The magnetic circuit is a closed orbit of the magnetic field lines. When those lines are maintained with the magnetomotive force we get magnetic flux. The magnetic flux is a set of field lines that are passing through a surface. We distinguish between the main magnetic flux ( useful ) and leakage magnetic flux ( useless ). With these two phenomena in the coil of the magnetic circuit of the electrical machine we induce the electromotive force, which we need in order to achieve energy conversion. With the calculation of the magnetic circuit we can find out what exactly is happening within the circuit and determine how the strength of the magnetic field is distributed to the core and air gap, also determine the total flow and power within circuit. The basic shapes of the magnetic circuit of electrical machines depend on the construction of the electric machine, and are divided into magnetic circuits with a rectangular magnetic core and magnetic circuits with a round magnetic core. The materials used to produce the magnetic core must be magnetic materials, the most common type of materials that are used are ferromagnetic materials. Depending from which material is core made of, there can be major or minor losses within it. The losses are divided into hysteresis losses ( depending on the magnetic properties of the material ) and losses due to eddy currents ( depending on the electrical properties of the material ).