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How does a frequency inverter (VFD) work?

An induction motor's speed depends on frequency. A VFD turns the mains into DC, stores it in capacitors, then uses IGBTs and SPWM to make a new supply for the motor. Switch states, tap blocks to measure, then try the fault-finding mode.

🔍 Explore🛠 Find the fault

▶ Show me

⚙️ 50 Hz full speed⏸ Stopped (ready)📈 Accelerating (20 Hz)🛑 Braking (slowing down)

⛶ Enlarge

Grid input

Three phases from the panel's breaker or motor protection switch (small models: single-phase 230 V, L N). They land on the R, S, T screws. Mixing them up with the output screws (U V W) destroys the drive on first power-up.

Oscilloscope

MultimeterR–S 400 V AC, 50 Hz

Tap a block to see what it does and what you'd measure there.

🔧 Inside a frequency inverter: what's where?

This drawing isn't a photo; it follows the typical layout of 2.2–7.5 kW wall-mount drives. The control board sits on top as a separate layer; below it is the power board and under that the heatsink and fan. Tap a region, then switch the energy path above. ⚠ Before opening the cover, wait for the CHARGE LED to go out (5–10 min) and measure 0 V between P/+ and N/−.

Energy path:

⚙️ Running (motor)🛑 Braking (brake resistor)🔌 Charging (power-up)

▶ Show me

⛶ Enlarge

Tap a region to see what it does and how to spot it on the board.

📷 Real-world view

Fotoğraf: C J Cowie, CC BY-SA 3.0, Wikimedia CommonsFotoğraf: Sthdhk, CC BY-SA 4.0, Wikimedia CommonsFotoğraf: Daniel*D, CC BY-SA 3.0, Wikimedia Commons

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⚠ Safety: The DC bus capacitors hold a lethal voltage for 5–10 minutes after switch-off. Even with the CHARGE LED out, touch nothing until you measure 0 V between P/+ and N/−. Don't disconnect the motor cable while the drive runs, and don't put a contactor on the output.