The air-to-air cooling system is the cooling system where the thermal exchange is made by the air-to-air heat exchanger, usually implanted in the upper part of the machine. With this system the machine is completely closed and can be used in more rigorous environments.

In both systems the internal fan establishes the circulation of the air through the hot parts of the motor and between the tubes of the heat exchanger. The external fan represses the external air circulation inside the heat exchanger tubes.

When these two air flows are adverse, the exchanger is of opposite currents and when they are in parallel direction the exchanger is called parallel chains. As a rule, the exchanger used is opposite chains, also called countercurrents, because they have greater efficiency in the thermal exchange.

In accordance with IEC-60034-6 the cooling system is called IC 0611 when the two fans, internal and external, are placed on the axis of the machine composing a self-ventilated system. According to this standard, when the two fans are connected by independent motors the cooling system is called IC 0666

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