DC GENERATORSIntroduction

• The outstanding advantages of dc machines arise from the wide variety of operating characteristics which can be obtained by selection of the method of excitation of the field windings.

• There are four basic kinds of dc generators, namely “separately excited”, “shunt”, “series” and “compound” generators

DC Separately Excited Generators• The required field

current is a very small fraction of the rated armature current of the order of 1 to 3 percent in the average generator.

• A small amount of power in the field circuit may control a relatively large amount of power in the armature circuit, i.e., the generator is a power amplifier.

• Separately excited generators are often used in feedback control systems when control of the armature voltage over a wide range is required.

DC Shunt Generators

• The field is a function of the terminal voltage, Vt. Hence, voltage decrease at the armature terminals cause a corresponding decrease in the field flux and hence in the induced emf, Eg in return a decrease in Vt again. This continues until an equilibrium is found.

• The reverse of the above process is followed in building up V t.

DC Shunt Generators (Cont.)

For a field resistance rf, the operating point is the intersection point of the rf

line and magnetization curve.

DC Shunt Generators (Cont.)

Its output voltage sharply decreases with the armature reaction as it is ecpected. The effect of the armature reaction is more severe for this kind of generator.

t g L f aV = e - (I + I ) r

DC Series Generators

• The field current of a series generator is the same as the load (or armature) current, so that the air gap flux and hence the voltage vary widely with load.

• As a consequence, series generators are not very often used.

DC Series Generators (Cont.)• Example: An 8-pole, lap wound, dc machine has 60 armature conductors is

series between the brushes. The armature resistance, ra=0.04ohm and the field resistance rs=0.02ohm. When connected as a series generator and run at 1000rpm, it delivers a certain amount of current to a load at a terminal voltage, Vt=126V. If it is run at 700rpm, the terminal voltage decreases to 86V for the same load current. Calculate:

a) the direct-axis flux/pole

b) the power delivered to the load under consideration at a shaft speed of 1200rpm.

Soln: conductors and a=p=8 for this machine. So

Z60

aa a

240K =

t g a a sV = e - I (r + r )

1g a126 = e - I (0.04 + 0.02)

2g a86 = e - I (0.04 + 0.02) 1 2g ge - e = 40V

1g a d 1e = K w

2g a d 2e = K w 1 2g g a d 1 2e - e = 40 = K (w -w )

DC Series Generators (Cont.)

3g

240 1 1200 960e = 2 = =160V

60 60 6

d

40 10.0167Web

240 2 60(1000 700)60

1g

240 1 1000 800e = 2 = =133.3V

60 60 6

a

133.3-126 7.33I = = =122.2A

0.06 0.06

tV =160 -122.2x 0.6 =160 -7.33 =152.7V

outP =152.7 x122.2 =18.5kW

• In most of the usual applications of the dc generators the desired point is to have the same voltage at both no-load and full-load.

• A suitable combination of shunt and series generators gives the compound generator.

DC Compound Generators

DC Compound Generators (Cont.)• The required number of turns in the series field winding is relatively

small since the contribution of this winding is just to compensate the small terminal voltage drop with the loading. So this kind of compound generator is called the “cumulatively compound” generator.

DC Compound Generators (Cont.)

• The series field winding can be purposely connected such that its flux does not add to that of the shunt field, then the resultant airgap flux is the difference of the series field flux from the main shunt field flux.

DC Compound Generators (Cont.)Example: A cumulative compound dc generator delivers 15 A to a 10 ohms load. (ra+rs) and rf are equal to 1 and 250 ohms, respectively. The magnetization curves for shunt and series fields are shown in Fig. in piecewise linearized form for 1500 rpm. Find the shaft speed in rpm and the electrical torque acting on the shaft.