Ferrite magnet type motors, which use permanent magnets, are used for the wiper motors. The wiper motor consists of the motor itself and the gears, which reduce the speed output by the motor.
The ferrite type wiper motor uses three brushes: a low-speed brush, a high-speed brush, and common brush (for ground).
A cam switch is incorporated into the gear section so that the wipers will stop at the same position every time.
(2)Switching motor speeds
A counter-electromotive force is generated in the armature coils when the motor turns, which acts to limit the speed of motor’s rotation.
When current is flowing into the armature coils from the low-speed brush, a large counter-electromotive force is generated, with the result that the motor rotates at low speed.
When current is flowing into the armature coils from the high-speed brush, a small counter-electromotive force is generated, with the result that the motor rotates at high speed.
The wiper system has a function to stop the wiper blades at the fixed position. Owing to this function, the wiper blade surely stops at the bottom of the windshield glass when the wiper switch is turned off. The “Cam switch” performs this function.
This switch consists of a notched cam plate and three contact points.
When the wiper switch is in the LO/HI position, battery voltage is applied to the circuit and current flows into the wiper motor via the wiper switch causing the wiper motor to operate.
However, the moment the wiper switch is turned off, if the contact point P2 is at a point other than the notch, the battery voltage is applied to the circuit and current flows into the wiper motor via contact point P2 to P1, causing the wiper motor to continue operating.
After that, with the contact point P2 at the notch by turning the cam plate, the current does not flow into this circuit and the wiper motor should stop.
However, because of the inertia of the armature, the motor does not stop immediately and continues turning for a little while. As a result, the contact point P3 passes through the conducting point of the cam plate and the following close-circuit is formed:
Armature → Wiper motor terminal +1 → Wiper switch → Wiper motor terminal S → Contact point P1 → P3 → Armature. Since the armature generates counter-electromotive force in this close-circuit, the electric brake is applied to the wiper motor and the motor stops without delay at the fixed position.