GB781934A - A control arrangement for an electro-magnetic clutch - Google Patents

Abstract

781,934. Automatic current control systems. ELEKTRO-MECHANIK GES. Jan. 18, 1955 [Jan. 23, 1954], No. 1509/55. Class 38 (4). The starting current of a motor, for driving a machine through an electro-magnetic clutch, is maintained constant by including a magnetic amplifier in circuit with the energizing supply for the clutch, the magnetic amplifier being controlled for varying the motor load, in accordance with deviations of the motor current from a desired value. A squirrel-cage motor 10 has its three-phase stator winding energized over a switch 27, and the A.C. windings 15, 16 of an auto-excited magnetic amplifier 14 are connected to the motor supply in series with a transformer 13 supplying a clutch winding 11 over a rectifier 12. The magnetic amplifier has oppositely-wound control windings 19, 20, the former being energized over a rectifier 22 from the output of a transformer 21 representing the motor current, and the latter being energized over a rectifier 23 from the constant output of a saturating transformer 24 representing a desired reference value; the latter value may be set by resistors 25, 26. A desired degree of clutch excitation is obtained by varying a tapping on the primary of transformer 13. In operation, with the motor current at the desired value, windings 19, 20 carry equal currents, the degree of saturation of the amplifier core being such as to allow the required excitation of winding 11 ; deviations of motor current result in unequal currents in the control windings, with a consequent change in core excitation to adjust the clutch excitation in a sense as to restore the motor current; the non-attainment of the desired value of the latter may be signalled by passing the control current through opposed windings of a relay, which is switched in during starting of the motor. In a modification, the clutch winding 11 may be supplied directly through slip-rings from the amplifier, in which case transformer 13 is omitted, and amplifier 14 is supplied from an A.C. source through an input auto-transformer with a variable tapping. The control is also stated to be applicable to a separately excited clutch.