Technical Training
Why do three-phase four-wire residual current circuit breakers used in inverters frequently trip?
In inverter control systems, there are many instances where a three-phase four-wire residual current circuit breaker is used prior to installing the frequency converter. However, during normal operation of the frequency converter, the circuit breaker frequently trips. The reason is that, under normal operating conditions, the output voltage of a typical inverter contains harmonic components, which create parasitic capacitances between the motor windings and the motor housing, as well as between the wires and ground. These parasitic capacitances form leakage currents through the wires to ground and from ground to the chassis. When the leakage current exceeds the residual current rating of the circuit breaker, the residual current circuit breaker will trip. Therefore, in control systems using frequency converters, if the above-mentioned situation occurs, it is necessary to replace the original residual current circuit breaker or adjust the residual current setting value.
Why can’t the “ground wire” of a converter be connected to the neutral wire?
In a TT system (three-phase four-wire system), the neutral wire connected to the circuit contains a significant amount of harmonics and various types of noise, sometimes reaching several volts or even hundreds of volts. Once the "to" terminal is connected to the inverter's neutral wire, these harmonics and noise can enter the inverter, causing internal interference that disrupts its normal operation. Therefore, it is generally recommended that inverters be grounded independently.
Why does the motor casing have a static voltage when an inverter is used?
Since the inverter outputs a PWM waveform, the high-frequency pulse sequence approximates a standard sine wave, and its spectral envelope contains harmonic components. The transient voltage amplitude and frequency are extremely high, which can induce significant voltage across the motor winding capacitance between the high-voltage field and the motor casing, thereby generating even higher voltages (the converter casing itself carries a certain static voltage level). For this reason, when using a variable-frequency drive, it is crucial to heed a warning: ensure reliable grounding. Furthermore, in industrial environments, it is common for there to be no dedicated ground wire; instead, the neutral line is often shared with other circuits. As a result, many grounded equipment enclosures end up sharing the same grounding system as the VFD. This can lead to the buildup of high-voltage static electricity throughout the entire system. Such high voltages generate strong electric fields that may interfere with the normal operation of the VFD. If this situation occurs, simply integrating a single VFD control system into the overall setup can effectively eliminate the problem.
Why does the frequency of the frequency cause the starting current to decrease?
During direct-on-line starting, the power supply frequency is high, and connecting the motor to a power supply at industrial frequency generates a rotating magnetic field at rated speed. At this point, the motor rotor remains stationary, and the relative speed between the rotor windings and the rotating magnetic field is very high. Consequently, the induced electromotive force and induced current are substantial, and the stator current can reach 5 to 7 times the rated current. In contrast, during variable-frequency starting, the output frequency of the inverter is initially set very low. After startup, the frequency gradually increases according to a preset acceleration profile. As a result, both the rotor speed and the relative speed between the rotor windings and the rotating magnetic field are highly precise. Consequently, the starting current is significantly lower and can typically be kept within the range of fluctuations around the rated current.
What is acceleration time? How do you use it?
The acceleration time is the duration required for the inverter to ramp up from 0 Hz to the base frequency (50 Hz). This specification also applies when the termination frequency is arbitrary. For example, in multi-speed operation, if a particular speed class is set to run at 30 Hz and the acceleration time is set to 30 seconds, the actual acceleration time for this speed class (up to 30 Hz) would be calculated as follows: (30 Hz / 50 Hz) × 30 seconds = 18 seconds. Thus, it’s not logical to assume that reaching 30 MHz would take 30 seconds.
When setting the acceleration time, the following factors should be taken into account: The acceleration process takes time, and an excessively long acceleration time can reduce operational efficiency—especially in applications involving frequent start-and-stop operations. However, if the acceleration time is too short, the inrush current will increase significantly. Therefore, it’s important to strike a balance between minimizing the inrush current and maximizing production efficiency; provided that the inrush current remains within acceptable limits, the acceleration time should be shortened as much as possible. Furthermore, when the load equipment has a large moment of inertia, the acceleration time should be appropriately extended, and the moment of inertia of the load equipment can also be reduced accordingly.
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