Engineering · Motors & Drives · Stage 2
VFD Architecture and Working
How does a drive turn fixed 50 or 60 Hz mains into any speed you ask for?
A variable frequency drive does it in three steps: a rectifier turns the AC supply into DC, a capacitor bank smooths that DC into a stable bus, and an inverter chops the bus into a three-phase output whose frequency and voltage it chooses. The chopping is PWM. This page follows the power from the supply terminals to the motor, with things you can move at each stage: a rectifier and DC-bus model, an inverter switch explorer, a PWM generator and a V/f explorer. Typical figures; check your drive’s manual.
Power flows left to right along the top row. The control electronics sit underneath: they read the speed command and the feedback signals, run the control algorithm, and send gate pulses to the inverter. Everything that protects the drive (overcurrent, over and under voltage, over temperature) is decided there.
Time axis in milliseconds. Six diodes pick, at every instant, the highest line-to-line voltage, so the output is six humps per mains cycle: a 6-pulse rectifier, with a ripple at six times the line frequency. Without a capacitor the DC rises and falls between 1.22 and 1.41 times the line voltage. The capacitor holds the bus near the peak and the ripple that remains depends on the load and the capacitance. Drives that must send power back to the supply use an active front end of IGBTs in place of the diodes.
| Supply class | Line voltage | DC bus (peak) | Typical over-voltage trip |
|---|
Bus voltage = √2 × line voltage (about 1.41×), so a 400 V supply gives about 566 V, and 480 V gives about 680 V. Under load the bus sags a little toward 1.35×. The capacitors also store energy to ride through short dips and to absorb energy returned by the motor when it brakes, which is why a decelerating drive can trip on DC bus over-voltage unless it has a braking resistor or regenerative front end (see Braking & Regenerative Control). Big capacitors would draw a surge when power is first applied, so the drive charges them through a pre-charge resistor that a contactor or thyristor then bypasses. An optional DC choke or an AC line reactor smooths the current the rectifier draws and cuts supply harmonics.
Each of the three legs has a top and a bottom IGBT, and never both on at once. Pick which legs have the top switch on to see the output voltages.
| State (A B C) | VAB | VBC | VCA | Kind |
|---|
Three legs give eight switch states: six active ones and two zero states. A short dead time between turning one IGBT off and its partner on prevents a shoot-through. The diodes across each IGBT carry the motor current when the switch is off, and also pass braking energy back into the bus. By switching quickly between these states the drive builds any average voltage you want: that is PWM.
Sine-triangle PWM compares a sine reference with a fast triangle carrier. Where the sine is higher than the triangle the phase is connected to +Vdc; where it is lower, to 0. The pulses get wider where the sine is high, so the average over each carrier period follows the sine (red line). Raising the amplitude (modulation index) raises the output voltage; changing the sine frequency changes the output frequency; the motor’s inductance smooths the pulses into a near-sine current. Adding a third harmonic to the reference, which cancels in the line-to-line voltage, is how space-vector PWM reaches about 15% more output voltage from the same bus. Real carriers run at 2 to 16 kHz and the drive uses a carrier at least ten times the output frequency.
Frequency axis in hertz. Motor flux is proportional to voltage divided by frequency. Lower the frequency without lowering the voltage and the motor saturates and overheats; lower both together and the flux, and so the torque, stays constant. That is the V/f ratio (400 V at 50 Hz is 8 V per hertz). Below a few hertz the resistance of the stator winding eats a noticeable part of the voltage, so drives add a voltage boost at low speed. Above base frequency the voltage cannot rise any further, so flux falls and the available torque falls with it: constant power instead of constant torque. Fans and pumps use a squared V/f pattern because their torque falls with speed. The flux and torque figures here are estimates for a typical motor.
Same bus, same carrier. At low speed the line-voltage pulses are narrow and mostly zero because the average voltage is low, and each cycle is long. At high speed the pulses are wide and the cycle is short. The carrier is drawn much slower than a real one to keep the pulses visible.
| Control method | How it works | Key features | Typical applications | Speed range and accuracy |
|---|
Details and explorers for each method are in VFD Control Methods and Direct Torque Control.
| Parameter | Typical range or value |
|---|
Figures are typical. Voltage classes, switching frequencies, overload ratings and trip levels differ between makers and between drive series, so use the drive’s manual and rating plate for any real design.