Select a drive by output amps, not by kW
A variable-frequency drive (adjustable-speed drive) converts the AC supply into a motor output of controlled frequency and voltage. That allows smooth speed control, controlled acceleration and, depending on the motor, the drive and the control method, useful torque over a range of speeds. A drive is not selected by matching the kW on two nameplates: the critical electrical rating is the manufacturer's continuous output RMS current at the intended motor voltage and operating conditions.
1. What the calculator does
It treats a three-phase squirrel-cage induction motor and the output-current method only. It does not claim support for permanent-magnet or synchronous motors, single-phase motors or single-phase-input drives with three-phase output. Without a candidate drive it gives the current to look for; with a candidate's manufacturer data it compares each condition separately and never reports an overall PASS.
2. The primary sizing calculation
When the nameplate current is not available the calculator estimates it from the shaft power, voltage, efficiency and power factor of the reference operating point. That is a starting point, not a substitute for the real motor data. For the 15 kW, 400 V, 50 Hz IE3 reference motor, Imotor ≈ 28.46 A, so the minimum continuous drive output current to look for is at least 28.46 A after the manufacturer's derating. That says nothing about the transient overload capability or the mains input current. The motor's current comes from the Motor Current & Starting calculation.
Use the line current, not the delta winding current. The motor page may show 16.43 A as a conditional delta winding current. A drive with 20 A continuous output fails the 28.46 A requirement even though 20 A exceeds 16.43 A: a conventional drive's output lead current is compared with the motor's rated line current.
3. Continuous rating and overload rating are different
The ABB ACS580 catalog, as one example, distinguishes separate output-current columns:
- IN: continuous nominal current.
- ILd: continuous current under light overload, where 110 % is allowed for one minute in ten under catalog conditions.
- IHd: continuous current under heavy duty, where 150 % is allowed for one minute in ten under catalog conditions.
These are alternative model ratings, not three guaranteed values equal to the kW nameplate, and other manufacturers define theirs differently. For the example motor, 1.1 × 28.46 = 31.31 A and 1.5 × 28.46 = 42.70 A are illustrative thresholds if the selected drive's corresponding duty rating has a continuous current of at least 28.46 A. A candidate with ILd = 32 A and IHd = 24 A passes the light-duty continuous check and fails the heavy-duty one; the columns must never be treated as interchangeable. The load's real overload need cannot be derived from the kW, and no universal 110 % or 150 % requirement exists.
To compare an overload need with a documented overload, both must be expressed on a known current. In the calculator, the overload you need is a multiple of the MOTOR rated current, and the documented overload is a multiple of the drive current you name (continuous, light-duty or heavy-duty); the check compares amperes. For example, 1.6 × 30 A = 48 A required against a documented 1.5 × 40 A = 60 A is acceptable even though 1.6 is larger than 1.5. Without a named basis the check is NOT ASSESSED. The documented overload is assumed to hold under the same ambient and switching conditions as the continuous rating, and the manufacturer's derating is applied to it.
4. Ambient, altitude and switching frequency
Drives generate heat. Catalog ratings are tied to conditions such as the ambient temperature, the installation and enclosure, the altitude and the switching (carrier) frequency, and manufacturers give tables or curves for reducing the available current. There is no universal derating factor: apply only the selected model's documented factors, and only combine several of them in the way the manufacturer permits.
Worked example (a teaching case, not a recommendation). A hypothetical drive has a 35 A continuous output and a verified combined derating of 0.80:
It fails the continuous-current comparison even though its uncorrected rating is higher. With a valid factor of 0.90 the available current is 31.5 A and that check passes; the other checks are still needed.
5. Voltage, phase and motor winding compatibility
The drive has to accept the available input supply, and its rated motor output voltage has to suit the motor connection. Some drives accept a single-phase input and deliver three-phase output, but at a lower current than under a three-phase supply. A 230Δ/400Y and a 400Δ/690Y motor must be connected correctly for the drive's output voltage. Input and output phase and voltage ratings are separate. A candidate that fails the voltage check fails regardless of its current rating.
6. Motor-side current versus mains current
Do not use the drive's output amperes as if they were the mains supply amperes. The rectifier design, the drive efficiency and power factor, the harmonics, the DC-link behaviour, braking or regeneration and any filters change the mains current. Upstream cable and protection checks use the manufacturer's input rating and installation instructions together with the installation standards. See the LV Cable Sizing guide for the cable side.
7. Cable length, motor insulation and EMC
PWM outputs produce a high dv/dt and reflected-wave voltage peaks at the motor terminals, especially on long leads. The manufacturer may require dv/dt filters, output reactors, sine filters, screened motor cable, bonding and EMC measures, or bearing-current mitigation. None of these is implied by a one-number current result. Drive safety and EMC are covered by standards such as IEC 61800-5-1:2022 and IEC 61800-3:2022; comparing currents does not certify compliance with them.
8. Why there is no universal VFD starting-current number
The acceleration current follows the torque the load needs and the control strategy. A drive can deliver low torque at start and draw far less than the rated current, or a demanding load can call for current up to the drive's allowed overload. 1.0 × the motor rated current can be shown as a nominal motor-side reference, but never as a guaranteed or predicted current, and never as a feeder rating.
9. Common mistakes and questions
Is a 15 kW drive always suitable for a 15 kW motor? Not necessarily: compare the voltage, the continuous current, the derating, the overload rating and the motor characteristics. Do I always need a 150 % drive? No universal requirement follows from the kW; check the real duty and the drive documentation. Are 110 % or 150 % for one minute a safety margin on the motor current? No: they describe specific catalog duty ratings. Can I use the drive output current for the upstream breaker? Not as a substitute for the drive's input-current and protection requirements. Is a VFD the same as a soft starter? No: a drive controls the frequency and speed during operation; a typical soft starter controls the start and stop and then the motor runs at line frequency (see the Soft Starter Sizing guide). Sources: ABB ACS580 catalog and hardware manual, IEC 61800-3 and IEC 61800-5-1.