| 摘要 |
1,243,958. Control of A.C. motors. SULZER BROS. Ltd. 5 Feb.. 1970 [6 Feb., 1969], No. 5597/70. Heading H2J. [Also in Division G3] A servo system comprises a reversible induction motor which adjusts valve 5 through gear or hydraulic transmission, the valve spindle being coupled to a position transducer 7 supplying a signal to logic circuit 10. A comparator 44 receives the signal i as well as a reference signal s and forms a deviation signal a which is amplified to provide signal b. A transmitter 46 receives the signal and a preset signal e which adjusts the limit G1 required for the appearance of an output signal c for switching on the motor. A further correcting signal d depends upon the length of time the motor has been switched on to allow for the inertia of the moving parts. It is triggered by the signal c which is inverted and integrated. The sum of the signals b and d decreases in accordance with the duration of the signal c until the motor is switched off at the limiting value G2. The switching off time of the motor is displaced as the length of time the motor has been switched on increases. The signal c is applied through AND gate 49 to store 53 associated with AND gates 54, 55 for controlling SCR switches 57. 58 through trigger circuits 56, 59. The sign of the signal a determines which of the switches 57, 58 will be triggered. A D.C. source 25 is provided for braking the motor, the said source being connected to the stator winding when the A.C. supply is removed The D.C. voltage is reduced when the motor comes to a stop. A current measuring network 15 provides a signal for switching off the motor when a current limit is exceeded. The current limit is increased for a predetermined period when the motor is being started, this period being prolonged if the valve is near its closed position. If an over current is indicated when the valve is not near its closed position, an alarm is given. A limit 30 compares the instantaneous voltage in the supply lines S and T and provides a signal m for switching the motor on or off only if the instantaneous voltages are equal and positive. |
