the poles of the rotor moves under the armature conductor on the stator , field flux cuts the armature conductors. therefor voltage is generated in these conductors. the voltage is of alternating nature, since poles of alternate polarity successively pass by a given stator conductor. A 3-phase alternator has a stator with three set of windings. arranged so that is a matual phase displacement of 120 . these winding are connected in star to a 3-phase output.the voltage generated in each winding is given as equation is called emf equation.
Let,
Φ = useful flux per pole in webers
P = total number of poles
Zp = total
number of conductors
Tp = total number of coils or tuns per phase
n = speed of rotation of rotor in revolution per second (r.p.s.)
f = frequency of generated voltage ( Hz )
Since the flux per pole is Φ , each stator conductor cuts flux PΦ.
The average value of generated voltage per conductor
= flux cut per revolution in Wb
time taken for one revolution in seconds
Since n revolution are made in one second, one revolution will be made in 1/n second. therefore the time for one revolution of the armature is 1/n second.
Eav/ conductor = PΦ = nPΦ .....................................(1).
1/n
We know that
f = PN/120 = Pn/2
Pn = 2f
these value put in equation 1 we get ,
Eav/ conductor = 2fΦ ........................................(2).
sinc there are Zp conductor in series per phase , the average voltage generated per phase is given by
Eav/ phase = 2fΦ Zp
since one turn or coil has two sides Zp = 2Tp , the expression for the average generated voltage per phase can be written as
Eav/ phase = 4fΦTp
for voltage wave , the form factor is given by
Kf = r.m.s. value/ average value
For a sinusoidal voltage , Kf =1.11 . therefore . the r.m.s. value of generated voltage per phase can be written as
Er.m.s. / phase = Kf x Eav / phase = 1.11 x 4fΦTp = 4.44fΦTp
the r.m.s. value of generated voltage per phase is given by
Ep = 4.44fΦTp
Equation has been derived with the following assumptions:
- Coil have got full pitch.
- all conductor are concentrated in one stator slot.
Great article. EMF Protection is a must nowadays.
ReplyDeleteCan you add some examples of this topic
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