Three-Phase Motor Controller (VFD)

Three-phase SPWM generates three sinusoidal PWM waveforms displaced 120° from each other. Each phase uses complementary top and bottom IGBT switches. The DSP generates six PWM signals: S1–S6, where S1,S3,S5 are top-side and S2,S4,S6 are bottom-side (inverted) for legs A, B, C respectively. A deadtime of 2–4µs prevents shoot-through when complementary switches transition simultaneously.

The simplest motor control strategy maintains a constant ratio of output voltage to frequency. At 50Hz nominal, voltage is 415V (line-to-line). At 25Hz (half speed), voltage is 207V — maintaining constant air-gap flux and therefore constant torque capability. Below 5Hz, boost voltage to overcome stator resistance voltage drop. Implement a linear V/Hz profile with adjustable boost using a lookup table in the DSP.

Abrupt speed changes cause mechanical shock and electrical stress. Implement configurable linear ramp: acceleration time (time from 0 to 50Hz, typical 5–30 seconds) and deceleration time. During deceleration, the motor regenerates energy back to the DC bus, raising bus voltage. The braking resistor dissipates this energy when DC bus voltage exceeds 750V, preventing overvoltage trip.

Implement: overcurrent protection (trip if any phase current > 150% rated for >5 cycles), overvoltage (DC bus > 800V), undervoltage (DC bus < 400V — mains brownout), overtemperature (IGBT module > 90°C via NTC), ground fault detection (sum of three phase currents ≠ 0 indicates ground fault), and output short circuit (instantaneous current > 300% rated — sub-microsecond response via hardware comparator).

Program motor nameplate data: rated voltage, current, frequency, power, speed. The VFD uses these to set overcurrent thresholds and V/Hz profile breakpoints. Perform autotune: the VFD applies a series of DC pulses to the stopped motor to measure stator resistance (for IR compensation boost), then at low speed to measure leakage inductance. These parameters improve V/Hz control accuracy especially at low speeds.

For precise speed regulation, add a digital tachometer or encoder to the motor shaft. Compare actual speed with setpoint speed. A PI controller adjusts the output frequency to maintain setpoint despite load variations. This closed-loop speed control improves steady-state accuracy from ±3% (open-loop V/Hz) to ±0.01% (closed-loop with encoder). Implement anti-windup in the PI integrator for step load changes.

Add RS485 Modbus RTU for industrial network integration. Map registers: setpoint frequency, actual frequency, motor current, motor voltage, DC bus voltage, fault code, run/stop command, direction (forward/reverse). This allows PLC or SCADA systems to control the VFD remotely, monitor its status, and command fault resets without operator intervention at the drive panel.