Parallel resonant intermediate frequency power supply is the mainstream intermediate frequency power supply in China, which adopts parallel resonant inverter technology.This technology originated from the former Soviet Union. The power supply has low technical requirements for components, and there are many manufacturers of supporting components. The manufacturing cost of the power supply is low, and its performance is stable. Especially, the popularity of medium frequency furnaces with small and medium capacities is high. However, with the continuous improvement of energy conservation and consumption reduction requirements, the shortcomings of this technology are increasingly being concerned by people. Its main manifestations are:
(1) The power regulation of low power factor parallel inverters mainly relies on changing the rectified voltage of the three-phase fully controlled bridge. When the control pulse a of the rectifier bridge is ≤ 0 ° (DC voltage is lower than the maximum value), its power factor will decrease accordingly, and even reach a minimum of 0.3. The power factor will only reach 0.9 when a=0 ° (maximum DC voltage). However, most users find it difficult to maintain the highest DC voltage value during device use, especially during feeding and discharging, where the power factor is very low and the reactive power loss of the device is large, resulting in an increase in electricity consumption. China has strict requirements for reactive power loss. When the reactive power loss increases beyond the prescribed value, enterprises not only have to bear the economic expenses caused by reactive power, but also have to bear the fines imposed by the power supply department. Some users have invested in reactive power compensation devices to solve this problem. But the device cost is high and the usage is low.
(2) Harmonic pollution is also caused by the use of three-phase fully controlled rectification technology in rectification. The equipment will cause harmonic pollution to the power grid during operation, especially the 5th and 7th harmonics exceeding the national standard by 6-7 times. Due to harmonic pollution, it can cause damage to the reliable operation of other equipment.
(3) It is difficult to start a parallel inverter under heavy load, and its self excitation starting method is commonly used. Before starting the equipment, its excitation signal should be slightly higher than the actual oscillation frequency of the load, and the actual oscillation frequency of different loads may not be the same. Especially when there are significant differences in loads, different excitation frequencies need to be set for different loads. Therefore, parallel inverters have poor adaptability to loads and poor starting performance under low impedance loads, especially when the cold state is full of steel, the equipment may not be able to start.
(4) When there is a sudden change in load, the thyristor may be broken down. Similarly, due to the self-excited starting method used in parallel inverters, which uses forced commutation, overcurrent protection may occur when encountering sudden changes in load impedance (such as when adding materials to the furnace), which is also an important reason why overcurrent protection is prone to occur during the operation of intermediate frequency inverters. In extreme cases or when the parameters of the commutation inductance are inappropriate, it can even cause thyristor breakdown faults.
(5) Cooling water takes away a lot of heat and consumes slightly more energy. Parallel inverters belong to current type inverters, and the resonant current is Q times the working current. The working current of the inductor is the resonant current, and the heat generated by the high current is consumed by the cooling water.
(6) The power regulation of a single parallel inverter with a single load type is achieved by adjusting the rectified voltage, and the inverter is not adjustable, which determines that one power supply can only carry one furnace body.
The technical characteristics of constant voltage series resonant variable frequency power supply are opposite to those of parallel and series inverters, so the disadvantage of parallel inverters is exactly the advantage of series inverters.
(1) The power setting of a high power factor series inverter is adjusted by adjusting the operating frequency of the inverter, and its rectification uses fully conductive diodes. Regardless of the power level, the DC voltage Ud=1.35Ua remains constant, and the power factor remains constant at 0.92-0.94% within the range of 10% to 100%.
(2) Harmonic pollution is minimal due to the use of uncontrollable fully conducting diodes in the rectifier bridge, with no delay control angle and minimal harmonic distortion pollution, fully complying with relevant national standards.
(3) The working frequency of the series inverter with high startup success rate is always controlled by the main control system and is not affected by the high or low load impedance. The startup frequency is directly stable within the working range during startup. The startup is simple, fast, and stable, and can be directly started at any temperature and load, with a startup success rate of over 99%.
(4) Effectively protecting the thyristor series inverter during operation, the commutation process is natural commutation, and there will be no breakdown of the inverter thyristor or overcurrent protection caused by commutation failure.
(5) There are various forms of loads connected in series with inverters, and the rectified voltage is uncontrollable. By controlling the inverters, the power output can be changed. Therefore, the rectifier can simultaneously supply power to two or more inverters, and each inverter can also be independently controlled. Implemented dual or multiple power outputs for an intermediate frequency power supply, enabling one intermediate frequency power supply to carry two or more sensors.
(6) The series resonant inverter with low heat loss in the circuit is a voltage type inverter, and its load voltage is Q times the output voltage. During operation, the voltage value of the resonant circuit is higher and the current value is smaller. Its effect is equivalent to high-voltage transmission, where a smaller current reduces heat loss in the resonant circuit, thereby saving energy.
Wuhan UHV specializes in producingseries resonance (also known as variable frequency series resonance withstand voltage device)and has been engaged in the electrical measurement industry for many years. Its products have first-class quality and welcome customers to come and purchase.
