Patent GB-1106631-A (GB)
Title: Sound suppressor assemblies for electromagnetic induction apparatus
IPC: H02K5/24
Filed: 1966-05-11
Abstract: 1,106,631. Electromagnetic couplings; cooling. HEENAN & FROUDE Ltd. 11 May, 1966 [17 May, 1965; 28 Dec., 1965], No. 20825/66. Heading H2A. [Also in Division F1 A sound suppressor assembly includes a chamber adapted to surround the rotary elements of an electromagnetic induction apparatus, the assembly having at least one gas passage provided with an opening communicating with the interior and a first transverse wall having an aperture smaller than the opening and so spaced therefrom as to enhance the reflection of sound waves, set up in operation of the apparatus, from a limiting marginal portion of the aperture back towards the opening, to reduce the noise level of the apparatus. As illustrated in Fig. 3 (not shown), an eddy-current induction coupling has a constant-speed motor driving the inductor drum and a tachometer control generator, the rotor of which is carried by the output shaft and the stator of which is wired to control the field strength of a stationary annular field coil within the drum; a polar field member, having interdigitated pole-forming teeth, rotates around the field coil within the drum. Noise is generated by cooling fins on the end and side wall of the drum and by openings in its end wall. One side, 55, of the chamber 51, Fig. 8, surrounding the rotary elements is occupied by a box-like structure S<SP>1</SP> divided by a vertical partition perpendicular to side 55 into an inlet region (shown) and an outlet region. The inlet region is subdivided by preferably two horizontal shelves 67<SP>1</SP>, 69<SP>1</SP> into three inlet passages 73<SP>1</SP>, while the outlet region is subdivided by a single shelf into two outlet passages. Openings 83<SP>1</SP> in the side of the box adjacent the chamber provide communication for cooling air flow into (and out of) the chamber. To the exterior of side 55 is a further side 77<SP>1</SP>, spaced from it by a distance related to one wavelength of a peak frequency of the generated sound. For the outlet passages, the velocity of the outgoing cooling air is added to the sound velocity in calculating the wavelength (from the frequency) which determines the spacing. For the inlet passages the air velocity is subtracted: the resulting wavelength, being shorter, is allowed for by applying block material 95<SP>1</SP> of appropriate thickness to the inner face of the side 77<SP>1</SP>. Apertures 85<SP>1</SP> are provided in the second side and are smaller than, and aligned with, the openings of the first side. The portions around the apertures reflect outgoing sound backwardly and provide a resonant sound-cancelling effect. The second side may be the outermost side of the box; preferably, as shown, a third side 57<SP>1</SP> is provided, with still smaller apertures 81<SP>1</SP>, and is spaced outwardly from the second side by a distance related to the wavelength of a higher peak frequency. Preferably, the third side is located, instead, inside the box (Fig. 6, not shown), appropriately spaced from the second side, to enable the box to be shorter; the sizes of the openings and apertures are then adjusted to diminish outwardly, as before. In another embodiment (Figs. 11 to 14, not shown), the third side is omitted, the passages are fewer by one in each case, and the pairs of openings and apertures are not aligned; the inlet and outlet passages are equal in length.