One important component of a rib cooled motor is the rigid cast iron frame. The frame design is closely tied to factors such as mechanical stability, ability to withstand harsh environments, and cooling efficiency.
The high voltage cast iron motors are totally enclosed and provided with external fan and cooling ribs. The cooling area of the ribs is large and the form of the cooling rib is optimized. The form of the ribs also keeps the air flow close to the motor surface along its entire length which means efficient cooling and a good self cleaning capability.
The inner surface of the frame and the outer stator core surface are accurately machined to accommodate a shrink fit. This minimizes the thermal resistance between the stator core and the frame and thus improves cooling.
Unidirectional fans are standard on two and four pole motors in order to reduce the noise level and windage losses. Bidirectional fans are used on lower speed motors. The fans are normally made of welded steel plate.