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Transformer sizing

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deetz

Electrical
Joined
Nov 19, 2010
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89
Location
US
When calculating the size of transformer needed for a 3 phase machine, are you required to up the machine amperage by 125% for the KVA?
 
Hi Deetz
Sizing a transformer kVA depends on the following;
a)Size of the load ( inductive / Resistive)
b)Power Factor
c)Motor starting VD % Max should be between 15 and 20% depending on the mechanical load.
d)Motor starting (DOL,SS,VSD)
e)Impedance of transformer
f)Starting a large drive when other loads are operating.
G)And of course what will the VD be at the TX secondary Terminals.
H)The norm is stick with 80% TX loading oil and 90% dry type.
I have attached an Excel spreadsheet that may help you.
 
 https://files.engineering.com/getfile.aspx?folder=9c4ed774-3235-484d-8b30-76223abe2e9b&file=Transformer_Regulation_(002).xls
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