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    VFD Sizing

    Required continuous output current and candidate checks.

    EstimatePreliminary result, not a design verification.
    Before you calculate

    VFD output-current sizing

    The drive's continuous output current, after its own derating, must cover the motor's rated line current. Mains input current, overload need, PWM effects and EMC are separate checks that this tool does not assess.

    Learn more — complete guide →
    Step 1

    Motor

    Output-current method for a three-phase induction motor. The motor current comes from the nameplate when you enter it, otherwise from the built-in reference.

    Three-phase motor. This tool covers three-phase motors only.

    The mechanical rating on the nameplate, not the electrical input power. 1 HP = 0.745699872 kW.
    Advanced: nameplate and motor data (all optional)

    Anything you enter here replaces the built-in reference model for that quantity. Leave a field empty to use the reference where it applies.

    The starting method does not prove how the motor is wound for running. Follow the nameplate.
    Advanced: check a candidate drive (optional, manufacturer data)

    Enter only what the drive's documentation gives, after its own derating for ambient, altitude and switching frequency. Anything missing stays NOT ASSESSED, and there is never an overall PASS.

    A different rating column from the one above; they are not interchangeable.
    Only a factor from the manufacturer; none is assumed.
    Enter that current above. The check compares amperes: your multiple of the motor current against the documented multiple of this drive current.
    Step 2

    Results

    Motor nominal current28.5 AReference assumption or your efficiency and power factor
    Required continuous VFD output currentat least 28.5 AContinuous output current of the drive, after the manufacturer's derating, compared with the motor rated LINE current.
    110 % light-overload illustrative output31.3 AAn example catalog definition (one manufacturer: 110 % for 1 min in 10 min), not a requirement.
    150 % heavy-duty illustrative output42.7 AAn example catalog definition (150 % for 1 min in 10 min), not a requirement and not the load's need.
    Drive suitabilityNot assessedNOT ASSESSED until a candidate is entered.
    Motor-side current is not mains current. The drive's input current, harmonics and upstream protection come from its input rating. 1.0 × the motor current is a nominal motor-side reference only, never a predicted start current. The delta winding current (16.4 A, only if delta-connected) is not used: a drive's output lead current is compared with the LINE current.
    NOT ASSESSED by this tool:
    • Drive suitability, until a candidate with manufacturer data is entered
    • Mains input current, harmonics and the upstream protection (need the drive's input rating)
    • Overload need of the load and the starting torque
    • Motor insulation under PWM (dv/dt), long motor cables, output reactors or sine filters, bearing currents
    • EMC filter, RCD type, braking resistor, safety functions and ambient or altitude derating
    Reference model in use (not a nameplate): IE3, 4 poles, 50 Hz, 400 V. A plain arithmetic mean of two catalog series per rating (ABB General Performance, edition 10-2021, and WEG W22 Rev. 34 of 04/2024), taken at the listed rating. It is a two-series reference estimate, not a market average and not the data of your motor; locked-rotor ratios in particular differ widely between motors of the same size. ABB later editions print different power factors and Is/In for some small ratings. Both series were compared value by value with the original catalog pages (ABB 10-2021 page 15, WEG 50025712 Rev. 34 page 34).
    Engineering scope: preliminary screening, not a product selection or an IEC 61800-5-1 or 61800-3 compliance statement.

    1. Efficiency, power factor and locked-rotor ratio in use

    Entered values replace the reference. The reference (IE3, 4 poles, 50 Hz, 400 V family) is the plain mean of two catalog series. Efficiency: two-series reference; power factor: two-series reference; Is/In: two-series reference.

    η,cos⁡φ,kLR=Is/In\eta,\quad \cos\varphi,\quad k_{LR}=I_s/I_n
    η=0.922,cos⁡φ=0.825,kLR=7.6\eta=0.922,\quad \cos\varphi=0.825,\quad k_{LR}=7.6

    2. Rated line current (three-phase)

    Mechanical output power divided by the electrical quantities at the rated operating point.

    In=1000 P3 ULL ηcos⁡φI_n=\frac{1000\,P}{\sqrt{3}\,U_{LL}\,\eta\cos\varphi}
    In=1000×153×400×0.922×0.825=28.4633 AI_n=\frac{1000\times 15}{\sqrt{3}\times 400\times 0.922\times 0.825}=28.4633\ \text{A}

    3. Delta winding current (rated running, only if the motor runs in delta)

    In a balanced delta connection each winding carries the line current divided by √3. This is the rated RUNNING winding current; it is not a starting current and does not depend on the starting method. In-line starters and drives are sized from the LINE current, never from this value.

    Iw,Δ=Iline3I_{w,\Delta}=\frac{I_{line}}{\sqrt{3}}
    Iw,Δ=28.46333=16.4333 AI_{w,\Delta}=\frac{28.4633}{\sqrt{3}}=16.4333\ \text{A}

    4. Input, apparent and reactive power

    Sinusoidal rated operating point.

    Pin=Pη,S=Pincos⁡φ,Q=Pintan⁡(arccos⁡φ)P_{in}=\frac{P}{\eta},\quad S=\frac{P_{in}}{\cos\varphi},\quad Q=P_{in}\tan(\arccos\varphi)
    Pin=16.269 kW,S=19.72 kVA,Q=11.1444 kvarP_{in}=16.269\ \text{kW},\quad S=19.72\ \text{kVA},\quad Q=11.1444\ \text{kvar}

    5. Nominal VFD motor-side current reference

    A nominal motor-side reference equal to the rated current. It is NOT a start-current prediction and NOT the mains input current of the drive.

    Iref=1.0×InI_{ref}=1.0\times I_n
    Iref=1.0×28.4633=28.4633 AI_{ref}=1.0\times 28.4633=28.4633\ \text{A}

    Calculations use full precision internally; the displayed numbers are rounded for readability.

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