Description & Theory Of Operation: Alternator
The alternator is an electro-mechanical device used to change mechanical energy into electrical energy. It has two functions which it performs. One is to charge and maintain the battery voltage and the other is to supply voltage and current to all electrical components of the vehicle when the engine is running. With the aid of a voltage regulator which may be mounted either external or internal to the alternator, the voltage output of the alternator is controlled to prevent higher than proper operating voltages. The battery may supply operating voltage when peak electrical loads exceed the capability of the alternator or when engine speed is not sufficient to bring the alternator to a charging condition. When the engine speed is increased or electrical load decreased, the alternator will again supply all necessary operating voltage and current.
The alternator produces alternating current (AC) and voltage. This AC current must be changed to direct current (DC) before the battery can be charged or electrical components may be operated. The AC voltage is rectified (changed) to DC voltage by solid state devices called diodes which will permit voltage and current to pass through in one direction only.
As the alternator produces AC current, the voltage coming from the alternator goes through cycles. During one complete cycle the voltage goes from zero volts to approximately 12-15 volts positive (+). After reaching the maximum positive voltage (maximum voltage controlled by voltage regulator), the voltage then drops to zero to complete the first half of the cycle. All of this time current is flowing in a forward direction. As the voltage goes through the zero point, it continues the downward (negative) half of the cycle until it reaches the maximum negative (-) voltage. The voltage again returns to zero to complete one alternating cycle. During the negative half-cycle, the direction of current flow is reversed and opposite to positive half-cycle. As the automotive electrical system and battery are direct current (DC) devices, we must rectify (change) the AC current to DC current. When we pass current through a diode it will allow current to flow in one direction only. Therefore only one half of the alternating current will be allowed to pass through the diode. We now have produced a pulsating DC voltage and current which can be used by the vehicle electrical system.
The pulsating (part time) DC current does not produce sufficient current for the vehicle electrical needs, so we add more electrical windings (stator windings) to the alternator until we have created an almost continuous DC current. These windings (three in number) are constructed and installed in the alternator in such a manner that they form a three phase circuit. The three phases are timed so that all positive and all negative voltage half-cycles will occur at equally spaced time intervals, and when the current is rectified (changed), it will be changed to continuous DC current. As can be seen from the illumination, as the number of phases (windings) are increased the amount and quality of the direct current is increased until good operational DC voltage is achieved.
To understand how an alternator produces alternating current and voltage take a loop of wire and place a magnet inside of the loop. When the magnet is rotated inside the wire loop, the rotating magnetic field surrounding the magnet induces (causes) an electrical current to flow in the wire loop. As shown in the illustration, the magnet has a north pole (N) and a south pole (S). When the north pole is near one side of the loop and the south pole is near the other side of the loop, while rotating, the current will be induced to flow in one direction. When the magnet rotates the poles to opposite (alternate) ends, the current will reverse to the opposite direction. As the magnet is rotated, the voltage will rise and fall, go from positive (+) to negative (-) and reverse direction from forward to reverse during one complete revolution.
In an actual alternator, the rotating magnet is called a rotor. It is constructed with an iron core surrounded by many turns of wire to form an electromagnet. The ends of the winding are connected to a pair of slip rings. Power to the slip rings is provided by a pair of brushes connected to the DC power source (battery).
Two methods of connecting the rotor field winding are used; one is the grounded field and the other is the isolated field type. In the grounded field type, one of the slip ring brushes is connected directly to ground and the other brush is connected to the voltage regulator and battery positive terminal.
In the isolated field type, both slip ring brushes are insulated from ground. The ground side of the rotor winding is connected to the voltage regulator and receives its ground internally through the voltage regulator. The other slip ring brush is connected to the alternator and battery in the same manner as the grounded field type.
The wire loop is called the stator (Stationary winding) and is made up of three windings in a laminated iron core. These stator windings have two different ways in which they are connected in the alternator. One is a "Y" connected stator and the other is a Delta connected stator. The Delta stator is generally found in the high output type alternators. It is from these windings where the electrical current is produced that power from the alternator is taken.
When the alternating current comes from the stator windings it goes through a full wave bridge rectifier circuit made up of six diodes. These diodes change the alternating current to direct current. Three of these diodes are negative (-) diodes and are connected between the stator windings and ground. The other three diodes are positive (+) and are connected between opposite ends of the stator windings and the alternator output.
Another type of diode is also found in some alternators with transistorized regulators, it is called an isolation diode. Its primary function is to act as an automatic switch between the battery and the alternator. It will block any current flow from the battery back to the alternator and regulator when the alternator is not operating. Instead of a single isolation diode, a three diode assembly (Diode Trio) may also be found in the alternator performing the same function as the single isolation diode. Where the mechanical or electro-mechanical voltage regulators are used, no isolation diodes are required. The field relay contacts will block current flow when the alternator is not operating.