PURPOSE: To cool a rotary machine with air at an optimum flow rate by detecting outside air temperature and exhaust air temperature and adjusting the flow rate of air of a blower based on the outside air temperature and exhaust air temperature thereby preventing not enough cooling and excessive cooling, in the cooling control method for a separately ventilated rotating machine.

CONSTITUTION: For example, in a separately ventilated DC motor 21 for driving the roller 24 of a cold rolling mill for a thin steel plate, the casing is provided, at the intake of outside air from the blower 23, with a first temperature measuring device S1 and, at the exhaust port 27, with a second temperature measurement device S2. Moreover, the rotational speed of a motor 28 for a blower is measured by a speedometer 30. By an operation means, the required flow rate of cooling air is found from the temperature measured by the first temperature measuring device S1 and the quantity of heat on calculation of the DC motor 21 is found, and also the present calorific value is found from the difference of temperatures measured by the first temperature measuring device S1 and the second temperature measuring device S2. And the optimum flow rate of air corresponding to the required flow rate cooling air and the calorific value is calculated, and the number of revolutions of the blower 23 corresponding to this optimum flow rate of air is determined to control the revolution of the motor 28 for a blower.