reactor to the input side of the inverter.
Misunderstanding 5: Select the circuit breaker as the thermal overload and short circuit protection of the inverter, the effect is better than the fuse
The circuit breaker has a relatively complete protection function and has been widely used in power distribution equipment, which has a tendency to replace the traditional fuse. Nowadays, many sets of variable frequency speed control equipment produced by many manufacturers are basically equipped with circuit breakers (air switches). In fact, there are some hidden dangers.
Disadvantages: When a short-circuit fault occurs in the power cable, the circuit breaker protection action trips due to the inherent operating time of the circuit breaker itself. During this period, the short-circuit current is introduced into the inverter, causing component damage.
Coping strategy: As long as the cable is selected according to the rated current, the drive unit protects itself, the input and the motor cable to prevent thermal overload and does not require additional thermal overload protection. Configuring the fuse will protect the input cable in the event of a short circuit and reduce damage to the unit and prevent damage to the connected equipment when the drive is internally shorted.
Check that the configured fuse action time should be less than 0.5 seconds. The action time depends on the type of fuse (gG or aR), the impedance of the supply network, the cross-sectional area of ​​the power cable, the material and the length. When using a gG fuse for more than 0.5 seconds of action time, fast-blow (aR) can reduce the action time to an acceptable level in most cases. The fuse must be of no delay type.
Circuit breakers do not provide fast enough protection for the transmission equipment because they react more slowly than fuses. Therefore, fuses should be used instead of circuit breakers when fast protection is required.
Misunderstanding 6: Inverter selection only needs to consider load power
When many users purchase inverters, they usually 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 different.
Disadvantages: Due to the difference in load characteristics of the motor, if the comprehensive factors are not fully considered, the inverter may be damaged due to improper use, and the safety risk may be caused by not having the necessary braking unit and filter.
Coping strategy: Reasonable selection of the capacity and configuration of the inverter for the characteristics and type of the load.
1) Fan and water pump are the most common loads: the requirements for the inverter are the simplest, as long as the inverter capacity is equal to the motor capacity (air compressor, deep water pump, sediment pump, fast changing music fountain need to increase capacity) .
2) Crane type load: This type of load is characterized by a large impact at startup, so the inverter is required to have a certain margin. At the same time, if the elbow is placed under the weight, there will be energy feedback, so use the brake unit or use the common bus method.
3) Uneven load: Some loads are sometimes light and sometimes heavy. In this case, the inverter capacity should be selected according to the heavy load, such as rolling mill machinery, crushing machinery, mixer, etc.
4) Large inertia load: such as centrifuge, punching machine, rotary kiln of cement plant, such load has large inertia, so it may oscillate when starting, energy feedback when the motor decelerates... It should be accelerated by a slightly larger inverter. Start to avoid oscillation. The brake unit is used to eliminate the feedback energy.
Second, the conclusion
The inverter is equipped with other intelligent devices (PLC, DCS system) to achieve multiple control strategies and closed-loop adjustments, and it also has a relatively complete protection function. However, in the actual application and installation environment, there are many misunderstandings. Facing the contradiction, avoiding risks and using them reasonably is the key to improving the efficiency and service life of the inverter.
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