Motor current before nominal kW
Use the motor nameplate current as the final sizing check. The same kW motor can draw different current at different voltages and efficiency classes.
Variable frequency drive selection
VFD Drives for Industrial Motor Control
Find a variable frequency drive that matches the motor, supply and load—not just the kW printed on the label. We support new installations, replacement projects and multi-brand sourcing from fractional horsepower through large industrial drive systems.
- Low-voltage and medium-voltage options
- Exact-model and cross-brand replacement
- Worldwide quotation and delivery support
Selection foundation
Start with motor current, voltage and load behavior.
A VFD changes the frequency and voltage supplied to an AC motor so speed and torque can be controlled. Correct selection depends on rated motor current, input supply, overload duty, acceleration time, ambient conditions and the control signals already used by the machine. For a replacement, dimensions, I/O, communications and braking arrangements also need to be checked before an alternative is approved.
Normal or heavy-duty overload
Pumps and fans often use normal-duty ratings, while conveyors, mixers and crushers may require a higher overload rating and more current margin.
Control and communication
Confirm analog signals, digital I/O, encoder feedback, PID functions and protocols such as Modbus, Profinet or EtherNet/IP.
Installation environment
Enclosure, temperature, altitude, cable length, EMC requirements and line quality can change the drive, reactor and filter package required.
Video introduction · 3:09
What is a Variable Frequency Drive?
A concise visual introduction to how a variable frequency drive controls an AC motor and where VFD technology fits in an industrial system.
Video by Invertek Drives
VFD fundamentals
How a variable frequency drive controls an AC motor
A VFD is more than an electronic speed dial. It converts fixed-frequency incoming power into a controlled three-phase output, then adjusts frequency, voltage and switching pattern to produce the motor speed and torque the process requires. The four explanations below connect the motor physics to the power electronics and the real application.
01 · Motor principle
Frequency sets the magnetic-field speed; slip produces torque
In a three-phase induction motor, current in the stator windings creates a rotating magnetic field. Its synchronous speed is determined by supply frequency and the number of motor poles: Ns = 120f/p. A four-pole motor on 50 Hz power therefore has a 1,500 rpm synchronous speed; the rotor turns slightly slower when it is producing torque.
That difference is called slip. Relative movement between the rotating field and the squirrel-cage rotor induces rotor current, and the interaction of the two magnetic fields produces torque. By changing output frequency, a VFD changes the speed of the stator field and gives the motor a controllable operating speed instead of tying it directly to the utility frequency.
- Frequency controls base speed
- Lower output frequency reduces synchronous speed; higher frequency increases it within motor and load limits.
- Voltage must follow frequency
- Below base speed, a drive normally reduces voltage with frequency to maintain usable magnetic flux and avoid excessive current.
- Load still determines slip
- The rotor does not run exactly at synchronous speed. Required torque, motor design and control mode determine the actual slip.
02 · Power conversion
Inside the drive: rectifier, DC bus and inverter
Most conventional low-voltage VFDs use three functional power sections. The input converter rectifies AC into DC. The DC-link capacitors smooth the rectified waveform and create an energy buffer. The inverter then switches that DC bus onto the three motor phases in a carefully timed sequence.
The exact hardware varies. A small drive may use a diode bridge, while regenerative or low-harmonic systems can use an active front end. The selection consequence is important: the VFD input current, harmonic behavior and ability to return energy are not defined by the inverter alone; they depend on the complete front-end and DC-bus design.
- 1. Rectifier
- A diode or active input stage converts incoming AC to a DC voltage while defining much of the drive’s input-current behavior.
- 2. DC link
- Capacitors smooth the DC bus and temporarily store energy between the input and motor-side switching stages.
- 3. IGBT inverter
- Six semiconductor switches typically create the U, V and W motor outputs from the positive and negative DC rails.
- 4. Motor output
- The drive regulates the fundamental voltage and frequency needed for acceleration, steady speed, deceleration and torque control.
03 · PWM output
The motor sees pulses, but its current is much smoother
The inverter does not generate a perfectly smooth sine-wave voltage at its terminals. It uses pulse width modulation (PWM): the IGBTs switch the DC bus on and off rapidly, and the width and sequence of those pulses create the required fundamental three-phase voltage. Motor inductance filters much of the high-frequency content, so the phase current is far closer to a sine wave than the terminal voltage.
This distinction matters in an installation. Fast voltage edges can interact with long motor cables, motor insulation and grounding. Cable length, switching frequency, EMC rules and the motor’s inverter-duty rating determine whether a line reactor, output reactor, dV/dt filter or sine-wave filter should be considered.
- Carrier switching
- The semiconductor switching rate is much higher than the commanded motor-output frequency.
- Effective voltage
- Changing pulse width and timing changes the fundamental voltage available to the motor without changing the DC-bus source.
- Cable effects
- Long leads can amplify peak voltage at the motor. Cable construction, grounding and output filtering must be checked together.
04 · Application value
Use a VFD when variable speed improves the process—not by default
Pumps and fans are strong candidates because flow or pressure can often be controlled by reducing speed instead of wasting energy across a valve or damper. Conveyors benefit from adjustable line speed and gentler acceleration. Mixers, compressors and winding systems may use the drive for repeatable process control, torque management or coordinated motion.
