What Is a Variable Frequency Drive (VFD)? How It Works, Types, and How to Choose the Right One

A variable frequency drive (VFD) is a motor controller that varies the frequency and voltage supplied to an AC induction motor, so the motor runs at the speed and torque your process needs instead of one fixed speed. It is also called an AC drive, inverter, frequency converter, variable speed drive, or adjustable frequency drive.

Most guides stop at theory. This one continues into what buyers actually get wrong: sizing, regional voltage and frequency compatibility, and model number decoding across the six brands we stock. If you already know your drive family and need Siemens details, read our SINAMICS G120 complete guide instead.

What a Variable Frequency Drive Actually Does

Motor speed is set by supply frequency, not voltage:

n = 120 × f / p, where n is synchronous speed in RPM, f is frequency in Hz, and p is the number of poles.

Actual full load speed sits 2% to 5% lower because of slip. The table also explains why a 50 Hz nameplate motor spins roughly 20% faster on a 60 Hz grid.

Beyond speed, a VFD gives you six controls:

1. Speed setpoint, so output matches demand instead of throttling a valve or damper.
2. Ramps, so bottles stay upright on the conveyor and belts stop slipping.
3. Torque control, holding torque through load swings rather than tripping.
4. Soft start, cutting inrush to roughly rated current, which extends contactor and gearbox life.
5. Braking and reverse, giving controlled stops without a mechanical brake.
6. Motor protection: overcurrent, overvoltage, undervoltage, overtemperature, phase loss, and ground fault in one unit.

A VFD is not a soft starter, which only manages the start ramp, not a servo drive, which controls position, and not a phase converter, which changes phase count but not speed. If your only complaint is high starting current, you may not need a VFD at all.

How Does a VFD Work? Three Stages Inside Every Drive

Stage 1: the rectifier. A six pulse diode bridge turns incoming AC into pulsating DC. Selection implication: six pulse front ends generate 5th and 7th harmonics, so where drive capacity is a large share of transformer capacity, plan for a line reactor, a 12-pulse unit, or an active front end.

Stage 2: the DC bus. Capacitors and a choke smooth and store that energy, and a 400 V input gives roughly 565 V DC. Selection implication: that fixed ratio is why drives are sold in voltage classes and why a 400 V unit must never be fed from 480 V. The bus stays charged after power off, so nobody opens the cover for 5 to 10 minutes.

Stage 3: the inverter. IGBTs switch the DC bus thousands of times per second, and pulse width modulation shapes those pulses into current that behaves like a sine wave at any commanded frequency. Voltage rises with frequency to hold the V/Hz ratio constant, which is why the drive lowers voltage at low speed to protect torque. Selection implication: raising carrier frequency lowers motor noise but increases drive losses (often forcing derating) and worsens dV/dt stress on cables and bearings.

What changed by 2026: silicon carbide devices in compact frames, built in EMC filters, industrial Ethernet (PROFINET, EtherCAT, and EtherNet/IP) as standard, and STO functional safety on most new series.

VFD Types and Control Methods

By topology, voltage source inverters (VSI) are what you will buy in 95% of cases. Current source inverters (CSI) appear in specific high power installations and are rare on the spare parts market. Direct torque control (DTC) is a vendor specific implementation with very fast torque response. As a buyer you rarely choose topology; you choose control method.

The judgment rule: If your load is not hoisting, winding, or positioning, sensorless vector covers about 90% of cases. One drive usually supports several modes, so this table decides which series you buy. Everything below concerns low voltage drives (under 1000 V), where almost all spare part purchasing happens.

Do You Actually Need a VFD?

Four questions before you buy: does the process need different speeds while running (if not, a VFD is the wrong tool), does it need precise positioning (if so, look at servo drives), is the only issue high starting current (a soft starter is cheaper), and is the load variable or constant torque (this decides the rating).

When a VFD is the wrong answer: a motor at full load and constant speed all year (payback can exceed equipment life), very frequent start and stop cycles, long cable runs with no room for filtering, hazardous areas where extra certification outweighs the benefit, and self cooled motors running below half speed. If you fall into any of these cases, tell us the duty before you order, because we may tell you not to buy the drive.

Voltage, Frequency, and Regional Compatibility

50 Hz vs 60 Hz: What Changes and What Does Not

Separate the drive from the motor.

Input side: most drives accept 50 or 60 Hz, so this is rarely the problem. Output side: the drive synthesises its own output frequency, so your grid does not limit it. Motor side: this is where things break. A 50 Hz motor run at 60 Hz turns 20% faster, and if voltage does not rise with it, flux weakens and torque falls. A 60 Hz motor fed 60 Hz voltage at 50 Hz saturates and overheats. Dual nameplate motors (400 V/50 Hz and 460 V/60 Hz) exist for this reason. On centrifugal pumps and fans, 20% more speed also means roughly 1.7 times the shaft power, enough to overload a healthy motor.

