US2736828A - Commutation system for universal motors - Google Patents

Description

Feb. 28, 1956 A- H. MAGGS ET AL COMMUTATION SYSTEM FOR UNIVERSAL MOTORS Filed Aug. 3, 1953 United States Patent COMMUTATION SYSTEM FOR UNIVERSAL MOTORS Arthur Hemborough Maggs and Ambrose Eiiison Young,

Rugby, Englan assignors to The British Thompson- Houston Company Limited, a British company Application August 3, 1953, Serial No. 372,061

Claims priority, application Great Britain September 9, 1952 1 Claim. (Cl. 310-158) This invention relates to dynamo-electric machines and more particularly to direct or alternating current series commutator motors of the type usually described as universal motors.

Motors of this type when used on commercial voltages and frequencies are in general limited to applications requiring outputs not exceeding about one third of a horse power at speeds of the order 10,000 revolutions per minute. The reason for this is that at appreciably greater outputs or lower speeds commutation becomes increasingly unsatisfactory due to increased sparking at the brushes.

It is well known that in universal motors the tendency to increased sparking at the brushes when operating on alternating current, as compared with operation on direct current, is due principally to the electromotive forces induced in the armature coils by alternation of the magnetic flux, which cause circulating currents to flow in the coils short-circuited by the brushes. This E. M. E, which is known as the transformer E. M. F., in the coils undergoing commutation is additional to that which occurs equally on both direct and alternating current, and is known as the reactance E. M. F. because it is due to the change of the armature current in these coils. It is usual to neutralise substantially the reactance E. M. E, by backward shift of the brushes so that the coils being commutated come under the influence of the magnetic flux from the trailing tips of the field poles, whereby a rotation E. M. F., which opposes the reactance E. M. F., is generated in these coils.

As the transformer E. M. F., is substantially in time quadrature with the motor current, in contrast with the reactance E. M. R, which is in time phase with it, the transformer E. M. F. cannot be compensated merely by shifting the brushes and heretofore could only be minimised by operation of the motor at high speed and/or limited output.

The object of the present invention is to enlarge the scope of universal motors, i. e. their horse power and/or speed range, by means aimed at assisting commutation.

The invention consists in providing a series commutator motor of the universal type with a field system in which each pole incorporates a shading ring embracing a portion of the pole at the trailing edge together with a backward shift of the brushes suflicient to bring the whole or at least one side of each of the armature coils being commutated within the zone of influence of the shaded portions of the poles.

The accompanying drawing illustrates this invention diagrammatically. In the drawing A is the field magnet of a universal motor having two pole pieces B. The rotor C rotates in the direction indicated by the arrow and brushes D engage the commutator. The trailing edges of the pole pieces are provided with shading rings E, and the brushes are set so that at least one side of the armature coil being commutated is within the influence of the portion of the pole within the shading ring.

The effect of the shading ring is to delay in time phase the magnetic flux in the portion of the pole embraced by the ring, and thus also to cause a corresponding shift in time phase of the rotation E. M. F.s, generated in the armature coils within the zone of influence of the shaded portions. As thus changed in time phase these E. M. F.s can be employed to neutralise either partially or completely both the reactance and the transformer E. M. F.s in the coils undergoing commutation by setting the brushes so that these coils come within the zone of influence of the shaded portions.

As the magnitude of the magnetic flux in the shaded portions of the poles is low because of the well-known effects of armature reaction, the loss in the shading rings is not high, and indeed, is largely offset by the reduction in losses in the armature coils undergoing commutation and at the brushes which would otherwise be caused by the transformer E. M. F.s.

What we claim as new and desire to secure by Letters Patent of the United States is:

An electric motor of the universal type comprising a stator forming a field system and provided with at least one pair of salient poles, a field winding on each of said poles, each of said pair of poles incorporating a shading ring embracing a portion of the pole at the trailing edge thereof, an armature, and brushes co-operating with said armature, said brushes being positioned to bring at least one side of each of the armature coils being commutated within the zone of the influence of the shaded portion of said poles.

References Cited in the file of this patent UNITED STATES PATENTS 1,750,745 Conley Mar. 18, 1930

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