Cranking Motor Designs

The most common type of starter motor used today incorporates the overrunning clutch starter drive instead of the old inertia-engagement bendix drive. There are four basic groups of starter motors:

  1. Direct drive.
  2. Gear reduction.
  3. Positive-engagement (moveable pole).
  4. Permanent magnet.

Direct Drive Starters

Solenoid operated Delco MT series starter motor

FIGURE. Solenoid operated Delco MT series starter motor.

A common type of starter motor is the solenoid-operated direct drive unit. Although there are construction differences between applications, the operating principles are the same for all solenoid-shifted starter motors.

When the ignition switch is placed in the START position, the control circuit energizes the pull-in and hold-in windings of the solenoid. The solenoid plunger moves and pivots the shift lever, which in turn locates the drive pinion gear into mesh with the engine flywheel. When the solenoid plunger is moved all the way, the contact disc closes the circuit from the battery to the starter motor. Current now flows through the field coils and the armature. This develops the magnetic fields that cause the armature to rotate, thus turning the engine.

Gear Reduction Starters

Gear reduction starter motor construction

FIGURE. Gear reduction starter motor construction.

Some manufacturers use a gear reduction starter to provide increased torque. The gear reduction starter differs from most other designs in that the armature does not drive the pinion gear directly. In this design, the armature drives a small gear that is in constant mesh with a larger gear. Depending on the application, the ratio between these two gears is between 2:1 and 3.5:1. The additional reduction allows for a small motor to turn at higher speeds and greater torque with less current draw.

The solenoid operation is similar to that of the solenoid-shifted direct drive starter in that the solenoid moves the plunger, which engages the starter drive.

Positive-Engagement Starters

Positive engagement starters use a moveable pole shoe

FIGURE. Positive engagement starters use a moveable pole shoe.

A commonly used starter on Ford applications in the past was the positive-engagement starter. Positive-engagement starters use the shunt coil windings of the starter motor to engage the starter drive. The high starting current is controlled by a starter solenoid mounted close to the battery. When the solenoid contacts are closed, current flows through a drive coil. The drive coil creates an electromagnetic field that attracts a moveable pole shoe. The moveable pole shoe is attached to the starter drive through the plunger lever. When the moveable pole shoe moves, the drive gear engages the engine flywheel.

Schematic of positive-engagement starter

FIGURE. Schematic of positive-engagement starter.

As soon as the starter drive pinion gear contacts the ring gear, a contact arm on the pole shoe opens a set of normally closed grounding contacts. With the return to ground circuit opened, all the starter current flows through the remaining three field coils and through the brushes to the armature. The starter motor then begins to rotate. To prevent the starter drive from disengaging from the ring gear if battery voltage drops while cranking, the moveable pole shoe is held down by a holding coil. Hie holding coil is a smaller coil inside the main drive coil and is strong enough to hold the starter pinion gear engaged.

Permanent Magnet Starters

The PMGR motor uses a planetary gear set and permanent magnets

FIGURE. The PMGR motor uses a planetary gear set and permanent magnets.

The permanent magnet gear reduction (PMGR) starter design provides for less weight, simpler construction, and less heat generation as compared to conventional field coil starters. Hie permanent magnet gear reduction starter uses four or six permanent magnet field assemblies in place of field coils. Because there are no field coils, current is delivered directly to the armature through the commutator and brushes.

Planetary gear set

FIGURE. Planetary gear set.

The permanent magnet starter also uses gear reduction through a planetary gear set. Hie planetary geartrain transmits power between the armature and the pinion shaft. This allows the armature to rotate at greater speed and increased torque. The planetary gear assembly consists of a sun gear on the end of the armature and three planetary carrier gears inside a ring gear. The ring gear is held stationary. When the armature is rotated, the sun gear causes the carrier gears to rotate about the internal teeth of the ring gear. The planetary carrier is attached to the output shaft. The gear reduction provided for by this gear arrangement is 4.5:1. By providing for this additional gear reduction, the demand for high current is lessened.

Note: The greatest amount of gear reduction from a planetary gear set is accomplished by holding the ring gear, inputting the sun gear, and outputting the carrier.

Comparison of the electrical circuits used in field coil and PMGR starters

FIGURE. Comparison of the electrical circuits used in field coil and PMGR starters.

The electrical operation between the conventional field coil and PMGR starters remains basically the same.