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Sophisticated Starter Contactors

October 8, 2020

Figure 1

Generic Starter Relay
Generic starter delay

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 (Figure 1).

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.

Figure 2

Contact Bounce
Contact bounce

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.

Meeting the Technical Challenges

Target electrical and mechanical performance specifications for a high-current contactor capable of operating in start/stop mode for a high-performance ICE would include the following:

  • Inrush handling of 950 A
  • Steady-state current handling of 200 A
  • Contact and mechanical lifetime of 300,000 to 500,000 closures
  • Mass of less than 350 g

Contact material and geometry are the keys to achieving both long contact life and high inrush handling. The materials of the mating contacts in extant automotive solenoids are usually pure copper. High contact life and high inrush handling need a material which would limit the arcing damage, reducing wear and debris. Such materials have the effect of increasing contact resistance to some degree. Increasing contact resistance would imply an increase in the heat generated within the relay, so this would have to be considered.

Figure 3

Internal Power Dissipation vs Resistant and Current
Internal power dissipation versus resistance and current

The internal resistance of the relay is dominated by the contact resistance. In testing of the high-performance contact materials, a 150% increase in contact resistance was noted, which implied an increase of 150% in dissipated power. Figure 3 shows the internal power dissipation as a function of steady-state current for various internal resistances. The typical contact resistances for copper and high-performance composite contacts are shown. At 200 A current draw, power dissipated rose from 16 W to approximately 26 W.

Of greater concern was the heat generated during the inrush spike. Resistive heating increases as the square of the current. An inrush current of 950 A across a resistance of 0.62 milliohms generates 560 W of resistive heating. The very short duration of this spike was found to contribute less than 1% of the dissipated heat energy over a 2-second start cycle, therefore inrush heating was not found to be a significant factor in this specific contactor design.

Evaluation of contact wear for the composite material versus copper was conducted using 950 A inrush current and steady-state current of 180 A. Based on observed wear rates, the contact life of the composite contactor was estimated to be > 250,000 cycles.

The life cycle of the tested contacts was therefore in the range required for start/stop operation while handling the very high inrush currents in a lightweighted system. Debris generation was dramatically reduced (> 95%), but for long-life contactors, debris management tactics such as isolation of the solenoid and contactor and/or provision of debris traps in the contactor would be desirable.

Figure 4

High Peformance Starter Contactor
High-performance contactor design

With contactor failure modes adequately addressed, solenoid mechanical life becomes the limiting factor in the lifetime of the starter relay assembly. Low-cost solutions used for non-start/stop applications generally do not have the mechanical life for start/stop operation, however mitigations such as lubrication, hard/high-lubricity coatings, or high-lubricity sleeves can dramatically improve performance. The cost penalty for a properly implemented solution is typically moderate, about 5% to 10% of the total cost of the contactor.

A final challenge for modern starter relays is lightweighting. Extant high-current relays have masses ranging from 300-750 g, with 350-400 g being typical. Targets for lightweighted designs were roughly 50% of these masses.

Case Study: A Lightweight, High-Performance Starter Contactor

A key customer required a starter contactor needing the following key characteristics:

  • 950 A inrush handling
  • 175 A steady-state current handling
  • 30,000 cycle durability
  • 1 meter/5 minute water immersion survival
  • Replace an incumbent part weighing 120 g

The incumbent part failed due to contact welding at inrush currents above 600 A and had no water immersion protection. The incumbent part utilized a copper contact system, and the high-risk failure mode of contact welding was realized during early testing with the actual vehicle powertrain. Development of an improved contactor to meet the key requirements while maintaining a minimum part mass was undertaken.

The resulting design (Figure 4) entered production in mid-2020 in an on-road recreational vehicle. The mass of the production design is less than 160 g and meets the target performance requirements. Current-handling performance meets or exceeds the specification of contactors weighing more than four times as much.

This article was originally published by International Design Engineer in October 2020

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