PURPOSE: To perform an online test economically.

CONSTITUTION: The current of respective phase is fed to analog OR-circuit 11a-11c, and in the case of an operation, the magnitude of the current signals is decided by a microprocessor 6, and whether an abnormality arises or not is decided. In the case of a test, to the analog OR-circuit of the phase to be tested, a testing signal is fed too, and the signal having the larger level than the ones of the other two phases is outputted for the test of the phase to be performed. But, since the current of the other two phases is monitored, an on-line test can be performed.