Leakage flux :
The flux that links the coil only partially passes through the core does not link the coil as the moving or rotating device is called leakage flux.
This kind of flux through small links, each of coil only as a result, the leakage flux of any one member does not interact with current of the other member, and so it does not produce mechanical torque, i.e. it does not take part in energy conversion process. So it only cause voltage drops.