Motor Starting Current Calculator
Find the line starting current and relative torque of a motor started direct on line, star-delta or with an auto-transformer, with formula and worked example.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
Use this to see how much the starting method changes the current drawn from the supply and the torque available at the shaft. Enter the full-load current and the locked-rotor multiple from the data sheet, then compare direct on line, star-delta and auto-transformer starting.
Limits of this calculator
The model assumes motor torque varies with the square of the voltage and that the supply does not sag during the start. Star-delta suits light or no-load starts only. Check the real supply voltage dip with the motor starting voltage dip calculator.
Cross-check: 30 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Torque and current ratios assume no voltage sag.
What this calculator checks
- Starting current and torque ratios for DOL, star-delta and auto-transformer starting.
What remains unverified
- Supply voltage sag during the start.
- The motor's real locked-rotor current and torque curve.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 60034-12, 2024 Ed 4.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Starting performance of single-speed three-phase cage induction motors is relevant. The in-app DOL/star-delta/autotransformer ratios are simplified estimates; use motor-manufacturer starting-current and torque data. |
| Cross-check test cases | 30 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Full-load current | 140 A |
| Locked-rotor current (× full-load) | 6.5 |
| Starting method | Direct on line |
| Auto-transformer tap | 0.65 |
- Direct-on-line current = k × In = 6.5 × 140 = 910 A
- Method factor = 1
- Line current = 910 A, torque = 100 % of DOL
Assumptions
- Motor torque varies with voltage squared
- Supply voltage does not sag during the start
Star-delta suits only light or no-load starts. The changeover can cause a second current peak.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Full-load current | A | greater than 0 A | |
| Locked-rotor current (× full-load) | (number) | 1 or more, up to 12 | From the motor data sheet, typically 5 to 8 |
| Starting method | (choice) | Direct on line; Star-delta; Auto-transformer | |
| Auto-transformer tap | (number) | 0.4 or more, up to 0.85 | Used only for the auto-transformer method (fraction of line voltage) |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Line starting current | A | Shown to 1 decimal place | |
| Starting torque (% of direct-on-line) | (number) | Shown to 1 decimal place |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| Full-load current (A) | Line starting current (A) | Starting torque |
|---|---|---|
| 20 | 130 | 100 |
| 50 | 325 | 100 |
| 100 | 650 | 100 |
| 140 | 910 | 100 |
| 200 | 1,300 | 100 |
| 400 | 2,600 | 100 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60034-1 (rotating electrical machines, rating and performance)
- IEC 60034-30-1 (efficiency classes)
Where the motor nameplate or data sheet gives a value, use it in place of a calculated one. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter full-load current, locked-rotor current (× full-load), starting method and auto-transformer tap. Change the unit of a value with its selector and the result stays the same.
How much does star-delta starting reduce the starting current?
Star-delta divides the line starting current by 3, and the starting torque is divided by 3 as well. The changeover from star to delta can cause a second current peak, which this model does not show.
How does an auto-transformer reduce the starting current?
The motor sees the tap fraction of the supply voltage. The current and the torque both fall with the square of the tap, so a tap of 0.65 gives about 42% of the direct-on-line values.
Where do I find the locked-rotor current multiple?
It is on the motor data sheet, often as locked-rotor current over full-load current. Values of 5 to 8 times are common for induction motors; use the data sheet figure, not a typical one, for design.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 30 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Motor Starting Formulas: DOL, Star-Delta: Motor starting formulas for direct-on-line, star-delta, autotransformer and soft starter current and torque
- Soft Starter and VFD Sizing Formulas: Soft starter and VFD formulas: start current and torque at a reduced voltage, drive input power, kVA and current
- Star-Delta Conversion Formulas for Resistors: Star-delta (wye-delta) conversion formulas for resistors or impedances in both directions, equal-resistor shortcut
- Motor Acceleration Time Formulas: Motor acceleration time formulas: load inertia referred to the motor shaft, accelerating torque
- Contactor Sizing Formulas: Motor contactor formulas: the full-load current of the motor
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