Patent GB-1389512-A (GB)
Title: Electrical protection apparatus
IPC: H02H7/08
Filed: 1973-02-01
Abstract: 1389512 Protection arrangements for electric motors INTERNATIONAL BUSINESS MACHINES CORP 1 Feb 1973 [14 April 1972] 5029/73 Heading H2J Electrical protection apparatus for preventing an excessive current switching rate damaging an electrical circuit comprises storage means to store a quantity which is increased for either all or preselected switches of current in the electrical circuit, means to decrease the quantity stored in the storage means at a predetermined rate and threshold means which generates an inhibit signal to inhibit the switching of current in the electrical circuit whenever the quantity stored in the storage means exceeds a predetermined amount. Fig. 1 shows the application of the invention to a load 10 connected to a source 11 via a network 12 which is controlled by start/stop signals on conductor 13 to connect or disconnect the load to or from the source. The signals on conductor 13 are also applied to a network 14 which uses them to increase the quantity stored in a storage means 16 connected also to a network 18 which continuously decreases the quantity at a rate calculated to ensure that, for a desired maximum start/stop signal rate, the stored quantity will be approximately zero at the instant each start signal occurs. Should the rate increase for a sustained period causing the upper limit of a threshold means 20 to be exceeded, the latter applies an inhibit signal 21 to the network 12 to deenergize the load 10 if it is energized or prevent its energization for a predetermined time if it is de-energized. The latter action may follow the normal de-energization of the load by a stop signal after an inhibit signal has occurred. When the stored quantity has fallen to a lower limit, a signal 22 restores normal operation of the network 12. The limits are chosen to protect the most sensitive element of the circuit 10, 11, 12. Fig. 2 shows the application of the invention to protect a circuit including the motor (not shown) driving the capstan 37 of a magnetic tape unit and the servo-system 42 for this motor. A typical accelerate-run-decelerate cycle for the motor is discussed, Fig. 5 (not shown), in relation to the disposition of information along the tape, Fig. 4 (not shown), and it is explained how a particular disposition can cause a high start/ stop signal rate at terminal 46 with consequent overloading of the motor and/or its servosystem. To prevent this, the signals are also applied to a network 14 which forms, with members 16, 18, 50, a circuit similar to members 14, 16, 18, 20 of Fig. 1, except that the threshold means 50 produces only an upper limit signal to inhibit the response of the servo-system 42 to a start signal for a predetermined time. The inhibit signal also provides a "busy" signal to a tape control unit controlling the tape unit in response to commands from a central processing unit, Fig. 3 (not shown). If the motor is being protected, the start/stop signals are counted by network 14 according to how the motor is controlled, e.g. only the start or stop signal if the motor is allowed to coast during deceleration, but both signals if the motor is decelerated by "plugging". Two embodiments of the members 14, 16, 18, 50 are described, in the first of which, Fig. 6 (not shown), the number in a counter (103) is increased by the start signals and decreased by a constant frequency oscillator (104), and in the second of which, Fig. 7 (not shown), the start signal controls charge and discharge sources (122, 125) supplying an analog integrator (124) whose output controls a threshold device (127) having upper and lower limits as in Fig. 1. The first embodiment also includes AND gates, a timer and a bi-stable circuit which operate to inhibit a start signal occurring less than a predetermined time after the counter has produced a signal indicating that its threshold value has been exceeded. Otherwise the start signal is not inhibited and the counter is reset to its initial state.