PURPOSE: To prevent the abnormal heating and abnormal cooling inside a generator, by providing a sensor to detect the temperature of coolant for engine and an ON-OFF control valve in accordance with the output of the sensor in the passage through which a part of coolant flows into the generator of a rolling stock.

CONSTITUTION: An electromagnetic valve 9 is provided at a hose to make the coolant circulate from a water jacket 3 to the coolant passage inside a generator 8 and has got an exciting coil 10, a movable valve 11 and a valve sheet 12. A sensor 13 detects the temperature of the coolant inside the water jacket 3. A control unit 14 inputs the output of the sensor 13, compares it with the preset value and energizes the exciting coil 10 when the output of the sensor 13 gets to the preset value and closes the valve 9. A connecting line 15 connects the control unit 14 to the exciting coil 10. Another connecting line 16 connects the control unit 14 to the sensor 13. When the temperature of an engine 1 comes up, the sensor 13 detects the temperature and outputs it to the control unit 14. When this output exceeds the preset value, the unit 14 introduces the current to the exciting coil 10, closes the valve 9, stops feeding the coolant and prevents the abnormal cooling.