VFD course · Lesson 2 of 12

Inside the drive

From the outside a VFD is a box with a display and terminals. Inside it is 3 stages in a row: it turns AC into DC, stores it, then chops it up fast to make new AC exactly the way we want. Understand these stages and you will understand every VFD fault after this.

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1AC → DC → AC

  1. Rectifier: AC from the supply → DC with ripple.
  2. DC bus: capacitors smooth and store it, and its voltage is almost constant.
  3. Inverter (IGBTs): chops the DC fast and outputs AC at variable frequency and voltage to the motor.
Above them is the control board that decides all of this, and some drives have a braking chopper on the DC bus.

2Open the drive

Tap each part of the drawing, and change the supply voltage to see how the DC bus voltage changes. Under each part is the waveform at that point.

Inside the drive

3DC bus voltage

The capacitors charge almost to the peak of the wave. On three phase, under load, the well-known figure is:

VDC ≈ 1.35 × VLL
SupplyDC bus, roughly
Single phase 230 V≈ 325 V (230 × 1.41)
3 phase 400 V≈ 540 V (with no load it can reach 565 V)
3 phase 480 V≈ 650 V
Why does this number matter? The drive watches it all the time: if it drops too low ⇒ a UV (undervoltage) fault, meaning the supply sagged or a phase was lost. If it rises too high ⇒ an OV (overvoltage) fault, usually meaning the motor is sending energy back while braking. Many drives show it on the display as a monitor parameter (DC bus voltage).

Drill: work out the DC bus

4Safety

Danger: the capacitors stay charged for minutes after you switch off the power (usually 5 to 15 minutes, and the figure is printed on the drive). A dark display does not mean the voltage is gone. Wait the stated time and measure between DC+ and DC− with a multimeter before you touch any terminal. We will practise this in lesson 11.

?Lesson quiz

Get at least 5 of 6 right to complete the lesson.

The numbers and examples here are common training values, and parameter and terminal names differ from brand to brand and model to model: your drive's manual is the reference. Your progress is saved on this device.
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