Ans. For purposes where for instance direct currents of widely different voltages are to be obtained from an alternating current circuit, and it is desired to install but one set, a motor generator consisting of an alternating current motor such as an induction motor, and a dynamo must necessarily be employed.
In such sets, it is common to find both motor and dynamo armatures mounted on a common shaft, and the respective field frames resting on a single base, although for connection on a very high pressure alternating current circuit, separate armature shafts insulated from each other but directly connected together, and separate bases resting on a single foundation, are usually employed to afford the highest degree of insulation between the respective circuits of the two machines.
Ques. What is the objection to a set composed of alternating current motor and alternator?
Ans. The commercial field that would be naturally covered by such a set is better supplied by a transformer.
Ques. Why?
Ans. Because a transformer contains no moving parts, and is therefore simpler in construction, cheaper in price, and less liable to get out of order.
Dynamotors.—A dynamotor differs from a motor generator in that the motor armature and the generator armature are combined into one, thereby requiring but one field frame. Since the motor and generator armature windings are mounted on a single core, the armature reaction due to the one winding is neutralized by the reaction caused by the other winding. There is, consequently, little or no tendency for sparking to occur at the brushes, and they therefore need not be shifted on this account for different loads.
Ques. How is a dynamotor usually constructed?
Ans. It is usually built with two pole pieces which are shunt wound.
Ques. Why does the voltage developed fall off slightly under an increase of load?