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Chapter 4: DC Generators

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Armature Reaction

• Current flowing in the armature coils creates a powerful magnetomotive force that distorts and weakens the flux coming from the poles

• Considering the armature along, 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

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Armature Reaction

• 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

• 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 (positions 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

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Improving Commutation

• 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

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Commutating Poles

• 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

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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

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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 No-load Saturation Curve

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Equivalent Circuit Model

• Circuit model development– armature circuit

• windings containing a set of identical coils and possessing a certain resistance, which can be modeled as a series resistance w.r.t. the terminals

• total armature resistance R0 is measured between the terminals when the machine is at rest

• resistance is in series with the induced voltage, which is represented by a voltage source, E0

– field winding circuit

• winding containing a set of identical coils in series

• total field resistance Rf

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Loading Characteristics

• Consider the generator operating under constant speed and field excitation– the exciting current is controlled by

a potentiometer

– the induced voltage E0 is fixed

• The voltage at the terminals E12

– is equal to the induced voltage E0

at no-load current condition, I = 0

– decreases as the load current increases

– E0 also decreases under load because of pole-tip saturation

Load Characteristic Curve

( ) IRIEE ⋅−= 0012

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Shunt Generators

• A shunt-excited generator is a machine with the field winding 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, IX

– 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 E0, which feeds back to increase IX

– E0 increases until Rf and the saturation limits the feedback

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Voltage Control

• 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 E0 is determined from the saturation curve and Rf

– it is the intersection of the Rf line and the voltage curve

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Shunt Generator Under Load

• The terminal voltage of a self-excited shunt generator falls off more sharply with increasing load than that of a separately excited generator– the field current in a separately

excited generator remains constant under any load

– the field current in a shunt generator is a function of the terminal voltage

– increased loading causes a drop in terminal voltage and consequently a drop in excitation current

• For a self-excited shunt generator the voltage drop from no-load to full-load is about 15% of the full-load voltage– for separately excited

generators, it is less than 10%

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Compound Generator

• The compound generator prevents the terminal voltage of a shunt generator from decreasing with increasing load– a compound generator is

similar to a shunt generator except that it has additional field coils connected in series with the armature circuit

– these series field coils are composed of a few turns of heavy gage wire for carrying the armature load current

– the total resistance of the series coils is very small

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Equivalent Circuit

• At no-load, the current in the series coils is zero– the shunt coils carry the

excitation current, IX to produce the field flux

• As load increases– the terminal voltage tends to drop,

but the load current IC now flows through the series field coils

– the mmf developed by the series field coils acts in the same direction as the mmf of the shunt field coils

– the flux increases under rising load

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Differential Compound Generator

• In a differential compound generator, the mmf of the series field acts opposite to the shunt field– under load, the terminal voltage falls drastically with

increasing load

– the series field circuit is reversed in polarity to make a compound generator into a differential compound generator

– useful in welding applications

– limits short-circuit currents

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Loading Characteristics

• loading characteristics of several generator types