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Six Misunderstandings and Countermeasures in the Installation and Use of Inverters

Mar 26, 2021

Six Misunderstandings and Countermeasures in the Installation and Use of Inverters

In practical applications, a contactor needs to be used to switch the frequency converter in some occasions: for example, when the frequency conversion fails, it switches to the power frequency state, or when the one-to-two method is used, one motor fails, and the frequency converter turns to drive the other Motors, etc. 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 safe way to disconnect the power supply. In fact, this approach has greater hidden dangers.


   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 to operate, which will cause a certain impact on the main circuit of the rectifier and inverter. In serious cases, it may even damage the output module IGBT of the inverter. At the same time, when the inductive motor is loaded, the inductive magnetic field energy cannot be released quickly, and high voltage will be generated, which will damage the insulation of the motor and the connecting cable.


  Response strategy: Connect the output side of the frequency converter directly to the motor cable, and the normal start and stop of the motor can be realized by triggering the control terminal of the frequency converter to achieve the effect of soft start and soft stop. If a contactor must be used on the output side of the inverter, the necessary control interlock must be added between the output of the inverter and the action of the contactor to ensure that the contactor can only operate when the inverter has no output. .


   Misunderstanding 2: When the equipment is normally shut down, disconnect the AC input power of the inverter


   When the equipment is normally shut down, many users are accustomed to turning off the AC input power switch of the inverter, thinking that it is safer and can save energy.


   Disadvantages: This approach, on the surface, seems to be able to protect the inverter from the impact of power failure. In fact, if the inverter is not electrified for a long time, coupled with the influence of the on-site environmental humidity, the internal circuit board will be damp, causing slow oxidation and gradual short-circuit phenomenon. This is the reason why the inverter will frequently report soft faults when it is powered on again after a period of time after the inverter is powered off.


  Response strategy: In addition to equipment maintenance, the inverter should be kept in a live state for a long time. In addition, the upper and lower fans of the frequency conversion control cabinet should be turned on, a desiccant should be placed in the cabinet, or an automatic temperature and humidity control heater should be installed to keep ventilation and the environment dry.


  Misunderstanding 3: The inverter control cabinet installed in the open air or in a dusty environment adopts a sealed type


  The inverter control cabinets installed in some factories, mines, basements, and open air will withstand harsh environments such as high temperature, dust and humidity. For this reason, many users will choose a sealed type frequency conversion cabinet. Although this can play a role in preventing rain and dust to a certain extent, it also brings about the problem of poor heat dissipation of the inverter.


   Disadvantages: The tight sealing of the control cabinet will cause the internal components of the inverter to overheat due to insufficient ventilation and heat dissipation, and the protection of the thermal components will cause the fault to trip and the equipment will be forced to shut down.


  Response strategy: Install a ventilated rain cover on the upper part of the inverter control cabinet, with a dust-proof filter, and at the same time serve as an exhaust port. The lower part is also slotted to install a fan with a filter as an air inlet. It can form air circulation and filter the dust in the environment at the same time. Cooling air flow direction: from the bottom to the top. The horizontal installation distance between the inverters 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, you need to consider installing the inverter in a small room with air conditioning.


  In the control box, the inverter should generally be installed on the upper part of the box. It is absolutely not allowed to install heating elements or components that are easy to heat up close to the bottom of the inverter.


   Misunderstanding 4: In order to improve the voltage quality, a power factor compensation capacitor is connected in parallel at the output end of the inverter


   Due to the limitation of power capacity, some enterprises cannot guarantee the voltage quality, especially when large-scale electrical equipment is put into use, it will cause the bus voltage in the plant to decrease, and the load power factor will obviously decrease along with it. In order to improve the voltage quality, users usually connect power factor compensation capacitors in parallel at the output of the frequency converter, hoping to improve the power factor of the motor.


