INTRODUCTION OF AC GENERATOR A Generator is a device which converts mechanical energy into electrical energy. WORKING PRINCIPLE
A.C Generator works on the principle of electromagnetic induction (motional emf). In generator an induced emf is produced by rotating a coil in a magnetic field. The flux linking the coil changes continuously hence a continuous fluctuating emf is obtained. CONSTRUCTION
A.C Generator consists of the following parts. Powerful field magnet with concave poles.
Armature:
It is a rectangular coil of large number of turns of wire wound on laminated softiron core of high permeability and low hysteresis loss.
Slip rings:
The ends of the coil are joined to two separate copper rings fixed on the axle (S1 & S2).
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Carbon brushes:
Two carbon brushes remain pressed against each of the rings which form the terminals of the external circuit.
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PRINCIPLE OF A.C. GENERATOR A.C. generators or alternators (as they are usually called) operate on the same fundamental principles of electromagnetic inductionas D.C. generators. Alternating voltage may be generated by rotating a coil in the magnetic field or by rotating a magnetic field within a stationary coil. The value of the voltage generated depends onthe number of turns in the coil. strength of the field. the speed at which the coil or magnetic field rotates.
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WORKING OF AC GENERATOR In order to determine the magnitude and direction of induced e.m.f, let us consider the different positions of the coil which has ‘N’ turns of wire. In one revolution following positions can be considered.
When initially coil is vertical, the length arms AC and BD are moving parallel to the lines of force maximum number of lines link the coil, but rate of change of flux is zero, hence emf is zero.
As the coil moves from this position, sides AC and BD begin to cut the lines of force and induced emf is setup till it is maximum when the coil is horizontal. It has o
rotated 90 , 1st quarter is completed. nd
Further rotation decreases the value of emf, until at the end of 2 quarter, when
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o
coil is vertical, it has rotated 180 , the emf is again zero.
As the coil rotates further from position 3 to position 4, an emf is again induced, but in reverse direction, because AC and BD are cutting the magnetic lines in opposite direction. The reverse emf reaches to – ve peak value at the end of rd
o
3 quarter. The coil is horizontal and angle of rotation is 270 .
On further rotation, the emf again decreases and becomes zero when the coil o
returns back to its original position after rotating 360 .
This shows that the coil of generator produces induced emf which reverse its direction 2*f times in one cycle. Where f = frequency of rotation of coil.
EXPRESSION FOR EMF IN A.C. GENERATOR
Consider a coil ABCDA of ‘N’ turns rotating in a uniform magnetic field B with a constant angular speed ‘w’. Let the length of the coil is ‘l’ and its breadth is ‘b’. To calculate emf in sides AC and BD we proceed as follows:
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Motional emf = BvlSinq
Emf in side AC = BvlSinq = x 1 Emf in side BD = BvlSinq = x 2 Emf induced in the coil = x1 + x2 = BvlSinq + BvlSinq
x = 2 BvlSinq
If coil has ‘N’ turns, emf will increase N times
x = 2 BvlNSinq ----(1) If angular velocity of coil is ‘w’ and it takes time ‘t’ to cover angle q then q = wt also V = rw and r = b/2 V = b/2w Putting the value of q and V in eq. (1) x = 2B (b/2w)lNsin(wt) x = wB(b*l)sin(wt)
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x = NBw(b.l)sinwt x = NBwAsinwt – (2) For latest information , free computer courses and high impact notes visit : www.citycollegiate.com this is the expression for the induced emf in the coil of an A.C generator at any instant. If f = no. of rotation per sec. Then we have w=2pf x = V = NBwAsin(2pft) – (3) o
for maximum emf q = 90 or 270
o
or 2pft = p/2 or 3p/2 o
and sin90 = sin p/2 = +1 o
sin270 = sin3p/2 = -1 xo = Vo = NBwA(1) xo = Vo = +- NBwA +- = shows direction of induced current Relation b/w x and xo x = NBwAsin(2pft) x = xosin(2pft)
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THEORY OF OPERATION A simple AC generator consists of: (a) a strong magnetic field, (b) conductors that rotate through that magnetic field, and (c) a means by which a continuous connection is provided to the conductors as they are rotating (Figure 3). The strong magnetic field is produced by a current flow through the field coil of the rotor. The field coil in the rotor receives excitation through the use of slip rings and brushes. Two brushes are spring-held in contact with the slip rings to provide the continuous connection between the field coil and the external excitation circuit. The armature is contained within the windings of the stator and is connected to the output. Each time the rotor makes one complete revolution, one complete cycle of AC is developed. A generator has many turns of wire wound into the slots of the rotor.
Figure 3
Simple AC Generator
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The magnitude of AC voltage generated by an AC generator is dependent on the field strength and speed of the rotor. Most generators are operated at a constant speed; therefore, the generated voltage depends on field excitation, or strength. The frequency of the generated voltage is dependent on the number of field poles and the speed at which the generator is operated, as indicated in Equation (10-1).
where
In this manner, the units of frequency (hertz or cycles/sec.) are derived.
Losses in an AC Generator
The load current flows through the armature in all AC generators. Like any coil, the armature has some amount of resistance and inductive reactance. The combination of these make up what is known as the internal resistance, which causes a loss in an AC generator. When the load current flows, a voltage drop is
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developed across the internal resistance. This voltage drop subtracts from the output voltage and, therefore, represents generated voltage and power that is lost and not available to the load. The voltage drop in an AC generator can be found using Equation (10-2).
where
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