MAGNETIC, ELECTRIC, AND INSULATION MATERIALS FOR IM
Induction machines contain magnetic circuits travelled by a.c. and traveling magnetic fields and electric circuits flowed by alternative currents. The electric circuits are insulated from the magnetic circuits (cores). The insulation system comprises the conductor insulation, slot, and interphase insulation. Magnetic, electrical, and insulation materials are characterized by their characteristics (B-H curve, electrical resistivity, dielectric constant and breakdown electric field (V/m)), and their losses. At frequencies encountered in IMs (up to tens of kHz, when PWM inverter fed), the insulation losses are neglected. Soft magnetic materials are used in IM as the magnetic field is current produced. The flux density (B)/magnetic field (H) curve and cycle depend on the soft material composition and fabrication process. Their losses in W/kg depend on the B-H hysteresis cycle, frequency, electrical resistivity, and the a.c. (or) traveling field penetration into the soft magnetic material. Silicon steel sheets are standard soft magnetic materials for IMs. Amorphous soft powder materials have been introduced recently with some potential for high frequency (high speed) IMs. The pure copper is the favourite material for the stator electric circuit (windings), while aluminum or brass is used for rotor squirrel cage windings. Insulation materials are getting thinner and better and are ranked into a few classes: A (105°C), B (130°C), F (155°C), and H (180°C).