| 摘要 |
724,867. Control of D.C. motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. June 23, 1953 [July 26, 1952], No. 17340/53. Class 38 (3). Dynamic braking operation of a traction motor is provided by connecting a fixed resistor Rb across the armature of the motor and exciting the field winding MF from a variable voltage generator G. An over-voltage relay 6' is connected across the motor armature through an adjustable resistor R1 and is adapted to close contacts 6" for energizing a protective or indicating circuit when the braking- current exceeds a permissible value. A second protective relay 7, which is responsive to both motor speed and current, over-rides the relay 6 at high speeds, so that the protective circuit is rendered effective at lower values of the braking- current when the motor is working under unfavourable conditions of commutation. The relay 7 has an actuating coil 13 energized in accordance with the motor armature current, and an opposing coil 16 which is excited in response to the motor field current. In addition, a second opposing means, providing a strong, substantially constant biasing-force, consists of a tension-spring 17, this biasing-force being intermediate in value between the maximum and minimum values of the opposing force exerted by the coil 16. An advantageous response characteristic of the relay 7 is afforded by making the magnetic circuit 15 for the opposing coil become saturated before the magnetic circuit 12 for the operating coil, within the range of motor armature and field currents in which the differential relay is actuated, but the overvoltage relay 6 is not simultaneously actuated. |
