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 θ + Xsin θ)
3∅ Voltage Drop = √3 I ( Rcos θ + Xsin θ)
I = Actual load current
R = Actual Resistance
X = Actual Reactance
COSθ =
PF =
Power Factor of load
Pure Resistance PF = 1, MOTORS PF .65-.9 depending on size
Single Phase
Example
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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, Cosθ = .85, Sinθ = .527
I = KVA
KV
= 75KVA
.23 KV
= 362 Amp
R actual = 10 (2.24 %) (.23KV)^2
100 KVA
= .01185Ω
XL actual = 10 (3.44%) (.23KV) ^2 = .0182Ω
100KVA
Volts Drop = I
[R Cosθ + Sinθ]
Volts Drop = 362 A [.01185 (.85) + .0182 (.527) ]
= 362 A [.01966] Ω
= 7.116 Volts
Vd= (7.116 Volts) 100
230 Volt base
= 3.09% Voltage drop
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 Cosθ + X Sinθ ]
Load 150 KVA, P.F. = .8 = Cos θ, θ = 36°, Sinθ = .6
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