GB460364A - Improved means for obtaining variable speed control and/or power factor correction with asynchronous induction motors - Google Patents

Abstract

460,364. Alternating current motors ; transmitting mechanical power. GILL, M. A., [Representative of GILL, J. F.], and CHAVIARA, J. N. June 21, 1935, Nos. 17837 and 28683. [Classes 35 and 38 (iv)] An electric motor or power transmission unit comprises three machines A, B, C, each having two relatively rotating coacting members, whereof two of the machines A, B have each a polyphase wound element 13, 3, and an element 11, 7 adapted for continuous current excitation, and the third machine C has two polyphase wound elements 6, 3. A polyphase wound element of the machine C and a polyphase wound element of the machine B are constructed with their conductors in common to constitute a single winding 5 (of the squirrelcage, short-circuited or slip-ring type) and the coacting polyphase wound element 6 of the machine C is electrically connected to the polyphase-wound element 13 of the machine A, these two elements being fixed or mechanically connected while the elements 11, 7 which are adapted for continuous current excitation are either connected together or are fixed. The winding 5 is shown as a squirrel cage winding mounted on a rotor 3 which is acted upon by the polyphase wound element 6 and by the continuous current excited element 7. The synchronous machine A may surround the element 7. The member 7 has two field windings 8, 9, one of which is manually adjustable and the other may be supplied by an exciter on the shaft 4 whereby the motor has a variable speed with a shunt characteristic at each speed. The field member 7 has the same number of poles as the stator 6 and is rotated by the synchronous motor A at the same speed as the rotary field established by the stator 6 so that the currents induced in the rotor-winding by the two fields have the same frequency. To obtain this result, the field member 7 is coupled directly to the rotor 11 if elements 13, 6 have the same number of poles or through gearing having a speed ratio determined by the ratio of the number of alternator poles to the number of induction motor poles. These fields oppose each other at low speeds and assist at high speeds. The continuous current excited systems may be mechanically driven, in which case the machine A functions as an alternator supplying the stator 6 of the induction motor. To reduce weight, e.g. in motor road vehicles, the windings on the stators 13, 6 may be replaced by a common squirrel cage winding. The alternator A may be compounded to give greater voltage as the load increases. The angle between the continuous current excited elements or between the stators 13, 6 may be variable for power factor adjustment. For this purpose, the element 7 may be of the non-salient-pole type carrying two windings displaced by 90 electrical degrees and fed with separately adjustable currents. For non- reversing rolling mill drives, the induction motor is given a slight series characteristic and the shaft 4 carries a flywheel. A series characteristic may be imparted by providing a single winding on the member 7 excited through a rectifier from the secondary of a current transformer in series with the winding on the stator 6. Alternatively, an exciter mounted on the synchronous machine shaft 10 may be interposed. In the case of single phase traction motor equipment, the supply is connected to a single phase winding on the stator 6 carried in the same slots as the polyphase winding. Automatic regenerative braking is obtained in that power is returned to the line when coasting at a predetermined speed. Where the invention is applied solely for power factor correction, the elements 13, 11 and 7 may be small relatively to the induction machine.