Patent GB-733867-A (GB)
Title: Improvements concerned with the speed control of induction motors
IPC: B66C13/26
Filed: 1952-09-08
Abstract: 733,867. Induction motors. SCHWARZ, B. Aug. 11, 1953 [Sept. 8, 1952], No. 22514/52. Class 35. [Also in Group XXXVII] An induction motor has an eight-pole stator winding S8 connected to a three-phase supply as well as a rotor winding R8 wound for eight poles and connected to resistance Res through sliprings 1, 2, 3. Four-pole windings S4, R4 are accommodated in the same slots as the windings S8, R8, the rotor winding R4 being connected through slip-rings 3, 4, to a rectifier Re for providing a braking-action upon the motor. The four-pole stator winding S4 is connected to resistance Res, the star-connection of which is altered to change the circuit-resistance of the winding R8 in the opposite sense to that of the winding S4. In view of the ratio between the pole-numbers of the windings S8, R8, and S4, R4, the rotating flux due to one group of windings does not induce voltage in the other group. The braking-action upon the motor may be adjusted by altering the resistance in the circuit of the winding S4 or by regulating the D.C. excitation by means of resistor Res.A. Improved operation is obtained by adding a resistance to the circuit of the winding S8, thereby increasing the saturation effect produced by the D.C. excitation, Fig. 3 (not shown). A second rectifier may be placed in circuit with the winding R4 and supplied from a current transformer connected in series with the winding S4, so introducing a stabilizing effect. A similar effect is obtained by connecting the second rectifier across the terminals of the winding S4. Alternatively, the D.C. excitation can be provided by.a generator coupled to the induction motor shaft, Fig. 4 (not shown), a rectifier being connected in parallel with the generator to provide excitation at standstill and low speeds. The generator may have a shunt field winding, a separately-excited field winding supplied through a rectifier, and a load-responsive field winding energized in accordance with the current in the winding S4. In another arrangement, the generator is driven by an auxiliary motor which is excited in accordance with the current in the winding S4, Fig. 4a (not shown). A tachometer generator may provide a signal for the automatic adjustment of the resistance in the circuit of this winding. The windings S4, R4 can be used for motoring at high speed, Fig. 5 (not shown), a change-over switch enabling the winding R4 to be connected either to the rectifier Re or to the three-phase supply. The primary windings S8 and R4 may be connected simultaneously to the three-phase supply for operation at an intermediate speed, the winding R4 being replaceable by a single phase winding. It is possible to provide tapped windings for simultaneous use for both number of poles, the windings S8, S4 being combined in this manner, for' example. In Fig. 6 the winding S8,4 is wound for eight poles in star connection, and a three-phase resistance Res4 is connected to the centre tappings of each phase. The second star-point Sp for the fourpole system is replaced by the three-phase supply which represents a short circuit for the voltage produced by the four-pole rotating field. The eight-pole winding W8.4 may be used as the secondary winding of the motoring system in connection with resistor Res 8, the four-pole connection being used to introduce the D.C. excitation by means of rectifier Re2 connected to Res<SP>1</SP>. The windings R8 and S4 may be replaced by windings of the short-circuited type.