A polyphase mercury-arc rectifier is a type of electrical device that was historically used for converting alternating current (AC) power into direct current (DC) power. It was commonly used in industrial applications before the advent of modern solid-state rectifiers.
Here's how a polyphase mercury-arc rectifier works:
Mercury Arc: The rectifier contains a pool of mercury, which serves as the cathode (negative electrode). Anodes (positive electrodes) are positioned above the mercury pool.
Phases: Polyphase rectifiers have multiple AC input phases. For example, a three-phase rectifier has three separate AC inputs, each 120 degrees out of phase with the others. This design allows for smoother DC output compared to single-phase rectifiers.
Arc Formation: When the AC voltage reaches a sufficiently high positive value, an arc (electric discharge) forms between the anodes and the mercury pool. This arc allows current to flow through the mercury vapor, ionizing the mercury and creating a conductive path.
Rectification: During the positive half-cycle of the AC input, the anode closest to the AC phase with the highest voltage becomes positive with respect to the mercury cathode. This causes the arc to form, and current flows through the arc. During the negative half-cycle of the AC input, the arc extinguishes due to the reverse voltage polarity, and little to no current flows.
Smoothing: To achieve relatively steady DC output, a filter circuit is often connected to the rectifier's output. This circuit may include capacitors to smooth out the pulsating DC generated by the rectifier's operation, producing a more continuous and stable DC voltage.
Polyphase mercury-arc rectifiers had several advantages and disadvantages:
Advantages:
Could handle high-power applications.
Provided a relatively smooth DC output (with proper filtering).
Used in industries such as electrochemical processes and early electric traction systems.
Disadvantages:
Use of toxic mercury vapor posed environmental and health hazards.
Efficiency was lower compared to modern solid-state rectifiers.
Required maintenance due to electrode wear and mercury vapor losses.
Bulky and heavy construction.
Due to the environmental and safety concerns associated with mercury, as well as advancements in semiconductor technology, polyphase mercury-arc rectifiers are no longer commonly used. Solid-state diode and thyristor-based rectifiers have largely replaced them, offering higher efficiency, reliability, and reduced environmental impact.