Industrial rectifier basics: ripple, regulation and cooling

An industrial rectifier takes a three-phase AC supply and delivers controlled DC to a process: an electroplating bath, an electrolytic cell, a battery bank, a DC motor or a magnet. Ratings run from tens of amperes to many thousands, mostly at DC voltages from a few volts to a few hundred. Three properties matter most when specifying one: how smooth the output is, how well it holds its setting, and how it gets rid of heat.

How the conversion works

A typical industrial rectifier has three stages:

1. Transformer: steps the 415 V three-phase supply down to the voltage the process needs and isolates the DC side from the mains.
2. Rectifier bridge: silicon diodes or thyristors convert the transformer's AC output to DC. Diodes give a fixed ratio; thyristors allow the output to be controlled by delaying the point at which each device turns on.
3. Control and protection: a control panel sets the output current or voltage, measures it and protects the equipment and the process.

Where fine control is needed with diodes, the voltage is adjusted on the AC side instead, by a tap changer or a variable transformer ahead of the bridge.

Ripple

Rectified DC is not perfectly flat. It carries a small AC component called ripple, whose size and frequency depend on the number of pulses in the rectifier circuit.

• A six-pulse bridge, the standard three-phase arrangement, produces ripple at 300 Hz on a 50 Hz supply. Its theoretical ripple is about 4% of the DC output before any filtering.
• A twelve-pulse arrangement uses two bridges fed from transformer windings phase-shifted by 30°. Ripple moves to 600 Hz and falls to around 1% in theory, and the harmonic current drawn from the supply is also reduced.

Thyristor control increases ripple as the output is turned down, because each device conducts for a shorter part of the cycle. A process that runs well below rated output for long periods should state that in the specification.

Why ripple matters

Some processes are tolerant: many electrolysis and battery-charging duties are not sensitive to modest ripple. Others are not: certain plating finishes, hard anodising and precision electrochemical work are affected by ripple, and the specification may set a limit. Ripple also adds heating in the load and in any DC cabling and busbars.

Regulation

Regulation describes how closely the rectifier holds its set output as conditions change: supply voltage, load resistance and temperature. A controlled rectifier normally regulates either current or voltage:

• Constant current holds the set current as the load resistance changes. Plating and electrolysis usually run this way, because current sets the rate of the electrochemical reaction.
• Constant voltage holds the set voltage. Battery float charging and some DC supply duties use this mode.

Closed-loop thyristor control can typically hold the setting within about ±1% across the stated range. The figure always depends on the measurement method and the conditions, so it belongs in the proposal with its conditions alongside.

Cooling

A rectifier rated for thousands of amperes dissipates a substantial amount of heat in its semiconductors, transformer and busbars, even at good efficiency. The cooling method follows from the rating and the environment.

Plating and electrolysis shops often have acidic or alkaline fumes. In such atmospheres an oil-immersed design, with the transformer and semiconductors sealed in a tank, keeps corrosive air away from the parts that matter. In clean, dry plant rooms, air cooling is simpler to maintain.

Efficiency matters too. The losses in the transformer, the semiconductors and the DC connections are paid for in energy every operating hour, and at very high currents the DC busbars and their joints can account for a large part of the total. Keeping connections short, clean and tight is part of the rectifier installation.

Information needed for a rectifier specification

• Process type and the electrochemical or electrical duty.
• DC output: maximum current and voltage, and the normal operating point.
• Control mode: constant current, constant voltage or both, with the required accuracy.
• Any ripple limit set by the process.
• Duty cycle: continuous, cyclic or timed.
• Installation environment: ambient temperature, fumes, humidity and space.
• Incoming supply and distance to the load, which affects DC cabling or busbar losses.

Where to go next

• Silicon, Industrial & AC-to-DC Rectifiers
• Air-cooled and oil-cooled rectifiers compared
• Discuss your requirement: share the DC output, duty and installation environment.