Starter Motors: Types, Operation, and History
Types of Starter Motors
Starter motors must provide high power at low speeds to crank an automotive engine at all temperatures and at the cranking speed required for the engine to start (60 to 250 engine RPM). Many starter motors are series-wound, which means that the current flows first through the field coils, then in series through the armature, and finally to a ground through the ground brushes.
Series Motors
A series motor develops its maximum torque at the initial start (0 RPM) and develops less torque as the speed increases. Because the power (torque) of the starter depends on the strength of the magnetic fields, the torque of the starter decreases as the starter speed increases.
Shunt Motors
Shunt-type electric motors have the field coils in parallel (or shunt) across the armature.
Compound Motors
A compound-wound, or compound, motor has the operating characteristics of a series motor and a shunt-type motor because some of the field coils are connected to the armature in series and some (usually only one) are connected directly to the battery in parallel (shunt) with the armature.
Armature and Commutator Assembly
Motor armatures are connected to the commutator in one of two ways. In a lap winding, the two ends of each conductor are attached to two adjacent commutator bars. The armature core, windings, and commutator are assembled on a long armature shaft. This shaft also carries the pinion gear that meshes with the engine flywheel ring gear. Most automotive starters have two grounded and two insulated brushes, which are held against the commutator by spring force.
Permanent-Magnet Fields
The permanent-magnet, planetary-drive starter motor is the first significant advance in starter design in decades. This eliminates the motor field circuit, which in turn eliminates the potential for field wire-to-frame shorts, field coil welding, and other electrical problems.
Caution: Do not hit that starter! If struck with a heavy tool, the magnets could be broken, with parts of the magnet falling onto the armature and into the bearing pockets, making the starter impossible to repair or rebuild.
Gear-Reduction Starters
The purpose of the gear reduction (typically 2:1 to 4:1) is to increase starter motor speed and provide the torque multiplication necessary to crank an engine.
Starter Drives
A starter drive includes a small pinion gear that meshes with and rotates the larger gear on the engine for starting. The ends of the starter pinion gear are tapered to help the teeth mesh more easily without damaging the flywheel ring gear teeth. If the engine starts and is accelerated to 2000 RPM (normal cold engine speed), the starter will be destroyed by the high speed (36,000 RPM) if the starter was not disengaged from the engine.
A Bit of History: Bendix Drive Is Now Called a Starter Drive
Older-model starters often used a Bendix drive mechanism, which used inertia to engage the starter pinion with the engine flywheel gear. The overrunning clutch, which is built in as a part of the starter drive unit, uses steel balls or rollers installed in tapered notches. The spring between the drive tang or pulley and the overrunning clutch and pinion is called a mesh spring, and it helps to cushion and control the engagement of the starter drive pinion with the engine flywheel gear.
Starter Drive Operation
A starter drive is generally a dependable unit and does not require replacement unless defective or worn. Intermittent starter drive failure (starter whine) is often most noticeable during cold weather.
Positive-Engagement Starters
The movable pole shoe is held down (which keeps the starter drive engaged) by a smaller coil on the inside of the main drive coil.
Solenoid-Operated Starters
A starter solenoid is an electromagnetic switch containing two separate, but connected, electromagnetic windings.
How Are Starters Made So Small?
A starter can be constructed smaller due to the use of gear reduction to achieve the same cranking torque as a straight drive starter, but using much smaller components.
Summary
- All starter motors use the principle of magnetic interaction between the field coils attached to the housing and the magnetic field of the armature.
- The control circuit includes the ignition switch, neutral safety (clutch) switch, and solenoid.
- The power circuit includes the battery, battery cables, solenoid, and starter motor.
- The parts of a typical starter include the main field housing, commutator-end (or brush-end) housing, drive-end housing, brushes, armature, and starter drive.