GB490926A - Improved method of control for electric vehicles, lifts, cranes and the like - Google Patents

Abstract

490,926. Field control of D.C. motors; exciting dynamo electric machines. MURPHY, L. Nov. 23, 1936, No. 31986. [Class 38 (iii)] [Also in Group XXXV] The shunt field 4 of a motor operating a vehicle lift or crane is supplied from an additional brush or brushes 3, which may be shifted around the commutator to vary the speed of the motor, while a series field 5 is under the control of a switch 9 operating automatically in accordance with the armature current to short-circuit or reduce the effective number of turns of the series winding for values of armature current within a certain range. Thus the machine operates as a compound-wound machine for values of armature current outside the said range, but as a shunt-wound machine or a compoundwound machine with a weak series field for value of armature current within the said range. The armature current passes through an operating coil 6 which is effective to open the switch 9 against the action of a spring 11 when the current attains a predetermined value. When a pedal or hand lever is operated the motor is brought up to say 30 per cent normal speed by resistance control or series-parallel control of field or armature windings, the shunt field being at a maximum. The switch 9 is effective to cause acceleration of the motor as a compound-wound machine while the current is in excess of the predetermined value, and as a shunt-wound machine when the current falls below that value. Further to increase the speed an accelerator pedal or thumb lever is operated to move the brush 3 against the bias of a spring, regenerative braking ensuing when the brush returns, or when the motor is over driven. The switch 9 may be polarized or non-polarized and may thus function either during motoring or regenerating conditions or both. A friction brake may be provided. The ends of the shunt field may span the portion of the commutator corresponding to the magnetically crowded portion of the armature during generator operation. The control system may be used in combination with a power unit such as a petrol engine which supplies an average amount of power, the controlled dynamoelectric machine acting to load or assist it as required.