US1776708A - Electric measuring instrument - Google Patents

Description

" '8. w.- sIT. CLAIR ELECTRIC umsunme 'iusrmm Sept. 23, 1930.

Filed s epf. 13,1929

Inventor. Byr-on W. St. Clair; 13

His A't'towney Patented Sept. 23,. 1.930

' UNITED STATES.

PATENT- OFFICE BYRON W. ST. CLAIR, OF LYNN, MASSACHUSETTS; ASSIGNOB TO GENERAL ELECTRIC COMPANY, A. CORPORATION NEW YORK ELECTRIC MEASURING INSTRUMENT Application fl1'ed8eptember 13, 1929. Serial No. 392,457.

My invention relates to electric measuring instruments and its principal object is to compensate alternating current instruments for variations in frequency. Another object is to provide a magnetically damped elec tric measuring instrument of improved design.

The novel features of the invention will be explained as applied to an alternating current instrument having a stationary magnetic circuit provided with an exciting winding and a magnetic vane armature. Such an instrument is suitable for use as an alternating current ammeter or voltmeter and the accompanying drawing illustrates the essentiahfeatures of an alternating current ammeter of this type to which I have applied my invention with satisfactory results.

Instruments of this character have been subject to frequency errors when employed on varying frequency circuits due to the iron losses, particularly to eddy current losses in the magnetic circuit. These losses increase with an increase in frequency and if uncom pensated, reduce the instrument torque with an increase in frequency and produce a corresponding error in the measurement. In a voltmeter the reactance of the'winding varies with frequency and produces a frequency error that maybe compensated by means of my invention. q

Eddy current losses maybe reduced by suitably laminating and. slotting the magnetic circuit and such expedients' are preferably included in my instrument but where the instrument is to be used with frequency variations, from 60 to 600 cyclesfor example, some frequency compensating feature such as the one herein described is necessary to bring the frequency error of the instrument within the allowable limit. In accordance with my invention I shunt a portion of-the instrument, flux around the magnetic vane armature and vary the shunted flux inversely with the frequency to the extent necessary to accom lish the results desired. Other features. 0 the I invention will hereinafter be explained and such features as are considered to be new and patentable will be pointed out in the claims.

appended hereto.

with a magnetic damper to steady the 'indications.

The stationary magnetic circuit is in the general shape of a U and comprises a laminated base or yoke portion 10, laminated pole piece portions 11, together with core pieces 12 joining the pole pieces 11 tothe base 10. The core pieces may also be laminated. For example, they may be formed by rolling up a sheet of magnetic material in a closely wound spiral. I have also found it helpful to slot the laminated pole pieces as shown at 22 .where they surround the core pieces 12. The energizing winding 13 is wound on thecore pieces in two sections connected in series as shown to produce a flux through the magnetic circuit and across the air gap between the pole pieces 11. In parallel with the pole pieces 11 and the armature air gap I provide a branch or shunt flux path through auxiliary pole pieces 14 integral with pole pieces 11.- There is a comparatively narrow air gap 15 between these auxiliary pole pieces and surrounding the auxiliary pole pieces 14 are short circuited coils 16 formed of a single turn of a heavy copper conductor. A non-magnetic yoke 17 is provided between the pole pieces for supporting the upper bearing of the instrument and the armature restoring spring. This yoke is broken away and the parts just mentioned are omitted from the drawing in order that the fre quency compensating shunt may. be more clearly seen. a

The movable vane armature 18 is a crescent-shaped sector 'of magnetic'.material and is mounted on the shaft 19 between the circular shaped faces of pole pieces 11. The instrument-pointer and scale are represented at 20 and 21 respectively. The armature 18 is shown in a position correspondin to a low instrument deflection and it will e underincreased the armature 18 will be moved in a clockwise direction against the torque of the control spring until the enlarged por- .stood that as the excitation of winding 13 is p tion of the armature sector is opposite the narrow portion of the air gap between the poles 11 and that'the latter position will correspond to maximum instrument deflection.

The material of which the magnetic circuit and armature is formed and the extent to which the circuit is laminated have a bearing upon the hysteresis and eddy current losses and the fre uency error of the instrument. Where it is own that the instrument will be used only on circuits having a comparatively narrow range of frequency variation a proper selection of the magnetic materials used and their proper lamination will sufiice to reduce the frequency errors to a negligible value.- Some instruments are employed on different frequency circuits or on circuits over which the frequency is varied several hundred per cent and in such cases it is necessary to employ frequency compensation and when frequency compensation is employed the choice of magnetic materials and the degree of lamination are not so im ortant since I have found that by means of my frequency compensating feature I may obtain any degree of compensation desired.

