Working Principle and Characteristics of Rectifier Transformers

Working Principle of Rectifier Transformers

The working principle of a rectifier transformer is the same as that of a conventional transformer. A transformer is a device that converts AC voltage based on the principle of electromagnetic induction. Typically, a transformer consists of two electrically isolated windings—the primary and secondary—wound around a common iron core. When the primary winding is connected to an AC power source, the alternating current generates a magnetomotive force, producing a varying magnetic flux within the closed iron core. This changing flux links both windings, inducing an AC voltage of the same frequency in the secondary winding. The voltage ratio between the primary and secondary windings is equal to their turns ratio. For example, if the primary has 440 turns and the secondary has 220 turns with a 220 V input, the output voltage will be 110 V. Some transformers may have multiple secondary windings or taps to provide several output voltages.

Characteristics of Rectifier Transformers

Rectifier transformers are used in conjunction with rectifiers to form rectifier systems, which convert AC power into DC power. These systems serve as the most common DC power sources in modern industrial applications and are widely used in areas such as HVDC transmission, electric traction, rolling mills, electroplating, and electrolysis.

The primary side of a rectifier transformer connects to the AC power grid (grid side), while the secondary side connects to the rectifier (valve side). Although the structural principle is similar to that of a standard transformer, the unique load—namely the rectifier—imparts specific characteristics:

• Non-sinusoidal Current Waveforms: In a rectifier circuit, each arm conducts alternately during a cycle, with conduction time occupying only a portion of the cycle. As a result, the current waveform through the rectifier arms is not sinusoidal but resembles a discontinuous rectangular wave. Consequently, the current waveforms in both primary and secondary windings are non-sinusoidal. The figure illustrates the current waveform in a three-phase bridge rectifier with YN connection. When using thyristor rectifiers, a larger firing delay angle results in steeper current transitions and increased harmonic content, leading to higher eddy current losses. Since the secondary winding conducts only part of the cycle, the utilization of the rectifier transformer is reduced. Compared to conventional transformers, rectifier transformers are typically larger and heavier under the same power conditions.

• Equivalent Power Rating: In a conventional transformer, the power on the primary and secondary sides is equal (neglecting losses), and the transformer's rated capacity corresponds to either winding's power. However, in a rectifier transformer, due to non-sinusoidal current waveforms, the primary and secondary apparent powers may differ (e.g., in half-wave rectification). Therefore, the transformer's capacity is defined as the average of the primary and secondary apparent powers, known as the equivalent capacity, given by S = (S₁ + S₂) / 2, where S₁ and S₂ are the apparent powers of the primary and secondary windings, respectively.

• Short-Circuit Withstand Capability: Unlike general-purpose transformers, rectifier transformers must meet stringent requirements for mechanical strength under short-circuit conditions. Ensuring dynamic stability during short circuits is thus a critical consideration in their design and manufacturing.