Disadvantages: Connect the power factor compensation capacitor and the surge absorber to the motor cable (between the drive unit and the motor). Their influence will not only reduce the control accuracy of the motor, but also form a transient voltage on the output side of the drive unit. Cause 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 will not be charged at the same time to prevent the surge voltage from damaging the inverter. The current of the inverter flows into the capacitor used to improve the power factor, and the inverter cannot be started due to the overcurrent (OCT) of the inverter caused by its charging current.


  Response strategy: Remove the capacitor and run it. As for improving the power factor, it is effective to connect an AC reactor on the input side of the inverter.


  Misunderstanding 5: Choosing a circuit breaker as the inverter's thermal overload and short-circuit protection is better than a fuse


  The circuit breaker has a relatively complete protection function, and has been widely used in power distribution equipment, and has a tendency to replace traditional fuses. Nowadays, the complete sets of variable frequency speed regulation equipment produced by many manufacturers are basically equipped with circuit breakers (air switches). In fact, this also has some potential safety hazards.


   Disadvantages: When a short-circuit fault occurs in the power cable, the circuit breaker protection action trip is delayed due to the inherent operating time of the circuit breaker itself. During this period, a short-circuit current will be introduced into the inverter, causing component damage.


  Response strategy: As long as the cable is selected according to the rated current, the inverter drive unit can protect itself, the input end and the motor cable to prevent thermal overload, and no additional thermal overload protection equipment is required. The configuration of the fuse will protect the input cable in the case of a short circuit, reduce device damage and prevent damage to the connected equipment when the transmission device is short-circuited.


   Check that the configured fuse action time should be less than 0.5 seconds. The operating time depends on the fuse type (gG or aR), the impedance of the power supply network, the cross-sectional area, material and length of the power cable. When the gG fuse is used for more than 0.5 seconds of operating time, fast-acting (aR) can reduce the operating time to an acceptable level in most cases. The fuse must be of no delay type.


   Circuit breakers cannot provide fast enough protection for transmission equipment because their response speed is slower than that of fuses. Therefore, when fast protection is required, a fuse should be used instead of a circuit breaker.


   Misunderstanding 6: Inverter selection only needs to consider the load power


  When many users purchase frequency converters, they usually only match the capacity of the frequency converter according to the power of the driving motor. In fact, the load driven by the motor is different, and the requirements for the frequency converter are also different.


   Disadvantages: Due to the differences in the load characteristics of the motors, if the comprehensive factors are not fully considered, the inverter may be damaged due to improper use. At the same time, because the necessary braking unit and filter are not equipped, it may cause safety risks.


  Response strategy: According to the characteristics and types of the load, select the capacity and configuration of the inverter reasonably.


1) Fans and water pumps are the most common loads: the requirements for frequency converters are the simplest, as long as the capacity of the frequency converter is equal to the capacity of the motor (air compressors, deep water pumps, sediment pumps, fast-changing musical fountains need to increase the capacity) .


  2) Crane type load: This type of load is characterized by a large impact when starting, so it requires a certain margin for the inverter. At the same time, if you put your elbow under a heavy object, there will be energy feedback, so you must use a brake unit or use a shared bus method.


  3) Uneven load: Some loads are sometimes light and sometimes heavy. At this time, the inverter capacity should be selected according to the heavy load, such as rolling mill machinery, crushing machinery, mixers, etc.


4) Large inertial loads: such as centrifuges, punch presses, and rotary kilns in cement plants. Such loads have a large inertia, so they may oscillate when starting, and there will be energy feedback when the motor decelerates. A frequency converter with a larger capacity should be used to speed up Start to avoid oscillation. Cooperate with the braking unit to eliminate the feedback power.


   2. Conclusion


   The frequency converter can realize multiple control strategies and closed-loop regulation after cooperating with other intelligent devices (PLC, DCS system), and it also has a relatively complete protection function. But in actual application and installation environment, there are many misunderstandings. Face up to the contradictions, avoid risks, and use them rationally are the keys to improving the efficiency and service life of the inverter.