Myth 1: Connecting electromagnetic switches and electromagnetic contactors to the output circuit of the frequency converter.
In practical applications, contactors are sometimes needed for switching frequency converters: for example, switching to mains frequency operation when the frequency converter fails, or in a one-to-two configuration where the frequency converter switches to drive the other motor when one motor fails. Therefore, many users believe that installing electromagnetic switches or contactors in the frequency converter output circuit is standard practice and a safe way to disconnect the power. However, this practice actually poses significant risks.
Disadvantages: When the inverter is still running, the contactor disconnects first, suddenly interrupting the load. The surge current will trigger the overcurrent protection, causing a certain impact on the rectifier-inverter main circuit. In severe cases, it may even damage the inverter's output module IGBT. At the same time, when driving an inductive motor load, the energy of the inductive magnetic field cannot be released quickly, generating high voltage and damaging the insulation of the motor and connecting cables.
Solution: Connect the inverter output directly to the motor cable. Normal motor start and stop can be achieved by triggering the inverter control terminals, achieving a soft start and soft stop effect. If a contactor must be used on the inverter output side, necessary control interlocks must be added between the inverter output and the contactor operation to ensure that the contactor can only operate when the inverter has no output.
Myth 2: Disconnect the AC input power supply to the frequency converter when the equipment is normally shut down.
When the equipment is shut down normally, many users are used to disconnecting the AC input power switch of the frequency converter, believing that it is safer and can save energy.
Disadvantages: While this approach may superficially seem to protect the inverter from power failures, in reality, prolonged periods without power, coupled with ambient humidity, can cause the internal circuit boards to become damp, leading to slow oxidation and eventually short circuits. This explains why the inverter frequently reports soft faults when power is restored after a period of shutdown.
Countermeasures: Except for equipment maintenance, the frequency converter should be kept energized for extended periods. In addition, the upper and lower fans of the frequency converter control cabinet should be turned on, and desiccants should be placed inside the cabinet or automatic temperature and humidity control heaters should be installed to maintain ventilation and a dry environment.
Myth 3: Variable frequency drive control cabinets installed in open-air or dusty environments should be of a sealed type.
Inverter control cabinets used in factories, mines, basements, and outdoor installations are subjected to harsh environments such as high temperatures, dust, and humidity. Therefore, many users choose sealed inverter cabinets. While this provides some protection against rain and dust, it also leads to poor heat dissipation for the inverter.
Disadvantages: A tightly sealed control cabinet can cause the inverter to overheat due to insufficient ventilation and heat dissipation, triggering the thermal protection devices, resulting in a fault trip and forced equipment shutdown.
Solution: Install a breathable rainproof cover with a dust filter on the upper part of the inverter control cabinet, which also serves as an exhaust vent. Similarly, install a fan with a filter in the lower part as an air intake. This allows for air circulation and filters dust from the environment. Cooling airflow direction: from bottom to top. The lateral installation distance between inverters should be no less than 5mm, and the temperature of the cooling air entering the inverters should not exceed +40 degrees Celsius. If the ambient temperature remains above +40 degrees Celsius for extended periods, consider installing the inverters in a small, air-conditioned room.
In the control box, the frequency converter should generally be installed on the upper part of the box. It is absolutely forbidden to install heat-generating or easily heat-generating components close to the bottom of the frequency converter.
Myth 4: To improve voltage quality, a power factor compensation capacitor should be connected in parallel at the output of the frequency converter.
Due to power capacity limitations, some enterprises cannot guarantee voltage quality, especially when large electrical equipment is put into operation. This can cause a drop in bus voltage within the plant, resulting in a significant decrease in the load power factor. To improve voltage quality, users typically connect power factor compensation capacitors in parallel at the inverter output, hoping to improve the motor power factor.
Disadvantages: Connecting the power factor correction capacitor and surge absorber to the motor cable (between the drive unit and the motor) not only reduces the motor's control accuracy but also creates transient voltages on the drive unit's output side, causing permanent damage to the ACS800 drive unit. If a power factor correction capacitor is connected in parallel on the three-phase input lines of the ACS800, it must be ensured that the capacitor and the ACS800 do not charge simultaneously to avoid surge voltage damage to the inverter. Current flowing into the power factor correction capacitor from the inverter can cause overcurrent (OCT) in the inverter, preventing it from starting.
Solution: Remove the capacitor and operate the inverter. As for improving the power factor, connecting an AC reactor to the input side of the inverter is effective.
Myth 5: Circuit breakers are better than fuses for protecting frequency converters from thermal overload and short circuits.
Circuit breakers have relatively complete protection functions and are widely used in power distribution equipment, showing a strong trend of replacing traditional fuses. Many manufacturers now also equip their complete sets of frequency converters (air switches) with circuit breakers, but this also presents some safety hazards.
Disadvantages: When a short circuit fault occurs in the power cable, the circuit breaker protection trips due to the inherent tripping time of the circuit breaker itself. During this period, the short circuit current will be introduced into the inverter, causing damage to the components.
Solution: As long as the cable is selected according to the rated current, the inverter drive unit can protect itself, the input terminal, and the motor cable to prevent thermal overload, and no additional thermal overload protection equipment is required. Installing fuses will protect the input cable in the event of a short circuit, reducing damage to the unit and preventing damage to connected equipment in the event of an internal short circuit in the drive.
The tripping time of the configured fuses should be less than 0.5 seconds. The tripping time depends on the fuse type (gG or aR), the power supply impedance, the cross-sectional area, material, and length of the power cable. When using gG fuses, a tripping time exceeding 0.5 seconds can be reduced to an acceptable level in most cases with fast-acting (aR) fuses. The fuse must be a non-delay type.
Circuit breakers cannot provide sufficiently fast protection for transmission equipment because their response speed is slower than that of fuses. Therefore, fuses should be used instead of circuit breakers when fast protection is required.
Myth 6: Only load power needs to be considered when selecting a frequency converter.
Many users typically match the inverter capacity to the power of the drive motor when purchasing inverters. In fact, different motors drive different loads, and therefore require different inverters.
Disadvantages: Due to the differences in load characteristics of motors, improper use of frequency converters may lead to damage if comprehensive factors are not fully considered. At the same time, the lack of necessary braking units and filters may cause safety risks.
Solution: Select the appropriate capacity and configuration of the frequency converter based on the characteristics and type of the load.
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, silt pumps, and rapidly changing musical fountains require larger capacities).
2) Crane-type loads: These loads are characterized by a large impact during startup, thus requiring the frequency converter to have a certain margin. Additionally, energy feedback occurs when lowering the heavy load, necessitating the use of a braking unit or a shared bus configuration.
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 situation, such as rolling mill machinery, crushing machinery, mixers, etc.
4) High-inertia loads: such as centrifuges, punch presses, and rotary kilns in cement plants. These loads have large inertia, so they may oscillate during startup, and the motor may regenerate energy during deceleration. A higher-capacity frequency converter should be used to speed up startup and avoid oscillation. A braking unit should be used to eliminate the regenerative energy.
II. Conclusion
When frequency inverters are used in conjunction with other intelligent devices (PLCs, DCS systems), they can achieve multiple control strategies and closed-loop regulation, and they also possess relatively comprehensive protection functions. However, many misconceptions exist in practical applications and installation environments. Addressing these issues, mitigating risks, and using them rationally are key to improving the efficiency and lifespan of frequency inverters.
