How a VFD Works: PWM and V/f Control Explained
What is PWM and V/f control in a VFD? A variable frequency drive generates its output not as a true sine wave but as a train of fixed-DC-bus-voltage pulses whose width varies to synthesize an effective AC waveform — this is pulse-width modulation (PWM). Underneath that switching pattern, most drives run scalar (V/f) control below base speed, holding the ratio of output voltage to output frequency constant so the motor's magnetizing flux does not collapse or saturate as speed changes. Get these two mechanisms wrong in your head and every other VFD topic — sizing, carrier frequency, cable length limits, control method upgrades — stops making sense. This article covers the inverter's switching behavior, the V/f relationship and its numbers, why V/f alone cannot hold torque at low speed, and where vector control and DTC take over.
The Inverter Stage: Where PWM Happens
A VFD has three power stages in series. The rectifier (diode bridge, or an active front end for regen capability) converts incoming line AC to DC. The DC bus — electrolytic capacitors plus, on many models, a DC choke — smooths that DC and stores energy. The inverter, built from IGBTs, is where PWM happens: it switches DC bus voltage on and off at high speed to reconstruct a variable-frequency, variable-voltage AC output. None of the three stages does anything without the other two, but the inverter is the one that decides what the motor actually sees.
The IGBTs do not output a smooth sine wave directly — they cannot, because they only have two states, on and off, at full DC bus voltage. What they output is a series of rectangular pulses, wide near the crest of the intended sine wave and narrow near the zero crossing. Stack enough of these pulses at a high enough rate and the motor's own winding inductance filters the pulse train into a current waveform close enough to sinusoidal for the motor to run smoothly.
Carrier Frequency: How Often the Inverter Switches
The rate at which the IGBTs switch is the carrier (or switching) frequency, typically 2-16 kHz depending on the drive and its power rating. Higher carrier frequency (8-16 kHz) pushes the switching noise above the audible range most people notice, giving a quieter motor and a smoother current waveform. It also means more switching events per second, which means more IGBT switching losses and, on some frames, a current derating requirement. Lower carrier frequency (2-4 kHz) runs cooler and more efficiently but the motor produces an audible whine at the switching rate. This is a trade-off the commissioning engineer sets, not a fixed drive property — see our breakdown of carrier frequency and switching losses for the derating tables.
What we see in the field: technicians raise carrier frequency to 16 kHz to chase a quiet installation, then wonder why the drive trips on overtemperature at full load in a hot cabinet. The two settings interact.
V/f (Scalar) Control: Holding the Volts-per-Hertz Ratio
Below base speed, most general-purpose drives run scalar, or V/f, control. The principle: a motor's magnetizing flux depends on the ratio of applied voltage to applied frequency, not on either value alone. If frequency drops but voltage stays fixed, flux rises and the motor saturates, drawing excess magnetizing current and overheating. If voltage drops proportionally with frequency, flux — and with it, available torque — stays roughly constant across the speed range up to the motor's rated (base) frequency.
Formula: Volts-per-Hertz Ratio — Source: IEC 61800-2, motor flux relationship
V/f = Vrated / frated
| Symbol | Description | Unit |
|---|---|---|
| Vrated | Motor nameplate voltage | V |
| frated | Motor nameplate frequency | Hz |
| V/f | Ratio the drive holds constant below base speed | V/Hz |
Worked example: a 400 V, 50 Hz motor gives V/f = 8 V/Hz. Command the drive to 25 Hz and it outputs roughly 200 V — half voltage for half frequency, same ratio, same flux. This is why a V/f drive can be commissioned in minutes with just the motor nameplate data and no encoder: it is an open-loop calculation, not a measurement.
Where V/f Control Falls Short
Scalar control has no feedback loop. It sends a voltage and frequency command based on a fixed ratio and a stator resistance compensation curve; it does not measure actual flux or actual torque delivered. At low frequency, the voltage drop across the stator winding resistance becomes a larger fraction of the (now small) commanded voltage, and the simple V/f ratio under-delivers flux unless the drive applies a boost correction. Even with boost compensation, V/f control cannot hold rated torque reliably below roughly 3-5 Hz, and it cannot hold any torque at all at zero speed — there is no mechanism to sense a stalled rotor and correct for it.
