Lecture 14Electro Mechanical System1  Current flowing in the armature coils creates a powerful magnetomotive force that distorts and weakens the flux coming from the poles  Considering the armature alone, the armature current produces a magnetic field that acts at a right angle to the field produced by the poles  The flux intensity depends on the current Armature Reaction

Lecture 14Electro Mechanical System2  Contrary to the field flux, the armature flux is not constant, but varies with the load  The flux in the neutral zone is no longer zero, and a flux is induced in the coils shorted by the brushes Armature Reaction  The armature mmf distorts the flux produced by the poles  The neutral zones have shifted in the direction of rotation  Flux is concentrated at the far end of the poles (pole tip 2 & 3 in fig.)  Increase in flux causes saturation to set in the far ends  The total flux produced by the poles is less than when the generator runs at no load

Lecture 14Electro Mechanical System3  The shift in the neutral zone causes an increase in arcing  We can move the brushes in the direction of rotation to reduce the arcing  For time varying loads, the fluctuating current raises and lowers the armature magnetic- motive force and the neutral zone shifts back and forth  It is not practical to continuously move the brushes to minimize the arcing  For small machines the brushes are set in an intermediate position to ensure reasonably good commutation at all loads Improving Commutation

Lecture 14Electro Mechanical System4  In larger machines, a set of commutating poles are placed to counter the effect of armature reaction  Narrow poles carry windings that are designed to develop a MMF equal and opposite to the MMF of the armature  As load current varies, the two MMF’s rise and fall together  The vertical component of the field is nullified and the neutral zone is restored Commutating Poles

Lecture 14Electro Mechanical System5 Separately Excited Generators  Instead of using permanent magnets to create the magnetic field, pairs of electromagnets called field poles are employed  Separately excited field poles are supplied by an independent current source  Batteries or another generator  The current source is referred to as the exciter

Lecture 14Electro Mechanical System6 Machine Saturation Curve  In a separately excited, no- load, generator a change in excitation current causes a corresponding change in the induced voltage  The saturation curve relates the flux produced to the current  For small currents, the flux is linearly proportionate  At higher currents, the flux output decreases due to iron saturation  The segment from a to b is the saturation knee  The induced voltage curve is identical to the flux curve

Lecture 14Electro Mechanical System7 Shunt Generator  A shunt-excited generator is a machine with the fieldwinding in parallel with the armature terminals  This eliminates the need for an external source of excitation  The generator becomes self-exciting  Starting the self-excitation  Remanent flux in the pole induce a small armature voltage when there is rotation  The voltage produces a small exciting current, I X  This results in a small mmf, acting in the same direction as the remanent flux and causing the flux per pole to increase  The increased flux raises E O, which feeds back to increase I X  E O increases until R f and the saturation limits the feedback

Lecture 14Electro Mechanical System8 Controlling the voltage of shunt generator  The induced voltage of the shunt generator is easily controlled by varying the excitation current by means of a rheostat connected in series with the shunt field coil  The no-load value of E O is determined from the saturation curve and R f  It is the intersection of the R f line and the voltage curve