GB2048497A - A moving-coil indicating instrument for measuring an electric current - Google Patents

Abstract

A moving-coil ammeter comprises first and second flat pole-pieces 2 and 5 and has the movable assembly 12 rotatably mounted about a pin 15 secured to the first pole-piece 2. The assembly 12 comprises a C-shaped support 13 and a coil 14 embracing a toroidal portion 3 of the pole-piece 2; balancing means 17 are provided at one end, and terminals 18, 19 at the other end for connection to conductive springs 20, 21. The magnetic system further comprises a permanent magnet 6, and a ferromagnetic plate 7 parallel to the pole-pieces: a bridge plate 28 thereon has a ferromagnetic screw 29 for zero adjustments. <IMAGE>

Description

SPECIFICATION A moving-coil indicating instrument, more particularly for measuring an electric current The invention reiates to a moving-coil indicating instrument, more particularly for measuring an electric current, of the kind comprising: A first flat pole-piece having a laterally protruding, hereinafter referred to as a heel-shaped portion, toroidal or functionally equivalent shape, hereinafter referred to as a toroidal portion; A second flat pole piece extending parallel to the first and having the opposite polarity, and A movable assembly rotatable through an angle around the axis of the toroidal portion and comprising a coil connected to the toroidal portion.

In order to obtain a highly accurate reading, more particularly a highly uniform scale, with the aforementioned devices, the pole-pieces and movable assembly must be coupled with high accuracy in accordance with the prescribed geometrical and kinematic relation.

More particularly there must be little or no misalignment between the axis around which the movable assembly rotates and the axis of the toroidal portion.

In existing instruments, owing to the large number of components for coupling the pole-pieces and the movable assembly, the coupling is not normally obtained with the required accuracy. More particu larlythere is always a misalignment between the axis around which the movable assembly rotates and the axis of the toroidal portion.

The result is that the known instruments are inaccurate unless very expensive methods of manufacturing and adjustment are used.

The invention is based on the problem of devising an indicating instrument having structural and functional characteristics which meet the aforementioned requirements while overcoming the aforementioned disadvantage of the prior art.

This problem is solved by an instrument of the specified kind, characterised in that the movable assembly is rotatably borne at the end of a pin securely mounted on the first pole-piece and extending along the axis of the toroidal portion and through it so as to be substantially symmetrical therewith.

Other features and advantages of the indicating instrument according to the invention will be clear from the following description of a preferred embodiment, given by way of non-limitative example with reference to the single accompanying drawing, which is an exploded view of the instrument according to the invention.

The single accompanying drawing shows an indicating instrument 1 according to the invention comprising a first pole-piece 2 which is flat and made up of a toroidal portion 3 having an axis X-X and a heel-shaped portion 4.

Instrument 1 also comprises a second, plateshaped pole-piece 5 extending parallel to the first pole-piece 2.

A permanent plate-shaped magnet 6 is disposed between the heel portion 4 of the first pole-piece 2 and the second pole-piece 5 and is stuck thereto.

Aferromagnetic plate 7 extends parallel to the second pole-piece 5 so as to hold the first pole-piece 2 between them.

Plate 7 and pole-piece 5 are interconnected by two columns 8,9 of equal height. More particularly, one end of columns 8 and 9 is keyed and force-fitted on to plate 7. The second pole-piece 5 is secured to the other ends of the columns by screws 10 and 11.

Instrument 1 also comprises a movable assembly (general reference 12) rotatable through an angle around axis X-X along arrow M, comprising a substantially C-shaped support 13 and a coil 14 connected to the toroidal portion 3 of the first pole-piece 2 and initially mounted thereon along a slot 4a formed in the heel portion 4.

The movable assembly is rotatably held at ends 15a 15b of a pin 15 securely mounted on pole-piece 2.

Pin 15 extends along the X-X axis of the toroidal portion 3 and through it, so as to be substantially symmetrical thereto, and is firmly held at its centre 15c by a projection 16 extending from the heel portion 4 of pole-piece 2.

One end of the C-shaped support 13 has conventional balancing means (general reference 17). At the other end of the C there are electric terminals 18, 19 for connecting to the internal ends of spiral springs 20,21 made of conductive material.

The outer ends of springs 20, 21 are connected to electric terminals 22, 23 for supplying an electric current to be measured.

Terminals 22, 23 are held by a ring 24 of insulating material secured to plate 7 by sheets 25, 26.

Assembly 12 has a seat 27 adapted to receive a pointer (conventional and therefore not shown).

Instrument 1 also comprises a bridge plate 28 secured at its ends to plate 7 at columns 8 and 9.

At its centre, plate 28 has a screw 29 made of ferromagnetic material and adjustable as necessary, for adjusting the bottom of the scale of the instrument.

The operation of the instrument according to the invention will now be described. Terminals 23, 24 of the instrument are connected to an electric power system in orderto measure the current, whereupon the current for measuring goes through coil 14.

Since the coil is in the magnetic field produced between the two pole-pieces, the movable assembly is subjected to a torque of electromagnetic origin. It therefore rotates against the action of springs 20, 21 until it reaches an angular position in which the torque is balanced by the resilient opposing torque of the springs. In the last-mentioned position, the pointer indicates the current flowing through the instrument.

As is known, the electromagnetic torque is directly proportional both to the current flowing through the instrument and to the intensity of the magnetic field, which depends on the permeability of the magnetic circuit.

In the instrument according to the invention, owing to its structural characteristics, both the intensity of the magnetic field and the permeability of the magnetic circuit are subject to only small fluctuations from the average value along the entire angular motion of the movable assembly. This is mainly because the axis around which coil 14 rotates has little or no misalignment relative to the axis of the toroidal portion. This is easily brought about, since pin 15 is secured to the first pole-piece 2.

Secondly, the presence of ferromagnetic plate 7 parallel to pole-piece 5 fixes the path of the magnetic flux, so that the magnetic circuit has a constant magnetic permeability along the entire angular motion of the movable assembly, even if the first and second pole-pieces are not perfectly parallel, owing to errors in parallelism between the surfaces of the permanent magnet 6 and/or faults in sticking.

Plate 7 can easily be made parallel to the second pole-piece 5, owing to the coupling means, which comprises columns having equal length.

Consequently, the instrument according to the invention has the advantage of highly accurate reading and, more particularly, a very uniform scale.

The aforementioned indicating instrument has the further advantage of being compensated for variations in temperature. This is because the angular position ofthe movable assembly at equilibrium does not vary with temperature, because when the temperature increases (or decreases) there is a proportional decrease (or increase) in the flux provided by the permanent magnets and consequently in the electromagnetic torque, and also in the rigidity of the springs and consequently in the resilient opposing torque thereof.

Claims (4)

1. A moving-coil indicating instrument, more particularly for measuring an electric current, of the kind comprising: a first flat pole-piece having a heel-shaped portion and a toroidal portion; a second flat pole-piece extending parallel to the first and having the opposite polarity, and a movable assembly rotatable through an angle around the axis of the toroidal portion and comprising a coil connected to the toroidal portion, characterised in that the movable assembly is rotatably borne at the end of a pin securely mounted on the first pole-piece and extending along the axis of the toroidal portion and through it so as to be substantially symmetrical therewith.

2. An indicating instrument according to Claim 1, characterised in that the pin is firmly held at its centre by a projection extending from the heel portion of the first pole-piece.

3. An indicating instrument according to Claims 1 and 2, characterised in that it also comprises a plate of ferro-magnetic material extending parallel to the second pole-piece so as to hold the first polepiece in position between them.

4. An indicating instrument substantially as hereinbefore described with reference to, and as shown in the accompanying drawing.