GB328661A - Improvements in electric motors - Google Patents

Abstract

328,661. Hoseason, D. B., and Associated Electrical Industries, Ltd. Nov. 5, 1928. Motors, cooling and ventilating; coils; insulation; cores, yokes, and carcass structures.-The output per unit weight of induction motors of the type in which the cooling is effected by external air passing through axial ducts in the cores is increased without exceeding a permissible temperature rise by a number of correlated features of design comprising (i) means for increasing the cooling air velocity to high values defined in the Specification, (ii) the employment of high-current density values, also defined in the Specification, together with methods of arranging and insulating the conductors in the slots such that the conductor heat is rapidly conveyed through the adjacent core metal to the cooling ducts, and (iii) core and frame constructions such as to give an increased heat conduction and radiation. The product of the air velocity in feet per cu. in. and the duct area in sq. ft. has a minimum value of 3000 per kw. to be dissipated and the slot insulation has a thermal conductivity not less than 0.001 watts per cu. in. per degree cent. difference between the opposed faces. The limits beyond which the output per unit weight is increased are defined by curves such as C, Fig. 13, which shows the minimum co-ordinates for motors made according to the invention. The curve D shows a typical, but not binding, upper limit and the curves A, B show for comparison the range within which known motors of standard make fall. Similar curves are given showing the limiting relationships between the motor horsepower and the stator current density and the pressure of the cooling air employed in machines running at given speeds and made according to the invention and the Specification describes methods for deriving similar curves for machines running at different speeds. Fig. 1 shows a slip-ring motor with the stator laminae exposed to the air and secured in a frame, as shown in Fig. 12, by end-rings 13 comprising discs 53, Fig. 8, having perforations 54 coincident with the stator air ducts 20 and having welded radial fingers 55 to keep the bunched ends of the stator coils from occluding the ducts. The rotor laminae have air ducts 23, the total area of which is less than that of the stator, so as to proportion the cooling in accordance with the heat generated in the two members, and they are clamped by end-rings of the form shown in section in Fig. 11. These comprise a frusto-conical hub 57 which has its smaller face in contact with the core and is connected by narrow spokes 60 to a substantially frusto-conical rim 61 provided with fingers 62 for bearing on the laminae. A fan 31, drawing air into the motor at 38 through the core ducts, and expelling it at the opposite end 37, has deep radial blades 33 shrouded at 34, 35 and an annular baffle 39 may be used to assist in finding the air over the end windings. No air passes through the rotor spider. The frame, Fig. 12, consists of end-rings 63, 64 of angle section united by narrow T-bars 65 against which the stator laminae bear and one of which is grooved at 70 for the key of the laminae. In place of a skeleton frame a " solid " frame may be used with the stator laminae making good thermal contact with its inner periphery. The conductors are of such form and so disposed in the slots as to convey as much of their heat as possible directly to the slot wall through a minimum thickness of insulating material and without passing through adjacent conductors. Fig. 6 shows an arrangement having two tiers of flat conductors each having one major face exposed to the slot wall,. and such that the areas so exposed are as nearly as possible equal for all the conductors. Alternatively, the conductors may be arranged one above the other ribbonwise in one or two banks. The heat-conducting capacity of the insulation 44, 46, 49 is made as high as possible by vacuum impregnation with a material of high thermal conductivity to the exclusion of substantially all air. In the case of coils consisting of a number of circular wires the spaces in the slot not occupied by the conductors are similarly completely filled with an insulating material of high thermal conductivity. The Specification contains examples of the dimensions and output of motors made according to the invention.