An electromagnetic shield for alternating current induction railway motors

Abstract: An electromagnetic shield for an AC motor comprises a stack including a plurality of ferro-magnetic shield layers. Each one of the plurality of shield layers is at least partially electrically insulated from and substantially aligned with the others of the plurality of shield layers. The plurality of shield layers are positioned over a portion of the AC motor near an area where an axle will be positioned.

Description: This invention relates generally to alternating current (AC) motors useful with AC traction motors in railroad or transit propulsion applications having AC voltages induced in the axles, for example. In AC railway propulsion motors, the close proximity of traction motors to railway axles results in AC voltages which are induced in the axles and which can adversely affect railway signal systems. Accordingly, the present invention seeks to reduce electromagnetic (EM) radiation and electromagnetic interference (EMI) which is radiated from alternating current (AC) traction motors in railroad or transit propulsion applications. Generally the needed protection is for frequencies in the 0 to 3 kHz range in contrast to more typical shielding requirements in the giga-Hz and mega-Hz frequency ranges. According to the invention, there is provided an electromagnetic shield for an AC motor, comprising a stack comprising a plurality of ferro-magnetic shield layers, each one of the shield layers at least partially electrically insulated form the others of the plurality of shield layers, each one of the plurality of shield layers being substantially aligned with the others of the plurality of shield layers; and positioning means for positioning the plurality of shield layers over a portion of the AC motor near an area where an axle will be positioned. Each of the shield layers may be at least partially electrically insulated from the others of the plurality of shield layers and has at least two openings. Each of the openings of one of the plurality of shield layers may be substantially aligned with corresponding openings in others of the plurality of shield layers to form a set of aligned openings. At least two fasteners may be provided with each fastener extending through a respective set of aligned openings and into a corresponding opening in the AC motor for fastening the plurality of shield layers over a portion of the AC motor near an area where an axle will be positioned. The invention will now be described in greater detail, by way of example, with reference to the drawings in which: FIG. 1 is a sectional side view of an AC motor and an axle having an electromagnetic shield of the present invention there between; FIG. 2 is front view of one of the shield layers shown in FIG. 1; and FIG 3 is a side view of the shield layer of FIG. 2 further showing a coating of non-magnetic material. FIG. 1 is a sectional side view of an AC motor 10 and an axle 12 having an electromagnetic shield of the present invention there between. The electromagnetic shield includes a stack of a plurality of shield layers 18. In one embodiment, the stack includes five shield layers. Each shield layer comprises a ferro-magnetic material such as low carbon electric motor laminate steel or transformer laminate steel, for example. In one embodiment, each shield layer has a thickness of about 0.356 mm (0.014 inches). Each shield layer is at least partially electrically insulated by a non-magnetic material from the other shield layers. In one embodiment, the insulation is provided by at least partially coating the shield layers with a thin coating of electrically insulative, non-magnetic material, shown as thin coating 38 in FIG. 3, so that the ferro-magnetic materials of adjacent shield layers are not in substantial contact. In this embodiment, the thickness of the thin coating ranges from .025 to .05 mm (1 to 2 mils). The thin coating may comprise, for example, inorganic varnishes that are conventionally already coated on both sides of stator and rotor laminations. Although in the preferred embodiment the thin coating is on both sides of each shield layer, the thin coating may alternatively be coated only on a single side of each shield layer. In another embodiment, an insulation layer of electrically insulative, non-magnetic material is inserted between each of the shield layers. The insulation layer may comprise a material such as an adhesive, for example. Complete electrical isolation of shield layers is not necessary. The thin coatings or insulation layers are necessary, however, to increase the resistance between the shield layers and to create eddy currents, which lead to magnetic fields operating in the opposite direction of the fields induced in the axle, to reduce the resulting electromagnetic field. The shield layers are bent to fit in the area between AC motor 10 and axle 12. In one embodiment, each shield layer is individually bent before the shield layers are positioned adjacent each other in the stack. Each shield layer has approximately the same shape as the others with adjustments made so that the shield layers can be positioned adjacent each other in the stack. The shape of the bend of the shield layers shown in FIGs. 1 and 3 is for example only. Any appropriate shape which allows positioning of the shield layers between motor 10 and axle 12 can be used. Shield layers 18 can be held in position with any appropriate positioning means. In one embodiment, the shield layers are held in place on first and second shield support brackets 14 and 16 with first and second mechanical fasteners 28 and 30 and first and second clamping plates 20 and 22. In this embodiment, each shield layer has at least two shield layer openings 32, each support bracket is a part of motor 10 and has at least one motor opening 32a, and each clamping plate overlies a portion of the shield layers and has at least one plate opening 32b. The mechanical fasteners can be inserted through the plate and shield layer openings and into the motor openings to position the shield layers. The mechanical fasteners may comprise bolts and the material of the mechanical fasteners may comprise a metallic material, such as stainless steel, for example, or a nonmetallic material, such as nylon. In one embodiment, the support brackets may comprise mild steel, for example, and can be welded to the motor frame. The use of two shield support brackets is for purposes of example only. In alternative embodiments, a large single support bracket can be welded to the motor frame or more than two supports brackets could be positioned on the motor frame. The clamping plates can be non-magnetic if they are situated outside of the shield layers, as shown in FIG. 1. Although not required, the clamping plates have advantages over the use of a fastener alone or a fastener with a washer because the clamping plates help to protect the shield from damage due to impact from foreign objects. Preferably, first and second plastic edge guards 24 and 26 are present on edges of the respective first and second clamping plates to improve the seal between the clamping plates and the shield layers. In another embodiment, the shield layers are held in position with a weld. The weld can be formed in any conventional manner, such as, for example, using welding material between the motor and shield layers under appropriate temperature and pressure conditions. In a related embodiment, an adhesive bond, such as a glue or an epoxy which can withstand high temperature applications, is used. The stack of shield layers need not extend completely around the motor or axle, but need only subtend an arc of the motor in the area of the axle which is big enough so the motor's own ferro-magnetic structure is adequately shielded. Furthermore, the shield layers do not need to extend the entire length of the motor. FIG. 2 is front view of one of the shield layers 18 with openings 32, and FIG 3 is a side view of the shield layer of FIG. 2 with an exaggerated thickness along lines 3-3 further showing a coating of non-magnetic material 38. In a preferred embodiment, the coating comprises a material with a lower electrical conductivity than the conductivity of the shield layers and the coating does not extend over the portions of the shield layers surrounding the openings 32. A notch 34, which is shown in FIG. 2, is present in one embodiment to leave an area free for placement of motor leads near the axle. One convenient feature of the shield is that it can be installed on a motor which has already been attached to a gear box and mounted on an axle. This eliminates the costly process of removing the motor from the gear unit and axle to install the shield and then remounting it. Modifications are possible within the scope of the appended claims.

