starter motor
An electrical motor, part of a modern car or truck, etc., which is used to initially turn the internal combustion engine of the vehicle to make it start.
starter motor: electric motor that spins your engine to life
A starter motor is an electric motor mounted on or near the engine block that engages with the flywheel (or flexplate on automatics) to crank the engine into motion. Its sole job is to turn the crankshaft fast enough, typically 200 to 300 revolutions per minute, for the engine to draw in fuel and air, compress them, and ignite. Once the engine fires and runs under its own power, the starter disengages and plays no further role until the next ignition cycle.
Modern starters are direct-current (DC) motors drawing 150 to 250 amps from the battery at 12 volts (24 volts on heavy trucks and diesel engines). The motor drives a pinion gear that meshes with teeth on the flywheel rim. A solenoid, energized by the ignition switch, both engages the pinion gear and closes the main power contacts to the motor simultaneously. The pinion must disengage immediately once the engine fires; if it does not, the spinning flywheel will drive the pinion backward and destroy the starter in seconds.
Starter failures fall into two categories: mechanical and electrical. The pinion may stick on the shaft, or the one-way clutch (overrunning clutch) that prevents backdriving may wear and allow the motor to be driven backward by the engine. Electrical failures include burnt solenoid contacts, failed field coils, or dead spots on the commutator where carbon brushes no longer make contact. A weak battery will not supply enough current to turn the motor fast enough to start the engine, and the starter will click repeatedly but not crank.
Starter motors on passenger cars weigh 2 to 4 kilograms; heavy-duty diesel engines use units up to 8 kilograms. Reduction-gear starters employ an internal gearbox to step down motor speed and step up torque, allowing a smaller, lighter motor to do the same cranking work. Permanent-magnet starters replace field coils with rare-earth magnets, reducing weight by 30 to 40 percent and improving efficiency, though at higher material cost.
Hybrid and electric vehicles use starter motors little or not at all; their engines (if present) may be restarted by the main traction motor or integrated starter-generator systems. The term 'starter motor' has remained consistent in automotive trade since the early 20th century, when it replaced hand-crank methods. Heavy trucks and construction equipment distinguish between light-duty starters and heavy-duty industrial starters rated for repeated high-load starting in cold conditions.