SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

In addition to measuring voltages with respect to the low side or the midpoint, the magnitudes of
differential voltages (such as the voltage between a terminal and the load neutral) are also
specified in terms of VDC and VBUS as shown in Figure 4.4.
Figure 4.4 – Differential voltage measurement.
The neutral point of the load is marked N and the midpoint of the DC bus is marked M. Voltage
at the output of the inverter (which is the input to the load) is called pole (or terminal) voltage;
voltage across the phase of the load is called the line-neutral (or phase) voltage; voltage between
terminals is called the line-line (or line) voltage. Examples of notation are v AM , v AN , and v AB ,
respectively. The neutral voltage is measured with respect to the midpoint: v NM .
Gating and Leg Control Schemes
At the highest level we can classify any three-phase VSI as either a 120° inverter or 180°
inverter. These definitions can be established according the way each leg is controlled or
according to the period over which the transistors are not OFF. These definitions are essentially
the same but both will be examined, in the order listed.
In a 180° inverter the top and bottom transistors in one leg are switched in a complementary
fashion. Each pole voltage is equal to either high (+VDC) or low (–VDC). In a 120° inverter the top
and bottom transistors in one leg are controlled separately, thus it is possible for both transistors
to be OFF. Each pole voltage may be high (+VDC), low (–VDC) or undefined. (When both
transistors in one leg are OFF the pole voltage will be determined by the current flowing trough
that phase of the load into the clamping / free-wheeling diodes.) The allowable leg states for a
120° inverter are shown in Figure 4.5, where the transistors are symbolized by switches and
phase-A is used as an example.
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