US2872626A - Time-overcurrent relays - Google Patents

Description

Feb. 3, 1959 E. PADDISON TIME-OVERCURRENT RELAYS Filed Jan 7, 1957 FIGJb United States PatentO TIME-OVERCURRENT RELAYS Eric Paddison, Stalford, England, assignor to The English Electric Company Limited, London, England, a British company Application January 7, 1957, Serial No. 632,845

Claims priority, application Great Britain January 11, 1956 5 Claims. (Cl. 317-167) This invention relates to electrical protection relays of the time-overcurrent kind which operate on the in duction principle. Such relays have a drive magnet which is energized in accordance with the current in a protectedcircuit and an induction armature which is arranged to move in an air gap in the drive magnet.

In such a relay the operating torque applied to the induction armature varies in approximate direct proportion to the square of the current energising the magnet and by arranging for suitable damping means to act upon the induction armature the relay can be caused to have an inverse time-overcurrent operating characteristic.

There are applications in which it is desirable to have a relay operating characteristic which is very inverse. For the purposes of this specification the meaning of a "very inverse relay operating characteristic is one which approaches closely to a curve having the form I t=constant, where I is the operating current and t is time. For some applications a very inverse operating characteristic is ideal. However, in practice there are several causes which are particularly important on low and high operating currents and, which prevent such a characteristic from being obtained and it is desirable to provide in a time-overcurrent relay some simple means by which the form of the characteristic can be modified. The present invention relates to a relay which incorporates means for eifecting one form of such a modification.

According to the invention, in an electrical time-overcurrent relay of the kind comprising an electroconductiv'e' armature, a shaded pole drive electro-magnet adapted to urge said armature in a relay operating direction by electro-magnetic induction when energised by an operating current, and permanent-magnetic damping means acting on said armature, said drive electro-magnet has a readily-saturable magnetic reluctance path in series with a main air gap in which said armature moves, and a lessreadily-saturable magnetic reluctance path directly shunting said main air gap.

According to a feature of the invention, said lessreadily-saturable magnetic reluctance path includes an air gap.

According to a preferred feature of the invention, in an electrical time-overcurrent relay of the kind comprising an electroconductive armature, a shaded pole drive electro-magnet adapted to urge said armature in a relay operating direction by electro-magnetic induction when energised by an operating current, and'permanent-magnetic damping means acting on said armature, said drive electro-magnet has a subsidiary air gap in series with a main air gap in which said armature moves, with a saturable magnetic bridge member accross said subsidiary air gap and a further saturable magnetic bridge member across said main air gap, the latter bridge member being designed to saturate at a higher magnet-energising current than the first-mentioned bridge member.

According to a further feature of the invention, an element having the magnetic character of an air gap 2,872,626 Patented Feb. 3, 1959 described with reference to the accompanying drawing a bridge electrical contacts in which Fig. la shows a front elevation view and Fig. 11') shows a plan view of a time-overcurrent relay.

Referring now to the drawing, a time-overcurrent relay is shown to include anelectro-magnet 10 which is arranged to be energised by a magnetising coil 11. An induction armature in the form of a metal disc 12 is mounted so as to turn about its centre on a spindle 13 and is biased towards a neutral position by the spring 14. An arm 15 carried by the spindle is arranged to 16 when the disc 12 moves a predetermined distance against the bias of the spring 14. Damping means in the form of a permanent magnet 17 are also provided as shown in Fig. la and Fig. lb.

An electro-magnet 10 has a main air gap 10:! in which the disc 12 moves. There is another air gap 101: which is bridged by a saturable member 10c, this member having a very high permeability and therefore being easily saturated. The coil 11 carries the operating current and produces the magnetic flux through the magnet 10, some of this flux linking the disc 12. Shading rings 10a cause the torque to be exerted on the disc in the contact closing direction. Thus, when the current through the coil 11 is small, the saturable member 100 being unsaturated, the operating flux produced by this current will be relatively greater than it is when the current is large and the member 100 is saturated. There is, therefore, a tendency for the relay to operate relatively faster on small operating currents due to the provision of this 7 saturable member 10c.

The electro-magnet 10 also has a member 10d across the main air gap 10a. However, this bridge member 10d is separated from the electro-magnet 10 by two non-magnetic spacers 10 This arrangement is such that at medium overcurrents this bridge member 10a is not saturated and therefore serves to divert a considerable proportion of the flux set up by the coil 11 from the working air gap 10a- The proportion diverted is substantially constant up to a particular value of current but for currents exceeding this the bridge member 10d saturates. This saturation limits the amount of flux diverted from the main air gap 10a and results in a relative increase of the operating torque applied to the disc 12. Again, this serves to decrease the operating time of the relay, but this time on high overcurrents.

