Fig. 1,892.—Fynn's shunt conductive single phase motor. In order to supply along the stator axis a constant field, suitable for producing the cross flux to which the torque is due by its action on the circuit perpendicular to the stator axis, the "armature circuit," as it may be called, has a neutralizing coil in series with it, so that the armature circuit and neutralizing coil together produce no flux. In addition to this, there is a magnetizing coil along the same axis, which is connected across the mains and so produces the same flux as the primary coil in a shunt induction machine. Fynn has proposed a number of methods of varying the speed and compensating this machine. It is, however, complicated in itself, and is only suited for very low voltages, so that on ordinary circuits it would need a separate transformer.
It is necessary to use laminated construction in the field circuit to avoid eddy currents, which otherwise would be excessive. Fig. 1,890 is a diagram of a simple shunt commutator motor.
Repulsion Motors.—In the course of his observations on the effects of alternating currents, in 1886-7, Elihu Thomson observed that a copper ring placed in an alternating magnetic field tends either to move out of the field, that is, it is repelled by the field (hence the name repulsion motor), or to return so as to set itself edgeways to the magnetic lines.
The explanation of the repulsion phenomenon is as follows:
When a closed coil is suspended in an alternating field so that lines of force pass through it, as in fig. 1,893, an alternating pressure will be induced in the coil which will be 90° later in phase than the inducing flux, and since every coil contains some inductance the resulting current will lag more or less with respect to the pressure induced in the coil.
Fig. 1,893.—Effect of alternating field on copper ring. If a copper ring be suspended in an alternating field so that the plane of the ring is oblique to the lines of force, it will turn until its plane is parallel to the lines of force, that is, to the position in which it does not encircle any lines of force. The turning moment acting upon the ring is proportional to the current in it, to the strength of the field, and to the cosine of the angle ß. Hence it is proportional to the product sin ß cos ß. The tendency to turn is zero both at 0° and at 90°; in the former case because there is no current, in the latter because the current has no leverage. It is a maximum when ß = 45°. Even in this position there would be no torque if there were no lag of the currents in the ring, for the phase of the induced pressure is in quadrature with the phase state of the field. When the field is of maximum strength there is no pressure, and when the pressure reaches its maximum there is no field. If there be self-induction in the ring causing the current to lag, there will be a net turning moment tending to diminish ß. The largest torque will be obtained when the lag of the current in the ring is 45°.
The cosine of this phase relation becomes a negative quantity which means that the coil is repelled by the field.
It is only when the ring is in an oblique position that it tends to turn. If it be placed with its plane directly at right angles to the direction of the magnetic lines, it will not turn; if ever so little displaced to the right or left, it will turn until its plane is parallel to the lines.