SYNCHRONOUS MOTORS AND CONVERTERS
Theory and Methods of Calculation and Testing

Starting of Machine with Laminated Field Poles

Besides these general methods, there is still another, which can be used only in the case of motors having laminated field-poles. The method consists in sending alternating currents through the fieldcoils and through the armature-coils, which are then provided with a commutator. The latter can be adapted either to the principal winding, or to a supplementary winding, which serves only for starting and for excitation. This arrangement has been used by the Fort Wayne Company in the United States. At the time of starting, the current is sent through the armature and field-circuits connected in series; the machine then operates like a series-motor having a commutator; the direction of the current changes, both in the armature and in the field, and consequently the torque is always in the same direction and can thus cause the motor to start.

Both the field and armature of the Fort Wayne motor are laminated; the field has two windings, one being a fine wire winding for synchronous operation, the other being a coarse wire winding which serves for starting. The armature itself also has two circuits wound on a core composed of sheet-iron disks of the form indicated in Fig. 55. The principal winding, whicn serves for synchronous operation, is placed in the round holes, there being as many coils as there are field-poles, and the winding is connected with two collector-rings. The other winding, distributed in the slots, is similar to that of an ordinary B.C. drum winding with its commutator.

The operation of starting is accomplished by means of a single lever controlling the switches. If it is first placed in the position indicated in Fig. 56 the motor will be connected to the source of current like a series-motor, and it will start as such. When its speed attains synchronism, a centrifugal device closes the circuit with a third collector-ring on the right, causing an incandescent lamp to be lighted; the switch-lever is then moved, causing the normal connections to be made; the armature is then connected directly with the circuit; and the commutator circuit is connected with the fine wire excitation-circuit. The inductance in circuit is adjusted in such a manner as to permit the maximum allowable current to pass through the circuit when starting. This maximum current is about two to two and a half times the full load-current.

The efficiency of this type of motor is 70 per cent for a 21/2 H.P. motor, and 80 to 85 per cent for motors of 10 to 15 H.P. They start readily under load, without excessive sparking at the brushes, the latter being of carbon and having a fixed position. The power-factor, which is very high when starting, does not fall below 0.95 to 1 under load.

For small motors, of 1 to 5 H.P., the Gramme commutator just mentioned can be replaced by a less complicated commutator having only one segment per pole of the motor. The small Ganz motors are of this kind. Their fields are excited by commutated currents obtained from a transformer connected in shunt to the line. The even segments are connected to one terminal of the field-circuit, and the odd segments to the other. To start the motor, which can only be done without load, only one set of brushes need be used on the commutator, the others being put off. The current thus changes direction in the field-coils and in the armature coils, as already mentioned; but there is much sparking at the brushes during the whole time that the motor is starting. When the synchronous speed has been attained, which requires at least a minute, the brushes previously removed are again put on, by hand or else by a centrifugal regulator, or they may be put in circuit by means of a switch ; sparking is then reduced to an allowable amount. Fig. 57 gives the diagram of the arrangement actually used (Blathy system). The field and armature windings are in parallel. The armature-current, coming from the transformer at 100 volts, through the wires I, II, enters into the armature by the terminal A, leaves by the terminal B and passes through the secondary-winding S1, S2 of the transformer E, called compensator, which plays the role of self-inductance for throwing the armature-current out of phase. The field-circuits are wound for a low voltage (25 volts) supplied by the secondary circuit S1, S2 of another transformer F.

At the time of starting, to overcome the induction of the fieldcircuit, the potential-difference at its terminals is raised by the primary winding of the compensating transformer E, whose secondary has the armature-current passing through it, as already stated. The field-current also passes through a fixed resistance, ah, in the rheostat, R, the object of which is to reduce the lag in the field-circuit, and to put it in phase with the armature-current. The motor starts. When it attains synchronism, the current in the field is commutated ; the connections are then changed by means of the double-throw switch D, which shortcircuits the secondary of E, connects the two neighboring brushes together and sends the secondary current of F into the variable resistance cd before it reaches the field-coils.

A motor of the same kind was put on the market some years ago by the firm of Brown-Boveri (Fig. 58). The motor starts by simply coupling in parallel the armature and the field-circuit and a resistance which is in series with it. To bring the phases of the two currents into proper relation, the brushes are moved an angle equal to π/2 during the operation of starting. When synchronism is attained, the brushes are brought back to their place, and the excitation-circuit is connected in shunt to one coil alone of the armature, so as to reduce the voltage.

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