Patent GB-872374-A (GB)
Title: Improvements in or relating to electric control systems
IPC: H02P7/282
Filed: 1959-06-29
Abstract: 872,374. Exciting dynamo-electric machines. IGRANIC ELECTRIC CO. Ltd. June 29, 1959 [July 3, 1958], No. 22201/59. Class 35. [Also in Group XXXVII] An electric motor M is operated in the forward direction by operating switch FS to energize contactor F which closes contacts F1, F2, and completes a circuit for shunt fieldwinding SF1 from transformer PT 1 through rectifier RB1, shunt field-winding SF2 being energized in parallel with the winding SF1. When the shunt field-winding SF1 is fully energized, relay 1FR closes to energize relay FR whereby resistor R8 is removed from the field circuit and timing relay IT energizes contactor 1A. Power is supplied from auto-transformers AT1-AT3 to the motor armature A through rectifier network RN; the armature voltage being increased by tap-changing of the transformers under the control of timed relays. When full voltage is applied to the armature, relay VR operates to interrupt a shunt across field rheostat SFR and to allow field weakening. Current limiting is provided by relay FA responsive to the armature current and having vibrating contacts in parallel with the rheostat SFR. The speed of the motor is decreased by strengthening the field, the resultant decrease in energization of signal windings S1-S3 of magnetic amplifier MA causing braking relay RB to close contacts 1RB1. A braking circuit is completed through armature A and resistors DBR, RBR. Braking operation is terminated when the motor speed has decreased to a constant value. To stop the motor, switch SS is actuated to disconnect the armature from the network RN and to shunt the field rheostat. A braking loop is formed comprising resistor DBR, this circuit being opened by a timing relay when the motor approaches zero speed. When the motor is operating in the forward direction and reverse switch RS is actuated, the armature is disconnected and the field windings are energized in the reverse direction whereby a reverse-current braking circuit is formed through resistors RBR, DBR, the field being regulated to provide a predetermined value of armature current. When the motor approaches zero speed, power is re-applied to the armature to effect rotation in the opposite direction. A capacitor C is connected across the centre taps of resistors R4, R6, forming part of the field bridge network, to reduce the current by-passed through rectifier RB2 when the field windings are being supplied from rectifier RB1. In another arrangement, suitable for series motors, a high resistor is inserted in the by-pass circuit to reduce the current therein, Fig. 2 (not shown). The series motor is controlled by a drum switch and is supplied from a power magnetic amplifier associated with a magnetic pre-amplifier. The speed is determined by comparing the load current and the armature voltage, the pre-amplifier having windings for this purpose. The output of the power amplifier is reduced when a currentlimiting winding is energized due to the motor current exceeding a predetermined value. Reverse operation of the motor is obtained by reversing the field while the armature is energized in the same direction. A discharge path for the field windings includes a rectifier bridge and a resistor. In a further embodiment, for shunt motors, a field failure relay disconnects power from the armature in the event of a failure of the field energization, Figs. 3A-3D (not shown). The armature is supplied through a three-phase rectifier from three power magnetic amplifiers with compensation windings for controlling the armature voltage as a function of the load current. Current-limiting windings of the power amplifiers are connected to a further amplifier having the necessary reference and signal windings. A pre-amplifier is provided with current-signal windings and a reference winding in series with a speed-setting rheostat. A braking amplifier energizes a relay for instituting a rheostatic braking circuit, or, alternatively is connected to an eddy-current brake. A field-strengthening network is provided with a field-supply amplifier, pre-amplifier and decelerating current-limiting amplifier, whilst a field-weakening network is similarly provided, field-weakening being prevented until full armature voltage has been applied. When stopping, the field-weakening network is rendered ineffective but, during reversal, the network acts automatically to limit the armature current.