Rectifier/transformer semiconductor rectifiers

Solid-State DC Drives. The controlled-thyristor rectifier and separate-field DC motor is the solid-state motor drive in greatest use. The combination provides control over at least a 10 1 speed range, plus an additional two to three times by field weakening. Depending upon the power level, the rectifier is operated directly from the AC supply lines, or via a transformer. Typical speed regulation of 2% can be accomplished with a single control system. The horsepower and speed limitations are set by the DC motor, not by the semiconductor rectifiers. The DC motor and rectifier can be combined to any required power level. 

When a transformer-rectifier operates at full current but below its rated output voltage, its power efficiency declines. This is because the losses remain virtually unchanged while the power output falls proportionately with voltage. If the same transformer-rectifier operates at full voltage but below its rated output current, the reverse is true. The power efficiency increases because the resistive losses decrease with the square of the current while the power output falls only linearly. The efficiency improvement is not as great as might be expected from this statement, because the no-load (iron) losses of a transformer do not reduce at all and the rectifier losses are only partly resistive. The latter reflects the fact that semiconductor devices have fixed voltage drops in addition to their resistive losses. 

Fig. 10.29 Cathodic protection using impressed current, (a) A circuit showing the principles the signal from the reference electrode is passed to a power-unit control where it is compared with a preset level. The resultant error signal is amplified and used to control semiconductor power devices which allow a controlled current to pass through the anodes, (b) A typical layout of components in various types of vessel, (c) Transformer/rectifier power units for marine use. (d) Platinized titanium or lead-silver alloy anodes being installed on a ship s hull. The anodes are insulated from the hull and have special insulating, backing shields which help to improve potential distribution and prevent over-protection, i.e. too negative a potential (Photographs courtesy Corrintec (UK) Ltd.)...

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