Engineering reference – source controlled

VFD Continuous and Overload Rating Comparator

For one motor, one identified variable-frequency drive and one documented operating configuration. The tool compares current ratings that you copy from an engineering load study and the current manufacturer data; it does not select a VFD model or frame size.

No kW shortcutDerated catalog currentOne overload point only

Do not use the former result. The old page treated a three-phase sinusoidal power estimate as motor FLA, applied the same formula to a “single-phase” option, displayed duty factors that were not used, and recommended a universal kW size. IEC 61800-2:2021 defines rating specifications and selection criteria for AC power-drive systems; it does not provide a universal motor-kW-to-VFD-size table. Use the exact current, voltage, duty, derating and overload data for the selected product.

Controlled comparison record

All enabled fields are required. Decimal point and decimal comma are accepted; grouped numbers, exponents and unit suffixes are rejected.

Current unit The same unit is used for all current fields.

Overload comparison A point check is not a full duty-cycle or curve check.

Required continuous output current From the documented load/torque study, in the selected current unit.

Available continuous VFD output current Catalog value after every applicable manufacturer derating.

Required overload current Required current at the checked overload point.

Allowed VFD overload current Exact product/duty rating under the same conditions.

Time unit The same unit is used for both duration fields.

Catalog recurrence/cooldown record Record only; not interpreted by this limited comparator.

Required overload duration

Allowed overload duration

Manufacturer source record Do not combine a continuous rating from one voltage/duty table with an overload rating from another.

Electrical and operating-condition record

Required confirmations I copied every rating from the same exact VFD product, voltage range and duty table, and the continuous rating already includes all applicable manufacturer derating. I understand that one overload current-duration point does not validate the full torque-speed envelope, acceleration, regenerative/braking duty, repetition/cooldown, motor heating, low-speed cooling or harmonic/EMC behavior. A qualified engineer will verify the motor, drive, supply, protection, cable, filters/reactors, grounding, EMC, functional safety, thermal limits and driven machine before use.

No result until the form is submitted.

Limited comparison outcome

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This wording is deliberately not “approved”, “selected” or “recommended”.

Continuous current ratio

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Available derated VFD current / required current.

Continuous current difference

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Available minus required; may be negative.

Continuous current margin

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Relative to the recorded requirement.

Overload current ratio

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Allowed / required at this one point.

Overload duration ratio

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Allowed / required for this one point.

Boundary checks

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Equality is accepted only for this arithmetic screen.

Not a final VFD selection. A result that meets the recorded values proves only those arithmetic comparisons. It does not establish compatibility, safe torque, starting/acceleration capability, overload repetition, braking/regeneration, thermal life, protection coordination, electrical safety, EMC or compliance of the complete power-drive system and machine.

Equations and interpretation

Continuous current

Rcont = Idrive,derated / Irequired
DeltaI = Idrive,derated – Irequired
Margin% = 100 x DeltaI / Irequired

Irequired is the continuous output-current requirement from the documented application study. Idrive,derated is the selected product’s continuous output-current rating after every applicable manufacturer derating. The arithmetic boundary is Rcont >= 1. Equality leaves no numerical current margin and is not an engineering approval.

One overload point

Rcurrent = Idrive,overload / Irequired,overload
Rtime = tdrive,allowed / trequired

The point meets this limited arithmetic screen only when both ratios are at least 1. The recurrence, cooldown, shape of the current/torque profile and the complete manufacturer overload curve still require separate review. A drive can satisfy one point and fail the actual duty cycle.

Why motor kW is not converted into a “required VFD current” here

The familiar expression P/(sqrt(3) V power-factor efficiency) is a balanced three-phase sinusoidal power relation under stated assumptions. It is not a universal VFD selection formula, does not apply unchanged to single-phase input, and cannot replace motor nameplate data, a torque-speed study or product-specific output-current and derating limits. Input current and output current are also different rating quantities.

