3 Sensorless drive method
This section explains the sensorless drive method, which
utilizes characteristic of the armature current waveform of
the PM motor with voltage source inverter and determine
the commutation timing [5].
Two notable current increase regions are present in this
armature current waveform as shown in Fig. 7. The first
current increase region is decided by the applied voltage,
the back electromotive force and the armature impedance,
it becomes LR response waveform toward the armature
impedance voltage drop. The second current increase
region is present because of the range that the back
electromotive force decreases and that the armature applied
voltage increases. Meanwhile, the mild current variation
region is recognized between two notable current increase
regions. In this region, a q-axis current contained the
instantaneous current increases. In other words, it is shown
that the motor produces an effective torque.
The basic composition of the sensorless drive utilized
the characteristic of the armature current waveform is shown
in Fig. 8. The characteristic of the armature current waveform
appears in the waveform of DC part as well. Therefore, we
detect the waveform of the current of DC part, which means
that the armature current waveform of three phase is
detected in the lump. The current of DC part is converted in
value of voltage. Sampling is done in the mild variation
region of the instantaneous current as shown in Fig. 7, and
this sampling value is regarded as the torque current
estimation value I, . The value, which the torque current
estimation value is multiplied by commutation marker current
coefficient, becomes a commutation command value I,, .
The commutation reference CTe, is produced every time
that the instantaneous current I, of DC part reaches a
commutation command value I, .
In the proposed sensorless drive system, it is impossible
to obtain the commutation command value in the first
current wave. In order to avoid such a case, this system