6 misunderstandings and countermeasures of ABB inverter applications

Myth 1: Connect electromagnetic switches and electromagnetic contactors in the output circuit of the inverter

In practical applications, some occasions need to use contactors for frequency converter switching: such as switching to the power frequency state when the frequency conversion fails, or when the one-to-two mode is adopted, one motor fails, and the inverter turns to drag another motor. Therefore, many users will think that installing electromagnetic switches and electromagnetic contactors in the output circuit of the inverter is a standard configuration and a way to safely disconnect the power supply, but in fact, there are great hidden dangers in this practice.

Disadvantages: When the inverter is still running, the contactor is disconnected first, and the load is suddenly interrupted, and the surge current will make the overcurrent protection action, which will have a certain impact on the main circuit of the rectifier inverter. Seriously, it can even cause damage to the IGBT of the inverter output module. At the same time, when the inductive motor is loaded, the inductive magnetic field energy cannot be released quickly, which will generate high voltage, 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 realized by triggering the inverter control terminal to achieve the effect of soft start and soft stop. If the contactor must be used on the output side of the frequency conversion governor, the necessary control interlock must be made 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 out of operation normally, many users are accustomed to disconnecting the AC input power switch of the inverter, thinking that it is safer and can also save energy.

Disadvantages: This approach seems to protect the inverter from the impact of power failure. In fact, the inverter is not charged for a long time, coupled with the influence of on-site environmental humidity, which will cause the internal circuit board to be damp and cause slow oxidation and gradual short circuit. This is the reason why after the inverter is out of power and out of operation for a period of time, soft faults 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 turned on, desiccant should be placed in the cabinet or an automatic temperature and humidity control heater should be installed to keep the ventilation and environment dry.

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

The inverter control cabinet installed in some factories and mines, basements and open air will withstand the harsh test of harsh environments such as high temperature, dust and humidity. For this reason, many users will choose a sealed type of 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 of the inverter to overheat due to insufficient ventilation and heat dissipation capacity, and the protective action of the thermal components will cause the fault to trip and the equipment will be forced to stop operation.

Solution: Install a breathable rain cover on the upper part of the inverter control cabinet with a dust filter and act as an exhaust outlet. The lower part is also slotted to install a fan with a strainer as an air intake. It creates air circulation and filters dust from the environment. Cooling air flow direction: from the bottom to the top. The transverse 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, consider installing the drive 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 install the heating element or the component that is easy to generate heat close to the bottom 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 limitation of power capacity, the voltage quality of some enterprises cannot be guaranteed, 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 decrease significantly. In order to improve the voltage quality, users usually connect power factor compensation capacitors in parallel at the output end of the inverter, hoping to improve the power factor of the motor.

Disadvantages: Connecting the power factor compensation capacitor with the surge absorber on the motor cable (between the transmission unit and the motor), their effects 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 must be ensured that the capacitor and the ACS800 are not charged at the same time to avoid damage to the inverter by inrush voltages. The current flowing into the inverter is used to improve the power factor, but the charging current of the inverter causes the inverter to overcurrent (OCT), so it cannot be started.

Solution: Remove the capacitor and run it, as for improving the power factor, it is effective 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 protection of the inverter, and the effect is better than that of the 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 the traditional fuse. At present, the complete sets of frequency conversion speed regulation equipment produced by many manufacturers are basically equipped with circuit breakers (air switches), in fact, there are some potential safety hazards.

Disadvantages: When the power cable is short-circuited, the circuit breaker protection action trips due to the inherent action time of the circuit breaker itself, which will introduce the short-circuit current into the inverter, causing component damage.

Strategy: As long as the cable is selected according to the rated current, the inverter drive unit can protect itself, the input and the motor cable against thermal overload, and no additional thermal overload protection equipment is required. Configuring a fuse will protect the input cable in the event of a short circuit, reduce unit damage and prevent damage to connected equipment in the event of a short circuit inside the drive.

Check that the configured fuse action time should be less than 0.5 seconds. The operating time 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 material and the length. When the operating time is exceeded by 0.5 seconds with a gG fuse, the fast melt (aR) reduces the operating time to an acceptable level in most cases. The fuse must be of the non-delay type.

Circuit breakers do not provide fast enough protection for transmission equipment because they react slower than fuses. Therefore, when fast protection is required, a fuse should be used instead of a circuit breaker.

Myth 6: The inverter selection only needs to consider the load power

When purchasing a frequency converter, many users usually only match the capacity of the frequency converter 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: Due to the difference in the load characteristics of the motor, if the comprehensive factors are not fully considered, the inverter may be damaged due to improper use, and at the same time, it may cause safety risks due to the lack of necessary braking units and filters.

Coping strategy: According to the characteristics and types of loads, the capacity and configuration of the inverter should be reasonably selected.

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

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. At the same time, when the elbow is placed under the heavy object, there will be energy feedback, so it is necessary to use a braking unit or use a common bus method.

3) Uneven load: some loads are sometimes light and sometimes heavy, at this time, 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 inertial load: such as centrifuge, punch, rotary kiln of cement plant, such load inertia is very large, so it may oscillate when starting, and there is energy feedback when the motor decelerates...... A slightly larger inverter should be used to speed up start-up and avoid oscillation. Cooperate with the braking unit to eliminate the regenerative energy.

2. Conclusion

After the inverter cooperates with other intelligent equipment (PLC, DCS system), it can realize multiple control strategies and closed-loop adjustment, and it also has a relatively complete protection function. However, there are many misunderstandings in the actual application and installation environment. Facing up to the contradictions, avoiding risks, and rational use are the keys to improving the efficiency and service life of inverters.
6 misunderstandings and countermeasures of ABB inverter applications

6 misunderstandings and countermeasures of ABB inverter applications

6 misunderstandings and countermeasures of ABB inverter applications

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