CHAPTER 2 BASIC THEORIES OF DC MACHINES

1. MODES OF FIELD EXCITATION（励磁方式） For generators :

1). Separately excited（他励） --- The field winding is

supplied from a separate power source.

2). Self-excited（自励） --- The field winding is supplied by armature induced voltage.

Exciting in shunt （并励） Exciting in series （串励） Exciting in compound （复励）

shunt

armature他励

shunt

并励

series

串励

shunt

series

复励

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

Exciting in shunt （并励） Exciting in series （串励） Exciting in compound （复励） Separately excited（他励）

shuntshunt

armature 他励 并励

series

串励

shunt

series

复励

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

2. ARMATURE REACTION （电枢反应）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 mmf’s fall down on the airgap. (ii). As armature conductors are in the Airgap field, we will just pay our attentionto the airgap field and discuss its features.

2). No-load Airgap FieldS N

fB

aB

B

n

ox x

fF

aF

(Generator) At no-load, armature current Ia=0. Airgap field is established

only by the exciting current offield windings .

Mmf distribution is a rectangular wave.

fF

Flux density distribution is

similar to mmf, except in the

interpolar region where there

are some slopes due to large

reluctance.

fB

3). Airgap Field at Load At load, armature current is not zero. The airgap field isexcited by both the field windings and armature winding. airgap field established only by armature current

• Mmf distribution is a triangular wave.aFFrom Ampere’s Law,

idlH

Draw a closed path and the mmf acting on the path is

AxFax 2'

)/( mAD

NIA

--- electric load（电负荷）

I --- conductor current N --- number of conductors D --- armature diameter

For each air gap,

AxFax

Features: 1). Triangular wave

3). The is at the point where the direction

of current is changed. max)( axF

AFx ax 2)(,

2 max

2). At

--- pole pitch

• Distribution of armature flux density Ba is a saddle curve

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

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 （补偿绕组）• Be mounted on the main pole face and connected in series with armature winding.• The mmf produced opposes the armature mmf.