Claims: An electromagnetic shield for an AC motor (10), comprising: a stack comprising a plurality of ferro-magnetic shield layers (18), each one of the shield layers at least partially electrically insulated form the others of the plurality of shield layers (18), each one of the plurality of shield layers (18) being substantially aligned with the others of the plurality of shield layers (18); and positioning means (20,22,28,30) for positioning the plurality of shield layers (18) over a portion of the AC motor near an area where an axle (12) will be positioned. The shield of claim 1, wherein each of the shield layers (18) is at least partially coated by a respective thin non-magnetic, electrically insulative coating layer (38). The shield of claim 1, wherein the stack of shield layers (18) includes a plurality of insulation layers (38), each one of the plurality of insulation layers situated between respective pair of shield layers (18). The shield of claim 1, 2 or 3, wherein the AC motor comprises an AC induction railway motor. The shield of any preceding claim, wherein each of the shield layers (18) has at least two shield layer openings (32), each of the shield layer openings (32) of one of the plurality of shield layers (18) being substantially aligned with corresponding shield layer openings (32) in others of the plurality of shield layers (15) to form a set of aligned openings: and wherein the positioning means includes at least two fasteners (28,30), each fastener extending through a respective set of aligned openings and into a corresponding motor opening (32a) in the AC motor for fastening the plurality of shield layers (18) over a portion of the AC motor near an area where an axle will be positioned. The shield of claim 5, wherein the AC motor (10) includes at least one shield support bracket (14,16) and wherein the motor openings in the AC motor are located in the at least one shield support bracket (14,16). The shield of claim 4, wherein the positioning means comprises a weld of the shield layers to the motor. The shield of claim 4, wherein the positioning means comprises an adhesive bond of the shield layers to the motor. The shield of claim 1, wherein each one of the shield layers (18) is at least partially coated by a respective thin electrically insulative coating layer (38) and has at least one shield layer opening (32), the shield layer opening (32) of one of the plurality of shield layers (18) being substantially aligned with the shield layer openings (32) in others of the plurality of shield layers (18) to form a set of aligned openings: and said positioning means includes at least one fastener (28,30) extending through the set of aligned openings and into a motor opening (32a) in the AC motor for fastening the plurality of shield layers (18) over a portion of the AC motor near an area where an axle (12) will be positioned. The shield of claim 9, wherein the AC motor (10) includes at least one shield support bracket (14,16) situated on the motor near which the axle (12) will be positioned and wherein the motor opening (32a) in the AC motor (10) is located in the shield support bracket (14,16), and wherein the fastener includes at least one clamping plate (20,22) overlying a portion of the shield layers (18) and having at least one plate opening (32b) through with the fastener (14,16) extends.

Inventors: STACY LEN ERIC, WNUKOSKI GEORGE J, STACY, LEN ERIC, WNUKOSKI, GEORGE J.
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1995
Main classification: H02K5
IPC codes: H02K11, H02K5