How to choose a variable frequency drive: VFD vs soft starter vs DOL, sizing by motor FLA, duty ratings, control modes, cable length and harmonics, braking, and the installation mistakes that destroy drives.
A variable frequency drive (VFD, also called an inverter or AC drive) converts fixed-frequency mains power into a variable-frequency, variable-voltage output. Because an induction motor's synchronous speed is proportional to supply frequency, controlling frequency controls speed - and that single capability buys three things at once: energy savings on variable-torque loads, mechanical soft starting without inrush current, and process control (conveyor pacing, pump pressure, fan airflow).
VFDs pay for themselves fastest on centrifugal fans, blowers and pumps, where torque follows the square of speed and power follows the cube: running a fan at 80% speed needs only about half the power. On constant-torque loads such as conveyors or extruders the savings are smaller, but soft starting and speed control still justify the drive in most installations.
All three start a motor, but they are not interchangeable. A soft starter only ramps voltage during start-up and then hands the motor back to full mains voltage; it cannot vary running speed. A DOL starter applies full voltage directly - cheapest, but with 6-8x inrush current and full mechanical shock.
| Option | Speed control while running | Starting current | Energy saving on fans/pumps | Typical use |
|---|---|---|---|---|
| DOL starter | No | 6-8x FLA | None | Small motors, occasional starts |
| Soft starter | No | 2-3x FLA (ramped) | None | Large motors, infrequent starts |
| VFD | Yes (0 to ~2x base speed) | Rated current, no inrush spike | 20-60% on variable-torque loads | Process control + energy saving |
Rule of thumb: if the motor must run at one speed only and starts a few times a day, a soft starter is the economical choice. If speed must vary, or starts/stops are frequent, a VFD is the only correct tool.
Size the drive to the motor's full-load current (FLA), not to its kilowatt rating. Motors of the same kW can draw very different currents at different speeds/poles, and the nameplate FLA is the number the drive must carry continuously. Two rules:
Also match the supply: single-phase 220 V input drives exist for small motors, but above roughly 2.2-3 kW a three-phase input drive is the norm - and a 220 V single-phase motor cannot be run from a standard three-phase output VFD.
Typical 4-pole 380-400 V three-phase motors. Always verify against the actual motor nameplate, and never use this table as a substitute for it.
| Motor power | Approx. FLA (400 V) | Suggested drive rating |
|---|---|---|
| 0.75 kW | ~2.2 A | 1.5 kW drive (2.2 A class) |
| 1.5 kW | ~3.7 A | 1.5 kW drive |
| 2.2 kW | ~5.0 A | 2.2 kW drive |
| 3.0 kW | ~6.9 A | 3.7 kW drive |
| 4.0 kW | ~8.8 A | 5.5 kW drive |
| 5.5 kW | ~11.5 A | 7.5 kW drive |
| 7.5 kW | ~15.5 A | 11 kW drive |
| 11 kW | ~21.5 A | 15 kW drive |
| 15 kW | ~29 A | 18.5 kW drive |
| 18.5 kW | ~35 A | 22 kW drive |
V/F (scalar) control holds a fixed voltage-to-frequency ratio. It is cheap, stable, and runs several motors from one drive - fine for fans, pumps and most conveyors. Its weakness is low-speed torque: below about 5 Hz it cannot hold full torque.
Sensorless vector control estimates rotor flux and delivers full torque down to very low speeds, with faster response - the right choice for hoists, winding machines and any load that must hold position or torque at low speed. It requires drive commissioning (motor parameter auto-tuning) and a motor cable within the drive's recommended length.
A VFD protects itself and the motor against overcurrent, overvoltage, undervoltage, phase loss, overload and overtemperature - but it is not an isolating device. A complete installation still needs branch-circuit protection upstream (an MCB or MCCB sized to the drive's input current), a local isolator, and EMC-compliant earthing with a short, wide earth bond from drive to panel backplate.
For the motor circuit itself, thermal protection is best handled inside the drive (electronic thermal relay). Where a contactor or motor protector is used upstream of the drive for safety switching, it must only be operated when the drive's output is disabled - never as a run/stop device. The wider motor-control panel around the drive - AC contactors for bypass or auxiliary circuits, industrial control devices for signalling - stays standard practice; only the switching rules change.
Most standard three-phase induction motors can run on a VFD, but two checks matter. First, the motor's insulation must withstand the drive's PWM waveform - very old or rewound motors may need an output reactor or a motor with reinforced insulation (class F or better). Second, running a motor well below its base speed raises current temperature: verify the motor's continuous torque rating at the lowest planned speed, and use forced cooling (a separately fan-cooled motor) if it will crawl at low speed for long periods.
Opening a contactor on the drive's output while it is running breaks a live inductive circuit with no path for the current - the resulting overvoltage spike can trip the drive or damage its output stage. Motor switching must happen on the supply side, and only after the drive's output has been disabled. If a bypass contactor is genuinely required, interlock it so transfer is impossible while the drive is modulating.
On centrifugal fans and pumps the saving is large because power scales with the cube of speed - 80% speed needs roughly half the power, so 20-40% total energy saving is common on duty cycles that actually vary. On constant-torque loads such as conveyors the saving is small at full speed and comes mainly from eliminating throttling or damping losses. Drives with poor control loops, oversized motors run at fixed full speed, or constant-speed duty will save little - measure the load profile before promising numbers.
Only when the load returns energy to the drive faster than losses absorb it: descending hoists, unwinders, centrifuges and high-inertia fans that must decelerate quickly. The symptom of needing one is overvoltage trips during deceleration. Size the brake unit and resistor from the braking torque and duty cycle, not from the motor kW - an undersized resistor overheats and fails, while an oversized one never reaches its rated dissipation.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.