Alternator: Description and Operation
Exploded view:
Basic construction
The main parts of an alternator are:
- a three phase stator winding
- a rotor with excitation winding and slip rings
- six power diodes, three exciter diodes and two car bon brushes
Alternators can be rotated in either direction.
The direction of rotation is only determined by the type of fan used.
Principle
The construction of an alternator is based on the principle that an electrical potential (Electromotive force = EMF) is induced in a conductor when the conductor is moved in a magnetic field. In an alternator, the conductor (stator winding) is stationary and the magnetic field (rotor winding) is moved.
As the rotor turns, the magnetic poles and hence the magnetic field move causing an alternating current to be generated in the stator winding. The electrical potential varies in both polarity and size.
The deflections of the voltmeter pointer indicate maximum values after each half revolution. The same effect is obtained regardless of whether the magnetic field remains stationary while the conductor moves or vice versa.
Sine wave curves
If an oscilloscope is used to register the electrical potential and the rotor is turned at a constant speed, a sine wave curve will be obtained between the max. and min. values.
The strength of the induced electrical potential is proportional to the strength of the magnetic field and the speed with which the lines of force are cut. The magnetic field in an alternator is produced by means of an electrical current supplied to the rotor winding (excitation winding). As long as this current flows and the rotor turns an alternating current will be generated in the stator winding.
Three phase alternating current
In order to use the alternator as efficiently as possible the stator winding is split into three separate windings spaced 120 degrees apart. The alternating current produced in each winding is called a phase. The three phases are denoted by "u" , "v" , "w".
Stator winding connections
The three windings are joined to each other in either a "delta or a wye" connection. The "delta" type provides a higher potential whereas the "wye" type provides more current without overheating.
Power diodes
Positive and negative power diodes are used in the alternator to rectify current. They differ from each other in that the rectifying material (silicon) is mounted in the casing in opposite ways. The diode casing is secured in a metal support (heat sink) to facilitate cooling; the heat sink is then connected to the positive or negative terminal of the battery. Positive diodes are called PN diodes and are marked in red. Negative diodes are called NP diodes and are marked in black.
Function of diode
Diodes allow current to flow in one direction only, the direction of the arrow, and block current in the opposite direction. They can be compared to a non-return valve which allows a liquid to flow in one direction only.
Voltage loss
Rectification does not however take place without any losses. At each diode a voltage drop of 0.6 - 1.2 V, depending on the strength of current, occurs. The max. inverse voltage a diode can block is approx. 100 V. If this voltage is exceeded, current flows through and the diode is ruined.
Rectification
The purpose of a diode is to rectify an alternating current phase. The negative half waves are blocked and a pulsating direct current is produced.
Full wave rectification
To use the alternating current efficiently full wave rectification is required. This means that each phase must have two diodes, a positive and a negative. A pulsating direct current is still however produced.
Three phase bridge connection
In this type of bridge connection one diode per phase is connected on the positive side and one per phase on the negative side. This means that the charging current from the alternator must pass through six power diodes. The direct current is not completely rectified but this is of no practical consequence since the battery acts as a buffer in taking up any variations.
Claw-pole rotor
The name of this type of rotor is derived from its shape. It consists of two halves (claws) which interlock and envelope the excitation winding (rotor winding). Each claw has six poles, which makes a total of twelve (six north and six south).
Magnetic field of rotor
A current flows through the excitation winding, a magnetic field is formed between the poles of the rotor. The magnetic field's lines of force cut each of the three stator windings as the rotor turns. Twelve pole passes occur in one revolution of the rotor (360°). Each pole that passes induces a voltage half wave which is either positive or negative.
Induced half waves
During one revolution of the rotor 36 half wave (12 poles x 3 phase windings) are induced In the three stator windings. By comparison a two pole rotor would only induce six half waves.
Cooling
The heat produced in the alternator is proportional to the current taken out and must be removed to prevent damage to the insulation and diodes. The alternator is therefore air cooled and equipped with a fan.
There are two types of fan, one with straight vanes and one with off-set vanes. The off-set type can only be driven in one direction where as the straight type can be driven in either direction.
Charging circuit
Current for the battery charging circuit and electrical consumers is taken from the B+ terminal on the alternator. The diagram shows that when phase "u" is at a 180° phase angle;
- Phase "u" has zero potential
- Phase "v" is positive
- Phase "w" is negative
The current flows from MP (the neutral center point of the phase windings) through phase "v" to the B+ terminal on the alternator, where it continues to flow to the car's electrical consumers and eventually to earth. The current returns from earth to the alternator via D- and continues through the phase winding "w" to MP. Consider a 270 ° phase angle at "u".
None of the phase are at zero potential and current flows from the windings "v" and "w" to the car's consumers. It returns via earth through D- and phase "u" to MP.
It can be seen that all of the diodes are in operation at any one point in time. The individual phase currents vary in size and direction (polarity) before entering the diodes. At the diodes the current is rectified and the output current produced flows in the same direction.
Pre-excitation circuit
Alternators are self exciting, that is the exciter current is obtained from the alternator itself by tapping the main current. The exciter current is the current required to magnetize the rotor.
When starting, there isn't any current available to tap from the alternator. It is therefore necessary to take current from the battery via a charging warning warning light, which must have a sufficiently high current draw, min. (12 V) 2 W.
Can the alternator begin to charge without pre-excitation?
There are two diodes in series in the exciter circuit, an exciter diode and a negative power diode. Self excitation can only start when the alternator provides (2 x 0.6 V) = 1.2 V. This is because there is a voltage drop of 0.6 V at each diode i.e. the voltage required by a diode to pass current in the forward direction. Normally there is sufficient residual magnetism in the rotor to provide this voltage but only at high revolutions.