Global Voltage Classes

Three rules follow. The 380 to 480 V class covers the most export markets. Never feed a 400 V unit from a 480 V grid because the numbers look close. Where grid voltage sags or fluctuates, common across parts of South Asia, Africa, and the Middle East, confirm the undervoltage trip threshold before ordering.

With a single-phase supply, you have three routes: a dedicated single phase input model (usually 2.2 kW and below), a three phase model fed single phase with substantial derating (check that model's manual, not a generic percentage), or a phase converter if you do not need speed control.

Certification by Market

EU Ecodesign Regulation 2019/1781 also pushes the motor side toward IE3 and above, which affects what you import alongside the drive. Documents matter more than claims, so request them from any supplier, including us. See our about us page.

How to Size and Select a VFD: A Seven Step Worksheet

Step 1: Identify the load type. Variable torque covers centrifugal pumps, fans, and blowers, where the affinity laws apply: 50% speed gives 50% flow, 25% head, and roughly 12.5% shaft power. Constant torque covers conveyors, extruders, positive displacement pumps, and compressors; constant power covers winders and spindles. A drive sized on variable torque ratings will trip on overcurrent starting a constant torque load.

Step 2: Size by motor full load current, not kW or HP. Two motors of identical rated power can differ 10% to 20% in full load current depending on poles, efficiency class, and voltage. Copy six nameplate fields: rated voltage, rated frequency, full load current, rated power, poles or rated RPM, and connection. The drive-rated output current must be equal to or greater than the motor FLA.

Step 3: Check overload rating and duration. Typical figures are 110%/60 s (variable torque) and 150%/60 s (constant torque). Extruders, loaded conveyor starts, and positive displacement pumps often need one frame larger. Always ask which rating a quoted current refers to, because the same model appears with two current figures in different documents.

Step 4: Apply derating. Above 40°C ambient, expect roughly 1% to 2% current derating per degree, and above 1000 m altitude, roughly 1% per 100 m. Higher carrier frequency derates further, and exact slopes come from that model's derating curve. Field note: Panel temperature usually runs 10°C to 15°C above workshop temperature.

Step 5: Choose the enclosure rating. IP20 or IP21 inside a cabinet (roughly NEMA 1) and IP54 or IP66 for field mounting (roughly NEMA 12 and 4X), remembering that sealed enclosures dissipate heat poorly and often need further derating.

Step 6: Confirm motor and cable compatibility. Existing 400 V class motors with short cables usually run fine. At 480 V class or with long cables, insulation stress rises sharply, and NEMA MG1 Part 31 inverter duty motors are safer. About 30 m (100 ft) of cable is the threshold for evaluating output filtering.

Step 7: Match communication and safety. Siemens points to PROFINET, Rockwell to EtherNet/IP, Beckhoff and Omron to EtherCAT, and older systems to Modbus RTU. Order numbers within one series differ here, so verify and confirm STO if there is a safety circuit. Our PLC, HMI, and PLC module pages cover the control side.

Worked Example: 15 kW Centrifugal Pump at 400 V / 50 Hz

Assumptions: 15 kW, 4 pole, 400 V / 50 Hz, nameplate FLA 29.5 A, cabinet mounted at 45°C, altitude 800 m, Modbus RTU required.

The load is variable torque, so 110%/60 s overload is sufficient and required current starts at 29.5 A. Running 5°C above 40°C adds roughly 5% to 10% derating, taking the requirement to about 32 to 33 A; 800 m altitude needs no correction. The 400 V class and short cable make the existing motor acceptable.

Specification: 380 to 480 V class, 33 A or more rated output, variable torque duty, IP20, Modbus RTU. Candidates include Siemens SINAMICS G120 PM240-2 18.5 kW, ABB ACS580-01 18.5 kW, and Mitsubishi FR-A800 18.5 kW. All three step up one power size, which is normal once derating is applied.

Send us those six nameplate fields plus ambient temperature and we will return a model number and stock status within one working day.

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How to Read a VFD Model Number

Series names are not orderable. One Siemens G120 family holds hundreds of order numbers with different voltages, ratings, filters, and interfaces, so a series name alone tells your supplier almost nothing.

• Siemens 6SL3210-1PE23-8UL0: 6SL3 is SINAMICS, 210 the power module type, PE the frame and filter variant, 23-8 the rating. Trap: power unit and control unit are ordered separately, so this is not a complete drive.
• ABB ACS580-01-033A-4: series, construction type (01 = wall mounted), 33 A output, voltage code 4 = 380 to 480 V. Trap: current, not power, defines the model.
• Mitsubishi FR-A840-00126-2-60: family, voltage class 840 = 400 V, then a current code. Trap: older FR series used K for kW, so FR-E740-7.5K is 7.5 kW, different logic from the same maker.
• Omron 3G3MX2-A4022-V1: series, A4 = three phase 400 V, 022 = 2.2 kW, V1 the version. Trap: the version suffix determines available functions.
• Schneider ATV630D18N4: ATV630 series, D18 = 18.5 kW, N4 = 380 to 480 V. Trap: power and voltage codes sit adjacent and get transposed.
• Allen-Bradley 25B-D030N104: 25B = PowerFlex 525, D = 480 V, 030 = 30 A. Trap: frame and enclosure digits decide whether it fits your cutout.

