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Working Principle and Introduction of Rectifier Transformers

Working Principle of Rectifier Transformers

The working principle of rectifier transformers is the same as that of ordinary transformers. A transformer is a device that transforms AC voltage based on the principle of electromagnetic induction. Generally, a transformer has two mutually independent windings: the primary winding and the secondary winding, which share a common core. When the primary winding of the transformer is connected to an AC power source, an alternating current flows through the winding, generating magnetomotive force and thus creating alternating magnetic flux in the closed iron core. The primary and secondary windings cut the magnetic flux lines, and an AC voltage with the same frequency is induced in the secondary winding. The voltage ratio between the primary and secondary windings is equal to the turn ratio. For example, if a transformer has 440 turns on the primary winding and 220 turns on the secondary winding, with a primary input voltage of 220V, the secondary output voltage will be 110V. Some transformers can have multiple secondary windings and taps, allowing for multiple output voltages.

Characteristics of Rectifier Transformers

Rectifier transformers are used with rectifiers to form rectifier equipment, allowing AC power conversion to DC power. Rectifier equipment is the most commonly used DC power source in modern industrial enterprises, widely used in fields such as DC transmission, electric traction, rolling mills, electroplating, and electrolysis.

The primary side of a rectifier transformer is connected to the AC power system, the grid side, and the secondary side is connected to the rectifier, the valve side. Although the structural principle of rectifier transformers is similar to that of ordinary transformers, they have specific characteristics due to their distinct load, the rectifier, compared to general loads:

1. Non-Sinusoidal Current Waveforms: Each arm of the rectifier conducts in turns within one cycle, with the conduction time occupying only a portion of the cycle. Thus, the current waveform flowing through the rectifier arm is not a sine wave but resembles a discontinuous rectangular wave. The current waveforms in both the primary and secondary windings are also non-sinusoidal. The figure shows the current waveform in a three-phase bridge YN connection. When using a thyristor rectifier, the greater the delay angle, the steeper the current fluctuation and the more harmonic components are present in the current, increasing eddy current losses. Since the conduction time of the secondary winding only occupies part of the cycle, the utilization rate of the rectifier transformer is reduced. Compared to ordinary transformers, rectifier transformers are larger and heavier under the same conditions.

2. Different Power Equivalence: In ordinary transformers, the power of the primary and secondary sides is equal (ignoring losses), and the transformer's capacity is that of the primary or secondary winding. However, for rectifier transformers, the power of the primary and secondary windings may be equal or unequal (e.g., half-wave rectification), depending on the current waveforms. Therefore, the capacity of a rectifier transformer is the average of the apparent power of the primary and secondary windings, known as the equivalent capacity, given by S=(S1+S2)/2, where S1 is the apparent power of the primary winding, and S2 is the apparent power of the secondary winding.

3. Short-Circuit Strength: Unlike ordinary transformers, rectifier transformers must strictly meet requirements for withstanding short-circuit forces. Hence, ensuring that the product has short-circuit dynamic stability is a critical topic in design and manufacturing.

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