US1968996A - Induction motor - Google Patents

Description

W. DOLGOFF INDUCTION MOTOR Aug. 7, 1934.

Filed April 7, 1934 INVENTOR ATTORNEY Patented Aug. 7, 1934 UNITED STATES ATENT OFFIC Application April 7, 1934, Serial No. 719,507 In Great Britain December 2, 1933 9 Claims.

This invention relates to squirrel cage induction motors and has for its object the provision of improved starting devices for such motors.

The starting current taken by a motor having a squirrel cageor short-circuited rotor is proportional to the ohmic resistance of the rotor winding and therefore in order to obtain the desirable condition of a high starting torque coupled with a reasonably low initial starting current it is necessary to provide some means for artificially increasing the resistance of the rotor winding when starting and subsequently reducing this resistance to the normal value when the rotor has attained suflicient speed. This is effected automatically according to the present invention by connecting the ends of the rotor conductors or cage bars by a wire resistance of relatively high ohmic value encircling the ends of the said conductors and transversed by the induced currents in the rotor during the starting period, and providing means for short-circuiting the said conductors when the rotor obtains a predetermined speed.

In the preferred construction according to the invention the resistance in the rotor circuit comprises a wire binding encircling a ring of spaced elements or segments to which the conductor bars-are individually secured and a switch element movable against a spring under the influence of. centrifugal means to short-circuit the said segments.

According to another feature of the invention the short-circuiting switch element comprises laminations of copper or other highly conducting material arranged in the form of a cone and adapted to contact with the similarly shaped interior surface of the ring of segments.

. 'According to a further feature of the invention 'the resistance wire binding is disposed or spaced longitudinally of the rotor to afford a large surface for cooling. Preferably the wire is arranged in a single layer helix on the outer periphery of the ring of segments. The wire may be laid in grooves formed in the segments which are connected to the rotor bars by threaded means and rest on a plate of insulating material interposed between the segments and the laminated rotor core.

The short-circuiting switch may be carried on a sleeve member slidable axially on the shaft and is movable to cause engagement of the switch ele- 'ment with the cage bar segments by pivoted weights actuated by centrifugal force.

' Referring to the accompanying drawing:

Fig. l is a longitudinal section more or less diagrammatic of a portion of a rotor for a squirrel cage induction motor embodying a starting device in accordance with the invention.

Fig. 2 is a section taken on the line 22 in Fig. 1.

In carrying the invention into effect according to one convenient mode as applied by way of example to a squirrel cage induction motor as shown in the accompanying drawing, 1 represents one end of the laminated core of a squirrel cage motor having the usual cage bars 2 embedded in the laminations. The bars 2 are each'threaded into or otherwise secured to a segment 3 of a series which are arranged in a circle at one end of the rotor core as shown, these segments replacing the usual short-circuiting end ring. The segments 3 which are separated from one another by radial slots are mounted upon a discor plate l of insulating material so that there is no direct electrical connection between them, the segments .75 being seated in a shouldered portion of the disc. Surrounding the segments is a layer of binding wire 5 of material having a considerably higher resistance than the segments or cage bars. This wire binding'serves the double purpose of providing an electrical connection of relatively high resistance between the bars and aifording mechanical strength and rigidity to the assembly. The binding acts to hold the segments firmly in position in their shouldered seating in the plate 4. In the example illustrated the binding consists of a single helix of wire laid in grooves formed in the periphery of the ring of segments, this arrangement offering a relatively large cooling surface to the resistance.

A sleeve 6 preferably of insulating material is mounted for sliding movement axially on the rotor shaft 7 and has a flanged portion 8 upon which a short-circuiting switch element 9 is secured by means of rivets 10 or other suitable means. The switch element 9 is built up of laminations of red or electrolytic copper in the shape of a cone and the inner side of the ring of segments 3 is correspondingly shaped so that good electrical contact may be made between the 1 short-circuiting element and the ring of segments, aided by the resilience due to the laminated construction of the switch element. Between the sleeve 6 and a collar 12 on the rotor shaft is located a strong helical spring 11 which tends to hold the switch element in its inoperative position away from the segments. x

In order to actuate the switch element two or more weights 13 are arranged on pivots 14 carried by a supporting member 15 mounted on the shaft. The weights 13 are provided with inwardly extending fingers 16 which engage the outer edge of the sleeve 6. The support 15 for the weights may conveniently be mounted on a sleeve 17 secured to the shaft in such a way that the said support may be readily removed from the shaft in order to obtain access to the shortcircuiting device.

When the motor is at rest. the various parts of the device occupy the positions shown in the drawing, that is to say, the switch element 9 is held out of contact with the segments 3 and consequently the circuit through the rotor cage bars is completed through the high resistance connections represented by the binding; wire 5. It follows therefore, that when the motor is switched on to the line the current that flows in therotor bars is limited by the resistance-in series therewith, and an unduly heavy current surge is prevented so that there is no necessity to reduce the applied voltage at starting and consequently a good starting torque may be obtained.

