Side-by-side comparison
| Parameter | Single Phase | Three Phase Inverter |
|---|---|---|
| Bridge Topology | Half-bridge (2 switches) or full-bridge (4 switches, H-bridge) | 6-switch three-phase bridge (3 legs, 2 switches per leg) |
| Output Phases | Single-phase 230 V (or 120 V) | Three-phase 415 V line-to-line (240 V phase) |
| DC Bus Voltage (typical) | Full-bridge: V_dc ≈ 320–400 V for 230 V AC output | V_dc ≈ 560–700 V for 415 V three-phase output |
| Switching Devices | 2 or 4 IGBTs (e.g. IRG4BC40W for < 3 kVA) | 6 IGBTs; often intelligent power module (IPM) e.g. IRAMS10UP60B |
| Output Ripple Frequency | 2× switching frequency | 6× switching frequency (lower ripple at same f_sw) |
| Transformer Requirement | Isolation transformer often needed for galvanic isolation | Less common; three-phase output can drive motors directly |
| Total Harmonic Distortion | Higher THD at same switching frequency | Lower THD due to 3-phase symmetry and higher ripple frequency |
| Power Range | 0.1 kVA – 10 kVA typical | 1 kVA – MW range |
| Control Complexity | SPWM with 2 or 4 signals; simpler DSP code | Space vector PWM (SVPWM) preferred; 6 signals with deadtime |
| Typical Application | Residential solar inverter, UPS for PC, single-phase VFD | Industrial VFD, grid-tie solar (> 5 kW), 3-phase UPS |
Key differences
A single-phase full-bridge (H-bridge) inverter needs four IGBTs and produces a bipolar PWM output at twice the switching frequency ripple; total harmonic distortion for a 230 V, 50 Hz output at 20 kHz switching is typically 3–5%. A three-phase bridge uses six IGBTs and inherently produces a balanced three-wire output with 6× ripple frequency, reducing filter inductor size by roughly 60% at the same switching frequency. Three-phase inverters use SVPWM rather than simple SPWM because SVPWM achieves 15% higher DC bus utilisation (modulation index up to 1.155 vs 1.0 for SPWM), directly reducing required DC bus voltage. Single-phase inverters driving inductive loads need a negative DC rail or split DC bus to handle reactive current, while three-phase bridges handle reactive current naturally through the other two phases.
When to use Single Phase
Use a single-phase inverter for residential solar systems, PC UPS, and single-phase motor drives up to about 5 kVA. Example: a Growatt 3000TL single-phase grid-tie inverter uses a full-bridge IGBT stage (STGW30NC60WD) with SPWM at 16 kHz to feed 230 V, 50 Hz to the domestic grid.
When to use Three Phase Inverter
Use a three-phase inverter for industrial motor drives, commercial solar plants above 5 kW, and any application requiring balanced three-phase supply. Example: a Danfoss FC302 VFD uses a three-phase IGBT bridge IPM with SVPWM at 4–16 kHz to control a 22 kW induction motor with < 3% THD.
Recommendation
For anything above 5 kVA or requiring three-phase motor control, choose a three-phase inverter — harmonic performance, power density, and motor compatibility are all better. Below 5 kVA on a single-phase grid, the single-phase bridge is simpler and cheaper. Never connect a single-phase inverter output to a three-phase motor; the result is unbalanced heating and eventual winding failure.
Exam tip: Examiners ask students to draw the gate signal sequence for a three-phase bridge inverter operating in 180° conduction mode and derive the output line voltage waveform — know the six-step sequence and that V_L = (2/3)V_dc for each 60° segment.
Interview tip: A placement interviewer at a VFD or solar inverter company will ask why SVPWM is preferred over SPWM in three-phase inverters — answer: SVPWM uses 15.5% more DC bus voltage for the same AC output (m_a up to 1.155), reducing DC link capacitor size and improving THD.