The benefit must still be checked against the complete system. Energy savings depend on the load curve, operating hours, static head, mechanical losses and existing control method. A VFD can reduce starting current and mechanical shock, but it also introduces harmonics, high-frequency common-mode effects, heat and additional commissioning requirements. At sustained low speed, a self-cooled motor may need separate ventilation or temperature monitoring.
- Process control
- Match flow, pressure, speed or torque to demand using a sensor, PLC command or the drive’s own PID function.
- Energy opportunity
- Variable-torque loads can offer substantial savings at reduced speed, but calculations should use the real system curve and duty profile.
- Mechanical protection
- Controlled acceleration and deceleration can reduce belt shock, water hammer and repeated stress on couplings and gearboxes.
- Engineering checks
- Review harmonics, EMC, motor cooling, insulation, bearing currents, bypass needs and safe stopping before ordering.
Product range
Product types within this category
These groups are a practical way to narrow the requirement. Final model selection still depends on the electrical, mechanical and installation data shown on this page.
Browse VFD model reference libraryCompact and micro VFDs
Space-efficient drives for small motors, simple machines and local speed control where the required I/O and overload duty are limited.
Typical fit: Small pumps, fans, conveyors and OEM equipment
General-purpose VFDs
Flexible low-voltage drives with common control modes, programmable I/O and communication options for everyday industrial loads.
Typical fit: Plant utilities and standard machinery
Pump and fan drives
Variable-torque products with PID, sleep, multi-pump and building-control features selected around the hydraulic or airflow system.
Typical fit: Water systems, HVAC and cooling towers
High-performance vector drives
Higher torque accuracy, faster response and optional encoder feedback for demanding speed or torque-control applications.
Typical fit: Conveyors, winders, mixers and process lines
Regenerative and low-harmonic drives
Drive systems designed for frequent returned energy or defined input-current performance, subject to a complete system review.
Typical fit: Cranes, test stands and high-cycling machinery
Medium-voltage VFD systems
Engineered drive packages for large motors where voltage class, transformer arrangement, cooling and site integration require project-level selection.
Typical fit: Large pumps, fans, compressors and mills
Quick selection
Send the VFD data you already know
Choose any known values. You can also send the page with no selection and add a model number or nameplate photo in WhatsApp.
Typical specification scope
What we can match
- Typical power range
- 0.1kW to multi-megawatt systems
- Common input classes
- 200-240V, 380-415V, 440-480V
- Motor types
- Induction, PM and selected synchronous motors
- Control modes
- V/f, sensorless vector, closed-loop vector
- Common interfaces
- Digital I/O, analog I/O, RS-485 and Ethernet options
- Supply formats
- Drive only, accessories, panel or motor-drive package
Common applications
Where selection changes with the load
Pumps and HVAC fans
Variable-torque control, PID, sleep mode and building-management communication can reduce energy use and mechanical stress.
Conveyors and material handling
Starting torque, speed regulation, braking and overload capability are matched to the loaded conveyor profile.
Compressors and process machines
Acceleration, minimum speed, current margin and process control signals are reviewed before the drive is selected.
Retrofits and discontinued models
We compare the original unit with a current-series successor and a compatible alternative, noting wiring or programming changes.
Decision guide
Choose the right product path before comparing models
Single-phase input VFD
- Choose when
- Only single-phase supply is available for a compatible three-phase motor.
- Verify before ordering
- Input-current rating, allowed motor size and manufacturer derating rules.
Standard three-phase VFD
- Choose when
- The load has ordinary acceleration and normal-duty or moderate overload requirements.
- Verify before ordering
- Motor current, overload class, minimum speed and control interfaces.
Heavy-duty vector VFD
- Choose when
- The application needs strong low-speed torque, fast response or repeated loaded starts.
- Verify before ordering
- Current margin, encoder need, braking energy and motor thermal limits.
Regenerative or low-harmonic system
- Choose when
- Energy returns frequently to the DC bus or the project has a defined harmonic target.
- Verify before ordering
- Duty cycle, point of common coupling, system impedance and compliance basis.
From requirement to quotation
A short path to a verifiable product match
Share the page and nameplate
Send the motor or existing drive label plus any known power, voltage and application details.
Confirm the electrical match
Current, duty, control method, communications and installation conditions are checked against available models.
Receive a supply-ready quotation
The quotation identifies the proposed model, included accessories, availability and delivery basis.
Frequently asked questions
Practical answers before you request a quote
What information is needed to select a VFD drive?
The minimum useful data is motor rated current, motor voltage, supply voltage and load type. For an exact replacement, also provide the existing model code, control wiring, communication protocol and any braking resistor or filter connected to it.
Can a VFD be sized only from motor horsepower or kW?
Horsepower or kW is a good starting point, but output-current capacity is the final check. Derating may be necessary for heavy overload, high ambient temperature, altitude, single-phase input or a high carrier frequency.
Can you replace a discontinued VFD with another brand?
Often yes, provided the electrical rating, overload duty, I/O, communications, dimensions and motor-control method are compatible. We identify any wiring, parameter or enclosure changes before proposing the substitute.
Do VFD installations always need a line reactor or filter?
No. The need depends on supply impedance, harmonics, cable length, EMC limits, nearby sensitive equipment and the drive manufacturer’s guidance. The accessory package should be selected for the installation rather than added automatically.