It will thus be evident that the characteristics of the magnetic materials, the degree of lamination, and the necessity-for compensation and the extent of such compensation will depend to some extent upon the use for which the instrument is intended. The cost of the instrument should alsobe considered in selecting the design.

For the magnetic vane armature I have found that an annealed alloy of iron and nickel, the nickelcontent being in the neighborhood of'78%, is satisfactory. The material to which I refer is prepared substantially in accordance with the teachings of Elmen Patent 1,586,883, June, 1926, although the commercial material which I have used may vary somewhatfrom that described in the patent. This material has high permeability and low iron losses. The poles 1] and the yoke plate 10 may be made of laminated silicon steel and the core pieces upon which the coils are wound may be made of annealed Norway iron. My experiments indicate that such an instrument without compensation will have a frequency error in the neighborhood of -1% between 30 and 120 cycles, and 8% between 30 and 600 cycles. By compensation these errors may be changed to about i5 and +1% re spectively. I do not intend to limit my invention to this particular design but give'it as one practical illustrationo the application-of my invention in a low cost instrument.

The frequency compensation of the instrument is accomplished by varying the relative magnitude of the total flux allowed to cross the armature air gap. A certain amount of flux is shunted around the armature air gap through the poles 14. Due to the reactance increase in frequency. a

The instrument is preferably provided with a magnetic damper and in order to combine this in a compact instrument without interfering with the meter accuracy I have found that certain precautions are necessary. The magnetic damper comprises a stationary permanent magnet 23 with its pole pieces 24 and 25 opposite a magnetic keeper 26 and a sectorshaped conductor 27 carried by the instrument shaft 19 between the pole pieces 24, 25 and the magnetic member 26. The shaft 19 extends through the yoke plate 10 at right angles thereto parallel with the leg or core ortions of the instrument magnetic circuit.

t willbe noted that the yoke 10 is slotted at 28 so that the instrument flux path therethrough is to the rear of the shaft as pictured and quite remote from the pole pieces of the permanent magnet. The pole pieces 12 where they join at right angles with the yoke 10 are quite remote from the permanent magnet pole pieces and what is more important they are mechanically and magnetically symmetricaL ly placed with respect to the pole pieces of the permanent magnet. The permanent magnet is evenly spaced away from the yoke 10 by a double function. It carries the instrument flux and serves as a magnetic screen between the permanent magnet and instrument.

The importance of the symmetrical arstrument pointer from zero were experienced after removal of the instrument current.

This'was caused evidently by leakage flux from the permanent magnet combining with nuts 29 at front and back. The yoke 10 serves the residual flux in the magneticcircuit of the instrument. The direction and magnitude of the residual instrument flux of course depends upon the'flux direction and magnitude when the instrument circuit is interrupted and it may be in either direction in varying amounts or it may even be zero. The resid: ual flux alone is insufiicient to cause any material departure of the instrument pointer an uncertain and troublesome zero indication from its true zero, but with an unsymmetri; 7

was experienced. After this symmetrical arrangement illustrated in the patent drawing was used this trouble was eliminated.

The permanent magnet' flux crosses from one pole piece 24 to the keeper 26, then to the pole piece 25 and then around the yoke of the permanent magnet which. is largely hidden in the illustration'by the instrument yoke 10. This damping flux thus cuts the conductor 27 which is suspended in the air gap between the poles of the permanent magnet and keeper 26 and if the conductor is moving eddy currents are produced which tend to damp the movement.

In accordance with the provisions of the patent statutes I have described the principle of operation of my invention, together with the apparatus which I now consider to represent the best embodiment thereof, but I desire to have it understood that the apparatus shown and described is only illustrative and v that the invention may be carried out by other quency comprising means for varying the proportion of the flux of saidelectromagnet which influences the armature.

2. An alternating current electric measuring instrument comprising a stationary magnetic circuit provided with pole pieces spaced apart to form an air gap, a winding on said circuit for producing a flux across said air gap, a magnetic vane armature actuated in proportion to the air gap flux, a flux path in shunt to said armature air gap, and a closed circuited conductor linked with said shunt flux path.

3. An alternating current measuring instrument comprising an electromagnet provided with pole pieces spaced apart to-form an air gap, a movable magnetic vane armature member in said air gap, auxiliary pole pieces spaced apart to form an auxiliary air gapin shunt relation to said first mentioned pole pieces and air gap, and short circuited coils surrounding said auxiliary pole pieces.

