Starting Solenoid for Electric Starting Motors
A solenoid is an electromagnetic switch that does two basic operations.
- Close the high current starting motor circuit with a low current start switch circuit.
- Engage the starting motor pinion with the ring gear.
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| Illustration 1 | g00285112 |
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Typical solenoid | |
The solenoid has windings (one or two sets) around a hollow cylinder. There is a spring-loaded plunger inside the cylinder. The plunger can move forward and backward. When the start switch is closed and the electricity is sent through the windings, a magnetic field is made. The magnetic field pulls the plunger forward in the cylinder. The motion moves the shift lever to engage the pinion drive gear with the ring gear. The front end of the plunger makes contact across the battery and the motor terminals of the solenoid. The starting motor begins to turn the engine flywheel.
When the start switch is opened, current no longer flows through the windings. The spring pushes the plunger back to the original position. The spring simultaneously moves the pinion gear away from the flywheel.
When two sets of windings in the solenoid are used, the windings are called the hold-in winding and the pull-in winding. Both of the winding have the same number of turns around the cylinder. However, the pull-in winding uses a wire with a larger diameter to produce a greater magnetic field. When the start switch is closed, part of the current flows from the battery through the hold-in windings. The rest of the current flows through the pull-in windings to the motor terminal. The current then goes through the motor to the ground. When the solenoid is fully activated, current is shut off through the pull-in windings. Only the smaller hold-in windings are in operation for the extended time. This time is the amount of time that is needed for the engine to start. The solenoid will now take less current from the battery. The heat that is made by the solenoid will be kept at an acceptable level.
The starting motors are used to turn the engine flywheel to get the engine running.
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| Illustration 2 | g01192489 |
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Cross section of the starting motor (typical example) (1) Field (2) Solenoid (3) Clutch (4) Pinion (5) Commutator (6) Brush assembly (7) Armature | |
The starting motor has a solenoid. When the start switch is activated, electricity will flow through the windings of the solenoid. The solenoid core will move to push the starting motor pinion with a mechanical linkage. The pinion engages with the ring gear on the flywheel of the engine. The starting motor pinion engages the ring gear before the contacts in the solenoid close the circuit between the battery and the starting motor. When the circuit between the battery and the starting motor is complete, the pinion will turn the engine flywheel. A clutch gives protection to the starting motor. The engine cannot turn the starting motor too fast. When the start switch is released, the starting motor pinion will move away from the flywheel ring gear.
The starting motor is protected from damage in two ways:
- The starting motor is protected from engagement with the flywheel ring gear when the engine is running. The control feature will not allow the starting motor to engage if the speed is above 0 rpm.
- The starting motor is protected from continued operation by holding the key in the "start" position after the engine starts. The starting motor solenoid is disengaged after engine speed reaches 300 rpm. This is a configurable parameter.