Motor Starting Voltage Dip Calculator
Voltage dip at the motor bus while it starts, and the torque still available. With the formula, a worked example and typical values.
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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
This page calculates voltage dip, starting apparent power and torque available vs full voltage from motor full-load current, starting current, line voltage and fault level at the motor bus. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
Many specifications limit starting dip to 10 to 15% at the motor bus. Check lights, drives and contactors on the same bus.
Cross-check: 8 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: Arithmetic impedance addition, starting power factor ignored.
What this calculator checks
- Voltage dip at the bus from motor starting kVA and source fault level.
What remains unverified
- Starting power factor and the true impedance angles.
- The effect on other loads on the bus.
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; IEC 60909-0, IEC 60034-12:2024 Ed 4.0; IEC 60909-0:2026 Ed 3.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Motor starting performance and supply-system short-circuit strength are relevant context. The voltage-dip equation is a simplified source-impedance estimate, not a complete motor-starting or network study. |
| Cross-check test cases | 8 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 |
|---|---|
| Motor full-load current | 140 A |
| Starting current (× full-load) | 6.5 |
| Line voltage (L-L) | 415 V |
| Fault level at the motor bus | 10 MVA |
- Sst = √3 × 415 × 910 = 0.6541 MVA
- dip = 0.6541 / (10 + 0.6541) = 6.139 %
- Torque = (1 − 0.0614)² = 88.1 % of full-voltage torque
Assumptions
- Source and motor impedances add arithmetically
- Starting power factor ignored (slightly conservative)
Many specifications limit starting dip to 10 to 15% at the motor bus. Check lights, drives and contactors on the same bus.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Motor full-load current | A | greater than 0 A | |
| Starting current (× full-load) | (number) | 1 or more, up to 12 | |
| Line voltage (L-L) | V | greater than 0 V | |
| Fault level at the motor bus | MVA | greater than 0 MVA |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Voltage dip (%) | (number) | Shown to 2 decimal places | |
| Starting apparent power | MVA | Shown to 3 decimal places | |
| Torque available vs full voltage (%) | (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.
How the result changes with motor full-load current
| Motor full-load current (A) | Voltage dip | Starting apparent power (MVA) | Torque available vs full voltage |
|---|---|---|---|
| 14 | 0.65 | 0.065 | 98.7 |
| 35 | 1.61 | 0.164 | 96.8 |
| 70 | 3.17 | 0.327 | 93.8 |
| 140 | 6.14 | 0.654 | 88.1 |
| 280 | 11.57 | 1.308 | 78.2 |
| 560 | 20.74 | 2.616 | 62.8 |
| 1,400 | 39.54 | 6.541 | 36.5 |
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 motor full-load current, starting current (× full-load), line voltage (L-L) and fault level at the motor bus. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Source and motor impedances add arithmetically. Starting power factor ignored (slightly conservative).
Which calculators should I use next?
Common next steps are Motor Starting Current and Fault Level (kA to MVA). The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 8 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 Start Voltage Dip in the Cable Formulas: Motor starting voltage drop formulas: the starting current
- Voltage Change and Flicker Formulas: Voltage change formulas: step change from a load, the dip as the ratio of load to short-circuit level
- Motor Starting Formulas: DOL, Star-Delta: Motor starting formulas for direct-on-line, star-delta, autotransformer and soft starter current and torque
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