The high-speed, low-torque permanent-magnet planetary-drive motor operates the drive mechanism through gear reduction provided by a simple planetary geared. Figure 9-49 shows the Bosch gear reduction design, which is similar to that used in Chrysler starters. Figure 9-50 shows the drive pinion. An internal ring gear is keyed to the field frame and held stationary in the motor.
The armature shaft drives the sun gear for the planetary geared. The sun gear meshes with three planetary pinions, which drive the pinion carrier in reduction as they rotate around the ring gear. The starter driveshaft is mounted on the carrier and driven at reduced speed and increased torque. This application of internal gear reduction through planetary gears delivers armature speeds in the 7,000-rpm range.
The armature and driveshaft ride on roller or ball bearings rather than bushings. Permanent-magnet, planetary-drive starters differ mechanically in how they do their job, but their electrical wiring is the same as that used in the field-coil designs (Figure 9-51). Although PMGR motors are lighter in weight and simpler to service than traditional designs, they do require special handling precautions. The material used for the permanent magnet fields is quite brittle.
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A sharp impact caused by hitting or dropping the starter can destroy the fields. Regardless of the type of starter motor used, when the engine starts and runs, its speed increases. The motor must be disengaged from the engine as soon as the engine is turning more rapidly than the starter motor that has cranked it. With a movable-pole-shoe or solenoid-actuated drive, however, the pinion remains engaged until power stops flowing to the starter.
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In these applications, the planetary gear set between the motor armature and the starter drive reduces the speed and increases the torque at the drive pinion. The compact gear set is only 1/2 to 3/4 inch (13 to 19 mm) deep and is mounted in line with the armature and the starter is protected by an over-running clutch (Figure 9-52).
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