A slot conductor is a copper coil wound in the armature slots of an induction motor. It is insulated from other coils, from the core, and from the rotor iron part. As the rotor spins, the magnet passes through the low reluctance rotor metal parts. Since the magnetic flux density is quite high there, little cut on the conductors inside the slots is made. As a consequence, no power is lost on the conductors and the rotor can be turned with little effort. Gboslot

However, there are some losses associated with the insulating material and the conductors themselves. To reduce these losses, the conductors can be shaped to better suit the slot geometry. A common method is to use rectangular rather than round conductors. This can also lead to a higher slot fill factor, which is important for good power transmission.

The slot fill factor is an expression of the number of turns per coil that fit within a single slot, divided by the total number of core teeth. It can be defined as an integer or fraction superior to 1. If it is an integer, it is called an integer-slot winding. If it is a fraction, it is called a fractional-slot winding. The winding layout is very important for the machine performance, because it is influencing the harmonic content present in the dc-link currents.

To estimate the slot impedance, a simple analysis technique is used that takes into account the conductor shape and the slot size. This can be compared with the actual machine performance in order to evaluate how accurate this analysis is. The results show that the technique is accurate enough for this purpose.

In the case of a copper rotor cage, the results are very close to those obtained for an aluminum rotor cage. This is in spite of the fact that the copper has a lower skin depth than the aluminum, which leads to higher frequency responses. The results are very encouraging and suggest that the technique is well suited for the estimation of rotor performance in permanent-magnet synchronous machines with a fractional-slot concentrated winding.

The analysis technique is based on the representation of the rotor bar as a ladder network. This is a simple circuit model consisting of inductance representing cross-slot flux and resistance, representing the current carried by each section. This is a very convenient way to determine the rotor leakages and resistances, because they are functions of both rotor speed and current. In addition, the ladder network is frequency dependent and thus a good representation of the rotor dynamics. This makes the technique very useful for rotor design and optimization. The disadvantage is that it is not as accurate as a full finite element analysis, which could be more complex but also less time consuming. The ladder network approach is nevertheless a very promising tool for the development of new induction machines. In the future, it might even replace some of the more complex models currently being used.