Magnetism can be used to produce electricity and electricity can be used to produce magnetism. Much about magnetism cannot be explained by our present knowledge. However, there are certain patterns of behavior that are known. Application of these behavior patterns has led to the development of generators, motors and numerous other devices that utilize magnetism to produce and use electrical energy. The space surrounding a magnet is permeated by magnetic lines of force called "flux". These lines of force are concentrated at the magnet's north and south poles. They are directed away from the magnet at its north pole, travel in a loop and re-enter the magnet at its south pole. The lines of force form definite patterns which vary in intensity depending on the strength of the magnet. The lines of force never cross one another. The area surrounding a magnet in which its lines of force are effective is called a "magnetic field." Like poles of a magnet repel each other while unlike poles attract each other.
All conductors through which an electric current is flowing have a magnetic field surrounding them. This field is always at right angles to the conductor. If a compass is placed near the conductor, the compass needle will move to a right angle with the conductor. The following rules apply:
- The greater the current flow through the conductor, the stronger the magnetic field around the conductor.
- The increase in the number of lines of force is directly proportional to the increase in current flow and the field is distributed along the full length of the conductor
- The direction of the lines of force around a conductor can be determined by what is called the "right hand rule." To apply this rule, place your right hand around the conductor with the thumb pointing in the direction of current flow. The fingers will then be pointing in the direction of the lines of force.
This is opposite the "electron" theory, which states that current flows from negative to positive.
An electromotive force (EMF) or voltage can be produced in a conductor by moving the conductor so that it cuts across the lines of force of a magnetic field. Similarly, if the magnetic lines of force are moved so that they cut across a conductor, an EMF (voltage) will be produced in the conductor. This is the basic principal of the revolving field generator. The figure below, illustrates a simple revolving field generator. The permanent magnet (Rotor) is rotated so that its lines of magnetic force cut across a coil of wires called a Stator. A voltage is then induced into the Stator windings. If the Stator circuit is completed by connecting a load (such as a light bulb), current will flow in the circuit and the bulb will light.
A SIMPLE AC GENERATOR
The figure below shows a very simple AC Generator. The generator consists of a rotating magnetic field called a ROTOR and a stationary coil of wire called a STATOR. The ROTOR is a permanent magnet which consists of a SOUTH magnetic pole and a NORTH magnetic pole. As the MOTOR turns, its magnetic field cuts across the stationary STATOR. A voltage is induced into the STATOR windings. When the magnet's NORTH pole passes the STATOR, current flows in one direction. Current flows in the opposite direction when the magnet's SOUTH pole passes the STATOR. This constant reversal of current flow results in an alternating current (AC) waveform that can be diagrammed as shown in Figure 1-5. The ROTOR may be a 2-pole type having a single NORTH and a single SOUTH magnetic pole. Some ROTORS are 4-pole type with two SOUTH and two NORTH magnetic poles. The following apply:
- The 2-pole ROTOR must be turned at 3600 RPM to produce an AC frequency of 60 Hertz, or at 3000 RPM to deliver an AC frequency of 50 Hertz.
- The 4-pole ROTOR must operate at 1800 RPM to deliver a 60 Hertz AC frequency or at 1500 rpm to deliver a 50 Hertz AC frequency.
A MORE SOPHISTICATED AC GENERATOR
The figure below represents a more sophisticated generator. A regulated direct current is delivered into the ROTOR windings via carbon BRUSHES AND SLIP RINGS. This results in the creation of a regulated magnetic field around the ROTOR. As a result, a regulated voltage is induced into the STATOR. Regulated current delivered to the ROTOR is called "EXCITATION" current.
The revolving magnetic field (ROTOR) is driven by the engine at a constant speed. This constant speed is maintained by a mechanical engine governor. Units with a 2-pole rotor require an operating speed of 3600 RPM to deliver a 60 Hertz AC output. Engine governors are set to maintain approximately 3720 RPM when no electrical loads are connected to the generator.
NOTE: AC output frequency at 3720 RPM will be about 62 Hertz. The "No-Load" is set slightly high to prevent excessive RPM, frequency and voltage droop under heavy electrical loading.
Generator operation may be described briefly as follows:
- Some "residual" magnetism is normally present in the Rotor and is sufficient to induce approximately 7 to 12 volts AC Into the STATOR's AC power windings.
- During startup, an engine controller circuit board delivers battery voltage to the ROTOR, via the brushes and slip rings.
(a) The battery voltage is called "Field Boost."
(b) Flow of direct current through the ROTOR increases the strength of the magnetic field above that of "residual" magnetism alone.
- "Residual" plus "Field Boost" magnetism induces a voltage into the Stator excitation (DPE), battery charge and AC Power windings.
- Excitation winding unregulated AC output is delivered to an electronic voltage regulator, via an excitation circuit breaker.
(a) A "Reference" voltage has been preset into the Voltage Regulator.
(b) An "Actual" ("sensing") voltage is delivered to the Voltage Regulator via sensing leads from the Stator AC power windings.
(c) The Regulator "compares" the actual (sensing) voltage to its preset reference voltage.
(1) If the actual (sensing) voltage is greater than the preset reference voltage, the Regulator will decrease the regulated current flow to the Rotor.
(2) If the actual (sensing) voltage is less than the preset reference voltage, the Regulator will increase the regulated current flow to the Rotor.
(3) In the manner described, the Regulator maintains an actual (sensing) voltage that is equal to the preset reference voltage.
NOTE: The Voltage Regulator also changes the Stator excitation windings alternating current (AC) output to direct current (DC).
- When an electrical load is connected across the Stator power windings, the circuit is completed and an electrical current will flow.
- The Rotor's magnetic field also induces a voltage Into the Stator battery charge windings.
(a) Battery charge winding AC output is delivered to a battery charge rectifier (BCR) which changes the AC to direct current (DC).
(b) The rectified DC is then delivered to the unit battery, to maintain the battery in a charged state.
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