GB652188A - Control systems for electric motors - Google Patents

Abstract

652,188. Control of two motors. BRITISH CELANESE, Ltd. Sept. 9, 1948, No. 23758. Convention date. Sept. 9, 1947. [Class 38(iii)] Control apparatus for use with a plurality of electric motors individually driving devices adapted to perform related operations, comprises an element responsive to changes in current supplied to one of said motors, a timing device adapted to be set in operation upon said element responding to an increase in said current to a predetermined value above normal and to be reset on said element responding to a return of said current to normal, and means for disconnecting the supply of power to a second such motor without interrupting the supply of power to the first-mentioned motor, said means being arranged to be operated by said timing device after the elapse of a set period during which the current supplied to the first-mentioned motor remains above said predetermined value. The Figure relates to a system for a pulverizing mill having a hammer mill motor M1 and a feed screw motor M2. When an overload occurs on the motor M1 an inductive coil 3, which is linked with one of the supplylines to the motor, becomes effective to disturb the balance of opposing primary windings 9, 11, of transformer 4. The winding 9 is connected across winding 3, winding 11 being supplied with a reference voltage from secondary winding 6 of mains-connected transformer 7. If the degree of substance is sufficient, a section 14a of a double triode valve becomes effective to energize a relay-coil 21 and open contacts 31, 32, thereby removing high negative potentials, derived through rectifier valve 34 and transformer secondary winding 36, which are applied to the grids of valve-sections 33a, 33b. Upon opening of contact 32, a condenser 37 commences to discharge through resistors 38, 39, and, after a predetermined short time interval section 33a passes sufficient anode current to energize relay-coil 43. If the overload on motor M1 has persisted during this period, contacts 54, 55 open the power circuit to motor M2 and no additional material is fed thereby to the mill. Opening of contact 31 causes condensers 47, 48 to discharge through resistors 45, 46, and, if the overload on motor M1 persists for a predetermined period after the motor M2 has been stopped, contacts 66, 67, of relay 49, become effective to disconnect the motor M1. If the duration of the overload on the motor M1 is such that the feed motor has been disconnected but the mill motor has not, then, provided the mill motor current returns to its no-load value, contacts 77, 78 close to re-energize the feed motor M2. This is effected by the primary winding 11 overcoming the winding 9 to render valvesection 14b operative to energize relaycoil 29. The sensitive device 2 may be responsive to rate of change of mill motor current and/or predetermined value of current. Apart from indicating the magnitude of current, a suitably connected set of coils, similar to the pick-up coil 3, may be used to obtain a signal proportional to the unbalance of current in a polyphase power system.