GB1390666A - Control systems for electrical motors - Google Patents

Abstract

1390666 Control of A.C. motors ODESSKOE SPETSIALNOE KONSTRUKTORSKOE BJURO SPETSIALNYKH STANKOV 16 May 1972 22835/72 Heading H2J A method of braking an A.C. motor having at least two windings comprises the steps of disconnecting the windings from their normal power supply, short circuiting at least a portion of at least one but not all of the windings and feeding rectified current through at least a portion of the other winding or windings but not through the winding or windings which include(s) a short circuit, whereby to induce a current in the shorted winding or windings, which current acts to oppose rotation of the motor. Embodiments are described wherein a motor having five stator windings is braked by connecting two of them in series through a diode to an A.C. supply and short circuiting the remaining three through a single contact set, Fig. 1 (not shown), or by connecting them one across another through two contact sets. Fig. 2 (not shown). The two groups of windings are inductively coupled through the rotor, which is of squirrel-cage type, so that the current in one group induces a current in the short-circuited group producing a braking torque whose value varies with the motor speed. Typical characteristics, Fig. 3 (not shown), relating the braking moment and motor speed and torque are discussed to explain the coupling action of the winding groups and the forces (14, 15) produced by the A.C. and D.C. components of the rectified current, their resultant force (16) with windings not short circuited, the force (17) produced by short-circuiting and the resultant of forces (16, 17) which provides a braking moment over the whole range of motor speeds. Fig. 4 shows the application of the invention to a single-phase capacitor induction motor having stator windings 19, 20 energized from an A.C. supply through a capacitor 23 and the contact sets 21, 22 of a starting contactor. To brake, the contact sets 21, 22 are opened and the contact sets 24, 25 of a braking contactor are closed. In Fig. 5, a three phase induction motor 26, 27, 28 is driven in one or other direction by closure of starting contact sets 29, 30, 31 or 32, 33, 34 and is braked by opening of the latter contact sets and closure of the contact sets 35, 36, 37 of a braking contactor. A current-limiting resistor may be connected in series with the contact set 35, Fig. 6 (not shown). Contact sets 35, 36 may be'replaced by sets (43, 39) respectively connected in the primary and secondary circuits of a stepdown transformer (41), whose secondary winding (40) supplies the series combination of windings 26, 28 through the diode 7, Fig. 7 (not shown). This diode may be replaced by a controlled rectifier in the embodiment of Fig. 5. Also discussed are the control of a motor with delta-connected windings and the use of fullwave rectified current to increase the braking at high motor speeds.