The two fields that most often cause a delivered unit not to fit are voltage class and frame or enclosure code, so check those twice. Our model library lists the numbers we hold in stock.

Real Drawbacks and How to Solve Each

Harmonics. Six pulse rectifiers distort supply current. Once total drive capacity exceeds roughly 20% of transformer capacity, check against IEEE 519 limits and add line reactors, DC chokes, or an active front end.

dV/dt and reflected wave. Above about 30 m of motor cable, voltage doubling at the motor terminals damages winding insulation, and 480 V systems are more sensitive than 400 V. Fit a dU/dt filter, an output reactor, or a sine wave filter.

Bearing currents. Larger frames, long cables, and high carrier frequency raise shaft voltage and cause fluting. A shaft grounding ring is the low cost fix; insulated bearings cost more.

Motor cooling at low speed. A self cooled motor running continuously below roughly 50% speed loses airflow while still carrying current. Fit forced ventilation, use an inverter duty motor, or set a minimum frequency.

Panel heat and capacitor aging. Drive losses run about 2% to 3% of rated power, so a 30 kW drive adds 600 to 900 W to the cabinet. Electrolytic capacitors are wear parts, and spares stored over a year should be powered up periodically to reform them.

Payback. Annual savings equal kWh saved multiplied by energy price, set against drive, installation, and filter cost. A variable torque pump at part load most of the year typically pays back in one to three years. A motor at full load and constant speed may never pay back within its life.

Where VFDs Are Used

Buying a VFD: Genuine, Refurbished, and Obsolete Series

Six checks before you pay: nameplate data matches carton and packing list; the serial number can be verified with the manufacturer; firmware and date code match the claimed batch; warranty holder and duration are in writing; packaging and accessories are complete; and a power up test report or video is provided.

Risk allocation matters more than labels. With new original stock, manufacturer and distributor carry warranty and batch risk. With refurbished units the reseller carries it, and downtime cost sits with you. With grey market goods, nobody carries it. This checklist applies to any supplier, including CHENTUO.

Obsolete replacements: MICROMASTER 440 maps to SINAMICS G120 or G120C, ACS550 to ACS580, FR-E700 to FR-E800, ATV31 to ATV320, PowerFlex 40 to PowerFlex 525. These are series level matches, so confirm the exact model by current rating and options, and check mounting dimensions, parameter migration, and communication cards. Some discontinued models are still available as original stock, so replacement is not the only route.

Factory production slots often run 8 to 16 weeks while stocked units ship immediately, which is a project risk, not a logistics detail. Hold a 1:1 spare for single point critical equipment; for several motors in the same voltage and power band, one shared spare is enough. We ship brand new original products from stock within 48 hours door to door, with one year after sales support and a 30 day return window.

FAQ

 

 

Is a VFD the same as an inverter or a frequency converter?

In everyday industrial use, yes. One real difference: frequency converter sometimes means a fixed output unit converting 50 Hz to 60 Hz power for equipment supply, which does not vary motor speed. Confirm which meaning your supplier uses.

Can a VFD run a 50 Hz motor on 60 Hz power, or the reverse?

Yes, because the drive generates its own output frequency and accepts either input. The limit is the motor, not the drive: above nameplate frequency torque falls, and on pumps and fans load power rises steeply.

Can a VFD convert single phase power to three phase?

Yes, it can drive a three phase motor from single phase input, usually with derating unless you buy a dedicated single phase model. Its output is not a general purpose three phase supply.

How far can the motor be from the VFD?

About 30 m (100 ft) of shielded motor cable is the common threshold. Beyond that, reflected wave effects stress motor insulation, so specify an output reactor or dU/dt filter. 480 V systems reach that limit sooner than 400 V systems.

Which VFD voltage class do I need for 380 V, 415 V, or 440 V power?

All three sit inside the 380 to 480 V three phase class, the most widely stocked export class. Confirm the model's input tolerance and undervoltage trip threshold if your grid fluctuates.

What replaces a discontinued MICROMASTER 440 or ACS550?

SINAMICS G120 or G120C, and ACS580 respectively. Both are series level matches, so confirm current rating, frame size, and communication options. Check the model library for stock of the original model itself.

Get the Right VFD for Your Application

If you have the nameplate, send us the six fields and we will return a model and stock status. If you know the brand but not the series, use the control method table above. If you are replacing a discontinued unit, search the model library first.

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