The mixed frequency testing is basically used for determining the temperature rise a machine for which direct loading is not possible due to orientation or test plant limitations. In this method, the electrical power at two different frequencies is fed to the motor. Mixing of two frequencies is motor results into a flow of rated current to the machine winding. The current is adjusted by adjusting the voltage and frequency of auxiliary source. The test is covered in IEC-37 / IEEE-12 / Draft IEC.