Inverter

The traction inverter converts battery DC into variable-frequency AC for the motor. Learn the three-phase voltage-source inverter, IGBT vs SiC MOSFET switches, PWM and space-vector modulation, field-oriented control, DC-link capacitors, losses and efficiency, and cooling.

Six power switches chop 400–800 V DC tens of thousands of times per second to synthesise the three-phase sinusoidal currents the motor needs — with precise control of torque and speed, in both driving and regenerative braking.

Silicon-carbide switches have pushed inverter efficiency above 98–99 %, adding several percent of range.

An analogy

An inverter is like a very fast light-switch orchestra: by flicking six switches in a pattern, it paints smooth AC waves out of a flat DC battery.

Three-phase bridge

Three half-bridges (six switches) connect each motor phase to either DC+ or DC−. By varying the duty cycle of each leg sinusoidally (PWM), the average phase voltages become three-phase AC whose frequency sets motor speed and whose amplitude and phase set current and torque.

Switches: IGBT vs SiC

Silicon IGBTs are robust and cheap; silicon-carbide MOSFETs switch faster with lower losses, especially at light load and at 800 V — improving efficiency by ≈ 1–3 % over drive cycles. GaN is emerging for lower power.

Field-oriented control

Motor currents are transformed (Clarke/Park) into a rotating d–q frame: i_q produces torque, i_d controls flux (negative i_d for field weakening). PI current loops at 10–20 kHz generate voltage references for SVPWM, using rotor position from a resolver or sensorless observer.

Losses, thermal and safety

Losses: conduction, switching, diode recovery, DC-link capacitor ripple. Double-sided cooled power modules and pin-fin baseplates handle hundreds of amps. Safety functions: active short-circuit or safe pulse-off on faults, active discharge of the DC link, ISO 26262 torque monitoring.

Formulas

Worked example: Maximum output voltage

Given: V_dc = 400 V, SVPWM linear range

Find: Max line-to-line RMS voltage.

Answer: ≈ 283 V RMS

Worked example: SiC inverter losses

Given: 300 A RMS per phase, R_ds(on) = 5 mΩ; E_sw = 5 mJ per switch at 10 kHz; Output ≈ √3 × 283 × 300 × 0.9 = 132 kW

Find: Losses and efficiency.

Answer: ≈ 1.65 kW loss, ≈ 98.8 %

Space-vector PWM duty calculation (min–max injection, C)

#include <math.h>

/* va, vb, vc: desired phase voltages (V); vdc: DC-link voltage.
 * Outputs duty cycles 0..1 for the three half-bridges. */
void svpwm_duty(float va, float vb, float vc, float vdc,
                float *da, float *db, float *dc)
{
    float vmax = fmaxf(va, fmaxf(vb, vc));
    float vmin = fminf(va, fminf(vb, vc));
    float voff = -0.5f * (vmax + vmin);          /* common-mode injection */

    *da = 0.5f + (va + voff) / vdc;
    *db = 0.5f + (vb + voff) / vdc;
    *dc = 0.5f + (vc + voff) / vdc;
}

Adding the min–max common-mode offset is equivalent to space-vector modulation and extends the linear range ≈ 15 % beyond sine PWM.

Common mistakes

Applications

Key takeaways

Going further