Patent GB-604816-A (GB)
Abstract: 604,816. Electromagnetic ammeters and voltmeters; power meters. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Dec. 21, 1945, No. 34678. Convention date, Dec. 27, 1944. [Class 37] A moving coil instrument is so arranged that the movable system can be withdrawn without disturbance of the magnetic system. As shown in Fig. 2, the two sides 34, 36 of the moving coil 28 move in the airgaps 8, 10 between the polepieces 4, 6 of the electromagnet 12 and a magnetic core consisting of two parts or inner polepieces 24, 26. These are separated so as to leave a gap through which the coil 28 may be inserted and withdrawn wnen it nas been rotated into the position shown in dotted lines. In order to avoid the increased reluctance due to the airgap between the parts 24, 26, the inner polepieces 38, 40, Fig. 3, corresponding thereto, may be connected by a magnetic conductor 42 arranged so as to be clear of the moving system when this is turned into the dotted-line position for removal or replacement. In a modification, Fig. 4, the polepieces 4, 6 are connected to the opposite internal polepieces 40, 38 by a magnetic conductor 46. The flux in the air gap 8 is in the same direction as in Figs. 2 and 3, but that in the gap 10 is reversed so that if a single moving coil were employed the torques applied to the coil would be in opposite directions. A pair of moving coils 50, 52 are therefore provided, one moving in each of the gaps 8, 10. They may be connected so that their torques are either cumulative or differential. In the construction shown in Fig. 5, the air gaps 8, 10 are in parallel, the polepieces 4, 40 being connected magnetically to one end of the yoke 62 and the polepieces 6, 38 to the other end. The flux in the air gaps 8, 10 is in the same direction as in Figs. 2 and 3, so that a single moving coil 16 may be employed. In a modification, Fig. 6, the outer polepieces 4, 6 are connected to one end of the yoke 68 and the inner polepieces 38, 40 to the other end, so that the air gaps 8, 10 are in parallel but have the flux directions through them reversed. In the construction shown in Fig. 7, each of the air gaps 8, 10 forms part of an independent magnetic circuit. By reversing the excitation of one circuit the moving system may include two' coils and may serve as a ratiometer by tapering the airgaps 8, 10. When two moving coils are employed, they may be connected in series or parallel and either aiding or opposing, or they may be separately brought out to terminals for independent connection. An instrument in accordance with Fig. 7 is shown in detail in Fig. 16. The rotor assembly comprises a coil 3 secured to a shaft 9 mounted in bearings 11, 13. Connections are made through flexible strips 15, 17. Also attached to the shaft 9 are a damping disc 35, an arm 39 carrying a pen 41, and a control spring 31. The stator assembly is formed in two parts 51, 67 with separators 79, 79a between them. The part 51 is formed with main polepieces 55, 57 corresponding to 4, 6, Fig. 7, and inner polepieces 59, 61 corresponding to 38, 40, Fig. 7. Air gaps in which the coil 3 moves are formed between the faces of the polepieces 55, 59 and 57, 61. The polepieces 59, 61 are spaced apart to form a passage 63 through which the coil 3 may be withdrawn. The fluxes passing through the two air gaps are generated by coils 47, 49. Each of the polepieces 59, 61 tapers from a large cross-section adjacent to the rim portion 53 to a smaller cross-section at the tip and accordingly a torque due to solenoid action tends to rotate the coil 3 clockwise. The second portion 67 of the stator assembly is formed with main polepieces 73, 75 and inner polepieces 69, 71 which are arranged on the opposite side of the shaft 9 to -the corresponding polepieces 61, 57 so that the torque due to solenoid action tends to rotate the coil 3 counterclockwise, thus counteracting the torque due to the portion 51 of the stator. The coil 3 is inserted by aligning its lower side with the passage 63 and passing it through the stator portion 51, then rotating the coil until its lower side is in alignment with the passage 77 in the stator portion 67 and lowering until the lower side of the coil has passed through the stator portion 67, the coil then embracing the polepieces 59, 61, 69, 71 of the complete stator. The spacers 79, 79a space the portions 51, 67 apart sufficiently to permit rotation of the lower side of the coil 3 between them during this operation. A twoelement instrument may be similarly constructed with two moving coils spaced axially along the shaft 9 and two complete stator assemblies. The vertical spacing is sufficient to allow one coil to be passed through the upper assembly into the space between the two assemblies during insertion of the moving system. An insulator for the fixed windings 47, 49 comprises two U-shaped parts 125, 127, Fig: 20, which are similar in construction, but are inserted from opposite faces of the magnetic structure 45. The part 125 has two tubular legs 125a, 125b which project and surround portions of the winding 47. Similarly, the part 127 has tubular legs 127a, 127b which engage the corresponding parts 125a, 125b with spigot and socket joints. The exposed ends of the parts 125, 127 are left open to facilitate winding the coil 47 therein.