Home › Guides › Variable Frequency Drives

Variable Frequency Drives (VFDs) Explained: Speed & Saving

For decades, the standard way to control the output of a pump or fan was to run its motor at full speed and then waste the excess: throttle a valve, close a damper, bypass flow back to the tank. A variable frequency drive takes the opposite approach — it changes the motor's speed itself, so the machine only does the work actually needed. The result is smoother control, softer starts, and, on the right loads, dramatic energy savings. Understanding how a VFD works, where it saves energy and where it creates new problems, is essential for anyone selecting, installing or maintaining motor control equipment.

How a VFD controls speed

An AC induction motor's speed is set by the frequency of its supply: synchronous speed is proportional to supply frequency, and the motor runs slightly below that speed by an amount called slip, which grows with load. A VFD exploits this directly. It takes the fixed-frequency mains supply, rectifies it to DC, and then inverts it back to AC at a frequency chosen by the control system — and at a voltage scaled to match, so the motor's magnetic flux stays roughly constant. At low frequency the voltage is low; at full frequency it is full voltage. This voltage-to-frequency ratio is the fundamental control law of the basic VFD, and it is why a motor can run smoothly from a crawl to full speed without losing torque at low speed, within limits.

Inside the drive, three stages do the work. The rectifier converts AC to DC, usually with diodes, sometimes with an active front end that can also return energy to the supply. The DC link smooths that DC with capacitors and, in larger drives, an inductor. The inverter rebuilds AC from the DC using fast-switching transistors, typically insulated-gate bipolar transistors, switched thousands of times per second in a pattern called pulse-width modulation. The motor does not see a clean sine wave; it sees a rapid train of voltage pulses whose average over each cycle approximates a sine wave of the desired frequency. The motor's inductance smooths the current, so the motor runs smoothly even though the voltage waveform is not sinusoidal. That pulse train is also the source of most VFD side effects, which we return to below.

Control methods range from simple to sophisticated. Basic volts-per-hertz control is open-loop: the drive sets frequency and voltage and trusts the motor to follow, with slip causing a small speed error under load. Sensorless vector control estimates the motor's flux and torque from measured currents and adjusts in real time, giving much tighter speed and torque control without a speed sensor. Closed-loop vector control adds an encoder on the motor shaft for the most precise control, used where positioning or exact speed holding matters. The right method depends on the application: a fan does not need closed-loop vector control, while a winder or a crane does.

Where the energy savings really come from

The famous VFD energy saving follows the affinity laws for fans and centrifugal pumps: flow is proportional to speed, pressure to the square of speed, and power to the cube of speed. Reduce a fan's speed by 20 percent and its power falls to roughly half. That is why replacing a throttling valve or damper with a VFD on a variable-flow fan or pump system can cut energy consumption dramatically — the motor simply does less work instead of doing full work and throwing part of it away. The saving is real, but it depends on the duty: a system that always runs at full flow saves almost nothing with a VFD, because there is no excess work to remove. Honest energy claims therefore start from the load profile — how much time the system spends at part load — not from a generic percentage.

On constant-torque loads such as conveyors, mixers and positive-displacement pumps, power falls roughly in proportion to speed rather than with the cube, so the saving is smaller. Here the VFD's value is more about control and mechanical gentleness: soft starting, smooth acceleration, adjustable speed for different products or recipes, and the ability to hold a precise speed or torque. Soft starting deserves emphasis on its own. A direct-on-line start draws a large inrush current and applies a sudden mechanical shock to belts, couplings and gearboxes; a VFD ramps the motor up smoothly, reducing electrical stress on the supply and mechanical stress on the driven equipment, which extends the life of both.

A VFD also enables control strategies that a fixed-speed motor cannot offer. A pump can follow a pressure setpoint from a calibrated pressure transmitter, a fan can follow a temperature or air-quality signal, and a process can be controlled by a PID loop whose output is motor speed rather than a valve position. That last point is worth pausing on: replacing a control valve with a variable-speed pump removes a throttling loss, a maintenance item and a source of noise, and it often improves control quality at the same time. It is also why VFDs appear in almost every modern automation design, whether the supervisory system is a PLC or a DCS.

Harmonics, motor heating and bearing currents

Every benefit of a VFD comes with a side effect to manage. The rectifier draws current from the supply in pulses rather than smoothly, which injects harmonic currents — multiples of the supply frequency — back into the network. Harmonics distort the supply voltage, heat transformers and cables, and can disturb sensitive equipment. Mitigation starts with the drive itself: a DC link choke or an AC line reactor reduces harmonics significantly, and larger installations may use active front ends or harmonic filters. The supply authority or the plant standard usually sets a limit on total harmonic distortion, and compliance should be checked at design stage, not discovered after commissioning.

On the motor side, the inverter's fast voltage pulses stress the motor insulation, particularly the first turns of the winding, which see the steepest voltage rise. Older motors and long cable runs between drive and motor make this worse, because voltage reflections on a long cable can double the peak voltage at the motor terminals. Mitigation includes output reactors or filters, shorter cables, and motors rated as inverter-duty with reinforced insulation. The same fast pulses can also induce shaft voltages that discharge through the motor bearings, causing microscopic pitting known as fluting and eventually premature bearing failure; insulated bearings, shaft grounding rings or common-mode filters address this on larger motors. Finally, a motor cooled by a shaft-mounted fan loses cooling at low speed, because the fan slows with the motor while the motor may still be producing significant torque and heat. Below a certain speed, a separately powered cooling fan or a motor derating may be needed. None of these problems is a reason to avoid VFDs; all of them are reasons to select and install them properly.

Selecting the right drive

Selection starts with the motor and the load, not with the drive catalogue. Record the motor's full-load current, voltage, power and speed, and the load type: variable torque (fan, centrifugal pump) or constant torque (conveyor, mixer, hoist). Size the drive by current, not just by power rating — a motor with a high current for its power needs a larger drive. Check the overload duty: a drive rated for a variable-torque load may only allow a small overload, while a constant-torque application needs a drive rated for a heavier overload, often 150 percent for a minute. Consider the environment: ambient temperature, altitude, dust, humidity and ventilation all affect the drive's rating and life, and a drive installed in a hot, dusty enclosure without cooling will fail early no matter how good its specification. Check the control interface: analogue inputs, digital inputs and outputs, fieldbus options, and whether the drive will be commanded from a PLC, a DCS or a local panel. And confirm the motor's suitability: inverter-duty insulation for new installations, and a careful review — with the motor manufacturer if needed — for retrofitting a VFD to an older motor, especially with long cables.

Commissioning is where selection meets reality. Before first run, verify the motor data entered in the drive, the direction of rotation, the acceleration and deceleration times, the minimum and maximum frequencies, and the protection settings: motor overload, overcurrent, overvoltage, undervoltage and earth fault. Run the motor uncoupled first if possible, then coupled at low speed, checking current, vibration and temperature before ramping to full speed. Record the settings. A drive whose parameters were never documented is a drive that cannot be restored after a failure without guesswork — and guesswork on a production line at midnight is how a good installation becomes a long outage.

Frequently asked questions

Educational content only — always follow your project specifications, manufacturer instructions and local safety regulations.