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Monday, April 1, 2019

Electrical Motor Starting Calculation Excel Sheet


File Size: 104 Kb



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. Modeling
This spread sheet allows the user to model a given network as follows:
A. Generator/s: The details of one generator shall be entered. If there are more than one generator of identical rating, these can be modeled simply by enter the number of generator in parallel.
B.Grid:
The details of grid shall be entered

C. Transformer/s: The details of one transformer shall be entered. If there are more than one transformer of identical rating, these can be modeled simply by enter the number of transformer in parallel.
D. The spread sheet 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.
E. Starting Motor: The details of the largest motor connected to the switchboard shall be entered.
F. 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.

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

5. Calculation: The spread sheet performs a static motor starting analysis using the iterative method. The re-acceleration of motors is not considered. Ensure that the following options are enabled:
Tools=>Options=> Calculation=> Automatic=> Iteration=>Max iteration= 100=> Max change = 0.001
Note: Change of any variable in the spread sheet, automatically initiates the iteration and recalculated the results. It may happen that due to incorrect details, the iteration diverges and erroneous results are displayed. In such case, check and correct the input and press "Ctrl+Shift+H" to reset.

6. Results: Two sets are of results are displayed on the spread sheet as follows:
Prestart: These voltages and power flows results are for "Pre-start" condition. i.e. before the starting of the "Starting motor"
During: These voltages and power flows results are for "Starting" condition. i.e. at the time of starting of the "Starting motor"
The voltage variations shall be highlighted if they exceed the permissible limits Also power flow over the cable-1 shall be highlighted if it exceeds the generator capacity

7. Application: This spread sheet can be useful for following checks  Accessing normal running and starting voltage drops  Deciding the number/rating of the generator for starting a given rating of largest motor  Checking adequacy of selected cable sizes etc.



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