CHAPTER 2 BASIC THEORIES OF DC MACHINES

1
CHAPTER 2 BASIC THEORIES OF DC MACHINES
1. MODES OF FIELD EXCITATION(????)

• For generators

1). Separately excited(??)--- The field winding
is supplied from a separate power
source.
2). Self-excited(??)--- The field winding
is supplied by armature induced voltage.

• Exciting in shunt (??)
• Exciting in series (??)
• Exciting in compound (??)

2

• For motors Both field windings and armature
  winding are
• supplied with external supplies.

• Exciting in shunt (??)
• Exciting in series (??)
• Exciting in compound (??)
• Separately excited(??)

• Note
• Generators and motors have different referring
  directions.
• Shunt field winding has large turns but smaller
  current.
• Series field winding is contrary to shunt
  one.

3
2. ARMATURE REACTION (????)
1). Main Flux and Main Magnetic Circuit
Main flux --- passing through airgap and linking
with both stator and rotor.

Main magnetic circuit --- the path that main flux
flows through. 8 parts --- 2 main poles, 2
airgap, 2 rotor teeth, 1 rotor core
and 1 stator york.
Note (i). Since the reluctance in airgap is
much greater than the other parts, we can
assume that the total mmfs fall down on the
airgap.
(ii). As armature conductors are in the Airgap
field, we will just pay our attention to the
airgap field and discuss its features.
4
2). No-load Airgap Field
At no-load, armature current Ia0. Airgap
field is established only by the exciting
current of field windings .
5
3). Airgap Field at Load
At load, armature current is not zero. The
airgap field is excited by both the field
windings and armature winding.

• airgap field established only by armature
  current

I --- conductor current N --- number of
conductors D --- armature diameter
6
Features 1). Triangular wave

• Distribution of armature flux density Ba is a
  saddle curve

--- Having a dip in interpolar region due to
large reluctance
7

• Armature Reaction

Definition The effects of armature mmf to the
no-load field is referred to as armature
reaction.
(1). Making airgap field distortion
--- making flux density increase under one
half of the pole and decrease under the other half
(2). Producing demagnetizing effect if saturation
occurs.
--- If saturation occurs, the flux per pole
will be decreased
(3). Making the zero flux density point departure
from q-axis
--- leading to poor commutation
Compensating winding (????)

• Be mounted on the main pole face and connected
  in series with armature winding.

• The mmf produced opposes the armature mmf.