Motor voltage dip calculator

Use this page when the starting current is already known and the real question is how much the motor start will depress the supply at the point being reviewed. The value is only useful when the source basis is visible beside it. A 400 V start at 0.08 ohm has a different meaning from the same current on a weaker feeder or from a transformer-only shortcut.

The practical job here is to produce a traceable record for a motor schedule, start-dip note, switchboard discussion, generator conversation or early design review. The calculator does not judge whether the dip is acceptable. It shows the estimate and keeps the basis explicit so the next person can see what still needs checking.

Field use cases

The source basis is the part of the result that makes the percentage usable. It tells the reviewer whether the dip came from an equivalent impedance, a transformer shortcut or a more complete project review.

Source-basis matrix

The same motor can be described with different levels of certainty. Source impedance is the strongest basis when it is tied to the point being reviewed. Transformer kVA and impedance is useful when that is all the project has, but it should not be presented as the whole supply network unless the wider source contribution is already included.

Review workflow

1. Confirm the motor reference used on the drawing, schedule, asset register or maintenance note.
2. Confirm the starting-current source. If the value came from the motor starting-current workflow, keep that basis visible.
3. Choose the supply arrangement and nominal voltage. Use 230 V for single phase and 400 V for three phase unless the project data says otherwise.
4. Choose the source basis that matches the information available. Use equivalent source impedance where a point-of-connection value exists. Use transformer kVA and %Z only as a transformer shortcut.
5. Enter a criterion only when there is a real project, equipment or supply value to compare against.
6. Read the retained voltage and the margin together. If the margin is negative, or if the retained voltage is physically implausible, stop and recheck the inputs and the source basis before relying on the record.
7. Export the result only after the source basis, current source and criterion source are visible in the note or record pack.

This workflow keeps the page in the right place. It estimates the start event and records the basis for later review; it is not a final equipment decision sheet.

How to read the result

The state is a comparison aid, not a final declaration. A low calculated dip can still be the wrong project answer if the source basis is weak, the current came from the wrong starting method, or equipment tolerance has not been checked.

Worked scenarios

These examples show the difference between a numerical answer and a usable engineering note. The 3.16% result on the pump is a traceable estimate, not a final decision. The 9.82% case is a prompt to check the current source, feeder strength and equipment data that govern the next decision.

Boundary lines

The boundary is deliberate. Motor full-load current gives running current. Motor starting current gives starting current. This worksheet turns starting current into a voltage-dip estimate. When the question changes to generator capability, final protection settings or final project decision, the voltage-dip record should stop being the main tool.

Australian context

Australian motor-start work is usually read against 230/400 V a.c., 50 Hz project context, but the actual site story still comes from the local authority, DNSP conditions, equipment data and current Australian standards. A calculator like this is helpful because it keeps the start event visible in a format that can be carried into a job pack, a design note or a handover record.

That said, the current being used, the source impedance being entered and the transformer data being assumed must still come from somewhere real. If the site has a weak supply, a long feeder, a soft starter, a drive, a high-inertia load or a sensitive adjacent installation, the start dip is only one part of the conversation. The numbers here can guide that conversation, but they do not replace manufacturer instructions or project-specific electrical judgement.

Stop points

• The starting current source is not known or cannot be matched to the motor reference.
• The source impedance value is being copied without a documented basis.
• The transformer shortcut is being used as if it described the whole supply network.
• The entered criterion is being treated as a final threshold rather than a comparison value.
• The retained voltage looks physically implausible for the selected data.
• The real question is generator selection, final equipment decision, motor torque behaviour or running voltage drop rather than motor-start dip.

When one of those points appears, stop treating the calculator as the answer and return to the source data, equipment data or wider project review.