The terminal voltages of respective phase coils of a steering shaft driving motor
constituted by a three-phase brushless motor are separately detected. When the
terminal voltages detected at at least three different conducting terminals u,
v, and w are made Vu, Vv, and Vw, respectively, and the values when those detected
values Vu, Vv, and Vw are arranged in order from largest to smallest voltage are
made V1, V2, and V3, respectively, provided that
V1V2V3, a calculation is performed
to determine if V1+V3 coincides with 2*V2 within
a predetermined allowable range, and a failure is determined based on results obtained
from such a calculation.
