magnetisation characteristics of dc shunt motor
purifier said:
Sorry...all this magnetisation characteristics are given under the topic DC Shunt Generator...
In DC shunt generator the field and armature circuit are in parallel.
Now say E is the generated armature EMF.
E=KΦω, where Φ is field flux and ω is generator speed or turbine speed and K is a proportionality constant.
Now Φ is dependent on I which is the field current.
Therefore E = GIω where G is a proportrionality factor which varies as the current varies. It depends on the µ of the core material.
Also I ≈E/Ra where Ra is the armature resistance.
Now you can generate two graphs for E vs I. One will look similar the B-H curve since B is proportional to Φ and hence depends on E while H is proportional to I.
The other is a straight line graph with a slope of Ra. The point of intersection of the two graphs gives us E and I.
Now if Ra is too high the straight line graph will coincide with the linear portion of the B-H curve so the resulting intersection will be at a low E and low I. So to avoid this situation Ra should be lower than a particular critical value. If you draw the two graphs you will understand it better.
Also in a DC generator some residual magnetism should be present or else when the generator is started Φ = 0 and hence E=0 irrespective of speed and therefore I is always 0. Another important point is that the field magnetism should aid rather than oppose the residual magnetism direction or else once again we'll end up with a Φ = 0 situation when the DC generator starts.