Patent GB-1393361-A (GB)
Title: Electrical control system for a plurality of loads
IPC: G08C15/06
Filed: 1971-07-12
Abstract: 1393361 Control of A.C. motors; protective arrangements DAVY POWERGAS Ltd and ROTRACO SYSTEMS Ltd 12 Oct 1972 [12 July 1971] 32544/71 Headings H2J and H2K An electrical control system for a plurality of loads such as A.C. motors comprises for each motor 14 a contactor control unit 3 to connect or disconnect the motor to or from its supply and a switching unit 2 which is periodically connected to the unit 3 to pass to the latter a signal representative of the condition of the unit 2, the periodic connection and operation of all the units 2, 3 being effected by a drive unit 1 which supplies them with a plurality of signals each at a different reference level and a coded interrogator signal divided into periods each allocated to a respective pair of units 2, 3. In the described embodiment, the unit 1 produces a fixed frequency stepped square wave signal providing a repeating 8 bit code to control the units 2, 3 of eight motors, a reference signal of zero level and two complementary reference signals in the form of square waves of “ supply frequency having the same positive and negative amplitude with respect to the zero reference level. These four signals are supplied through the cores 4, 5 of a control cable to a decoder module 6 and a push button module 8 of each switching unit 2 and to a module 10 of each unit 3, which latter module is arranged to operate in a synchronous identical manner with the module 6. Thus the same bit of the interrogator signal causes the modules 6, 10 to simultaneously close static switches 7, 11 to connect the module 8 via a control link 9 with a module 12 controlling the solenoid-operated contactor 13 of the motor 14, the coil being energized, de-energized or maintained in its existing condition according to the signal generated by the module 8, namely:- (1) Start push-button pressed-alternating pulses. (2) Stop push-button pressed-no output signal. (3) Neither push-button pressed-unidirectional pulses. An overload detection module 15 produces a signal condition identical to that of condition (2), if temperature-responsive resistors fitted in the motor indicate overheating. The drive unit 1 comprises a Schmitt trigger, flip-flops, an amplifier, signal inverters, switches and an auto-transformer, Fig. 1 (not shown), and typical waveforms of the signals it produces are described, Fig. 2 (not shown). The control system provides the following operations:-On/offcontrol from local/remoteposition. Start/stop control from local/remote position. Forward/stop/reverse control from remote position. Inching forward/reverse control from remote position. Overtemperature tripping or alarm from remote position. Fail-safe operation is provided by de-energizing the contactors 13 in the event of failure of the unit 1 or short or open circuits on the control cables, which may be run as a ring main with spurs so that a damaged section may be isolated to leave the system operating as two radial sections. The modules have self current limiting protection and, if failure occurs, are designed to produce fail-safe operation of the system. They are encapsulated and mounted on printed circuit cards for fitting in standard push-button enclosures and starter cubicles. Modification of the cabling to control 16, 24 or 104 motors is discussed.