SYNCHRONOUS MOTORS AND CONVERTERS
Theory and Methods of Calculation and Testing

Excitation of Synchronous Motors

Excitation of Synchronous Motors

Synchronous motors may be excited in several ways:

  1. A small Exciter-dynamo mounted on the same shaft

  2. Shell-commutator

  3. Series-excitation

  4. Shunt-excitation

  5. Armature winding plus continuous-current commutator

Exciter-dynamo mounted on the same shaft

As in the case of alternators, it is possible to use the current produced by a small exciter-dynamo mounted on the same shaft. The machine is put in operation by one of the methods indicated later, or else by running the exciter-dynamo as a motor, by means of current from a storage battery. This method of excitation is advantageously used for large motors.

Shell-commutator

To avoid the complication of the exciter-dynamo, especially for small motors, the machine is often made self-exciting by means of cornmutated currents, by sending through the field-circuit a portion of the current obtained from the mains. The oldest method of commutating this current consisted in using a simple shell-commutator by which the current was reversed at each change of polarity. Such was the Ganz commutator shown diagrammatically in Fig. 15 for a six-pole motor.

The inducing circuit connects with three segments p, p', p" connected in parallel with one pole, and the other end of the circuit connects with three other segments n, n', n", which are connected in parallel with the other pole. The alternating current to be commutated is brought by two double brushes bb', b1b', which can be shifted to a suitable extent by means of the collar supporting them, the object being to reduce sparking to a minimum. The use of double brushes has for its object to reduce the sparking by short-circuiting the field windings at the time when the induced current is reversing; the extra current which is then produced in the exciting coils prolongs the current in the field-circuits and diminishes the undulations of the inducing magnetic flux. Nevertheless, these fluctuations are quite perceptible and interfere very much with the efficiency and the stability of the machine.

These commutating arrangements can only be used for low voltages. For this reason, when the voltage at the terminals of the motor exceeds no volts, the commutator is supplied by the secondary winding of a transformer which is connected in multiple with the mains or else by a special low-voltage circuit wound on the same armature, as in the case of the old Westinghouse alternators (Fig. 17).

Commutated currents can, theoretically, be used for excitation either in series or in shunt (even compound) as indicated in the diagram of Figs. 18 and 19.

Series-excitation

Series-excitation allows the use of large wires for the field-coils, and insures coicidence in phase between the currents of the inducing and induced circuits during the operation of starting, which operation is thereby facilitated. It has, however, the great objection of making the induced E.M.F. vary with the load, whereas, according to theory, the most advantageous E.M.F. is fixed a priori, and should remain practically independent of the load. With series-excitation, strong wattless currents occur at very light and very heavy loads (which may, it is true, be reduced by shunting the field-coils, as shown in Fig. 19), and, besides, there is a lack of stability of operation at light loads because the field is then too weak. Morevoer series-excitation causes very bad sparking at the brushes.

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