conductor moves relative to magnetic field, voltage is induced in the conductor.
Particularly, if a coil is spinning in a magnetic field, then the two sides of the coil move in opposite directions, and the voltages induced in each side add. Numerically the instantaneous value of the resulting voltage (called electromotive force, emf) is equal to the minus of the rate of change of magnetic flux Φ times the number of turns in the
coil: V=−N•∆Φ/Δt. This relationship has been found experimentally and
is referred to as Faraday's law. The minus sign here is due to Lenz law, which states that the direction of the emf is such that the magnetic field
from the induced current opposes the change in the flux which produces this emf. Lenz law is connected to the conservation of energy.
For clarity in the above animation a single rectangular conductor loop is shown instead of an armature with a set of windings on an iron core. Since the rate of magnetic flux change through the coil that spins at a constant rate changes sinusoidally with the rotation, the voltage generated at the coil terminals is also sinusoidal (AC). If an external circuit is connected to the coil's terminals, this voltage will create current through this circuit, resulting in energy being delivered to the load. Thus, the mechanical energy that rotates the coil is converted into electrical energy. Note that the load current in turn creates a magnetic field that opposes the change in the flux of the coil, so the coil opposes the motion. The higher current, the larger force must be applied to the armature to keep it from slowing down. In the animation the coil is rotated by the hand crank. In practice, the mechanical energy is produced by turbines or engines called prime movers. In a small AC generator a prime mover is usually a rotary internal-combustion engine. In commercially available devices an alternator is integrated with this engine into a single appliance. The resulting device is referred to as engine-generator set or genset, although casually it is often called just a generator. A genset is the most common and probably the cheapest emergency backup power source for home use. Cheap generators sell for as low as $100 per kilowatt.
Note that the production of the voltage depends only on the relative motion between the coil and the magnetic field. EMF is induced by the same physics law whether the magnetic field moves past a stationary coil, or the coil moves through a stationary magnetic field. In the animation, the magnetic field is produced by a fixed magnet while the coil is revolving. Today's AC gensets are usually brushless. They have spinning field and a stationary power-producing armature. This armature comprises of a set of coils that form a cylinder. Also, in practice, the magnetic field is usually induced by an electromagnet rather then a permanent magnet.
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The electromagnet consists of
so called field coils mounted on an iron core. A current flow in the
field coils produces the magnetic field. This current may be obtained
from an external source or from the system's own armature. Regulation
is achieved by sensing the output voltage, converting it to a DC, and
comparing its level to a reference voltage. An error is used to control
the field in order to maintain a constant output. Most modern AC
sources with field coils are self-excited:
the current for field coils is supplied by an additional exciting winding in the armature.
How does self excitation works? The exciter's output voltage is
rectified by a diode bridge and usually is fed into a voltage regulator. When output AC current is generated, a portion of it flows
into field coil to generate magnetic field. The initial magnetic field before the device started is produced by residual magnetism in electromagnet's cores or is created by a electric current driven from a battery during engine cranking.
The residual magnetism of the exciter's core may be lost or weakened by external magnetic fields from any source, or by
non-operation for a long time. Some genset models provide automatic field flashing. Otherwise, if the electromagnet's core lost its residual magnetism, the rotor will spin, but no AC output voltage will be produced. In this case, to start the device you may need to do so-called generator field flashing. Here is a typical field flashing
procedure: stop the engine, disconnect exciter field leads from the voltage regulator (note the polarity of the leads), and turn the
circuit breaker off. Then briefly apply voltage from an external battery or another DC source in series with a 10-20 Ohm 25W limiting resistor or a bulb to the field coil while observing polarity. Allow the field to be flashed for some 10 seconds, then remove the external voltage source, and finally reconnect the exciter coil. For a particular model consult your owner's operation manual for the recommendations. |
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