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AC Variable Frequency Drives

AC variable frequency drives regulate the speed and torque of AC induction motors by converting a fixed-frequency mains supply into a variable-frequency, variable-voltage output using pulse-width modulation. PLC Direct supplies AC VFDs from leading automation manufacturers as surplus sealed, refurbished, and used hardware, supporting replacement and maintenance of installed AC drive systems across manufacturing, process, utilities, and infrastructure environments.

Where Are AC Variable Frequency Drives Used in Industrial Environments?

AC variable frequency drives convert a fixed mains supply into variable-frequency, variable-voltage output using pulse-width modulation to regulate the speed and torque of AC induction motors across standard industrial motor circuits. They are specified wherever a three-phase induction motor requires variable speed operation, controlled ramp starting, or energy-efficient load matching, without the closed-loop encoder feedback that servo applications require.

Facilities and operations that depend on AC variable frequency drives include:

Water and wastewater treatment facilities running AC VFDs on pump and blower motors to match output to demand and reduce energy consumption at distributed pump stations and treatment plants.

Mining and mineral processing operations using 3-phase AC drives on conveyor motors, crusher drives, and slurry pumps, where controlled starting and variable speed reduce mechanical wear on high-load equipment.

Food and beverage processing plants where AC inverter drives govern mixer, conveyor, and filling line motor speeds in washdown and temperature-cycling production environments.

HVAC and building infrastructure installations using AC variable speed drives on air handling unit fans and chilled water pump motors to deliver energy savings through speed matching to building load.

General manufacturing and assembly operations where AC motor speed control on conveyor, lift, and process motor circuits reduces starting current and enables flexible line speed adjustment.

When an AC VFD fails, sourcing a matched replacement by part number is the fastest path to restoring motor control without modifying existing control wiring or fieldbus configuration.

Which Brands of AC Variable Frequency Drives Does PLC Direct Stock?

PLC Direct stocks AC variable frequency drives from major industrial automation manufacturers.

Siemens SINAMICS G120C and MICROMASTER 440: The SINAMICS G120C AC VFD is available from 0.55 kW to 75 kW with PROFINET and PROFIBUS fieldbus variants; legacy MICROMASTER 440 AC drives are also available for maintaining older Siemens drive installations.

Schneider Electric Altivar 61 and Altivar 71: The Altivar 61 AC variable frequency drive is optimized for pump and fan torque characteristics; the Altivar 71 covers high-power machine and complex load applications, both available across multiple current ratings.

ABB ACS550: The ACS550 AC drive family covers general-purpose motor control with a 380-480 V 3-phase input, in NEMA 1 and NEMA 12 enclosure configurations across multiple horsepower ratings.

Yaskawa GA800, GA500, V1000, and A1000 Series: Current-generation GA800 and GA500 AC VFDs and legacy V1000 and A1000 AC variable speed drives are available across a wide range of power ratings for general-purpose and application-specific motor control.

All AC variable frequency drives at PLC Direct are available as surplus, sealed, refurbished, or used, depending on current inventory.

What Should You Know Before Ordering AC Variable Frequency Drives?

Hardware condition options: PLC Direct supplies AC variable frequency drives as surplus, sealed, refurbished, and used hardware. Surplus sealed units are factory-sealed stock in original packaging; refurbished units have been tested and restored to operational condition. Used hardware has been previously installed and carries the same 1-year PLC Direct warranty as all other condition grades.

Compatibility and part identification: A replacement AC VFD must match the installed unit's input voltage, output current rating, enclosure type, and communication interface, all of which are encoded in the part number. The motor nameplate full-load current is the primary sizing reference; the installed drive's part number is the most reliable basis for sourcing a like-for-like replacement that fits the existing panel, control wiring, and fieldbus configuration without modification.

Warranty: All AC variable frequency drives hardware purchased from PLC Direct carries a standard 1-year warranty covering defects and functionality, applicable to surplus sealed, refurbished, and used products.

PLC Direct supplies AC variable frequency drive hardware for replacement and maintenance, and does not provide system design, programming, or integration services.

Frequently Asked Questions

An AC variable frequency drive rectifies the incoming AC mains supply to DC, then uses an inverter stage with pulse-width modulation to synthesize an AC output at the commanded frequency and voltage. Reducing the output frequency below the mains frequency slows the motor; increasing it above the mains frequency allows operation above rated speed in a field-weakening range, where available torque decreases.

V/Hz (volts-per-hertz) control maintains a fixed ratio between output voltage and frequency, which is adequate for constant-torque loads like conveyors but provides limited torque accuracy at low speeds. Vector control calculates and regulates the motor's flux and torque-producing current components independently, providing significantly better torque response and low-speed performance, and is required for applications such as hoists, winders, and high-dynamic machine drives.

Standard AC variable frequency drives are designed for 3-phase AC induction motors and are not compatible with single-phase induction motors, which have different starting and running winding configurations that a standard VFD cannot properly drive. Single-phase input to the drive supply is a separate matter; some AC VFDs accept single-phase input power while still outputting 3-phase to the motor; confirm input supply compatibility from the drive's specification sheet.

PWM inverter output from an AC drive produces fast voltage rise times that can stress motor winding insulation, particularly on older motors not designed for inverter duty. This is most significant on long cable runs, where voltage reflections at the motor terminals can create spikes above the motor's insulation rating. Installing an output reactor or dV/dt filter between the drive and motor mitigates this risk on affected installations.

AC variable frequency drives are rated for either Normal Duty (typically 110% overload for 60 seconds) or Heavy Duty (typically 150% overload for 60 seconds), reflecting the type of load for which the drive is sized. Constant-torque loads, such as conveyors and compressors, require a Heavy Duty rating; variable-torque loads, such as pumps and fans, can typically use a Normal Duty rating, allowing a smaller drive frame for the same motor power rating.