Charging System Principles of Operation
Charging Systems use the principle of electromagnetic induction to generate electrical power. Electromagnetic induction occurs in a generator when a magnetic field is rotated within a stationary conductor. The magnetic field can be generated by permanent magnets or as in the case of the automotive generator by powerful electromagnets.
Passing electric current through a wire or winding causes a magnet field to surround the wire or winding. The number of turns in the winding and the magnitude of the current flowing through the winding determine the magnetic field strength.
The strength of the field is further increased by surrounding the coil with pole pieces. The poles will take on the polarity (North or South) of the side of the coil they touch or the ones they are closest to. The combined windings (field coil) and pole pieces are referred to as the rotor. Output of the generator is regulated through control of field coil intensity.
An examination of single phase AC voltage generation, will aid in the understanding of 3 phase AC voltage generation.
Notes
Single-Phase AC Voltage Generation
Single Phase AC Voltage Generation requires one stator frame with windings and one magnetic field (North and South pole pieces).
Scheme 1
As the North pole of the rotor pole piece approaches the winding of the stator, induced voltage level in the stator begins to rise. The closer the North pole gets to the stator winding the higher the induced voltage. As the North pole reaches 90° to the stator winding the maximum amount of flux lines are acting on the winding, induced voltage is at its highest positive value (1/4 turn).
Scheme 2
The rotor continues to rotate and the North pole gets further a- way from the winding. The voltage drops, until 0 voltage is induced through the stator (1/2 turn).
Scheme 3
With the South pole now approaching the winding voltage begins to increase negatively. When the South pole reaches 90° to the winding, again the maximum amount of flux lines are acting on the winding and induced voltage is at its greatest negative value (3/4 turn).
Scheme 4
The South pole continues to travel farther away from the winding decreasing the negative voltage value until 0 voltage is again reached (1 full turn).
This comprises one cycle or 360° rotation of the magnetic field.
Scheme 5
Sine wave produced by a single winding of the stator during a single revolution of one pair of pole pieces is called single phase voltage.
Three-Phase AC Voltage Generation
Most AC generators use either a twelve or a fourteen pole rotor. Each pair of poles (North and South) produce one complete sine wave in each winding per revolution.
Scheme 6
Voltage of each stator winding is added together to create three-phase voltage.
During one revolution a fourteen pole rotor will produce seven sine waves. (The stator has one winding (coil) for each pair of rotors.)
The rotor generates three overlapping sine wave voltage cycles in the stator (one rotor three sets of windings in the stator).
The total output would be twenty one sine wave cycles per revolution. (3 sets of stator windings, each with 7 coils).
Scheme 7
Sine wave cycle of a fourteen pole rotor and three phase stator.