How To Choose Perfect Transformer

How To Choose Perfect Transformer |Easy Steps to Calculate Required Capacity of KVA Rating. 

This article will explain how to select a perfect transformer in your substation and also discuss here calculations of kVA rating.

For example, you can easy to understand the calculation.  Here I choose connected load around 77.84kVA and consider maximum demand 58.7kVA. Large motor load is very important for transformer calculation because of the motor will consume a large amount current in starting time 

Collect the details from load calculation sheet.

Connected load: 77.84 kVA

Max. Demand: 58.7 kVA

Ratings of largest motor: 5.595KW(7.5HP)

Starting method of Largest Motor: Star-Delta Starter

2. Calculate Continuous Steady State Capacity in kVA

=Max. Demand/Derating factor

= 58.7/0.9

= 65.22kVA

3.  Calculate the Starting load of the largest motor

= (0.746*k*Pmhp)/(PFm*Eff.m)

= (0.746*3*7.5)/(0.8*0.8)

= 26.22 kVA

4  Calculate the total load present during the starting of the largest motor

P1= P - PmFL + Pmst

= 58.7-5.595 + 26.22

= 79.325 kVA

5  Calculate Rated Continuous Capacity of  Transformer during staring of the            largest motor

= P1/Overload Factor of Transformer

= 79.325/1.25

= 63.46 kVA

6  Comparison & Selection of Rating

Continuous steady-state capacity = 65.22kVA

Rated continuous capacity during starting of largest motor = 63.46KVA

65.22kVA is higher and it is selected as the required capacity.

The nearest available size above the required rating = 100 kVA

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