EV-07 — Electric Drive Control (FOC)

ASSUMPTION — This lesson is advanced; it introduces the mathematical framework of motor control. Implementation details (sampling, gains) are OEM-specific and not given here.

1. What Is FOC?

Field Oriented Control (FOC) controls an AC motor’s current in a rotating (dq) reference frame, managing torque and flux separately. It gives DC-motor-like simplicity: the torque-producing component (Iq) and the field/flux component (Id) are decoupled.

FACT — FOC aligns the current vector with the rotor flux, giving high dynamic performance and efficiency in PMSMs and induction motors.

2. Clarke Transform

The three-phase (abc) currents are transformed into a two-axis stationary (αβ) frame, using the fact that a balanced 3-phase system has only two independent components.

3. Park Transform

The αβ components are transformed into a (dq) frame rotating with the rotor position. AC quantities become DC quantities in the rotating frame, which PI controllers handle easily.

FACT — The Park transform needs rotor position (encoder/resolver); the dq axes rotate with the rotor flux.

4. Id/Iq Current Control

• Iq — the torque-producing component (q axis).
• Id — the field/flux component (d axis); in a PMSM often zero or negative (for field weakening).

5. Inverse Park and SVPWM

The controller output (Vd, Vq) is transformed back to αβ via the inverse Park transform, then to inverter switching signals via SVPWM (Space Vector PWM).

6. MTPA and MTPV

• MTPA (Maximum Torque Per Ampere) — the Id/Iq combination giving maximum torque at a given current.
• MTPV (Maximum Torque Per Volt) — maximum torque under the voltage limit; applies in the field-weakening region.

7. FAQ

What is the difference between FOC and scalar V/f control?

FACT — V/f control only holds the voltage/frequency ratio and does not control torque dynamically; FOC controls current in the dq frame, managing torque and flux separately.

Does FOC work without an encoder?

FACT — Sensorless FOC estimates rotor position from current/voltage, but performance is limited at low speed and standstill, so traction motors mostly use an encoder/resolver.

8. Summary

• FOC decouples torque (Iq) and flux (Id) control.
• Clarke does abc→αβ; Park does αβ→dq.
• SVPWM turns the voltage reference into switching signals.
• MTPA/MTPV optimize efficient torque production.

9. Sources and Verification Note

Control theory is established textbook/IEEE material; no OEM-specific parameters.

• IEEE, SAE J1715 — motor control and terminology.

Next Lesson

• EV-08 — Regenerative Braking: brake blending, limits, and winter conditions.