GB365695A - Improvements relating to electric motor control gear - Google Patents

Abstract

365,695. Master control of A. C. motors; thermal switches. ELECTRICAL APPARATUS CO., Ltd. and WALTON, J. R., Vauxhall Works, South Lambeth Road, London. Nov. 12, 1930, No. 34061. [Classes 38 (iii) and 38 (v).] Sequential contactors in a starting system are controlled by thermal devices, and a master thermal device is provided to ensure the lapse of sufficient time for the contactor thermal devices to return to their normal condition, after being cut out of circuit upon the completion of the starting operation, before fresh operation of the system can take place. Upon depression of a starting button ST, a switch C is closed, and maintained over auxiliary contacts 17. The primary b of a transformer T is energized, and thermal devices MT, T<2>, T<3> receive current from its secondary so that contacts 21, 31 are closed at time intervals to energize contactors C<1> C<2> which cut out rotor resistance r<1>, r<2>, r<3> in two steps. The contactors C<1> C<2> have auxiliary contacts 26, 32 respectively by which they are maintained closed independently of the devices T2, T<3>. When the contactor C<2> closes, contacts 19, 25 are opened, which interrupt the primary circuit of the transformer T and the maintaining circuit of the contactor C<1> respectively. The thermal device MT is normally closed, and is arranged to ensure that the starting operation may be commenced by the button ST only after an interval following a previous actuation. The thermal devices comprise pronged bimetallic strips through which the current passes, and the normally fixed contact of each device is formed by the surface of a cam-shaped plate which is provided with a handle for setting purposes. A resistance coil may be connected in parallel with the bimetallic strip, to have a heating effect on it.