Tag: reactor

A、 When carrying out frequency conversion and energy-saving transformation for slip speed control motors, the original excitation box (referred to as the speed control box) and the original slip mechanism were retained due to the consideration of emergency speed control operation in the event of frequency converter failure. During operation, adjust the speed control knob on the speed control box to the full speed position, and the required speed on the load side is set by the frequency converter to achieve speed control and energy-saving operation. However, after such renovation, accidents occurred where the excitation coils in the speed control box or slip mechanism were repeatedly burned out. Why is it that the original power frequency speed regulation is not easily damaged, but it is repeatedly damaged after being modified to a variable frequency drive?

The analysis is as follows:

1. When the original power frequency excitation speed regulation is used, within a certain speed regulation range, the establishment of feedback voltage maintains the excitation current in the excitation coil within a small amplitude, and basically does not reach the maximum value, unless it can reach the maximum value under full speed operation. In variable frequency operation, the actual speed of the motor is controlled by the frequency converter, which may only reach half of the rated speed, and the speed feedback voltage only reaches half of the amplitude. At this time, the speed given by the speed control box is full speed. The speed control box assumes that the motor speed is lower than the given value, and therefore continuously outputs the maximum excitation current (voltage). When applied to the excitation coil, the temperature rise of the excitation coil increases, which is a factor that makes the excitation coil prone to damage.
2. The power supply of the excitation coil of the speed control box is on the same power supply branch as the incoming power supply of the frequency converter, which is essentially connected at the same location. The three-phase rectifier inside the frequency converter is a nonlinear component, and the significant absorption of rectified current leads to serious distortion of the voltage (current) waveform on the power supply side, forming an undeniable peak voltage and harmonic current. This may cause inter turn breakdown of the excitation coil, or breakdown of the freewheeling diode and voltage controlled silicon in the speed control box, which can also cause burnout of the excitation coil! This should be the main factor causing repeated burnout of the speed control box and excitation coil.
   2、 A low-power frequency converter was installed in a certain location, and there were faults that burned out the three-phase rectifier bridge. The frequency converter is 2.2kW, and the equipped motor is 1.1kW. The load is relatively light, and the operating current is less than 2A. The power supply voltage is around 380V, which is very stable. Therefore, no abnormalities were observed on site. But three frequency converters were replaced successively, and the operating time was less than two months. After inspection, it was found that the three-phase rectifier bridge was burnt out. What is the reason? Upon conducting a comprehensive on-site inspection, it was discovered that two other high-power frequency converters were installed in the same workshop and power supply line. The three frequency converters may operate simultaneously or start/stop at different times. The operation and start stop of high-power frequency converters may be the root cause of damage to low-power frequency converters!
   The reason is the same as before. The nonlinear current flowing into two high-power frequency converters greatly increases the distortion component of the voltage (current) waveform on the power supply side (equivalent to the installation of two capacitor compensation cabinets on site, resulting in oscillating capacitor switching current). However, for high-power frequency converters, due to their large internal space, the insulation treatment of the input circuit is easy to strengthen, so it is not easy to cause overvoltage breakdown. However, for low-power frequency converters, due to their small internal space, insulation withstand voltage is a weak link, and the surge voltage surge on the power supply side makes it difficult to escape.
   Furthermore, compared to the power supply capacity, the power of low-power frequency converters is clearly too mismatched. When the power capacity of the frequency converter is several times smaller than the power supply capacity, the harmonic components on the input side of the frequency converter are greatly enhanced. This energy is also an undeniable factor that endangers the three-phase rectifier bridge inside the frequency converter.
   3、 A certain chemical plant has installed several imported frequency converters, and their working current and operating status are normal. However, they have also repeatedly experienced faults that destroy the rectifier bridge, often bursting without warning during operation. On site survey and analysis: In order to compensate for reactive power consumption, the factory has installed several capacitor compensation cabinets in the electrical control room. The switching of large capacity capacitors creates extremely high amplitude surge voltage and surge current in the power grid. Observing the capacitor incoming line in the capacitor compensation cabinet, it was found that a surge suppression reactor was not installed as required. The function of this reactor is not only to suppress the surge current entering the capacitor, but also to improve the surge impact throughout the entire power grid.
   B\fter the frequency conversion transformation of the production line, the on/off (charging and discharging) current of the compensating capacitor and the harmonic current caused by the rectification of the frequency converter amplify each other, forming an instantaneous and turbulent voltage peak in the power grid system. This voltage peak far exceeds the power supply voltage, and it is natural to breakdown the rectification module in the frequency converter.
   How to solve the above problems? Overall, the above three issues are actually just one issue, that is, the distortion of the grid voltage waveform forms voltage spikes, causing electrical equipment to be damaged due to its impact. Therefore, the measures to be taken are also very simple.

On the power input side of the excitation coil of the speed regulating motor, local materials were taken and a “reactor” was connected in series with a BK type control transformer for secondary measurement of 12V or 24V windings; On the power input side of the low-power inverter, a cost-effective “three-phase reactor” made of XD1 capacitor surge suppression coil has also been connected in series; XD1 capacitor surge current suppressor has been installed for the capacitors in the reactive power compensation cabinet. After the above handling, these three issues have not reoccurred. The usage effect is excellent, and the renovation cost is low. And it eliminates the trouble of processing and purchasing materials from other places, shortening the renovation period. By analogy and adapting to situations, many tedious problems can actually be easily solved.