Can minimum braking resistance be calculated from motor power?

No. The drive or brake-chopper manufacturer specifies the minimum allowed resistance. Never use a lower value.

Can nominal AC line voltage be used as braking-chopper voltage?

No. Use the applicable chopper switch-on or specified braking voltage from the exact drive documentation.

Does cycle-average power select the resistor?

No. The resistor and chopper must also satisfy manufacturer peak power, pulse energy, time-window, temperature, duty and protection limits.

Drive-specific candidate arithmetic

VFD Braking Resistor Candidate Check

Check a candidate resistance against the drive maker’s minimum and calculate idealized chopper on-state/event demands from qualified input data. No resistor is selected automatically.

Use drive UonUse manufacturer RminNo AC-line estimate

Candidate arithmetic

Resistance check

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Full-on current Uon/R

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Full-on electrical power Uon²/R

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Required event-average power E/t

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Ideal average on-fraction during event

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Energy at full-on for entered t

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Cycle-average energy rate E·z/3600

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Electrical on-state arithmetic

Ion = Uon/R; Pon = Uon²/R

Uon is the documented chopper switch-on or braking voltage, not nominal AC line voltage and not a generic Vac√2 estimate. Pon is the ideal electrical power while the chopper is fully on at that voltage; the real chopper is pulsed and voltage varies.

Event and repetition demand

Pevent,avg = E/t; don,ideal = E/(Pon·t); Pcycle,avg = E·z/3600

The on-fraction is an ideal energy-balance check. A value above 100% means the entered R and Uon cannot dissipate E within t even at continuous on-state. A value at or below 100% does not prove acceptability because chopper ratings, voltage thresholds, resistor pulse-energy curves, temperature and duty limits still govern.

Primary manufacturer boundaries

A ABB ACS880-604 brake-unit manual warns never to use a resistance below the minimum for the drive/chopper and requires both maximum-power and time-window energy checks. The Siemens F-TM ServoDrive manual uses configured Uon for peak Uon²/R and separately requires peak and continuous capability checks.

No selection verdict: also verify drive/chopper maximum current and power, resistor minimum/maximum resistance range, pulse energy versus duration, ten-minute or other accumulation windows, continuous rating at ambient temperature, enclosure/protection, insulation voltage, wiring, fusing, thermal switch and safe mounting clearances exactly as specified by the manufacturers.