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EMF Equation of an Alternator and AC Generator

EMF Equation of an Alternator / AC Generator

An alternator or AC generator (dynamo) is a device which convert mechanical energy to electrical energy. When we supply the magnetizing current by DC shunt generator through two slip rings (in recent alternators, they use electronic starting system instead of slip rings and commutators) because the field magnets are rotating. keep in mind that most alternators use a rotating magnetic field with a stationary armature.

When the rotor rotates, the stator conductors which are static in case of alternator cut by magnetic flux , they have induced EMF produced in them (according to Faraday’s law of electromagnetic induction which states that if a conductor or coil links with any changing flux, there must be an induced emf in it.

Note: We will discuss the construction, Working & Operation. Types of Alternators in details in our next posts.EMF Equation of an Alternator and AC Generator

This induced EMF can be found by the EMF equation of the alternator which as follow:


P = No. of poles

Z = No. of Conductors or Coil sides in series/phase i.e. Z = 2T…Where T is the number of coils or turns per phase (Note that one turn or coil has two ends or sides)

f = frequency of induced EMF in Hz

Φ = Flux per pole (Weber)

N = rotor speed (RPM)

Kd= Distribution factor = Kd= Distribution factor for emf equation of alternator and ac generator

Where Distribution factor = Kd =distribution factor

Kc or KP = Cos α/2

If induced EMF is assumed sinusoidal then,

Kf = Form factor = 1.11

In one revolution of the rotor i.e. in 60/N seconds, each conductor is cut by a flux of ΦP Webers.

dΦ = ΦP and also dΦ = 60/N seconds

then induced e.m.f per conductor ( average) = EMF EQUATION OF AN ALTERNATOR OR AC GENERATOR ….. (i)

But we know that:

f = PN / 120 or N= 120f / P

Putting the value of N in Equation (i), we get,

Average value of EMF per conductor = EMF EQUATION OF AN ALTERNATOR(N= 120f/P)

If there are Z conductors in series per phase,

 then average e.m.f per phase = 2 f Φ Z Volts = 4 f ΦT Volts ….. (Z=2T)

Also we know that;

Form Factor= RMS Value / Average Value

= RMS value= Form factor x Average Value,

= 1.11 x 4fΦT = 4.44fΦT Volts.

(Note that is exactly the same equation as the EMF equation of the transformer)

And the actual available voltage per phase

= 4 Kc Kd f ΦT = 4 Kf Kc Kd f ΦT Volts.

Note: If alternator or AC generator is star connected as usually the case, then the Line Voltage is √3 times the phase voltage as derived from the the above formula.

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