Chapter 4: DC Generators. 9/8/2003 Electromechanical Dynamics 1

Transcription

1 Chapter 4: DC Generators 9/8/2003 Electromechanical Dynamics 1

2 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 9/8/2003 Electromechanical Dynamics 2

3 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 9/8/2003 Electromechanical Dynamics 3

4 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 9/8/2003 Electromechanical Dynamics 4

5 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 9/8/2003 Electromechanical Dynamics 5

6 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 9/8/2003 Electromechanical Dynamics 6

7 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 9/8/2003 Electromechanical Dynamics 7

8 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 R 0 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, E 0 field winding circuit winding containing a set of identical coils in series total field resistance R f 9/8/2003 Electromechanical Dynamics 8

9 Loading Characteristics Consider the generator operating under constant speed and field excitation the exciting current is controlled by a potentiometer the induced voltage E 0 is fixed The voltage at the terminals E 12 is equal to the induced voltage E 0 at no-load current condition, I = 0 decreases as the load current increases E 12 ( I ) R I = E0 0 E 0 also decreases under load because of pole-tip saturation Load Characteristic Curve 9/8/2003 Electromechanical Dynamics 9

10 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, 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 0, which feeds back to increase I X E 0 increases until R f and the saturation limits the feedback 9/8/2003 Electromechanical Dynamics 10

11 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 E 0 is determined from the saturation curve and R f it is the intersection of the R f line and the voltage curve 9/8/2003 Electromechanical Dynamics 11

12 Shunt Generator Under Load The terminal voltage of a selfexcited 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% 9/8/2003 Electromechanical Dynamics 12

13 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 9/8/2003 Electromechanical Dynamics 13

14 Equivalent Circuit At no-load, the current in the series coils is zero the shunt coils carry the excitation current, I X 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 9/8/2003 Electromechanical Dynamics 14

15 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 9/8/2003 Electromechanical Dynamics 15

16 Loading Characteristics loading characteristics of several generator types 9/8/2003 Electromechanical Dynamics 16