Figure 3-6 summarizes the basic scheme of torque control with FOC. Note that this description assumes a two-shunt sensing system to align with the figure, but single and three-shunt sensing schemes are also common. Three-shunt control uses a somewhat different implementation of the Clarke transformation, while single-shunt requires programmatic phase current reconstruction prior to the Clarke transform. In any of these instances, the scheme is identical beginning with the ⍺ and β output of the Clarke transform.

Figure 3-6 Basic Scheme of FOC for AC Motor

Two motor phase currents are measured. These measurements feed the Clarke transformation module. The outputs of this projection are designated i_sα and i_sβ. These two components of the current are the inputs of the Park transformation that calculates the current in the d,q rotating reference frame.

The i_sd and i_sq components are compared to the references i_sdref (the flux reference component) and i_sqref (the torque reference component). At this point, this control structure shows an interesting advantage: the structure can be used to control either synchronous or induction machines by simply changing the flux reference and obtaining rotor flux position.

In a PMSM, the rotor flux is fixed and determined by the magnets; there is no need to generate any flux. Hence, when controlling a PMSM, set i_sdref to zero. As an AC induction motor needs a rotor flux creation to operate, the flux reference must not be zero. This conveniently solves one of the major drawbacks of the classic control structures: the portability from asynchronous to synchronous drives.

The torque command i_sqref can be the output of the speed regulator when a speed FOC is used. The outputs of the current regulators are V_sdref and V_sqref; these outputs are applied to the inverse Park transformation. The outputs of this projection are V_sαref and V_sβref which are the components of the stator vector voltage in the (α, β) stationary orthogonal reference frame. These are the inputs of the Space Vector PWM. The outputs of this block are the signals that drive the inverter.

Note that both Park and inverse Park transformations need the rotor flux position. Obtaining this rotor flux position depends on the AC machine type (synchronous or asynchronous machine).