Fig. 2,067.—General Electric motor generator set consisting of 230 volt induction motor and 125 volt dynamo.
In many central stations where they supply a great variety of apparatus, the motor generator sets are employed as the generator is independent of the alternating current line voltage and any degree of voltage regulation can be performed.
Motor Generator Combinations.—The following combinations of motor generators are made and used to suit local conditions:
| Synchronous motor | dynamo |
| Induction motor | dynamo |
| Direct current motor | dynamo |
| Direct current motor | alternator |
| Synchronous motor | alternator |
| Induction motor | alternator |
Fig. 2,068.—General Electric generator set, as installed for the Cleveland Electric Illuminating Company, Cleveland, Ohio. It consists of 11,431 volt motor and 275 volt generator. Speed, 360 revolutions per minute.
Standard practice has adopted high tension alternating current for transmission systems, but direct current distribution is very frequently used. This is particularly true where alternating current apparatus has been introduced in old direct current lighting systems.
The synchronous motor or the induction motor connected to a generator stands next in importance to the rotary converter because it is easy to operate and the pressure may be changed by a rheostat placed in the field circuit of the generator.
The line wires carrying full voltage can usually be connected direct to the motor and thus do away with the necessary step-down transformer required by the rotary.