Circuit layout and changeover switch for braking AC commutator motor - has top-dead-centre toggle action switch with intermediate dwell position to prevent mains short-circuit by arcing at contacts during braking

摘要

The circuit for braking an AC commutator motor (6,7) as employed in electrical hand tools provides a four contact (I,II) changeover switch (4) to promote rapid deceleration by short-circuiting the motor windings at switch-off. The switch has a top-dead-centre i.e. toggle action which is largely independent of user inconsistencies during operation and provides a short dwell in the centre position (8). The changeover from device to braking condition thus incorporates a moving contact (S1,S2) travel time which is greater than the duration for inductive arcing to ensure mains isolation before motor short-circuit via the bridge (5) and limiter (2). USE/ADVANTAGE - Reliable and safe means of preventing mains short-circuit due to arc-over at switch contacts on changeover to braking. Reduces excessive commutator sparking and brush wear imposed by braking action.