Solid state high frequency welder technology and working principle

I.EARLY VACUUM TUBES HIGH-FREQUENCY EQUIPMENT INTRODUCTION

20 years ago, high-frequency power sources (commonly known as "high-frequency furnaces") used for welding pipes and heat treatment in China were mainly equipment with electronic tubes (also known as "vacuum tubes") as the main inverter components. The basic principle is shown in the following figure:

Normal industrial AC (three-phase four wires 380V/50HZ in China) is converted into adjustable AC ranging from Oto 380V(frequency unchanged)through thyristor AC voltage regulator, and then stepped up to nearly 1 OOOOV high voltage(frequency unchanged) by a transformer,and then rectified by a high voltage rectifier circuit to tens of thousands high voltage DC, and then oscillated by vacuum tube to HF and HV current, and then through LC parallel resonance to amplify oscillating current.and after voltage reduction supply to inductor used for heating of steel pipes and other workpieces etc.

AC voltage regulation purpose is to adjust input and output power to meet working requirements,and when adjust voltage also needs to make the voltage stable to ensure equipment output power stability under certain operating conditions.

Due to the use of vacuum tube(high voltage small current components),voltage must be increased or can not output large power.but at the same time.in order to adapt to the small impedance of the inductor, it is necessary to further reduce the voltage.

The below photos are 600KW Single-loop vacuum tube high frequency and 100KW triple circuit vacuum tube HF quenching equipment.

In the pipe welding industry.there are still a small number of high frequency vacuum tubes, while in the fields such as quenching and brazing.high frequency vacuum tubes are still widely used.

As this vacuum tubes belongs to the eliminated products, we will not introduce in detail here.

II.SOLID-STATE HIGH-FREQUENCY WORKING PRINCIPLE

The so-called "solid state high frequency" is because it uses transistors(MOS field effect transistors or IGBT)as the main inverter components.Unlike vacuum tubes are hollow(interior is full of noble gas,so can be called "gaseous"),transistors are solid.

Solid state high frequency is an updated product of vacuum tube high frequency,and its main circuit is similar to thyristor medium frequency,but different from vacuum tube high frequency. Its basic principle is as follow:

Normal three-phase AC(380V and frequency 50HZ in China) is converted to adjustable voltage pulsating DC by through rectifying circuit(SCR or diode and IGBT),this DC is filtered or flat wave become smoothing DC, and then go into inverter bridge(using large power transistor MOSFET or IGBT) to become high frequency current. This high frequency current is supplied to the tank circuit load resonance can be used for metal heating.Inverter bridge power units adopts modular structure.each pair power modules is same.but the number of power modules used varies depending on the equipment power.W hether the equipment is large or small, the structure is basically same.The resonant tank circuit is in series or parallel form.no high voltage and no output step-down transformer.

Compared with high frequency vacuum tubes, solid state high frequency equipment advantages as follow:

1. Good welding quality:comparison shows that the steel pipes welded by solid state high frequency equipment have uniform welded width and heat, and fewer internal and external burrs

2. Energy saving:tests shows that same specifications this welder can save more than 25% of electricity compared to vacuum tube equipment

3. Water saving:due to small self loss,does not require too much cooling water.so it consumes more than 50% less water than vacuum tube equipment of the same specifications

4. Small size and light weight:due to small size of main components(MOSFET) and also no welding transformer.filament regulators.matching coils.gate circuits.etc. So the overall volume is more than 50°/4。 smaller than vacuum tube equipment of the same specifications

5. Easy to operate: no voltage,peak voltage does not exceed a few hundred volts, so will not cause personal injury