Compensating Winding in DC Machines

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.

compensating winding in dc machines

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,

Then

Again,

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.

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