How does a VFD actually change a motor's speed?
It turns AC into DC, then builds new AC at the frequency and voltage the motor needs. Here is what happens inside, with numbers for a 415 V, 50 Hz supply.
1. Rectifier
Six diodes act like check valves. The output is always the largest gap between two phases: humps between 508 and 587 V, about 560 V on average (1.35 × 415), with a 300 Hz ripple.
2. DC bus
Capacitors smooth the ripple and store energy. A DC choke spreads out the input current spikes; ABB's data shows THDi falling from over 100% to about 40%. A pre-charge resistor limits the inrush at switch-on, and an optional brake chopper burns off the energy a motor feeds back while braking a high-inertia load.
3. Inverter + PWM
Six IGBTs switch the DC bus thousands of times a second (typically 2–16 kHz). The pulse widths follow a sine wave, so their average is a sine, and the winding's inductance smooths the current into almost a pure sine.
4. V/f control
Frequency sets speed: Ns = 120 × f ÷ P, so a 4-pole motor's field turns at 1500 rpm at 50 Hz and 750 rpm at 25 Hz. The voltage drops with frequency, to about 208 V at 25 Hz, to keep the flux constant. Keep 415 V at 25 Hz and the core saturates.
Why we put them on AHU fans and pumps: in the ideal fan law, power falls with the cube of speed, so 80% speed needs about 51% power. Real systems with static head or a pressure set-point save less.
One safety point for site teams: the DC bus stays charged for minutes after isolation. Wait the time on the drive's label and prove it dead with a meter.
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