| 摘要 |
A reversible d-c motor, driving a load such as a vehicular radio antenna, has its armature winding connected in a transistorized energizing circuit which is activated by a start circuit and locks operated as long as the motor speed surpasses a certain threshold, i.e. until a major obstacle or a terminal position is encountered. The threshold may be determined by a voltage sensor detecting the counter-e.m.f. induced in the armature winding, by a Hall-effect device in the magnetic field of the stator, or by a speed sensor. The armature winding lies in series with a current limiter which determines the maximum torque applied to the load and may also transmit a no-load signal to a protective circuit when the motor is found idling, the protective circuit including a timer which cuts off the motor after a predetermined idling period. Alternatively, the operating period may be determined by a pulse counter de-energizing the motor after a preferably settable number of rotor revolutions. |
