adhesion. Table I shows that the performance of manganese phosphate as slot coating is higher than zinc and close to the thin Fe3O4 film. The thick Fe3O4 film shows a good property to improve the motor efficiency when compared to bluing film and that can be associated to an increase in cross-resistance (better electrical insulation of rotor slot).
III. ROTOR ANNEALING
Rotor annealing is an important step of small rotor cage process if traditional high-pressure die-cast is used. The highpressure casting and fast aluminum solidification leads to very low rotor cage shrinkage, resulting in close mechanical and electrical contact of cage aluminum and rotor steel. If the coating of inner slot surfaces is poor, this contact produces multiple micro-welding points that cannot be eliminated by rotor annealing. Typically, the mechanical contact between rotor bars and steel can be relived, “detaching” the cage from steel. As a result, the electrical contact is decreased and the inter-bar resistance increased. Annealing cycle is very effective to reduce the inter-bar currents, leading to significant improvement in maximum torque and power factor. Additionally, annealing also relieves mechanical stress of cast aluminum, contributing to increase the aluminum conductivity. There are some standards types of rotor annealing. The thermal shock (also called hardening) consists in heating the rotor and next cooling it at cool water. The traditional heat treatment (also called tempering) involves in heating the rotor slowly to a desired temperature then holding at that temperature for long enough to enable the internal changes to take place and finally to cooling it slowly in air. The modern induction heating process relies on induced electrical currents within the material to produce heat. Advances in power electronics technology have made high-frequency induction heating a remarkably simple and cost-effective heating method for motor applications. To verify the effect of annealing in motor performance four ways of detaching rotor bars from steel were used: the thermal shock in cold water, inductive high frequency heating, heat treatment at 450°C (one cycle) and heat treatment at 450°C (several cycles). All assembled rotors have the laminates with thin Fe3O4 film and cage obtained by high-pressure die-casting process. Table II shows the obtained results. Table II shows that there is no significant difference among results obtained by annealing processes. Despite the minimal gain in results, the use of induction heating increases in motor industry because it is faster and cost effective.