Patent GB-653805-A (GB)
Title: Improvements in or relating to control for alternating current motors
IPC: H02P3/18
Filed: 1947-11-12
Abstract: 653,805. Control of A.C. motors. IGRANIC ELECTRIC CO., Ltd. Nov. 12, 1947, No. 30127. Convention date, Nov. 12, 1946. [Class 38(iii)] A method of controlling a polyphase motor for a hoist under overhauling load conditions consists in supplying A.C. to the several phases of the primary windings and selectively establishing within the motor different coil circuits in one phase of the primary windings and to vary simultaneously the impedance in the supply circuit to the primary windings. This method of control gives different speed-torque characteristics providing increased torque for given speeds. Fig. 2 shows the connections of a 3-phase motor primary winding when a controller of the standard drum type is on the first lowering position. In this position a terminal T<SP>4</SP> connected to a tapped phase winding is joined to line L<SP>2</SP>. A resistor R<SP>7</SP>-R<SP>8</SP> shunts the lead from the terminal T<SP>4</SP> to the line L<SP>2</SP>. Terminals T<SP>1</SP>, T<SP>3</SP> are connected to the line L<SP>1</SP> but a resistor R<SP>1</SP>-R<SP>2</SP> is inserted in the lead from T<SP>1</SP> to the line L<SP>1</SP>. In the first lowering position, the effective number of turns in the phase  winding N-T<SP>2</SP> is reduced to those embraced by N-T<SP>4</SP>. The flux and therefore the torque available for a given speed are greater when utilizing the reduced number of turns. In a second lowering position of the controller, the connections of T<SP>1</SP>, T<SP>3</SP> to L<SP>1</SP> are unchanged and the terminals T<SP>2</SP>, T<SP>4</SP> are connected to L<SP>2</SP>, the lead from T<SP>4</SP> to L<SP>2</SP> has the resistor R<SP>7</SP>-R<SP>8</SP> in series. In a third lowering position, T<SP>4</SP> is disconnected. T<SP>2</SP>and T<SP>3</SP> remain connected as before, and T<SP>1</SP> has now a second step of resistance included in series with R<SP>1</SP>-R<SP>2</SP>. In a fourth lowering position, no change takes place of the connections except the inclusion of a third step of resistance from T<SP>1</SP> to L<SP>1</SP>. Fig. 6 shows the connections when the controller is in its final lowering position, T<SP>2</SP>, T<SP>3</SP> are now connected to L<SP>2</SP>, L<SP>1</SP> respectively, and T<SP>1</SP> and R<SP>5</SP>-R<SP>6</SP> to L<SP>3</SP>. In an alternative arrangement shown in Fig. 7, a 3-phase dual-voltage motor 40 has sections of each of two phase windings in series and sections of the third phase winding in parallel. The two phase windings with their respective sections in series remain so connected throughout all lowering positions, and the third phase sections are changed from parallel to series connections in the second and remaining lowering positions. In the first four lowering positions in all the above arrangements, all the resistors of the secondary winding except the end resistor sections near their star connection are included in the secondary circuit. In the fifth lowering position all the secondary winding resistors are included in the secondary circuit. The controller is also provided with hoisting segments.