PURPOSE: To maintain high insulation property stably for a long period by suppressing the occurrence of the cracks of a cover for insulation while materializing the remarkable improvement of yield by enabling the injection molding and greatly lessening the breaking of a coil line at the time of injection molding.

CONSTITUTION: A cover 10 for insulation, which covers the periphery of the coil section 9 of a coil assembly 2, is made by the injection molding of thermoplastic rein such as LPC in which either fiber-shaped matter such as glass fibers, etc., or granular matter such as talc, etc., is mixed, and which is set to the melt viscosity of 200-500 poise under the condition of 250°C-340°C in molding temperature and 1000sec-1 in shearing speed.