Leakage Fluxes in Rotating Machines

Leakage Fluxes in Rotating Machines

 AU: May-11

• In machines, there are two parts to which leakage fluxes are associated which are,

i) Main poles and ii) Armature flux

• Leakage flux in main poles : The main poles of d.c. machines and synchronous machines are excited by d.c. supply. The main difference between the two is that the poles of d.c. machines are on stator while the poles of synchronous machines are on rotor. The flux which is generated by main poles, entering the armature passing through the air gap is called useful flux. While the flux which links with the poles only without linking with the armature is called a leakage flux. The possible paths of leakage flux in d.c. and a.c. machines are shown in the Fig. 2.13.1 (a) and (b) respectively.

• Due to leakage flux, the flux density at the root of the poles increases but without contributing to the useful flux.

• Leakage flux in armature : The armature carries the distributed winding and it is slotted hence the air gap is not uniform. Hence there are many types of leakage fluxes associated with an anrmature.

• Thus the total leakage flux associated with an armature is divided into following types.

• Slot Leakage: The leakage flux associated with stator and rotor conductors flowing across the slots, having path from tooth to tooth is called slot leakage flux.

• The slot leakage flux is shown in the Fig. 2.13.2

• This flux path is perpendicular to the path of the main flux. The slot leakage depends on the shape of the slot. If the slot openingis narrow, it is larger while it is lesser if slot opening is wide. Thus for semienclosed slots having narrow opening, slot leakage is more, which is common in induction machines while for open slots it is less, which is common in d.c. and synchronous machines.

• Tooth-tip Leakage: This type of leakage flux flows from tip of the one tooth to the adjacent tooth tip, surrounding all the conductors, as shown in the Fig. 2.13.3.

• This type of flux is more in the machines where air gap is larger.

• In induction machines, the air gap is very narrow hence tooth-tip flux is less in induction machines than the d.c. and synchronous machines.

• Zig-zag Leakage : In the machines like wound rotor induction machines, both stator and rotor are slotted for accomodating the windings. Thus flux takes the path which alternates between stator teeth and rotor teeth as shown in the Fig. 2.13.4. The path is zig-zag in nature hence it is called zig-zag leakage flux. This is the characteristics of the induction machines.

Overhang Leakage: While designing the stator and rotor windings, the end conductors are necessary. The flux associated with such end conductors is called overhang leakage flux. This is shown in the Fig. 2.13.5. Mainly its path is through the air but some times part of it may complete its path through the iron core. It is absent in the squirrel cage induction machines as there is no overhang.

• Rotor Bar Leakage: When the rotor uses the bars then the leakage flux exists across the rotor bar circuit as shown in the Fig. 2.13.6. This flux is called rotor bar leakage flux. This must be considered while determining the rotor circuit leakage reactance as it contributes to decide the starting performance of the machine.

• The leakage flux mainly responsible for the leakage reactance. In d.c. machines and synchronous machines, the d.c. supply is used hence leakage flux is insignificant for such machines. But in a.c. machines, it affects the steady state and starting performance hence must be considered at the time of designing the machine.

Review Question

1. Write short notes on various leakage fluxes.

AU: May-11, Marks 8