Induction
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Induction
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ELECTROMAGNETIC INDUCTION The prime link between electricity and magnetism is motion. Whenever a charged particle moves a magnetic field is created. Any current flow produces a magnetic field. ELECTROMAGNETISM’S LEFT-HAND RULE Left-Hand Rule for Conductors Grasp a conductor in your left hand with the thumb pointed in the direction of electron flow, negative to positive. Your fingers will indicate the direction of the magnetic lines of force. Conductors in parallel with current flowing in the same direction will create magnetic lines of force which merge together and create a stronger magnetic field. Conductors in parallel with current flowing in opposite directions will create magnetic lines of force which oppose each other. Left-Hand Rule for Coils A coil creates a situation where the magnetic lines of force are all in the same direction, and close together, which creates a stronger magnetic field. The coil will have north and sole poles like a permanent magnet. The magnetic polarity depends on the direction of the windings and the direction of current flow. Hold a coil with your left hand with your fingers pointing the direction of electron current flow. Your thumb will point to the north magnetic pole. THE LAWS OF INDUCED VOLTAGE Faraday’s Law Electromotive force is induced in a coil by changing magnetic flux lines. The magnitude of the induced emf is determined by the number of turns in the coil, the strength of the magnetic field, and the relative speed between the coil and the magnetic field. Lenz’s Law Current induced in a coil due to a change in the magnetic flux is such that it opposes the cause producing it. AC GENERATORS AC Generator Characteristics An AC generator, or alternator, converts mechanical energy into electrical energy by means of electromagnetic induction. Extend your thumb, forefinger, and middle finder of your left hand at right angles to one another. Point your thumb in the direction the conductor moves. Point your forefinger in the direction of magnetic flux, from north to south. Your middle finger will indicate the direction of current flow in the conductor. The Elementary AC Generator The armature is the loop, or loops, or wire that rotate within a magnetic field. Slip rings and brushed are located at the pivot point of the armature to allow electrical connections to be made to the armature. One complete rotation of the armature completes one cycle of a sine wave AC output. The Practical AC Generator All generators have a rotor and a stator. The rotor is the part that rotates. The stator is the part that remains stationary. High-voltage applications often use a rotating-field alternator which provides a DC voltage to the armature and induces the higher voltage to the stator. DC GENERATORS The Basic DC Generator A commutator is each half-used instead of the slip rings to reverse the connection of the output at cycle point. This provides a pulsating DC. The Practical DC Generator The armature may be manufactured to contain a number of coil segments. The commutator has an appropriate number of segments for the number of coil segments. The output has less ripple voltage because of the additional segments. GENERATOR LOSSES Copper Losses There is power lost to heat because of the resistance of the armature winding. Eddy Current Losses Laminated armature cores are often used to reduce eddy currents within the core of the armature. Eddy currents cause power to be lost as heat. Hysteresis Losses Molecule frictions due to the continually changing of the magnetic dipole in the armature core cause energy loss in the form of heat. This is called hysteresis loss. DC MOTORS A motor converts electrical energy into mechanical energy. The right-hand rule can be used for motors. Extend your right hand with your thumb, index finger, and middle finder at right angles to each other. Point your forefinger in the direction of magnetic flux, north to south. Point your middle finger in the direction of current flow. Your thumb will point in the direction of motion the conductor will take. Types of DC Motors Series Motors A series motor has field windings connected in series with the armature coil. The speed may vary widely based on the mechanical load. Shunt Motors A shunt motor has the field windings connected in parallel to the armature coil. The speed varies only slightly based on the mechanical load. Compound Motors A compound motor has one set of field windings connected in series with the armature. Another set of field windings in connected in parallel. The compound motor will have some characteristics of both the series and shunt motors. AC MOTORS Types of AC Motors Series AC Motors Multiphase voltages are supplied to the status windings which cause the rotor to be pushed and pulled. Synchronous Motors Synchronous motors maintain a constant speed. Induction Motors Induction motors are simple because they do not require an electrical rotor connection. Induction motors are very common. An additional starter circuit is required. |