In Figure 2, the red trace is the current in the starter motor circuit (limited to 100 A for testing). The blue trace is the solenoid current (right-hand scale). At point (1), the solenoid is actuated. First contact is made at (2), and starter current begins to rise. At (3) and (4) contact is momentarily lost again due to bounce. At (5) the solenoid armature has completed travel and is locked to the pole. At (6) the solenoid voltage is removed, and current falls rapidly. Once the magnetic field has decayed, the armature moves back toward its starting position, and at (7) the contacts are opened.
Vehicles using start/stop engine operation require up to 10 times the contact lifetime of non-start/stop vehicles (roughly 300,000 lifetime operations vs 30,000 operations). With high inrush current placing downward pressure on contact life, and start/stop operation requiring dramatic improvements, starter solenoid designs for modern applications face serious challenges.
Existing electromechanical contactors used in automotive applications will typically handle the high inrush currents without failure due to contact welding. However, the resulting contact lifetime is short, and mechanical failure of the actuator often results from the generation of large amounts of metal debris. This results in two predominant failure modes:
- Debris (mostly melted copper particles) mechanically jams the contactor.
- Contact wear is so severe that the contactor can no longer close.
The existing contactors studied had masses of between 350 g and 750 g. None of these contactors were able to achieve 30,000 cycles with an inrush current of 950 A.