Systems of single-phase regeneration for use with series type commutator motors

The alternating-current series motor may be adapted to regeneration in a number of important respects more readily than a similar direct-current series motor on account of the following features: (1) The a-c. motor always employs a cross-field winding which prevents armature distortion. (2) A relatively low impressed commutator voltage is used reducing the danger from flashing. (3) With the a-c. system a transformer can be used to obtain desired variation in motor voltage. The advantages of this feature are outlined at length with reference to its effect on regeneration. The several schemes of regenerative connections are divided into four classes as follows: (1) Series excitation; (2) self-excitation, or cross excitation; (3) shunt excitation; (4) separate excitation. The first two connections have not been commercially used. A number of fundamental assumptions as to the theory of the single-phase motor are given as a basis for vector diagrams, which are used to analyze the conditions for both motoring and regeneration for the four types of connections. A detailed presentation of the conditions which obtain with these connections is then given by the author. When using the first connection for regeneration it is necessary with series excitation to reverse the main field with reference to the motoring conditions. With the alternating-current series motor a sudden decrease in the line voltage will not increase the regenerated current as in the case of direct current, but will tend to decrease it. In a like manner an increased speed with any given impressed voltage will also decrease the regenerated current. Objectionable characteristics are: first, a strong tendency to pick up as a direct-current generator with the secondary transformer winding acting as a short circuit; second, a like tendency for low-frequency currents to be set up; third, undesirable speed torque characteristics. These features are cited as the reasons for not seriously considering this type of motor and connection for railway application with regeneration in this country. The second system employs an extra set of brushes located midway between the normal brushes on the commutator. This system is handicapped by the fact that armatures and also the commutators of the motors must have increased capacity because of the necessity for carrying exciting as well as load currents during regeneration. This is particularly objectionable on account of the space limitations existing in connection with railway application. Another disadvantage is the necessity for extra brushes around the commutator. This method of connection, however, has the advantage of giving a shunt speed torque characteristic and permitting power factor compensation during regeneration. The third connection using the so-called shunt excitation has the advantage of simplicity and reliability. The chief disadvantage lies in the fact that power factor correction cannot be made during regeneration. It is handicapped to some extent, due to the fact that the same continuous torque between armature and field winding cannot be obtained as was possible with the series connection. In a majority of commercial applications, however, this connection will provide sufficient tractive effort. Only two units are required to obtain regeneration: first, a substantial reactor and, second, a change-over switch for controlling the same. The fourth system utilizes separate exciters either of the (1) constant speed or (2) variable speed type. Under the constant speed systems, either phase converters or motor generator sets may be used. With the variable speed, either separate axle generators or one of the main motors is used as an exciter. This system, although in general requiring more apparatus for regeneration (hence costing and weighing more) than some of the other systems, is also more flexible. The desired speed torque characteristics can be obtained and, with a constant speed exciter, power factor compensation is available as well as power for driving auxiliary apparatus. The conclusion is reached that regeneration can be properly and successfully obtained with one or more of the several systems outlined. In the several appendices, explanation is made of the vector diagrams by means of which the author analyzes the conditions for each system of connection.