Abstract: The loss of control of current or voltage in traction motor feedback tests is analyzed based on voltage balance and power balance, and the restrictive conditions of non-loss of control is obtained. Since the loss of control is caused by the difference in magnetic field characteristics of motors, a method by regulating the magnetic field is put forward to eliminate the phenomenon of loss of control. The magnetic field regulating device and technical parameters are designed to meet the regulating requirements for all the motor tests.