Basic Theory of Electromagnetic Power Conversion



Generic structure of electrical machines


Electrical machines convert electrical energy to mechanical energy and vice-versa. In the motoring mode, the electrical power is converted into mechanical power. All the machines have a stationary part called as stator and a rotating part, called rotor. They are separated by an air gap thus allowing the rotor to rotate freely on shaft, supported by bearings. The stator is firmly fixed to a foundation to prevent it from turning. Both stator and rotor are made up of high permeability ferromagnetic material and the length of the air gap is kept as small as possible (of the order of 0.5-1 mm) so that the ampere turns required to establish the flux crossing the airgap are very small.
You should also note that to reduce eddy-current losses, the stator and rotor are consist of laminations of silicon steel (see photo below). These laminations are insulated from each other by a layer of thin varnish.



These laminations are stacked together, perpendicular to the shaft axis. Slots may be cut into these laminations to place the conductors.

Salient and non-salient pole machines

If rotor and stator are perfectly cylindrical, the air gap is uniform and the magnetic reluctance (similar to the resistance in electric circuits) in the path of flux lines crossing the air gap is uniform.



Machines with such structures are called non-salient pole machines as shown in figure above. Sometimes, the machines are purposely designed to have saliency so that the magnetic reluctance is unequal along various paths as shown in figure. Such saliency results in what is called, the 'reluctance torque', which may be the primary or a significant means of producing torque.

Principles of operation


There are two basic principles that govern the operation of electric machines : Now, consider two bar magnets pivoted at their center on the same shaft. There will be a torque, proportional to the angular displacement of the bar magnets, which will act to align them. This physical picture is useful in analysing the torque production in machines. Currents in the machine windings create magnetic flux in the air gap between stator and rotor. This condition corresponds to the appearance of magnetic poles on both the stator and the rotor, centered on their respective magnetic axes as shown in figure.



Torque is produced by the tendency of these two magnetic fields to align. The resulting torque is proportional to the product of the amplitudes of the stator mmf and rotor mmf , and sine of the angle measured from the axis of stator mmf wave to that of the rotor. Therefore the generalized expression for torque is given by



Do note that all the flux does not link both stator and rotor. It is the mutual flux (flux which links both stator and rotor) which is responsible for output power. The flux, which does not link both stator and rotor is called as leakage flux. Leakage flux does affect the machine performance.
From the diagram it can be seen that the above torque equation can be written as



A steady torque is developed only when both the fields are stationary with respect to each other. This is the essential condition for the machine to develop steady torque.