3508, 3512 and 3516 Engines Caterpillar


Electrical System

Usage:

3508 5PW
The electrical system has three separate circuits. The circuits are the charging circuit, the starting circuit, and the low amperage circuit. Some of the electrical system components are used in more than one circuit. The following components are common in each of the circuits:

  • battery

  • circuit breaker

  • ammeter

  • cable

  • wires for the battery

The charging circuit is in operation when the engine is running. An alternator makes electricity for the charging circuit. A voltage regulator in the circuit controls the electrical output in order to keep the battery at full charge.

The starting circuit is in operation only when the start switch is activated.

The circuit that has a low amperage is connected to the same side of the ammeter as the charging circuit. The starting circuit connects to the opposite side of the ammeter.

Charging System Components


NOTICE

Never operate the alternator without the battery in the circuit. Making or breaking an alternator connection with heavy load on the circuit can cause damage to the regulator.


Alternator




Illustration 1g00285111

Alternator Components (Typical Example)

(1) Regulator. (2) Roller bearing. (3) Stator winding. (4) Ball bearing. (5) Rectifier bridge. (6) Field winding. (7) Rotor assembly. (7) Fan.

The alternator is driven by a belt from an auxiliary drive at the front right corner of the engine. This alternator is a three-phase, self-rectifying charging unit, and the regulator is part of the alternator.

This alternator design has no need for slip rings or brushes, and the only part that has movement is the rotor assembly. All conductors that carry current are stationary. The conductors are the field winding, stator windings, six rectifying diodes, and the regulator circuit components.

The rotor assembly has many magnetic poles with air space between each opposite pole.

The poles have residual magnetism that produces a small amount of magnetic lines of force between the poles. As the rotor assembly begins to turn between the field winding and the stator windings, a small amount of alternating current (AC) is produced in the stator windings. This current is from the small, magnetic lines of force that are made by the residual magnetism of the poles. This alternating current (AC) is changed to a direct current (DC). The change occurs when the current passes through the diodes of the rectifier bridge. Most of this current charges the battery and the current supplies the circuit that has a low amperage. The remainder of the current is sent to the field windings. The DC current flow through the field windings (wires around an iron core) now increases the strength of the magnetic lines of force. These stronger lines of force increase the amount of AC current that is produced in the stator windings. The increased speed of the rotor assembly also increases the current and voltage output of the alternator.

The voltage regulator is a solid-state, electronic switch. The regulator feels the voltage in the system. The regulator turns on and the regulator turns off many times in one second in order to control the field current to the alternator. The output voltage from the alternator will now supply the needs of the battery and the other components in the electrical system. No adjustment can be made in order to change the rate of charge on these alternator regulators.

Starting System Components

Starting Solenoid

A solenoid is an electromagnetic switch that does two basic operations.

  • Closes the high current starting motor circuit with a low current start switch circuit.

  • Engages the starting motor pinion with the ring gear.




Illustration 2g00285112

Typical Solenoid Schematic

The solenoid has windings (one or two sets) around a hollow cylinder. There is a plunger with a spring load 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. This moves the shift lever in order 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 flywheel of the engine.

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 windings have the same number of turns around the cylinder. However, the pull-in winding uses a wire with a larger diameter in order 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 period of time. This period of 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.

Starting Motor

The starting motor is used to turn the engine flywheel in order to get the engine running.




Illustration 3g00285113

Starting Motor Cross Section (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 in order to push the starting motor pinion with a mechanical linkage. This will engage with the ring gear on the flywheel of the engine. The starting motor pinion will engage with the ring gear before the electric 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 can not turn the starting motor too fast. When the start switch is released, the starting motor pinion will move away from the flywheel ring gear.

Other Components

Magnetic Pickup




Illustration 4g00285114

Schematic of Magnetic Pickup

(1) Magnetic lines of force. (2) Wire coils. (3) Gap. (4) Pole piece. (5) Flywheel ring gear.

The magnetic pickup is a single pole, permanent magnet generator. The magnetic pickup is made of wire coils (2). The coils go around a permanent magnet pole piece (4) .

As the teeth of the flywheel ring gear (5) cut through the magnetic lines of force (1) around the pickup, an AC voltage is generated. The frequency of this voltage is directly proportional to engine speed.

Circuit Breaker

The circuit breaker is a switch that opens the battery circuit if the current in the electrical system goes higher than the rating of the circuit breaker.

A heat activated metal disc with a contact point completes the electric circuit through the circuit breaker. If the current in the electrical system gets too high the metal disc will get hot. This heat causes a distortion of metal disc. The disc opens the contacts. The disc breaks the circuit.


NOTICE

Find and correct the problem that causes the circuit breaker to open. This will help prevent damage to the circuit components from too much current.


Magnetic Switch

A magnetic switch is used for the starting solenoid circuit. The switch electrically operates in the same manner as the solenoid. The function of the switch is to reduce the low current load on the start switch. The switch also controls the low current to the starting solenoid.

Water Temperature Contactor Switch




Illustration 5g00322044

Water Temperature Contactor Switch

The contactor switch for the water temperature is installed in the regulator housing. An adjustment to the temperature range of the contactor cannot be made. The element feels the temperature of the coolant. The element then operates the microswitch in the contactor when the coolant temperature is too high. The element must be in contact with the coolant in order to operate correctly. The contactor switch will not operate if the engine is too hot due to low coolant level or no coolant level.

The contactor switch is normally connected to the electric shutoff system in order to stop the engine. The switch can also be connected to an alarm system. As the coolant temperature lowers back to the operating range, the contactor switch opens automatically.

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