Charge state: When command voltage 0 r U > ,
the duty cycle of the T1 and T2’s driving signal will be
greater than 50%, T1 and T2 close together for some time,
as shown in Fig 2 (a), a positive voltage with the amplitude
of DC V is applied to the bearing coil and the coil current
increases. The coil current after the switches T1 and
T2 on is expressed by