# 9 transformer voltage drop

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```					                      Transformer Voltage Drop
TRANSFORMER VOLTAGE DROP

Primary Voltage 12.7 KV

Secondary

Voltage:

120/240 1

From Transformer Manufacturer:

% Resistance, % R = 2.24%

% Reactance, % XL = 3.44%

R actual = 10(%R) (KV Secondary) ^2
KVA Transformer

XL actual = 10(%X) (KV Secondary) ^2
KVA Transformer

1 Voltage Drop = I (Rcos  + Xsin )

3 Voltage Drop = 3 I ( Rcos  + Xsin )

R = Actual Resistance
X = Actual Reactance

COS = PF =       Power Factor of load

Pure Resistance PF = 1, MOTORS PF .65-.9 depending on size

Single Phase

Example

6/20/02                   Chapter 9: Transformer Voltage Drop   1/3
Transformer 100 KVA, 1, 12.4 KV to 115/230V

% R = 2.24%

% X = 3.44%

Find voltage drop across transformer assuming 230 V base at no load

Load: 75 KVA @ .85 OF, Cos = .85, Sin = .527

I = KVA          = 75KVA       = 362 Amp
KV              .23 KV

R actual = 10 (2.24 %) (.23KV)^2      = .01185
100 KVA

XL actual = 10 (3.44%) (.23KV) ^2 = .0182
100KVA

Volts Drop = I         [R Cos + Sin]

Volts Drop = 362 A [.01185 (.85) + .0182 (.527) ]
= 362 A [.01966] 
= 7.116 Volts

Vd= (7.116 Volts) 100          = 3.09% Voltage drop
230 Volt base

Voltage drop on 3 is same except for 3 in formula.

Current I = KVA 3
KV 3

Three Phase
Example

3  Voltage drop = Vd = 3 I [ R Cos + X Sin ]

Load 150 KVA, P.F. = .8 = Cos ,  = 36, Sin = .6

6/20/02                     Chapter 9: Transformer Voltage Drop       2/3
6/20/02   Chapter 9: Transformer Voltage Drop   3/3

```
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