The Ingenious Use of Reactors in INVERTER Retrofits and the Hazards of Grid Harmonics to Electronic Equipment

In modern industrial production, INVERTERs, as key equipment for motor speed regulation and energy saving, are increasingly being applied. However, during the retrofit of INVERTERs, some overlooked issues often lead to equipment damage, particularly the significant impact of grid harmonics on electronic equipment. This article, through examples, delves into the ingenious use of reactors in INVERTER retrofits, the hazards of grid harmonics to electronic equipment, and proposes corresponding solutions.

I. Application of Reactors in INVERTER Retrofits

When performing frequency conversion energy-saving retrofits on slip-ring motors, the original excitation box (abbreviated as speed regulation box) and slip mechanism are usually retained for emergency speed regulation operations in case of INVERTER failures. After the retrofit, the speed regulation knob on the speed regulation box is set to the full-speed position, while the required speed on the load side is set by the INVERTER to achieve speed regulation and energy-saving operation. However, such retrofits often result in accidents where the excitation coil or slip mechanism in the speed regulation box is repeatedly damaged.

The reason lies in the fact that during the original line-frequency excitation speed regulation, the establishment of feedback voltage keeps the excitation current in the excitation coil fluctuating within a small range, generally not reaching its maximum value. However, during frequency conversion operation, the actual speed of the motor is controlled by the INVERTER and may only reach half of the rated speed, with the speed feedback voltage also reaching only half of its amplitude. At this point, the speed given by the speed regulation box is full speed, so the speed regulation box continuously outputs the maximum excitation current (voltage), leading to an increase in the temperature of the excitation coil and making it prone to damage.

Furthermore, the three-phase rectifier inside the INVERTER is a nonlinear component, and its significant absorption of rectified current causes severe distortion of the power supply side voltage (current) waveform, resulting in non-negligible peak voltages and harmonic currents. These harmonic currents and voltage peaks can cause inter-turn breakdowns in the excitation coil, or breakdowns of the freewheeling diodes and thyristors in the speed regulation box, further causing damage to the excitation coil.

To solve this problem, a reactor can be connected in series on the power input side of the excitation coil of the speed-regulating motor. The introduction of the reactor can effectively suppress harmonic currents, reduce voltage waveform distortion, and thereby protect the excitation coil from damage.

II. Hazards of Grid Harmonics to Electronic Equipment

Grid harmonics not only affect speed-regulating equipment but also pose a serious threat to electronic equipment such as INVERTERs. For example, a small-power INVERTER installed in a certain location experienced multiple failures where the three-phase rectifier bridge was damaged. Despite the INVERTER’s small power, light load, and stable power supply voltage, the failures could not be avoided. After on-site inspections, it was found that two high-power INVERTERs were also installed in the same workshop and on the same power supply line. These three INVERTERs may operate simultaneously or start and stop at different times, and the harmonic currents generated by the operation and start-stop of the high-power INVERTERs are the root cause of the damage to the small-power INVERTER.

The nonlinear currents generated by high-power INVERTERs lead to increased distortion of the power supply side voltage (current) waveform, forming harmonic components. For high-power INVERTERs, due to their large internal space, the insulation treatment of the input circuit is easy to strengthen, so they are not easily damaged by overvoltage breakdowns. However, for small-power INVERTERs, their internal space is limited, insulation withstand voltage is a weak link, and they are difficult to withstand the surge voltage impacts on the power supply side.

III. Solutions and Application of Reactors

To address the aforementioned issues, the most effective solution is to connect a reactor in series on the power input side of the electronic equipment. The reactor can suppress harmonic currents, reduce voltage waveform distortion, and thereby protect the electronic equipment from damage.

For the excitation coil of speed-regulating motors: Connecting a reactor with a secondary measurement winding of a BK-type control transformer in series on the power input side of the excitation coil can effectively protect the excitation coil.

For small-power INVERTERs: Connecting an economical “three-phase reactor” made of an XD1 capacitive inrush current suppressor in series on the power input side can significantly reduce the impact of harmonic currents.

For reactive power compensation capacitor banks: Installing an XD1 capacitive inrush current suppressor at the inlet end of the capacitors to suppress the inrush currents and harmonic currents generated by the charging and discharging of the capacitors.

Through the implementation of these measures, the aforementioned three issues have been effectively resolved. The application of reactors not only improves the operational reliability of electronic equipment but also reduces retrofit costs and shortens retrofit cycles. Therefore, the ingenious use of reactors in INVERTER retrofits and electronic equipment protection cannot be ignored. By analogizing and adapting to different situations, many cumbersome issues can actually be easily resolved.