fig4 a to 4c and 5a to 5c fig4 a to 4c show drive circuits for the motor 7 fig5 a shows a drive torque of the motor 7 fig5 b shows supply currents to the coils 15a and 15b of the motor 7 and fig5 c shows a time chart of base potentials in transistors of the motor drive circuit as shown in fig4 a to 4c the coils 15a and 15b are connected to mid point of a bridge circuit comprising four transistors a 1 to a 4 or b 1 to b 4 and reversal currents shown in fig5 b are supplied to those coils the coils are driven in the single phase excitation system and the torque characteristic thereof is shown by a solid line in fig5 a fig5 c shows a time chart of base potentials of the drive transistors a 1 to a 4 and b 1 to b 4 the abscissas in fig5 b and 5c are equal to that of the torque characteristic of fig5 a as seen from those time charts when one of the coils 15a and 15b is not energized it is shorted as shown in fig4 c when the coil 15a is excited the coil 15b is shorted and when the coil 15b is excited the coil 15a is shorted thus when.