Motor Starting Calculation

Updated: Jul 8

Motor Starting Case Studies


A motor starting case study for any given electrical network can be performed by reducing the network to a smaller equivalent network consisting of a source (generator/grid), the starting motor (largest motor), other large running motor and a lumped load.


Warning: The content of this article is cited from the Motor Starting Calculation MS Excel-based spreadsheet


This MS Excel-based spreadsheet is based on this concept and present a small model of the electrical network that can be adapted for such motor starting case studies.

This is very helpful especially for quick checks as follows:

  1. What is the largest motor size that can be started with a given generator rating

  2. What should be the rating of the generator (or nos of generator operated in parallel) for a given load and largest motor size

  3. Knowing voltage drop during starting conditions for given motor details and hence deciding the key parameter during design (say max allowable starting current)

  4. Deciding the cable size/number of runs etc.

The spreadsheet performs an iterative calculation based on pre-start load inputs and starting motor data. Detailed analysis may be performed with the help of available software.

Notes:

  1. Motor starting analysis are generally performed for the largest motor connected to a switchboard.

  2. For worst case results, motor starting analysis are generally performed considering the weakest source.

  3. Considering the above two facts, any given electrical network, where a starting analysis of a motor is required, can be reduced to a simplified network consisting of a source (generator/s or grid), the starting motor (largest motor), the running motor/s (second largest motor/s or motor of same rating as the largest motor under study) and other loads lumped together.

  4. Modelling This spreadsheet allows the user to model a given network as follows:

  • Generator/s: The details of one generator shall be entered. If there are more than one generator of identical rating, these can be modelled simply by entering the number of generator in parallel.

  • Grid: The details of the grid shall be entered

  • Transformer/s: The details of one transformer shall be entered. If there are more than one transformer of identical rating, these can be modelled simply by entering the number of transformer in parallel.

  • The spreadsheet considered either the “Generator/s” or the “Grid” supply for the motor starting analysis. The “source for the motor starting analysis” can be selected by the use via the drop-down menu. The selection automatically simulates switching “ON” or “OFF” the respective source breaker.

  • Starting Motor: The details of the largest motor connected to the switchboard shall be entered.

  • Running Motor/s: The details of the second largest running motor/s (or motor of rating same as the starting motor) connected to the switchboard shall be entered. This model can also be utilized by the user to check the voltage drop at the terminals of any remote motor/s.

  • Lumped Load: The details of the remaining load on the switchboard can be modelled as a lumped load. The % motor can be defined which shall be modelled as constant power load while remaining % of load shall be modelled as constant impedance load for the calculation.

  • Error in data entry of the “voltage” input shall be highlighted, if bus voltages are much different from the generator terminal voltage input of transformer voltage ratio inputs.

The excel spreadsheet has 2 sheets as follows:

  • Sheet title “Preface” with notes explaining the calculation spreadsheet and the summary about the software and

  • Sheet title ”Motor starting calculation” where actual data entry and calculations can be performed


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#motor #starting #calculations #error #grid #generator #transformer

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