The Y-configured motor windings, with terminals Y and Z driven and X open, are repeated below from Part 2:

In the Y configuration, phase-windings are driven in series pairs (YZ), with the open phase-winding (X) undriven.

The magnitude and phase components of the stator flux vector are the controlled variables of the motor. Rotor induced voltage is the sensed variable from which speed and rotor angle are derived. The induced-voltage waveforms for the three phase-windings cross zero twice per electrical cycle, resulting in six zero-crossings. These zero-crossings can be sensed by low-cost comparators to provide rotor phase information, for winding-sensed or "sensorless" control. Alternatively, position sensors, such as HEDs, can be used.

To simplify control, each of the three terminal-pairs can be driven with current of either polarity, which also results in 6 phases per electrical cycle. Phase control is thereby quantized into 60° phase intervals, or steps. Consequently, with only 6 phase values per cycle, the torque angle, d, will vary with rotor position by ± 30° from its desired 90° value. For field orientation, the phase-step that drives a given winding-pair is centered ± 30° around its induced-voltage peak, and begins 60° after its zero-crossing at 0° or 180° .

The induced-voltage waveforms of the phase-windings are offset from the terminal (winding-pair) voltages by 30° . The vector diagram shows the phase-winding voltages, X, Y, and Z, at their 0° positions (at positive zero-crossings), measured with respect to N and relative in phase to stator coordinates. The X phase-winding is set at the zero degree electrical position of the stator. If the motor is spun and the open-circuit voltages are measured on an oscilloscope, the terminal voltages are the vector sum of induced voltages of the series winding-pairs. The voltage from terminal X with respect to Y is X – Y = XY. XY lags X in phase by 30° with a magnitude of A× Ö 3, where A is the amplitude of the phase-winding voltages, X, Y, and Z.

If the vectors are rotating CCW, the positive zero-crossing (or 0° ) of XY (at –30° relative to X) occurs 60° before XZ. Field orientation is most closely approached by centering the drive step around the peak of the induced-voltage waveform. A drive step from 30° to 90° is centered around the positive peak of XY, which is at +60° , 90° after (CCW from) its positive zero-crossing. Fortunately, a zero-crossing of XZ occurs at +30° , to trigger the drive to advance to this phase-step. But sensing XZ from the Y terminal to ground includes PWM noise from the driven terminal.

The phase sequencing for the drive is shown in the 3-phase vector diagram below. All vectors rotate synchronously in the same direction with the rotor. The induced-voltage vector of phase-winding X is at 0° electrical relative to the stator coordinates. That is, the voltage at open terminal X relative to node N (which is X) is crossing zero volts in the positive direction at a stator phase of zero degrees electrical. At this point in time, drive is being applied to terminals Z (+) and Y (–), as indicated by "ZY"-denoted phase intervals on the diagram, during the 60° interval around 0° (±  30° ).

Why drive terminals ZY during this interval? Note that the ZY voltage vector is pointing downward, at –90° of the stator coordinates (or relative to X). The beginning of the ZY drive step is 60° later (going CCW), where XY crosses zero. For each of the six winding-pair voltage vectors, the corresponding drive step begins 60° later.

A positive zero-crossing (+zc) of XY can be sensed to advance from the ZX to the ZY driven step. YX can be sensed for a negative zero-crossing (–zc) because terminal X is grounded during drive step ZX (the step at the end of which XY is sensed) and Y is open. However, PWM noise from Z through the neutral node will interfere with sensing and must be filtered out. If the PWM frequency is much greater than the motor electrical frequency, then the filter phase delay will not be significant.

How does this vector approach to 6-step phase control relate to previously developed d-q axis motor theory? Drive applied to ZY around 0° puts the q axis on 0° while the d axis occurs 90° earlier, when rotor magnet flux reaches its peak for the given winding-pair. At this time, the change in flux is zero and the so is the induced voltage. The positive zero-crossings of the induced voltage electrically identify the +d axis.

From the previous vector diagram, a table can be constructed from which logic circuits can be designed to effect six-step phase control. Three position sensors (or zero-crossing comparators for winding-sensed phase), A, B, and C, have waveforms as shown in Part 2 (square-waves separated by 120° el). Their six allowable states are given in the following logic table, with electrical phase, polarity of induced voltages of the phase-windings, and polarity of drive of the three-phase full-wave bridge driver.

Note that for a p-pole motor, the mechanical position relates to the electrical phase by dividing the electrical phase by p/2. For an eight-pole motor, a 60° electrical step is a 15° mechanical step. Position sensors can be spaced accordingly.

The drive applied to the motor terminals opposes winding-pair induced voltages of the same phase. A separate table listing the induced voltages would be redundant. A zero entry in the table represents either an open phase-winding terminal or a zero-crossing induced voltage.

The combinatorial logic that implements the table has six outputs for driving the 3 "+" or high-side drivers and 3 ("–") low-side drivers of a 3-phase bridge driver. The six outputs are decoded from the sensor outputs. For example, XU, the X-terminal high-side (U for "upper" in XU) driver logic asserts for X driven "+":

XU = A× /B× C + A× /B× /C = A× /B

The two unused states could be designed, should they accidentally occur, to turn off all drivers. Six 4-input AND gates can be used to decode for the six outputs, and two two-input NAND gates could detect the invalid states and disable (via the third and fourth inputs) the other gates.

Control of the magnitude of the vector drive currents is a topic onto itself. Direct control of stator currents implements torque control, for torque is proportional to current in a field-oriented PMS motor. The usual approach is to switch the power rails to the motor terminals using power transistors. This all-or-nothing behavior can supply a range of average voltages by controlling the fraction of time during a switching period, T_s, that the switch is on, or t_on. This fraction is called the duty ratio, D:

The average voltage applied to the motor terminal is then

where T_s is the switching period (t_on + t_off) and V_s is the motor supply voltage. This scheme is called pulse-width modulation (PWM). Any waveform can be applied to the motor with a frequency much less than the switching frequency f_s = 1/T_s by making D a function of time, such as D(t) = D_max× sin(w_el×t).

In the fourth and final part of this brief introduction to motor-drive electronics, we will finally look at motor-drive circuitry – the power driver.