Patent GB-603287-A (GB)
Title: Control system for electric motors, especially for hoists
IPC: B66C13/26
Filed: 1944-10-31
Abstract: 603,287. Master controlof A.C. motors. WAYGOOD-OTIS, Ltd. Oct. 31, 1944, No. 21256. Convention date, Nov. 27, 1943. [Class 38 (iii)] A manual controller operates in association with a switch responsive to the load to cause a hoist motor : (a) to hoist or lower the load at a speed not exceeding certain value ; (b) to hoist or lower the load at a higher speed if the load be below a certain minimum, and (c) to move the load at a low speed (below the certain speed) but only in the lowering direction. For A.C., as described, the motor, which is of the kind described in Specification 603,352, [Group XXXV], is a 3-speed induction motor having one stator winding MST which can be connected to give for example 4 poles or 8 poles and another winding MSS which gives for example 36 poles. The maximum number of poles is an odd multiple of the minimum number and is indivisible by the intermediate number. The rotor has a definitepitch squirrel cage winding MRS. effective for 4 and 36 poles and a phase-wound 8 pole winding MRP connected to resistances RR through slip-rings. The control circuit is supplied through transformer T1 and rectifier RE1. At rest, relay AF is energized. Movement of the manual controller for hoisting connects +ve through contacts MUF to contacts MU1 to MU6 in succession, thereby energizing up switch U ; this energizes the brake relay BR, which in turn energizes magnet B to take the mechanical brake off the hoist, as well as normal speed switch NS and governor switch G. The latter is energized by the voltage across resistance R2 opposed by the lower voltage applied across R4 from the motor sliprings through transformer T2 and rectifier RE2, and in turn energizes auxiliary Up switch Z. With contacts U1 and U2 and NS1-4 closed the motor is connected for 8 poles ; accelerating switches AA-DA operate successively to short-circuit rotor resistance RR, with minimum timeintervals determined by the discharge of condensers C4-6 through the delaying coils of AA-DA, and the motor runs up to normal speed. If the controller be in the full speed position, relay BF is now energized through contacts MU6 and DA5 ; contacts BF1 open but are short-circuited, at full load, by contact LM of load switch L which is a torque motor with series winding LC and a winding LP connected to potentiometer R1. If the motor is, however, only lightly loaded windings LC and LP are in phase, no torque is exerted, contact LM is disengaged and relay AF drops, thus opening the circuit of the accelerating switches AA-DA and of the normal speed switch NS and closing that of the fast speed switch FS. The motor is then re-connected for 4 poles and can run up to high speed. When the controller is returned to the " off " position relay BF and fast speed switch are de-energized while relay AF and normal speed switch NS are re-energized ; the motor is re-connected for 8 poles and regeneratively braked. Relays CF and DF, energized by the fast speed switch, have sufficienttime delay in dropping out to ensure that switches AA-DA-connected to +ve by contacts DF1-4-operate in timed succession to short-circuit rotor resistance RR, and that opening of switches NS and U is delayed. Relay Z is maintained so that when Up switch U opens, a circuit is made from +ve through relay SA, Down switch D and contact U4. The Down switch D operates to apply an 8-pole reverse field to the motor for further braking. Relay SA energizes relay S which by opening contacts S2 and S3 and closing S1 and S4, connects governor switch G across a smaller part of resistance R3. When the motor reaches standstill, the voltage across R4 equals that across the part of R3 and switch G drops out; the remaining switches other than AF drop out, and the mechanical brake is applied. A similar result-lowering at either normal or fast speed, according to load, and subsequent regenerative braking and plugging-is obtainable by moving the manual controller to the lowering position except that accelerating switches AA-DA act entirely automatically ; relay X during lowering performs a similar function to relay Z during hoisting. Furthermore, the controller, by connecting contact MDF to only the contact MD1, can energize slow-speed switch SS to connect the supply through contacts SS1 and SS2 to 36-pole winding MSS for lowering at slow speed. Overload switches OA and OB are included. The Specification includes a diagram of the motor rotor windings.