Patent GB-308144-A (GB)
Title: Improvements in electric rotary converters
IPC: H02K47/16
Filed: 1928-02-09
Abstract: 308,144. Macfarlane, J. C., and Macfarlane, W. A. Feb. 9, 1928. Divided on 308,041. Rotary transformers; commutating poles and windings; cores; exciting.-A rotary transformer for producing a constant current of variable voltage from a constant-voltage supply or vice versa comprises an armature D, Fig. 2, having a normal full-span commutator winding and a field structure F composed of a number of bridge-pieces A<1>, A<2>, B<1>, B<2> of magnetic material, each bridgepiece having two polar projections at its ends and being carried by a non-magnetic support G. The field excitation for the poles on the field structure producing electromotive force in the constantcurrent or secondary supply C', C<2> is provided by armature reaction due to the commutator brushes K', K<2> connected to the constant-voltage or primary supply P', P<2>, while the excitation for the field poles producing back electromotive force in the constant-voltage or primary supply is provided by armature reaction due to the commutator brushes J<1>, J<2> connected to the secondary supply C<1>, C<2>. The fluxes set up by armature reaction return through bridge-pieces, as shown by the dotted lines and arrows, instead of through the yoke F, which maye be made of nonmagnetic material. Interpoles I', I<2> may be formed in the centre gaps of the bridge-pieces to assist commutation or provide regulation, the windings R, Q on these interpoles being arranged in series with the primary and secondary circuits as shown. The armature may be driven by a motor M of a size suitable for supplying the losses of the transformer, or, as shown in Fig. 4, the small motor M may be dispensed with and the windings X, Y may be fitted on certain of the field poles to hold the speed approximately constant. The winding X is connected in series with the constant-current circuit C<1>, C<2> and opposes the armature magnetomotive force producing the flux b while the winding Y is connected across the constant-voltage circuit P', P<2> and assists said armature magnetomotive force. When the machine is running at its correct speed, the windings X, Y balance one another. The speed of the transformer may be altered by altering the ratio of the turns of the windings X, Y, or by connecting a resistance in series with Y or across X. Where the primary and maximum secondary voltages must be different, and armature with two commutator windings H<1>, T<1> may be used as shown in Fig. 4, in which the circuit C', C<2> is entirely independent of the circuit P', P<2>. The two armature windings H', T<1> may, however, be '' auto-connected "; in this case the brushes K<2>, J<1> are joined together and the constant-voltage supply is connected to the brush K<1> and to the common connection of the brushes K<2>, J<1>, the constant-current supply being connected across the brushes K<1>, J<2>. The transformer may be made self-starting when a small driving motor is not employed by fitting suitable shunt and series windings on the field motoring poles and cutting them out when the machine has reached full speed. Transformers built according to the invention may be very small, allowing, for instance, a motor on, say, a crane to be fitted with its own transformer or groups of contiguous motors may be supplied from one transformer. Fig. 6 shows a motor W connected to the constant-current transformer D. The motor has a field winding Z<1> connected across the positive brushes K<1>, J<1> of the transformer and a field winding Z<2> connected across the negative brushes J<2>, K<2>. It is explained that, with this arrangement, the motor has a characteristic similar to that of a series motor on a constant-voltage circuit. Further speed control of the motor W is obtained by regulating resistances U<1>, D<2> in the two field circuits arranged to be cut in or out simultaneously as shown. Specification 308,041 is referred to.