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,

Î¦ = useful flux per pole in webers

P = total number of poles

Z

_{p = 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 Î¦ , each stator conductor cuts flux PÎ¦.

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 =

__P____Î¦____= nPÎ¦ .....................................(1).__
1/n

We know that

f = PN/120 = Pn/2

Pn = 2f

these value put in equation 1 we get ,

Eav/ conductor = 2fÎ¦ ........................................(2).

sinc there are Z

_{p}conductor in series per phase , the average voltage generated per phase is given by
Eav/ phase = 2fÎ¦ Z

_{p}_{}

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 = 4fÎ¦Tp

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 4fÎ¦Tp = 4.44fÎ¦Tp

the r.m.s. value of generated voltage per phase is given by

**Ep = 4.44fÎ¦Tp**

**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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