As soon as the rotor reaches a predetermined speed of'rotation-which is somewhat lower than its normal running speed, the centrifugal force acting on the weights 13 becomes sufficient to overcome the opposition of the spring 11 so that theweights assume the positions indicated by broken lines and the switch element 9 is pressed with considerable force into contact with the segments 3 which are thereby short-circuited andthe resistance of the rotor bars is brought back to its normal value, this being maintained under all normal running conditions of the motor.

When the motor is stopped or its speed reduced below a given point-the spring 11 disengages the switch element from the segments. and automatically'returns the parts to their position of rest ready for the next start.

It will be seen that the starting device described is exceedingly simple in construction and certain in operation, while the arrangement of the starting resistance in the form of a wire binding on-the periphery of the rotor oflers a large surface to the surrounding air and provides ellicient cooling of the resistance whenthe rotor is in motion. Owing to the improved cooling effect a smaller resistance may be used than would otherwise be necessary without exceedingithe permissible temperature rise and moreover by. employing a suitable alloy for the resistance a very considerable temperature rise may be permitted without harmful result, thus still further decreasing the size and cost of this part of the device.

It is to be understood that the invention is applicable to all types of squirrel cage motors and is not confined to the particular construction described which may be variously modified to suit different conditions met within the practice of the invention.

Having now particularly described and ascertained the nature of my said invention,'and in what manner the same is to be performed, I declare that what I claim is:--

1. In an induction motor having a squirrel cage rotor, cage bars forming conductors for the rotor, a wire resistance of relatively high ohmic value encircling the ends of said conductors and traversed by the induced currents in the rotor cage during the period of starting of'the rotor, and means for short-circuiting said conductors when rotor attains a predetermined speed.

2. In an induction motor having a squirrel cage rotor, cage bars forming conductors for the rotor, a wire resistance of relatively high ohmic surfaces and a resistance in the form of value encircling the ends of said conductors and traversed by the induced currents in the rotor cage during the period of starting of the rotor, and means for short-circuiting said conductors when rotor attains a predetermined speed, said resistance comprising a ring of electrically spaced segments connected to said bars and a binding about said segments, said binding being in electricalconnection with the segments.

3. In an induction motor having a squirrel cage rotor, cage bars forming conductors for the rotor, a Wire resistance of relatively high ohmic value encircling the ends of said conductors and traversed by the induced currents in the rotor cage, during the period of starting of the rotor, and means for short-circuiting said conductors when rotor attains a predetermined speed, said resistance wire being spaced longitudinally with respect to said rotor to provide a large cooling surface.

4. In an induction motor having a squirrel cage rotor, cage bars forming conductors for the rotor, a wire resistance of relatively high ohmic value encircling the ends of said conductors and traversed by the induced currents in the=rot0r cage during the period of starting, of the rotor, and means for short-circuiting saidconductors when rotor attains a predetermined speed, said resistance comprising a plurality ofelectrically spaced segments of electrical conductive material positioned to form a ring, and'a single layer of helix wound-Wire applied to the outer periphery of said segments.

5. In an induction motor having a squirrel cage rotor, cage bars forming conductors for the rotor, at wireresistance of relatively high ohmic value encircling the ends of said conductors and traversedby the inducedcurrents in the rotor cage during the period of starting of the rotor, and means for short-circuiting said conductors when rotor attains a predetermined speed,xsaid resistance comprising a plurality of electrically spaced elements of electrical conductive material, said elements'provided with grooves in their outer wire laid in said grooves. 6. In an induction motor having asquirrel cage rotor, cage bars forming. conductors'forthe rotor, a wire resistance of relatively high ohmic value encircling the ends of said'conductors-and traversed by the induced currents in' the rotor cage duringthe period of starting of the rotor, and'means for short-circuiting said conductors when rotor attains a predetermined speed, said resistance comprising a ring-of electricallyspaced segments connected to said barsand a binding about said segments, said binding being in electrical connection with thezsegments, a: disc of insulatingmaterial interposed between" said segments and the core'of said rotor.

7. In an. induction motorhavinga squirrel cage rotor, cage bars forming conductors'for the rotor, a wire resistanceof relatively high ohmic value encircling the ends of said conductors-and traversed by. the induced currents in the rotor cage during the period of starting'of the rotor and means .for short-circuiting said conductors when rotor attains a predetermined speed, andxa switch membermovableto short-circuit the cage bars at predetermined speeds of rotation of the rotor, and means actedupon by centrifugal action for operating said switch member to move it out of short-circuiting position.

8. In. an induction motor'having-a squirrel cage; rotor, cage bars forming conductors for the 9. In an induction motor having a squirrel cage rotor, cage bars forming conductors for the rotor, a wire resistance of relatively high ohmic value encircling the ends of said conductors and traversed by the induced currents in the rotor cage during the period of starting of the rotor, means for rendering said resistance ineffective, and means operated by centrifugal action for rendering said last named means ineffective.

WLADIMIR DOLGOFF.

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