GB596157A - Improvements in or relating to braking systems for induction motors - Google Patents

Abstract

596,157. Control of A.C. motors. IGRANIC ELECTRIC CO., Ltd. July 23, 1945, No. 18811. Convention date, July 22, 1944. [Class 38(iii)] A reversible induction motor is braked by the supply of D.C. to its primary winding or by a counter torque produced by connecting the primary winding to the D.C. supply whilst energised by the A.C. and the D.C. source may be a generator driven by the motor or may be rectified current controlled, e.g. by phase shifting control grids, in response to the speed of the motor. In Fig. 1, the motor of a hoist can have its primary winding 11 connected to the polyphase supply by hoisting contactor H or lowering contactor L in conjunction with main contactor M, and by the normally closed contacts of contactor LH to the D.C. generator G driven by the motor and having two field windings, viz. the self-excited winding f<1> and the separately excited winding f<11> supplied through rectifier 37. Resistances R1-R4 in the circuit of the secondary winding 12 are controlled by contactors 1A-4A. The relay SR, energised by the secondary current, controls contactor 2A. The rectifiers 37 also supply the timing relays 1CT, 2CT and 3CT, each of which has its operating coil in parallel with a resistance and a condenser. In each hoisting position of the master controller MS, Fig. 2, the contactor H is energised, and the contactor M (through auxiliary contacts of contactor H, contacts of relay 1CT and contacts of relay FL energised by the current through field winding f<11>), as well as the brake relay BR (controlling the release cf the brake 13, Fig. 1) through contacts of contactor M or relay ICT and the contacts of relay FL. The contactor LH is also energised through contacts of contactor M. The contactor 1A is energised only on the first step, contactor 2A is energised later and contactors 3A and 4A on later steps and after timing relay 1CT is opened by closure of contactor 2A. In the first lowering position, the contactor L is closed and also-after a delay if relay 1CT has just been open during hoisting- the brake relay BR. The motor is excited solely by the D.C. generator G which is fully excited by field windings f<1> and f<11>- the resistances r4, r7 and r6 in series with the latter being short-circuited by contactor L and relays 1FA and 2FA. The contactor 2A is also energised. In the second lowering position the contactor M is energised through auxiliary contacts on contactor L and contacts of relays 1CT and FL; the field switch 1FA is also energised through auxiliary contacts of contactor M, to insert resistance r7 in the circuit of field winding f<11>. The motor is now connected to the polyphase supply through the resistances r1 and r2, which prevent appreciable D.C. from flowing to the A.C. network, and to the D.C. generator G through the combination of inductance 25 with a condenser 26 to prevent appreciable A.C. from flowing through the generator; the generator excitation is greatly reduced by the insertion of resistance r7 and the disconnection of field winding f<11>. On the next step, field switch 2FA inserts further resistance r6 in series with field winding f<11>. On the last step, contactor LH operates and connects the motor solely to the A.C. supply. On return movement of the controller, field switch 2FA is temporarily maintained by timing relay 3CT (energised by contactor LH) and field switch 1FA by timing relay 2CT (energised by field switch 2FA). In the " off " position of the controller, or on A.C. supply failure, the motor, so long as it is running, is supplied by generator G which is almost fully excited, resistance r4 being however in the circuit of field winding f<11>. A low-voltage relay UV and overload relays 29, 30 are provided. In a modification, the motor is supplied by an independently driven D.C. generator excited by the machine G, the field winding f<11> being on either the independent generator or the exciter G. The generator G may be independently driven but a braking torque of the motor may not then be produced on failure of the A.C. supply. The generator G driven by the motor may have a permanent magnet field system and variable resistance in its armature circuit. The generator excitation again may be varied by saturable reactors controlled by the load current. Alternatively the D.C. supply may be by an electronic rectifier, grid-controlled, e.g. by phase-shifting means, as a function of motor speed means may be provided for unbalancing the primary voltages.