Starter Relay Overview
A starter relay, sometimes referred to as a starter solenoid, is used to switch the high current needed to turn the starter motor on an internal combustion engine. The starter relay comprises a set of high-current contacts and a linear actuator (solenoid) to move one or more of the contacts to make or break the circuit.
When actuated, the solenoid armature moves to the right, removing the spring-applied force that keeps the contacts open, closing the circuit. When de-energized, the solenoid return spring reapplies the opening force, opening the circuit.
Market Drivers and Associated Risks
Market changes, such as the need for start/stop functionality, are requiring improvements in starter relay performance—namely a much longer operating life (more operating cycles). High-compression engine designs may require higher currents for starting. These changes challenge existing high-current contactor designs. Contact degradation speeds up dramatically as current is increased. The resulting debris can affect mechanical operation of the switch, limiting component life.
Starting current is highest immediately upon closure. This inrush current peak can be more than five times the steady-state current during the start sequence. The advent of higher-compression over-square engines (bore larger than stroke) raises the torque that must be delivered, and therefore the current. Lightweighting has resulted in changes to the configuration of starting systems to remove excess cable length and reduce overall resistance of the starting circuit outside of the motor. Reduction of this series resistance raises the peak current. In one case, inrush current for a 1.3-liter ICE reached more than 900 A, a value 50% higher than expected for the starting system. It was discovered that the main driver for the high inrush current was the low series resistance.
High inrush current also drives the risk of contact welding. During high-current closure, there is always some contact welding due to arcing and contact bounce (see Figure 2). If enough melted area is present when the contacts close, the weld formed may be strong enough that the contacts will not open when the contactor is released. Raising the inrush current dramatically increases the risk of this dangerous failure mode.
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.