When an unbalanced load comes on, the voltage on the lightly loaded side rises and on the heavily loaded side drops. The machine on the light side then takes power from the line and runs as a motor driving the machine on the heavy side as a dynamo, supplying the extra current for that side. This action tends to bring the voltage back to normal and gives good regulation.
In some cases the field of each machine is connected to the opposite side of the system which gives a quicker action. This regulation is automatic and the set takes care of unbalanced loads in either direction without adjustment.
Balancing Coils.—Another method of balancing a three wire system which does away with any additional rotating machines makes use of balance coils.
Ques. Describe the type of dynamo used with balancing coils?
Fig. 801.—Diagram showing connections of balancing coil system. The dynamo used in this system is provided with both commutator and collector rings.
Ans. The regular two wire dynamo is used supplying power to the outside wires, but there are collector rings connected to the armature. These rings are much lighter than they would be for a converter as they carry only about ⅛ of the dynamo load. These rings being light are usually placed at the end of the commutator and are connected directly to the commutator bars.
Ques. How are the balancing coils constructed?
Ans. They are built of standard transformer parts, and are placed in cases similar to those of ordinary small transformers.