| 摘要 |
1,181,646. Control of D.C. motors. SINGER CO. 26 Nov., 1968 [1 Dec., 1967], No. 55996/68. Heading H2J. [Also in Division G3] In a speed control system for a permanent magnet type D.C. motor comprising a silicon controlled rectifier 17 in series with the armature 20 of the motor across the full-wave rectified output 15, 16 of an A.C. supply 33, speed variations caused by supply voltage variations in a range above a predetermined minimum value are prevented by a circuit comprising members 22, 23, 26, 31, 32 in which the predetermined minimum value is set by the breakdown voltage of the Zener diode 32. At this point, the transistor 26 is substantially cut-off so that it has a negligible shunting effect across the voltage divider 22, 23 which supplies a triggering voltage from tap 24 determined by the voltage at terminals 15, 16 and the back E.M.F. of the armature 20. With increase of supply voltage from the predetermined minimum value, the transistor 26 is driven into conduction to alter the impedance of the divider 22, 23 and so alter the firing angle of S.C.R. 17, the effect being to cause the rectifier 17 to fire later in the cycle to compensate for the increased supply voltage.
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