Motor Starting Voltage Dip Estimator — Model the Starting Sag
Estimate the first-pass voltage dip when a motor starts by comparing starting current with the source short-circuit current. Use the result to identify scenarios that deserve a more detailed system study.
Starting and source inputs
How the simplified voltage-dip estimate works
For a three-phase source, short-circuit current can be approximated from short-circuit MVA and system voltage. A simplified voltage-dip ratio can then be estimated from motor starting current divided by source short-circuit current.
Why source strength matters
A strong source has a large short-circuit capacity and therefore a larger short-circuit current. The same motor can create a smaller percentage voltage dip on a stronger source than on a weaker one.
What the review level means
The review level is your own screening threshold. It is not presented as a universal limit because acceptable voltage variation depends on the system and equipment.
Worked example
At 400 V, a 20 MVA source has an approximate short-circuit current of 28.9 kA. A 150 A motor starting current would therefore produce a simplified voltage-dip estimate of about 0.52% before feeder and transformer effects are separately considered.
Where this model becomes insufficient
For a final assessment, the impedance of the transformer, feeder, connection point and starting method may need to be modeled explicitly rather than collapsed into a single source MVA value.
Frequently asked questions
Does this calculate actual measured voltage sag?
No. It is an electrical source-strength estimate.
Can soft starters or variable-frequency drives change the result?
Yes. A lower starting current changes the estimated dip substantially.
Why is short-circuit MVA used?
It provides a convenient way to represent source strength without requiring a detailed impedance network.
Is the review level a code requirement?
No. It is a screening value selected by the user.