We use compensating winding in larger rated dc generators and dc motors. In large dc machines like motors of rolling mills, turbo generators, etc. there may be a large fluctuation of the load. The fluctuation of the load leads to a sudden change in armature current in the machine. Since there is a change in the armature current, the effect of the armature reaction also changes. As a result, there will be a forward or backward shifting of field flux at every event of changing the load. In other words, the angular position of the magnetic neutral axis changes. Practically the brushes rest on the commutator segments cannot follow this changing alignment of the magnetic neutral axis. On the other hand, the sudden shifting of flux produces statistically induced EMF in the armature winding. The magnitude of the statically induced EMF depends on the rapidity the flux shifting. Sometimes this EMF may be so large that can cause serious sparking in the commutator brush contacts. Because of that sparking, there may be flashover between adjacent commutator segments. The flashover ultimately leads to a short circuit in the armature winding.
To overcome this problem in larger DC machines we use compensating windings. Because the function of the compensating winding is to neutralize the armature reaction of the machines.
Construction of Compensating Winding
We place the compensating winding on the pole faces of the field system in the machine. There is a number of slots in pole shoes to hold this winding on the pole faces. We connect this with the main armature winding in series so that the same current can flow through the armature winding as well as the compensating winding. But the winding is wound on a pole-shoe-slots in such a way that the direction of current in compensating winding is exactly opposite to the direction of the currents in the armature conductors just under that pole shoe.
Considering the cost and the room required for the arrangement, we only use compensating winding in large DC machines where fluctuation of load creates a serious problem in commutation.
Number of Conductors in Compensating Winding
The total number of conductors on the armature of a DC machine is Z and P is the number of poles. Hence,
If I is the current flowing through the armature, then
Ampere turns per pole of the compensating winding is 70% of armature ampere-turns per pole.
- EMF Equation of a DC Generator Step by Step Derivation
- Working Principle of DC Generator with Single Loop Model
- Types of DC Generator Separately and Self Excited
- Different Characteristics of DC Generators
- Characteristics of Series DC Generator (Self Excited)
- Characteristics of DC Shunt Generator (Self Excited)
- Characteristics of a Separately Excited DC Generator
- Characteristics of Compound DC Generator
- Construction of DC Generator
- Armature Winding Pole Pitch Coil Pitch Commutator Pitch
- Armature Reaction in DC Machine i.e. Generator and Motor
- Compensating Winding in DC Machines
- Commutation in DC machine and Reactance Voltage
- Methods of Improving Commutation
- Losses in DC Generator Core Copper & Mechanical Losses
- Uses or Applications of DC Generators
- DC Generators in Parallel Operation