Modern industrial enterprises, the widespread use of DC power. In the metallurgical industry, the smelting of nonferrous metals such as aluminum, magnesium and zinc and the production of electrolytic copper are used in the chemical industry for the production of electrolytes, electroplating, chemicals and reagents. In the mining machinery, steel industry and transportation, cranes , Rolling mills, electric locomotives, electric trains, and trams for urban transport all require DC power.

Classification by purpose, the rectifier transformer is divided into three major categories of metallurgy, chemicals and traction. They are in the pressure regulating mode, the pressure regulating range and secondary phase voltage difference, the common characteristic is the secondary voltage is low, the current is large. In order to improve the rectification efficiency, the number of phases on the secondary side is generally not less than three phases, sometimes using six-phase, twelve-phase or plus phase-shifted coils.

Four-quadrant rectifier transformer and ordinary transformer in addition to the difference between the structure, the load characteristics are not the same. Power transformer secondary load is generally considered constant impedance, the output current is sinusoidal. Four-Quadrant Rectifier Transformer Due to the rectification of the rectifier, each anode conducts only for part of the time in each cycle. Therefore, the output current of each phase of the secondary side of the rectifier transformer is also only part of the time of the weekly wave. Therefore, the operating current waveform of the rectifier transformer coil is an irregular non-sinusoidal waveform. This non-sinusoidal current leakage leakage voltage drop, will affect the secondary side of the rectifier transformer terminal voltage, thus affecting the rectifier DC voltage characteristics.

Modern silicon rectifier, silicon controllable rectifier has been rapid development, the production of high-power thyristor, planar thyristor and various types of triacs, and widely used in production practice. Silicon rectifiers will gradually replace the mercury arc rectifiers because of the many advantages of silicon components, such as high rectification efficiency, small size, light weight and simple operation and maintenance. Silicon rectifier components require a wide range of rectifier transformer voltage regulator and stepless regulation of the characteristics, so there on-load regulator and reactor voltage regulator rectifier transformer, and sometimes the rectifier transformer and rectifier components together into a large rectifier . Rectifier transformer due to different uses to take a variety of wiring methods, different wiring methods, for the secondary side of the transformer phase voltage, current and transformer primary side have an impact. When we have to determine the DC side of the parameters, you can consider the transformer wiring and capacity issues. For the sake of simplicity, ignore the transformer excitation current, transformer impedance and arc voltage drop.

A basic rectifier circuit

When the transformer secondary coil a is positive, the rectifier element D conductive, resistance R has a current through; after a half cycle is negative, D off, no current through, so the transformer secondary coil current only in Half a cycle through. The same resistance on the current only exist in half a cycle, so called single-phase half-wave line. The average voltage across the resistor can be written as follows

In order to obtain a flat output voltage and improve the utilization of the transformer, often using three-phase bridge rectifier circuit. As more lines add three components, so the transformer secondary coil current flow in two directions, three-phase bridge rectifier is equivalent to the six-phase out of the mid-point rectifier circuit.

Large-scale rectifier equipment, sometimes using transformer star-shaped double-twists and turns of the three-phase rectifier connected to the line, also known as three-phase fork-type rectifier circuit. The obvious characteristic of this kind of line is that the external characteristic curve is steady, there is no surplus magnetic potential. Often used as a traction DC power supply, such as trams, electric trains. Regardless of transformer leakage resistance, loss and arc voltage drop transformer secondary side has three connected into a star group, a total of nine coils. Three of them meet at the midpoint. The current in the coil is different from the current in the other six coils.

Rectifier transformer capacity and utilization factor

Rectifier transformers standard capacitor closer to the output side of the DC output capacity, then the transformer utilization factor higher. The use of the level of the coefficient depends on the transformer wiring.

Based on the above, we can see that the capacity of the rectifier transformer and the wiring of the rectifier circuit is related to the operation of the rectifier transformer and the ordinary transformer is different, because the current flowing through the secondary coil has a DC component. This situation has an impact on transformer leakage flux and additional losses.

It should be noted that the rectification circuit output DC voltage, not pure DC, the waveform is pulsating to some extent. Also said that there are exchange components, obviously the more the number of DC voltage pulsation smaller. Generally, the number of commutating lines in practical application is not more than 12 phases. In order to reduce the pulsation of the DC voltage, a filter reactor and a shunt capacitor are connected in series in the rectifier circuit, so that the rectified voltage approaches the pure DC.

In the three-phase rectifier circuit, the secondary coil utilization factor, three-phase half-wave, K = 0.67, and six-phase half-wave, K = 0.55, are not high, so the industrial rectifier transformers are used three-phase bridge And double Y with balanced reactor rectifier circuit.

Three overlapping phenomenon, arc voltage drop, resistance voltage drop

Above we discuss the rectifier transformer are considered to be the ideal working condition, did not consider the transformer leakage reactance voltage, as well as the rectifier circuit in a variety of voltage drop. In practice, no matter what kind of rectifier line, there are transformer leakage reactance and voltage drop, due to the existence of these factors, will affect some of the rectifier circuit parameters.

All the rectifier transformer is actually a leakage reactance exist in the rectification process, when an anode rectified after another anode rectifier, the anode current changes will not change, but an anode current slowly weakened , While the other anode current is slowly increasing, resulting in the simultaneous rectification of two anodes, the so-called overlap phenomenon.

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