| 摘要 |
1295941 Control of DC motors SINGER CO 14 Jan 1970 [21 Jan 1969] 1771/70 Heading H2J A D.C. motor is energized from an A.C. supply via a drive S.C.R. and is dynamically braked by connecting it to a dissipative load via a brake S.C.R., and means are provided for preventing simultaneous conduction of both S.C.R.'s comprising means for delaying operation of the brake S.C.R. for a predetermined time after the termination of a stop command signal, and a monostable device responsive to triggering of the brake S.C.R. inhibits triggering of the drive S.C.R. When treadle T is in its relaxed position, motor 10 is energized through S.C.R. network 31 since trigger-line 32 is connected through resistor 33 to voltage-line 26. Capacitor 38 is short-circuited through contact 41 whereby brake trigger-line 42 is not energized, a monostable multivibrator 46 being in its stable state so that switching transistor 35 is cut-off. To stop the motor, the treadle is moved and switch 30 engages contact 53. The line 26 is earthed to remove voltage from trigger-line 32, the S.C.R.'s 14 remaining conductive until the end of the respective half-cycles of power voltage. To prevent premature firing of the brake S.C.R. 45, the voltage on trigger line 42 is increased at a rate determined by the charging of capacitor 38 through resistor 39. When diode D breaks down, S.C.R. 45 is triggered to connect braking resistor 44 across armature 11. The time-delay is chosen to be longer than the maximum time for the S.C.R. network 31 to turn off. The S.C.R. 45 is rendered non-conductive when the back EMF of the motor reaches a low value. To prevent the S.C.R. network 31 from being turned on, even though commanded to do so, before the brake S.C.R. 45 has turned off the trigger pulse from the threshold diode D is also applied through diode 52 to the multivibrator 46 which changes state to render the transistor 35 conductive. This earths the line 32 and inhibits triggering of the S.C.R. 31 for a pre-set period. Resistor 33 and capacitor 34 prevent the sudden application of full voltage to the line 32, so protecting the S.C.R. 31. The voltage on line 26 is increased in steps by operating switches 27, 28, 29, in sequence under the control of the treadle. |
