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
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 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 = PÎ¦   = 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  Zp  conductor in series per phase , the average voltage generated per phase is given by

Eav/ phase = 2fÎ¦ 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 = 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.

### 2 Comments

1. Great article. EMF Protection is a must nowadays.

2. Can you add some examples of this topic