Fig. 797.—Three wire double dynamo system having two separate windings on the same core and separate commutators A and B as shown.
Extension of the Three Wire Principle.—In order to attain still greater economy in copper, the principles of the three wire system may be extended to include four, five, six, and seven wire systems. The comparative weights of copper required by such systems are as follows:
| Two | wire | system | 1.000 | |||||
| Three | " | " | all wires of equal size | .370 | ||||
| Three | " | " | neutral wire one-half size | .313 | ||||
| Four | " | " | all wires of equal size | .222 | ||||
| Five | " | " | " | " | " | " | " | .156 |
| Seven | " | " | " | " | " | " | " | .096 |
The four wire system requires about two-ninths as much copper, and the seven wire system about one-tenth as much copper, as an equivalent two wire system; but neither is desirable, as their operation involves too much inconvenience, too many unavoidable complications, and create a possibility of accident, which more than offsets the saving in copper.
Fig. 798.—Diagram showing dynamotor connections when used as an equalizer in the three wire system. DM, dynamotor; G, generator side; M, motor side.
The Five Wire System.—This system is employed advantageously in many places in England and Europe, but has not as yet been introduced to any extent in America. It is very probable that in the future the three wire 440 volt system will be selected in preference to the five wire system.
Dynamotor.—This is a combination of dynamo and motor on the same shaft, one receiving current and the other delivering current, usually of different voltage, the motor being employed to drive the dynamo with a pressure either higher or lower than that received at the motor terminals.
The dynamotor in the direct current circuit corresponds to the transformer in the alternating current circuit.