Patent GB-781198-A (GB)
Title: Overheat electrical protection device
Inventor: PEARCE FREDERICK MELVILLE
IPC: H02H6/00
Filed: 1955-01-12
Abstract: 781,198. Protective cut-out arrangements. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Dec. 30, 1955 [Jan. 12, 1955], No. 986/55. Class 38 (5). An overload protective arrangement comprises a first means H for producing pulses at a rate proportional to the heat produced in a piece of electrical equipment, e.g. a motor, a second means N for producing pulses at a rate proportional to that part of the heat produced which is dissipated by cooling, and a third means responsive to the pulses and operable in accordance with the difference between the heat produced and the heat dissipated to disconnect the equipment from a source of current supply or to remove the load therefrom on this difference reaching or exceeding a predetermined value. The first and second means are conventional induction meter devices having impulse contacts H1, N1 respectively, the first means being energized from the supply to the motor and being arranged to have a disc speed proportional to the heat produced, and the second means being energized from an auxiliary A.C. supply through a tapped resistor R. The third means includes a three-bank electromechanical step-by-step switch S having operating coils M1, M2 connected in a pulse control circuit comprising telephone type relays A ... G. When the motor is started the contacts H1 commence to close and open and the first closing of the contacts establishes a circuit for a slow-tooperate relay A. During the operating lag of this relay, relay C is energized and closed contacts c1 to deliver a D.C. impulse to the coil M1 to move the switch wipers W1 ... W3 one step to the right. Relay A then opens contacts a1 and contacts H1 are re-opened. The second means N is now energized over the whole of the resistor R and its disc rotates slowly, the heat dissipation at starting being low. After several further closings of the contacts H1, resulting in the further stepping of the switch wipers and reduction of the resistance in series with the means N, with consequent increase in disc speed, the contacts N1 close, causing relay F to operate, lock in at contacts f and at contacts f2 cause the energizing of relay D. Relay D at contacts d3 locks in, at contacts d4, d5 prepares circuits for relays B, G and changes over contacts d6, d7 so that the resistance in series with the means N is increased by one step to reduce the disc speed. Slow-to-operate relay E then de-energizes relay F at contacts e1 and contacts N1 are then opened. Relay D remains energized so that if contacts H1 close next, relay C is not energized to cause the wipers to move a further step as contacts dl are open, but the energizing of relay B opens contacts b3 to deenergize relay D. Then a further closing of contacts H1 results in the further stepping of the wipers. If on the other hand, contacts N1 close again before the contacts H1, relay G is energized and at contacts g1 energizes coil M2 to move the wipers back one step. When the motor is operating under steady load conditions, the disc speeds of the devices N, H are substantially the same and the switch S will remain inoperative with the wipers at an intermediate position. If the motor is overloaded the wipers will move to the right and at contact 12 complete an alarm circuit and at contact 17 a tripping circuit.