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Machinery can be driven directly by a turbine as in traditional corn mills and many modern timber sawing mills, but converting the power into electricity does have several advantages. For instance, it enables the use of all types of electrical appliances from lighting to electric motors and the flexible positioning of the appliances to wherever a power point can be set up near or far from the turbine. A device which converts a generator. The most common type of generator produces alternative current and is known as an alternator.
As a brief reminder of simple
electrical theory, the flow of electricity or current (symbol I) is measured
in amps (A). The potential difference (V) or pressure of the electricity
is measured in volts (V). The power (P) measured in watts (W) or more often
kilowatts (1 kW=1000 W), is equal to volts× amps. The resistance
(R) of a circuit is a measure of how well the electricity is being conducted
(a poor conductor has a high resistance). Resistance is measured in Ohms
(W ) and is equal to the potential difference (or voltage drop) divided
by the current. Capacitance (C) expresses the degree to which energy is
being stored in an electric field rather than being available to do work
and inductance (L) is similar to capacitance but refers to magnetic fields.
AC and DC
Two types of current are produced by electrical generators, either alternating current (AC) or direct current (DC). In the case of AC a voltage cycles sinusoidally with time, from positive peak value to negative. Because the voltage changes its sign the resulting current also continually reverses direction in a cyclic pattern. DC current flows in a single direction as the result of a steady voltage. DC is not usually used in modern power installations except for very low-powered systems of a few hundred watts or less.
Alternating voltage can be produced in
a stationery coil or armature by a rotating magnetic field (figure18b),
but more usually a coil is rotated in a stationary magnetic field (figure
18a). The magnetic field can be produced either by a permanent magnet or
by another coil (i.e an electro-magnet) know as a field coil (as in figure
18c and 18d) which is fed by direct current known as the excitation current.
A generator supplying alternative current is described as an alternator
to distinguish it from a machine designed to supply DC current which is
know as a DC generator or dynamo.
Figure 18 Alternator configuration
Current flow when a voltage difference is place across a conducting body. In AC circuits the magnitude and timing of the current cycle relative to the voltage cycle will depend on whether the conductivity body is resistance, inductive, capacitive or some combination of these elements.
The first three cases in
figure 19 show that for different types of load, the current cycle either
(a) stays in phase with the voltage, (b) lags behind the voltage by a phase
angle of 90° or (c) runs ahead ("leads") by a phase angle of 90°
. Circuits in which the load causes the current and voltage to be out of
the phase are said to have reactive loads. Generally a load is a combination
of resistance, capacitance and inductance, described by a term impedance
(symbol Z) and causes a phase difference between current and voltage of
Figure 19d shows the current-voltage characteristic of a circuit where the load causes the current I to lag the voltage V by an angle f (the load in this case is predominantly inductive). Because the current and voltage are not perfectly in phase, the useful power available is reduced and is proportional to the cosine of the phase difference, so the power usefully consumed by the load is V× I× cosf , although the power supplied is V× I. The power not consumed is simply being shunted back and forth between supply and load. The ration of useful power to total supplied power is called the power factor and is numerically equal to cosf
Principles of reactance