GB424136A - Improvements in and relating to systems of electric motor control - Google Patents

Abstract

424,136. Induction motors. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London, and ADKINS, B., 13, Fisher Avenue, Paddox, Rugby. Aug. 15, 1933, No. 22806. [Class 35] In a speed control system for an induction motor, a commutator machine is provided to apply a voltage of slip frequency to the secondary circuit of the motor and to act alternatively as a machine having a low frequency output for feeding the induction motor during low speed operation. The secondary of a motor 2 is connected through slip rings 3 to the commutator 8 of a regulating machine 6 in series with a compensating winding 7. The machine is coupled to a motor 10 running at approximately constant speed. The exciting winding 9 can be fed either from a variable auto-transformer 12 connected in shunt to the slip rings 3 or through a switch 16 directly in shunt in series with a rheostat 17. The auto-transformer circuit also includes a frequency changing exciter 13. For the upper part of the speed range, the main motor is connected to the supply, switch 16 and the rheostat 17 are opened, switches 11, 18 are closed and the speed is controlled by varying the taps on the autotransformer. For the lower part of the speed range, the primary circuit of the main motor is disconnected from the mains and short-circuited through a switch 5. Switch 16 is closed and switches 11, 18 are opened so that the regulating machine 6 acts as a low frequency generator feeding the main motor, the speed being controlled by means of the rheostat 17. The machine 6 may be of the type described in Specification 420,230 and the main motor 2 and frequency changer 13 may be combined as described in Specification 402,498. In a modification, the machine 6 is replaced by a continuous current machine the brushes of which are connected to the commutator brushes of a rotary converter, the slip rings of which are connected through switches controlling the direction of phase rotation to the slip rings of the main motor. The speed can be varied down to about half the synchronous speed by varying the field of the continuous current machine. For low speed operation the main motor primary winding is disconnected from the mains and short-circuited, and the phase rotation of the connections between the main motor and the converter is reversed so that the continuous current machine acts as a generator feeding the rotary converter which supplies low frequency current to the secondary of the main motor. Specifications 19362/11 and 140,476, [both in Class 35], also are referred to.