4;. An alternating current measuring instrument comprising a magnetic circuit provided with two sets of pole pieces spaced apart to form air gaps in parallel relation with each other in said magnetic circuit, a movable magnetic vane armature member in one of said air gaps, short circuiting coils on the pole pieces adjacent the other air gap, and an alternating current energizing winding for said magnetic circuit.

5. An alternating current electric measuring instrument having a magnetic circuit provided with an air gap, a movable magnetic vane armature member in the air gap, a winding on the magnetic circuit for producin atorque flux in said armature, and means %or compensating said instrument for variations in frequency comprising means for diverting a portion of the flux of said magnet around the armature air gap, and means for varying the proportion of the flux thus diverted posite side of said base portion to the armature, and a permanent magnet for producing a damping flux in said damping vane, the

poles of said permanent magnet being symmetrically disposed with respect to the leg portions of said U-shaped magnetic circuit.

7. An electric measuring instrument having a U-shaped electromagnet provided with an air gap, a movable armature member in said air gap, a magnetic damper for saidmovable armature having a stationary permanent magnet, the yoke portion-of said U- shaped electromagnet lying between the permanent magnet and the armature and the pole pieces of the permanent magnet being symmetrically disposed with respect to the leg portions of the electromagnet.

8. An electric measuring instrument having a U-shaped electromagnet, pole pieces spaced apart between the open ends of the U- shaped electromagnet toform an air gap, a movable armature in said air gap, a shaft upon which said armature is mounted extending parallel with the legs of the U and be- )yond the yoke portion thereof, a vane of conducting material on the extended portlon of said shaft, a permanent magnet spaced apart from the yoke portion of the magnetic c1rcuit adjacent the said vane for producing a damping flux therein, said yoke pbrtion forming a magnetic screen between the permanent magnet and theremainder of the magnetic circuit of the instrument. x

9. An electric measuring instrument having an electromagnet formed by a laminated plate yoke portion, core pieces extending at right angles therefrom in one dlrection, an

pole pieces spaced apart to form an air gap between the outer ends of said core pieces, a movable armature in said air gap, a shaft on which said armature is mounted extending through the yoke portion of said electromagnet, a dam ing vane on the extended portion ofsaid shaft, a permanent magnet energizing winding on said core pieces, and

spaced apart from said yolk portion on t e side opposite said core pieces for producing a damping flux in the vane, the pole pieces of the permanent magnet being sym- 5 metrically placed with respect to the core pieces and the yoke forming a magnetic screen between the permanent magnet and measuring instrument.

10. An electric measuring instrument having a U-shaped magnetic circuit, a movable armature between the open ends of said U- shaped circuit, an energizing winding on the legs of the U-shaped circuit, the base or yoke portion of the magnetic circuit comprising a plate of inagnetic material, and a permanent .magnet damper for said instrument on the opposite side of said plate from said armature, the permanent magnet of which is symmetrically disposed with respect to the leg portions of the U-shaped magnetic circuit. I

11. An alternating current measuring instrument having a U-shaped electromagnet, a

magnetic vane armature between the open ends of the U-shaped electromagnet, an energizing winding on the legs of the U-shaped electromagnet, a permanent magnet adjacent the base or yoke portion of the U-shaped electromagnet and on the side 0 posite from the magnetic vane armature, sai d permanent 3o magnet forming a part of a magnetic damper for the instrument, the pole pieces of the permanent magnet being symmetrically placed with respect to the leg portions of the electromagnet. 12. An electric measuring instrument comprising a magnetic circuit provided with an 'air gap, the magnetic circuit being of a' U shape with the air gap between the open ends of the U, the base or yoke ortion of 40 the circuit comprising a laminate plate, an

energizing winding on the leg portions of the U-shaped magnetic circuit, a movable armature between the pole pieces, a shaft on which said armature is mounted extending through the yoke plate at right angles thereto, a

damping vane carried on the extended portion of the shaft, a permanent ma net secured to the yolk plane and space therefrom in a parallel plane on the side away so from the instrument, said permanent m et cooperating with the damping vane to orm a magnetic damper for the instrument, the

pole sieces of the permanent magnet being space awa from the flux path of the instrumentt roughthe yoke but the yoke having extensions toward the permanent magnet poles and formin a flux screen between the instrument an damper, the pole 4 pieces of the permanent magnet being symmetrically s aced from the leg portions of "the U-shape magnetic circuit of the instrument. I

In witness whereoi I have hereunto set my hand this 10th day of Se tember, 1929. BYRON W. T. CLAIR.

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