same motor · locked rotor · Δ reference to Y estimate

Idealized Star-vs-Delta Starting Ratio Worksheet

Compare winding voltage, locked-rotor line current and locked-rotor torque for one documented motor that runs in delta at the supply line voltage.

VY = VL/√3IY ≈ IΔ/3TY ≈ TΔ/3

Not a starter, contactor, protection or timer selector. The one-third current and torque relations are idealized locked-rotor comparisons for the same winding and supply. They do not predict acceleration, transition current, heating, voltage dip, load capability or equipment ratings.

Documented inputs and traceability

Supply line-to-line voltage V_L (V)positive RMS value for the documented start condition

Documented Δ DOL locked-rotor line current I_Δ,LR (A)positive manufacturer/test value at the stated voltage and frequency

Documented Δ DOL locked-rotor torque T_Δ,LR (N-m)positive manufacturer/test value for the same condition

Calculation / comparison record ID

Motor, nameplate and datasheet identifiers

Six-terminal winding diagram and connection revision

Line-voltage source, RMS method and uncertainty

Evidence of Δ continuous-run rating at line voltage

Locked-rotor current source, voltage, frequency and revision

Locked-rotor torque source, voltage, frequency and revision

Motor frequency, IEC design and starting-performance record

Load torque-speed curve, inertia and acceleration study

Supply capacity, voltage-drop and network-limit study

Open/closed-transition circuit and interlocking drawing

Contactor, overload and SCPD coordination record

Transition criterion and measured current/speed record

Responsible electrical engineer / reviewer

Review date

Notes and exclusionsOptional: saturation, supply impedance, open-transition transients, thermal limits and hazardous-area requirements.

The motor has six accessible winding terminals, is intended to run in Δ at the stated line voltage and is approved for star-delta starting by its manufacturer/project documentation.
I_Δ,LR and T_Δ,LR are documented for the same motor, Δ connection, line voltage, frequency and locked-rotor condition; neither value is inferred from rated current or power.
The one-third values are idealized standstill estimates. Saturation, supply voltage drop and winding-impedance variation can make measured results differ.
No transition time/current, acceleration, contactor, overload, short-circuit protection, thermal adequacy or acceptance verdict will be inferred.

Idealized locked-rotor comparison

Δ winding voltage

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Y winding voltage

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Documented Δ DOL locked-rotor line current

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Idealized Y locked-rotor line current

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Documented Δ DOL locked-rotor torque

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Idealized Y locked-rotor torque

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Y/Δ winding-voltage ratio

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Idealized Y/Δ current and torque ratios

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Formula, units and classification

V_Δ,winding = V_L [V RMS]
V_Y,winding = V_L/√3 [V RMS]
I_Y,line,LR ≈ I_Δ,line,LR/3 [A RMS]
T_Y,LR ≈ T_Δ,LR/3 [N·m]

Antal	Betydelse	Gräns
V_L	supply line-to-line RMS voltage	not automatically the voltage present at motor terminals during starting
I_Δ,LR	documented DOL Δ locked-rotor line current	not rated current multiplied by a generic factor
T_Δ,LR	documented DOL Δ locked-rotor torque	not a universal multiple of rated torque
I_Y,LR,T_Y,LR	idealized star-start standstill estimates	not acceleration curves, transition peaks, thermal duty or equipment ratings

The one-third relations are manufacturer-supported engineering approximations for the same motor winding at locked rotor. They are not themselves acceptance formulae from an IEC standard.

Removed unsafe selection claims

No invented starting torque. The former page hard-coded DOL torque as 1.5× rated torque. Starting torque must come from the motor manufacturer or a controlled test at the stated voltage/frequency.
No timer by motor kW. Fixed 3–5, 5–10, 8–15 and 10–20 second bands ignored load torque, inertia, acceleration, supply behaviour and transition method.
No contactor sizing from amperes alone. Device selection requires circuit position, utilization category, duty, coordination, overload/SCPD arrangement, switching frequency, ambient conditions and manufacturer data under current IEC 60947-4-1.
No universal changeover speed or transient. ABB notes high transition currents and requires the chosen transition point to be verified; this worksheet does not generate an 80%/75–85% rule or a 10–15× current claim.
No generic application verdict. Starting succeeds only if accelerating torque remains positive through the speed range and thermal/network limits are satisfied.
No hidden state. Power/rated-current defaults, multipliers, auto-calculation, dynamic tables, clipboard output and generated FAQ HTML were removed.

Source traceability

Claim	Klassificering	Evidence
IEC 60034-1:2026 Edition 15 is the current rating-and-performance base publication; IEC 60034-1:2022 was withdrawn on 13 March 2026.	Official IEC status and scope	IEC 60034-1:2026
IEC 60034-12:2024 Edition 4 covers starting-performance parameters for specified single-speed three-phase cage motors intended for DOL or star-delta starting, including qualifying dual-voltage motors.	Official IEC status and scope	IEC 60034-12:2024
IEC 60947-4-1:2023 Edition 5 covers electromechanical contactors and motor-starters and has Corrigendum 1:2026 incorporated in the corrected 2026-03 version.	Official IEC status and scope	IEC 60947-4-1:2023; COR1:2026
For a qualifying motor, ABB states that star-delta start reduces current inrush and motor torque to one third of direct-start values, warns of high transition currents, and requires delta motor voltage equal to mains plus six terminal windings.	Manufacturer engineering guide; page visually inspected	ABB Starting Combinations, p. 1/5 (PDF p. 7)

Accessed: 15 July 2026. Exact IEC starting-performance tables, device ratings, utilization categories and coordination requirements remain NEEDS_LICENSED_SOURCE.

Reference check

For V_L=400 V, documented I_Δ,LR=294 A and T_Δ,LR=150 N·m: V_Y,winding=230.940107676 V, I_Y,LR≈98 A and T_Y,LR≈50 N·m. The winding-voltage ratio is 57.735026919% and the idealized current/torque ratios are 33.3333333333%.

Questions

Can this choose a contactor or overload relay?

No. Use the manufacturer’s coordinated starter tables and the applicable current IEC 60947 documents for the actual circuit, duty and fault level.

Can I enter rated current instead of locked-rotor current?

No. This worksheet requires a documented delta-connected locked-rotor line-current value. It does not invent a DOL multiplier.

Does one-third torque prove the load will accelerate?

No. Compare the full motor and load torque-speed curves, inertia, supply voltage drop and thermal duty across the entire start.