Source and standard record

Source	Status checked 16 Jul 2026	What it supports here	What it does not support
IEC 61800-2:2021, Edition 3.0	Valid; published 2021-03-03; stability date 2027.	General rating specifications and criteria to consider when selecting AC power-drive-system attributes.	No universal motor-kW-to-drive-frame table and no fixed ND/HD multiplier for every product.
IEC 61800-5-1:2022, Edition 3.0	Valid; corrigenda 1:2023 and 2:2025 included in the official copy; stability date 2026.	Electrical, thermal and energy safety scope for power-drive systems.	Not a shortcut for application sizing or a substitute for the selected manufacturer’s ratings.
IEC 61800-3:2022, Edition 4.0	Valid; official copy includes Corrigendum 1:2025.	EMC is a separate system-level requirement that must be reviewed.	This comparator does not perform an EMC assessment.
ABB Technical Guide No. 7, Dimensioning of a drive system, 2011, pp. 26 and 31	Official manufacturer guide; used as an engineering explanation, not as a current product catalog.	Selection considers torque characteristics, converter current limits, supply voltage and derating; p. 31 illustrates a continuous-current requirement.	Its example values are not universal ratings and must not be copied to another application or product.
Danfoss VLT AutomationDrive FC 301/302 Design Guide, document shown as AJ286655760917en-000101 / 130R0301, 2019.10, pp. 94, 96, 98, 130-131	Official product guide retained as a product-specific illustration; use the current revision for an actual selection.	Continuous and intermittent output-current ratings vary by type code, supply-voltage range and duty; derating varies with altitude, temperature, switching frequency and enclosure.	The illustrated 110%, 150% or 160% values are not universal VFD rules.

NEEDS_LICENSED_SOURCE: the full normative text of IEC 61800-2:2021 and IEC 61800-5-1:2022 is not freely available in the audit record. This page therefore does not invent clause numbers, reproduce closed tables or claim that either standard supplies the comparison equations. The ratios above are transparent general arithmetic applied to user-entered, source-controlled ratings.

Published boundary example

The example button copies one product-specific catalog boundary from the Danfoss guide, page 98: FC 301/302 T4/T5 type P7K5 in the high-overload 380-440 V row lists 16 A continuous and 25.6 A intermittent; the table heading specifies one minute per ten minutes. The example sets both requirements equal to those catalog limits only to test equality handling. It deliberately leaves all confirmations unchecked. It is not a recommendation for that drive or for any real motor.

Limits that remain outside this comparator

Full torque-speed/loadability envelope, field weakening, acceleration time, motor and load inertia, starting and breakaway torque.
Overload curve, repetition/cooldown, cyclic duty, stall/locked-rotor behavior, regenerative power, brake chopper/resistor duty and DC-bus limits.
Motor inverter suitability, insulation stress, bearing currents, shaft grounding, low-speed cooling, thermal model and overspeed limits.
Input supply phase and fault level, transformer/reactor/cable voltage drop, harmonics, fuses/breakers, conductor sizing, filters and protection coordination.
Ambient temperature, altitude/air pressure, enclosure, contamination, switching frequency, motor-cable length, output filter and every manufacturer derating curve.
Electrical safety, EMC, functional safety, machine risk assessment, hazardous-area requirements and local regulations.

Frequently asked questions

Does a larger motor-kW label prove that a VFD is suitable?

No. Manufacturer kW/hp labels are conditional typical motor associations. Compare the exact continuous and overload current limits for the product, voltage range and duty, then complete the torque, thermal, supply, safety and system checks.

Can I always use 110% for normal duty and 150% for heavy duty?

No. Those values appear in some product families under exact time, recurrence, voltage, power and configuration conditions. Other products and ranges differ. Copy the exact rating from the current manual or selection software for the model under review.

Is motor nameplate current enough?

It is important input, but not always sufficient. ABB’s guide notes that it is only an indication when motor/application derating or a torque profile changes the dimensioning requirement. The complete load study and manufacturer limits govern.

If both ratios are at least 1, is the VFD selected?

No. It means only that the entered continuous current and one overload point do not exceed the entered ratings. The many system checks listed above remain mandatory.