For pumps and fans, this rarely matters: the affinity laws mean torque demand falls with the square of speed, so low-speed torque accuracy is not a design constraint. For a crane holding a suspended load at zero speed, it is disqualifying.
From V/f to Vector Control and DTC
Vector control (field-oriented control, FOC) and ABB's Direct Torque Control (DTC) both close the loop that V/f leaves open. Vector control mathematically decomposes stator current into a flux-producing component and a torque-producing component and regulates each independently, using either a motor model (sensorless) or an encoder. DTC skips the modulator stage entirely and selects inverter switching states directly from instantaneous flux and torque error, giving faster torque response than a modulated vector scheme. Both hold torque down to standstill in a way scalar control cannot. The trade-off: more tuning (autotune runs, sometimes encoder wiring) and, on encoder-based schemes, an added component to maintain. See our full comparison of V/f, vector and DTC control methods for the selection criteria by application.
This depends on the load, not on drive price tier. A high-end drive left in V/f mode on a variable-torque fan gains nothing over a cheaper V/f-only drive; the load never asks for what vector control provides.
Where This Shows Up Across Drive Families
Entry-tier drives — ABB's ACS180, Schneider's ATV12, Siemens' V20 — are built around V/f control and cost less because they skip the processing overhead of a full vector algorithm. Step up to ABB ACS580, Schneider Altivar ATV340 or Siemens SINAMICS G120C and vector control (and, on the ABB line, DTC) becomes available alongside V/f, selectable per application in commissioning. All three product families sit inside our variable frequency drives collection with in-stock availability and lead times by SKU.
The control method also interacts with the standards a drive is rated to — IEC 61800-2 covers the rating conventions referenced above, and drive selection often starts from a standards checklist before it gets to control method. Our guide to IEC and NEMA standards for VFDs covers that groundwork, and our broader explainer on what a VFD is and how it works is the right starting point if the power-stage basics in the first section need more background.
For the full architecture picture — sizing, harmonics, enclosure ratings, and how PWM and V/f fit into the rest of drive selection — see the VFD engineering guide.
Frequently Asked Questions
What does PWM stand for in a VFD?
Pulse-Width Modulation. It describes how the inverter varies the on-time of each DC bus voltage pulse to synthesize an effective AC sine wave at the motor terminals, rather than switching a fixed-width pulse.
Why does the inverter output pulses instead of a smooth sine wave?
IGBTs are two-state devices — fully on or fully off at the DC bus voltage. A smooth analog output is not physically available from that switching stage; the motor's winding inductance does the filtering that turns the pulse train into a usable current waveform.
Can V/f control hold a motor at zero speed under load?
No. V/f is open-loop with no flux or torque feedback, so it cannot reliably develop or hold torque as speed approaches zero. Applications needing zero-speed torque — hoists, some positioning tasks — need vector control or DTC instead.
What is the difference between V/f control and vector control?
V/f commands a fixed voltage-to-frequency ratio without measuring the result. Vector control splits stator current into flux and torque components and regulates each against a motor model or encoder feedback, giving accurate torque control across the speed range including near zero.
Does raising carrier frequency improve motor performance?
It reduces audible motor noise and smooths the current waveform, but it increases IGBT switching losses and can require a current derating on the drive. It is a trade-off setting, not a performance upgrade with no cost.
Do all VFDs use PWM?
Essentially all modern IGBT-based drives use some form of PWM in the inverter stage; the variation is in the modulation scheme and switching pattern, not whether PWM is used at all.
Conclusion
PWM is how the inverter stage builds a variable-frequency AC waveform from a fixed DC bus using rapid on/off switching. V/f control is the algorithm layered on top that decides what voltage to command at each frequency, holding the ratio constant to keep motor flux stable up to base speed. Together they explain why a basic drive can be commissioned from nameplate data alone, why carrier frequency is a trade-off rather than a free setting, and why low-speed, high-torque applications eventually need vector control or DTC instead. Everything downstream — sizing, cable length limits, standards compliance — builds on these two mechanisms.
