CONVERSION EQUIPMENT

206. The conversion of alternating current to direct current is usually effected with a motor-generator set, a synchronous converter, an electronic rectifier, a contact (barrier-layer) rectifier, or solid-state semiconductor rectifiers. frequently used for large-capacity installations than is the induction motor. The synchronous motor is inherently a constant-speed motor. Therefore a more nearly constant voltage can be maintained at the generator terminals than by the use of an induction motor. The field of the synchronous motor may be overexcited. The motor then acts as a synchronous condenser, thus improving the power factor.

207. A motor-generator set consists of a motor mechanically coupled to and driving one or more generators. The motor is usually alternating-current and is mounted on the same bedplate and on the same shaft with the generator or generators which it drives. A motor-generator set can be employed to convert electrical energy from one voltage or frequency to another voltage or frequency, to convert alternating current to direct current, or to convert direct current to alternating current. When an ac motor is used, it may be of either the induction or the synchronous type. The induction type has the advantages of low first cost and rugged construction. It may also be wound for voltages as high as 13,500 V. This will, in many cases, eliminate the necessity for transformers. This type has, however, poorer speed regulation and a lower power factor. A synchronous-motor drive is probably more frequently used for large-capacity installations than is...