SYNCHRONOUS MOTORS AND CONVERTERS
Theory and Methods of Calculation and Testing

Starting of Single-Phase Machine

The winding operates like a potentiometer by which any fraction whatever of voltage can be obtained at will. At the beginning, very low voltage is obtained by placing the switch handle to the right-hand side. The handle is then moved gradually toward the left in proportion as the speed increases, and gradually raising the voltage applied to the motor, while reducing the current consumed by it. The current I1 taken from the supply-source is a fraction of the current I2 , consumed by the motor, which is represented by the inverse ratio of voltages:

I1 = I2 * E2 / E1

The winding of the transformer is traversed by the current I1, between A and D, and by the current I2 - I1, between D and B. In the case of polyphase apparatus it is sufficient to install as many transformers with commutating switches as there are phases. All the levers of these switches are operated by one handle.

Starting of Single-Phase Machines

These machines can also be started by the first two methods, i.e., by means of an auxiliary D.C. motor, or by a small induction-motor which starts by means of an auxiliary phase such as, for example, shaded pole single-phase motors. It is desirable to be able to increase the speed of the machine gradually while keeping constant the speed of the small auxiliary motor, because the latter would be stalled if it were made to slow-up. To overcome this difficulty the motor is mounted on guides, and it transmits power to the larger machine by means of a friction-cone bearing against a large disk mounted on the shaft of the machine itself. The motor is gradually shifted on the guides in such manner that the friction-cone approaches the center of the disk until the moment when synchronism is attained.

The starting of the machine as an induction-motor is more difficult, but it can be accomplished by transforming temporarily the armature into a two-phase armature, by means of a second winding suitably provided for that purpose, and which can be of small wire, inasmuch as the time required for starting is so short. By sending into this auxiliary winding a current which is more or less out of phase with respect to that in the principal winding, the armature produces a revolving field which is more or less imperfect and pulsating, but which is, nevertheless, sufficient to start the motor.

To produce the phase-difference between the two currents, recourse may be had to some one of the methods knowrn for producing inductionmotors by means of a single alternating force. For example, use may be made of the Tesla method of introducing a self-induction into one branch of the circuit, and a resistance in the other, or use may be made of the Leblanc method, which is more perfect, and which consists in introducing a self-induction in one side and a capacity or a polarizer in the other.

But in order to reduce to a minimum the current taken from the line, there is again a great advantage in inserting a reducing transformer, built according to the principle already explained, and supplying current either for one phase or for both phases of the motor. One arrangement consists in supplying only the principal winding by means of the reducing transformer and in connecting the secondary winding directly to the switchboard bus-bars. In such a case, the secondary winding is made with a very high number of turns so as to have a very high self-induction and to absorb only a weak current having much lag with respect to the current of the other winding.

The arrangement employed by Labour (Fig. 52) is the reverse of this. In this case, it is the supplementary winding of the armature, made of fine wire, which is supplied by a reducing transformer, while the principal winding receives current direct from the switchboard through a rheostat. The double-throw switch C being moved to the right (starting position) as shown in the diagram, the supplementary winding will then be in circuit with the secondary of the reducing transformer, and at the same time the rheostat is inserted in the principal circuit, which is closed by the circuit-breaker 7. The motor starts as an induction-motor. When the speed approaches synchronism, the switch C is suddenly thrown to the left; by this operation the current in the supplementary winding is broken, and the rheostat of the principal winding is short-circuited, at the same time that the fields are excited. The motor is then in step. In large motors the circuit-breaker is replaced by a starting rheostat, the object being to avoid fluctuations in the lights supplied from the same distributing system. Sometimes an electromagnetic circuit-breaker is interposed in the excitation-circuit, its function being to open the main-circuit whenever the excitation -current fails.

The operation of starting and synchronizing requires about 30 seconds. The maximum current taken from the mains during this period does not exceed the normal current by more than 20 per cent; and it falls down to 1/4 or 1/5 the normal current when synchronism is attained.

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