How to Reduce Armature reaction in DC Machine
Armature Reaction refers to a phenomenon in DC machines where the magnetic field of the poles is distorted by the magnetic field produced by the armature current.
1) The armature reaction causes the distortion in main field flux. This can be reduced if the reluctance of the path of the cross-magnetizing field is increased.
- The armature teeth and air gap at pole tips offer reluctance to armature flux. Thus by increasing length of air gap, the armature reaction effect is reduced.
- By using special construction in which leading and trialing pole tip portions of laminations are alternately eliminated.
- It is always placed in series with armature winding.
- The armature ampere conductors under pole shoe must be equal to compensating winding ampere conductors which will compensate armature m.m.f. perfectly. This method is very popularly used in actual practice for d.c. machines.
Methods to reduce Armature reaction in DC Machine
- Interpoles are similar to the main field poles and located on the yoke between the main field poles.
- They have winding in series with the armature winding.
- Interpoles have the function of reducing the armature reaction effect in the commutating zone. They eliminate the need to shift the brush assembly.
2. Compensating Winding
- The compensating winding are basically used to neutralize the armature flux in the pole arc region which will otherwise cause sever distortion of main field flux.
- By using high reluctance pole tips, reduction in armature flux and using strong main field flux poles we neutralize the armature reaction.
- These compensating winding are of concentric type and are placed in axial slots in the pole faces.
In case of large machines having large fluctuations in load, the effect of armature reaction is produce and sudden shift of flux oscillate depending on change in the load. This will cause due to statically induced e.m.f. and dynamically induced e.m.f. in the armature coil.
In worst conditions these two e.m.f.s may become additive. This will occur when load is increased on motor and decreased from generator.
- If this e.m.f. is more than the breakdown voltage across adjacent commutator segments, a spark over may occur which can easily spread over as conditions near commutator are favorable for flash over.
- The maximum allowable voltage between the segment is 30 to 40 V. Thus there is always danger of short circuiting the whole armature if armature flux is not compensated.
- These winding are connected in series with the armature.
- The current in this winding flow in opposite direction to that in armature conductors below the pole shoes.
- This will counterbalance the cross magnetizing effect of armature reaction which may cause flash over between the segments.