In the air-to-air cooling system the thermal exchange is made by the air-to-air heat exchanger generally mounted on the top of the electric machine and with this system the machine is fully enclosed and can be applied in more aggressive environments. https://www.mrosupply.com/v-belts/cogged-v-belts/ax-section/181015_ax28_gates-rubber/ According to IEC-60034-6 the cooling system is called IC 0611 when the two internal and external fans are mounted on the motor shaft composed of a self-ventilated system. According to the standard, when the two fans are driven by autonomous motors the cooling system is called IC 0666.
In both systems the internal fan forces air circulation through the hot parts of the machine and through the heat exchanger tubes. The external fan will force external air circulation through the heat exchanger tubes. When these two air flows are adverse the exchanger is of opposite chains and when they are in the same direction the exchanger will be called parallel chains. Usually the exchanger applied is of opposite currents, also known as countercurrent, and present greater efficiency in the thermal exchange.