The result is that on low and high overcurrents, the operating time of the relay is reduced but on medium overcurrents it remains substantially unaffected.

The advantage of this will readily be understood by considering the eifects of the control spring 14, friction at the bearings of the spindle 13, the effects of the inertia of the disc 12, the dynamic braking effect of the driving electro-magnet 10 and the natural saturation tendency of the electro-magnet 10, when the operating currents are very high.

Although a very inverse characteristic may be expected from a consideration of the simple theory of an induction drive the above effects contribute to cause the operating characteristic to deviate from this.

If no control spring is used and if the damping magnet produces a constant flux the damping force will be directly proportional to the speed of the disc, but even if the torque from the drive magnet is exactly proportional to the square of the operating current the time characteristic would still not be inversely proportional to the square of the current over a wide current range.

saturable bridge The reasons for this are, firstly that at low currents the driving torque is small and more close in magnitude to the frictional torque which is practically constant. Friction has relatively no importance at high currents but its effect is to increase the operating time of the relay at low currents. Secondly, although at low currents, where the operating time is relatively long, the inertia effect of the disc is negligible, at high overcurrents Where the operating time is short the inertia of the disc becomes important and serves to delay the operation of the relay. Thirdly, on high overcurrents a dynamic damping effect is caused by the drive electro-magnet and whereas the normal damping action of the permanent magnet would lead to an ideal operating characteristic the dynamic damping action has the effect of increasing the operating time of the relay on high overcurrents. The effect of saturation of the electro-magnet on high overcurrents modifies the characteristic still further and it will be seen from these considerations that these various factors serve to increase the operating time of the relay on low and high overcurrents but not on medium currents. By utilising the invention a compensating effect can, therefore, be achieved since the function of the saturable bridge members utilised by the invention is to decrease the operating time of the relay, on low and high overcurrents but not on medium currents.

Whereas, in theory, it may seem possible to achieve the same technical result by using separate saturable magnetic devices in the energising circuit of the drive electromagnet as is achieved by the saturable features provided by the invention, the construction according to the invention is preferred for the following reasons. Firstly, the invention provides a cheap and simple means for introducing saturable features and, secondly, the performance of the relay is determined solely by the signal applied to the relay energising coil 11. If independent magnetically saturable devices were incorporated in the circuitof this coil, the performance of the relay would to some extent depend upon the nature of the power source energising the relay. For example, the nature of the operating characteristic of the relay would depend upon whether or not the relay was energised from a current transformer or a voltage transformer. This feature is of less consequence if the relay has a form according to the invention.

In its preferred form the invention provides two magnetically saturable features on a single drive magnet.

One of these features serves to modify the relay characteristic on low operating currents and the other feature serves to modify the relay characteristic on high operating currents. 7

What I claim as my invention and desire to secure by Letters Patent is:

1. An electrical time-.overcurrent relay of the kind comprising an electroconductive armature, a shaded pole drive electro-magnet adapted to urge saidtarmature in a relay operating direction by electro-magnetic induction when energised by an operating current, and permanentmagnetic damping means acting on said armature, said drive electro-magnet having a readily-saturable magnetic reluctance path in series with .a main air gap in which said armature moves, and a less-readily-saturable mag netic reluctance path directly shunting said main air gap.

2. An electrical time-overcurrent relay of the kind comprising an electroconductive armature, a shaded pole drive electro-magnet adapted to urge said armaturein a relay operating direction by electro-magnetic induction when energised by an operating current and permanentmagnetic damping means acting on said armature, said drive electro-rnagnet having a subsidiary air gap in series with a main air gap in which said armature moves, with a saturable magnetic bridge member across said subsidiary air gap and a further saturable magnetic bridge member across said main air gap, the latter bridge member being designed to saturate at a higher magnet-energising current than the first-mentioned bridge member.

3. An electrical time-overcurrent relay according to claim 1 wherein said less-readily-saturable magnetic reluctance path includes an air gap.

4. An electrical time-overcurrent relay according to claim 2 wherein an element having the magnetic character of an air gap is included in the magnetic circuit of said further saturable magnetic bridge member.

5. An electrical time-overcurrent relay according to I claim 4 wherein said element having the magnetic char- References Cited in the file of this patent UNITED STATES PATENTS Smith July 30, 1918 Goldsborough July 17, 1928

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