AC-AC Converters

VFD · matrix · cycloconverter · STS

AC-AC converters run the motors, fans and pumps of every gigafactory and data center. Variable frequency drives are the volume product; the rest are specialists.

Two ways to change AC into different AC. Indirect conversion through a DC link: variable frequency drives for motor speed and torque, and frequency converters for 50/60 Hz, 400 Hz and rail. Direct conversion with no DC link: matrix converters with nine bidirectional switches, cycloconverters for low-speed heavy drives, voltage controllers for soft starters and heaters, and static transfer switches that switch sources in milliseconds. VFDs dominate volume because motors use close to half of world electricity. TWO WAYS TO CHANGE AC INTO DIFFERENT AC Indirect: AC to DC to AC Direct: AC to AC, no DC link VFD motor speed and torque Frequency converter 50/60 Hz, 400 Hz, rail Matrix converter nine bidirectional switches Cycloconverter low-speed heavy drives Voltage controller soft starters, heaters Static transfer switch source switching in ms VFDs dominate volume: motors useclose to half of world electricity
Volume product
VFD
Motor share of electricity
~45%
Direct converters
niche
Hub
Supply Chain

Finding

Variable frequency drives are the AC-AC product that matters at scale, and the market is split between European and Japanese incumbents and fast-growing Chinese makers led by Inovance

Electric motor systems use roughly 45% of the world's electricity (IEA), and a VFD typically cuts fan and pump energy by 20-50% at partial load. ABB, Siemens, Danfoss, Schneider, Rockwell, Yaskawa and Mitsubishi lead globally; Inovance and INVT now lead general-purpose drives in China, and Delta is the main Taiwanese supplier.

Almost every modern VFD is indirect: a diode or active front-end rectifier, a DC link and an IGBT or SiC inverter. Direct converters survive in niches. Yaskawa sells the only matrix-converter drive line in volume. Cycloconverters ran mine hoists, cement mills and early ship propulsion, and are being replaced by medium-voltage voltage-source drives. Thyristor voltage controllers remain standard for soft starters and resistive heating. Static transfer switches (Vertiv, ABB, Eaton, Socomec) move critical loads between feeds in a quarter cycle in data centers and fabs.

Gigafactories, battery plants and AI data centers all draw on the same drive and switchgear suppliers, and the shared constraint is the power semiconductor module: IGBT modules from Infineon, Mitsubishi, Fuji and Semikron Danfoss, and Chinese modules from StarPower and CRRC Times Electric. Frequency conversion is covered separately in Frequency Converters.

Types

AC-AC converter types

Main AC-AC converter types
TypeCore jobHow it worksWhere used
Variable frequency drive (VFD)Motor speed and torque controlRectifier, DC link, PWM inverterPumps, fans, compressors, conveyors, HVAC, robots
Matrix converterDirect AC-AC with regenerationNine bidirectional switches connect any input phase to any output phaseElevators, cranes, compact regenerative drives (Yaskawa U1000)
CycloconverterLow-frequency AC from grid ACThyristor groups assemble output from input waveform segmentsLegacy mills, mine hoists, ship propulsion
Solid-state voltage controllerVaries RMS voltage at fixed frequencyPhase-angle control with thyristors or triacsSoft starters, heaters, lighting
Static transfer switch (STS)Moves load between two AC sourcesBack-to-back thyristors switch in under a quarter cycleData centers, fabs, hospitals, telecom
Frequency converterChanges supply frequencyRectifier, DC link, inverter at the new frequency50/60 Hz conversion, 400 Hz aircraft power, rail, shore power

Suppliers

Drive and converter suppliers by region

AC-AC converter suppliers
RegionDrives (VFD)Transfer switches and specialty
EuropeABB, Siemens, Danfoss, SchneiderABB, Socomec, Siemens
USRockwell, Eaton, GE Vernova (MV)Vertiv, Eaton
Japan and TaiwanYaskawa, Mitsubishi Electric, Fuji Electric, Toshiba, DeltaYaskawa (matrix), Fuji
ChinaInovance, INVT, HiconicsKehua, Huawei (data center power)

Constraints

Where the supply chain tightens

AC-AC converter supply constraints
AreaWhat gets tightEffect
Power modulesIGBT and SiC modules shared with EVs, solar and storageLead times and redesigns
Control siliconMCUs, DSPs, isolated gate driversProduction stops even when power stages are available
Magnetics and filtersLine reactors, dV/dt and sine filtersEMC and motor-insulation problems if substituted badly
Engineering talentDrive firmware and application engineersSlower commissioning, poorer tuning

Sources

Sources and scope

Scope
AC-AC Converters
Sources
IEA (electric motor systems); supplier disclosures
Updated
October 2026