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. shunt armature 他励 并励 series 串励 复励 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）

supplied with external supplies. 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） shunt armature 他励 并励 series 串励 复励 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.

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 attention to the airgap field and discuss its features.

2). No-load Airgap Field At no-load, armature current Ia=0. Airgap field is established only by the exciting current of field windings . Mmf distribution is a rectangular wave. Flux density distribution is similar to mmf, except in the interpolar region where there are some slopes due to large reluctance.

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 Mmf distribution is a triangular wave. From Ampere’s Law, Draw a closed path and the mmf acting on the path is --- electric load（电负荷） I --- conductor current N --- number of conductors D --- armature diameter

For each air gap, Features: 1). Triangular wave 2). At --- pole pitch 3). The is at the point where the direction of current is changed. 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.