US3037176A - Electrical inductors - Google Patents

Description

May 29, 1962 D. F. CHAPMAN ELECTRICAL INDUCTORS Filed Dec. 30, 1959 /A/l/E/VTO Q DOA/A LD F CHAPMAN Unite States atent 3,037,176 ELECTRICAL INDUCTORS Donald Frederick Chapman, London, England, assignor to Thorn Electrical Industries Limited, London, England, a company of Great Britain Filed Dec. 30, 1959, Ser. No. 862,978 5 Claims. (Cl. 33683) The present invention relates to electrical inductors.

One use of an electrical inductor is as a choke in circuit with a gaseous discharge device, such as, for example, a fluorescent lamp, supplied by alternating current.

It is often required to mount such an inductor in a shallow channel which forms parts of a fluorescent lighting fitting. It is also required that the inductor be substantially silent and that stray magnetic field which could cause vibration of adjacent steel or iron parts of the lighting fitting is not excessive.

It is well known that an inductor or choke having a magnetic core must be provided with an air-gap in the magnetic circuit in order to prevent flux saturation of the magnetic circuit. Considerable leakage flux is present in the vicinity of the air-gap and precautions may be necessary to shield the inductor so as to prevent this flux leakage and thereby prevent the stray alternating magnetic field from causing noise and vibration in adjacent steel or iron parts which may be located near the inductor.

The stray field will pulsate at 50 cycles per second on ordinary A.C. mains and may cause appreciable hum and noise to be generated by the lighting fitting. Such noise is undesirable.

In one known form of inductor construction the inductor is shielded with thick iron castings in the form of plates which are clamped around the inductor. These castings effectively form a magnetic screen for the inductor but add considerably to the cost and weight of the inductor.

It is an object of the present invention to provide an improved inductor suitable for use as a choke in a fluorescent lighting fitting and having, in operation, stray magnetic fields of such small value as not to cause appreciable noise to be produced in neighbouring parts of the fitting.

According to the present invention there is provided an inductor comprising a winding and a magnetic circuit of laminated ferro-magnetic material, the magnetic circuit 'being in the form of an inner core around which the winding is disposed and an outer shell co-operating with the inner core and comprising two laminated U-shaped yokes fitted one into the other, the limbs of each yoke bridging the gaps between the side edges of the limbs of the other and the transverse portion of each yoke bridging the gap between the ends of the limbs of the other yoke, the magnetic circuit including a series gap in the inner core or between the inner core and the outer shell or both.

An inductor embodying the invention will be described, by way of example, with reference to the accompanying drawings in which:

FIG. 1 is a somewhat diagrammatic exploded perspective view of the inductor, and

FIG. 2 is a side elevation of the assembled inductor.

The inductor includes a winding of enamelled copper wire on an inner magnetic core. This inner core comprises two stacks 11, 12 of strip-shaped laminations of silicon-iron, the two stacks being separated by a gap 13 filled with a non-magnetic cement. A suitable cement is one of the well known epoxy resin compounds. The laminations are coated in the usual way with an insulating material, such as, for example, varnish, in order to reduce eddy-current losses.

The magnetic circuit of the inductor is completed by an outer shell completely surrounding the winding and inner core. This shell comprises two elongated, U-shaped, laminated yokes 14, 15 so fitted into one another as to form a closed, elongated, box-shaped structure 16 (see FIG. 2). The limbs of the elongated U-shaped yokes 14, 15 constitute the four adjacent side faces respectively of the outer shell and their transverse portions constitute the two end faces. This box-like construction is achieved by fitting the U-shaped yokes together in the manner shown in FIG. 2, that is the limbs of each yoke bridge the gaps between the side edges of the limbs of the other yoke and the transverse portion of each yoke bridges the gap between the ends of the limbs of the other yoke.

Thus the outer shell serves to screen magnetically the gap in the inner core and to reduce flux leakage from the Although the U-shaped yokes 14, 15 need not necessarily be elongated, the form of construction described is particularly suitable for an inductor which is long and slim and is therefore of a convenient shape for mounting in assemblies such as fluorescent lighting fittings.

In order to reduce still further the flux leakage which would occur in the case where one central gap is provided in the inner core, two further gaps 17, 18 may be provided between the ends of the inner core and the end faces of the outer shell. These gaps may also be filled with a non-magnetic cement. It has been found that provision in this Way of three relatively small gaps considerably reduces flux leakage as compared with that occurring in an inductor assembly having one relatively large gap. All three gaps are, of course, completely screened by the outer shell.

Measurements obtained on an watt fluorescent lamp choke constructed as described and placed inside a sheetsteel channel showed that there was no detectable increase of vibration or noise due to stray magnetic field eifects.

This 80 watt choke, suitable for a 230/2/l0 volts 50,

cycles A.C. supply, was constructed as follows:

The two yokes 14, 15 forming the outer shell were each made from six strips of 19" x 1% x .02" dynamo iron bent at the centre to give a width of 1% between the limbs of the U-shaped member thereby produced. The centre core was made from two deep stacks 11, 12 of strips of 4 x x .02" dynamo iron. The coil 10 comprised 1900 turns of 25 S.W.G. enamelled copper wire layer wound in 6 layers round the core. The gap 13 between the two halves of the inner core and the gaps 17, 18 between the ends of the inner core and the end faces of the outer yokes were each 0.05 inch wide.

I claim:

1. An electrical inductor comprising an inner core of laminated ferromagnetic material having a series gap therein filled with a non-magnetic material, a winding disposed around said inner core, and an outer shell of laminated ferromagnetic material, said outer shell comprising two laminated, U-shaped yokes fitted together to form a hollow enclosed shell surrounding said winding and said inner core.

2. An electrical inductor according to claim 1 wherein gaps filled with a non-magnetic material are provided between the extremities of said inner core and the adjacent end-faces of said outer shell.

3. An electrical inductor comprising an inner core of laminated ferromagnetic material, a winding disposed around said inner core, and :an outer shell of laminated ferromagnetic material surrounding said winding and said inner core, said outer shell comprising two laminated, U- shaped yokes fitted together to form a hollow enclosed shell, gaps filled with a non-magnetic material being provided between the extremities of said inner core and the adjacent end-faces of said outer shell.

4. An electrical inductor comprising an elongated inner core of laminated ferromagnetic material having a cen- Wally-disposed series gap therein filled with a non-magnetic material, a Winding disposed around said inner core, and an elongated outer shell surrounding said winding and said inner core, said outer shell comprising two laminated, elongated U-shaped yokes of laminated ferromagnetic m aterial fitted one into the other, the limbs of each yoke bridging the gaps between the side edges of the limbs of the other yoke and the transverse portion of each yoke bridging the gap between the ends of the limbs of the other yoke, gaps filled with a non-magnetic material being provided between the extremities of said inner core and said transverse portions of said yokes.

References Cited in the file of this patent UNITED STATES PATENTS 2,948,871 Craige Aug. 9, 1960 FOREIGN PATENTS 647,995 Great Britain Dec. 28, 1950

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