| 摘要 |
A valve actuator is constructed to drive a valve member into either of two end positions, into one of which the valve member is biased by spring means, or a mid-position. The valve actuator includes an AC synchronous motor in which one of the rotor or stator windings is modified so that the output torque of the motor can be switched between a value in which it overcomes the return force of the spring means at any position of the valve member and a value which exactly balances the return force of the spring means at the mid-position. As illustrated the modified rotor or stator winding comprises two windings (18, 20) which are switched by means of a microswitch (12) between a first arrangement in which they are connected in parallel to provide maximum torque and an arrangement in which only the winding (20) is connected to the power supply to provide the reduced torque. <IMAGE> |
