Patent GB-666872-A (GB)
Title: Improvements in or relating to speed governors for a motor and an electric motor employing said speed governor
IPC: H02K23/66
Filed: 1948-10-05
Abstract: 666,872. Centrifugal governors. STEVENS, A. H. (Birtman Electric Co.). Oct. 5, 1948. No. 25950/48. Class 57. [Also in Groups XXII, XXXV and XXXVII] A speed governor for a motor 10 having a shaft 19 (Figs. 1, 4, 6) comprises a rotatable plate 12 attached to the shaft, a plurality of arms 13 pivoted at 40 to the plate, a plurality of levers 14 each contacting one of the arms at one end and each arranged to move - about a pivot 37 when centrifugal forces cause the arms 13 to move outwardly and further levers 15, at right angles to and contacting the levers 14 at the other ends for breaking electrical contact to the motor when the centrifugal forces exceed a certain predetermined value. The levers 15 are pivoted so that upon actuation of one arm 17 of the levers by the levers 14, the other arm 18 engages a thimble 20 on the shaft 19 to move it along the axis of the shaft to engage a button 36 on contact piece 25. Contact piece 24, adjustable by means of a plate 29 and knob 31, has a hole to allow passage of the shaft 19 and thimble 20 therethrough. The contact pieces are hinged together at the upper end and spring biassed. Thus when the speed of the motor exceeds a certain predetermined value the arms 13, levers 14, and levers 15 push thimble 20 along the shaft to engage the button 36 thereby pushing the contact piece 25 away from the contact piece 24 and breaking the circuit of the motor, previously closed by contact 23. The contacting edge of the arm 13 is shaped as a section of parabola so that the point of contact between arm 13 and lever 14 runs along a line joining their pivots, and it is shown that with this construction the end of the lever 14, not in contact with the arm 13 moves equal distances for equal changes in speed. In a modification (Figs. 9 and 11) the contact pieces 127, 130 have attached to the lower ends two extension pieces 124, 131 on which are mounted two pairs of contacts 125, 133 and 126, 134. The extension 124 bearing the contacts 125, and 126 is rigidly connected to the contact piece 127 whereas the extension 131 bearing the contacts 133 and 134 is formed in two parts pivotally mounted on the contact piece 130 at 132. The two ends of the extension pieces are connected by springs 135, 136 of different strength so that upon actuation of the contact piece 130 by the thimble 123 one pair of contacts are broken before the other. In order to ensure smooth operation the first pair of contacts to break are adapted to place in series with the field windings either more field windings or a resistance and the second pair of contacts break the circuit to the motor. As shown in Figs. 15 and 16 manual switches 146, 165 are provided for short circuiting part of the impedance inserted when the first pair of contacts 133, 125 open and are opened when the motor is to operate on no-load or on very light loads. The speed controlled motor may be applied to a variable speed electric drill (Fig. 17). The motor is supplied through a centrifugally operated contact 227 and contact 228 set in position by a movable trigger 234 against the action of a spring 231. The contact 227 is operated by levers 221 and thimble 222 as in Fig. 1. The higher the axial position of the trigger, therefore, the greater the speed at which the motor will be controlled. The trigger may be set in any desired position by lever 236 co-operating with the serrated edge 235 of the trigger.