Small Transformer Impedance
Small Transformer Impedance
(OP)
Do small transformers have more resistance than reactance?
I'm trying to calculate the reactance and resistance of my 25KVA generator neutral grounding transformer. There is no nameplate, and the impedance test data shows Impedance Amps = 1.88, Impedance Watts = 230, Impedance Volts = 460. From this, I calculated X=253.82 and R=920.
Can a transformer truly be more resistive than reactive?
I'm trying to calculate the reactance and resistance of my 25KVA generator neutral grounding transformer. There is no nameplate, and the impedance test data shows Impedance Amps = 1.88, Impedance Watts = 230, Impedance Volts = 460. From this, I calculated X=253.82 and R=920.
Can a transformer truly be more resistive than reactive?






RE: Small Transformer Impedance
RE: Small Transformer Impedance
RE: Small Transformer Impedance
jghrist: It's a 13,300V primary, 120/240V secondary. During operation, there is a 0.9ohm resistor on the secondary. During the impedance test, the secondary was shorted, so it does not include the resistor.
The test sheet has the following for the impedance test section:
"Impedance Test - SHORT X1-X2-X3-X4 HOLD IMPEDANCE AMPS ON H1 & H2 (13,300V CONN)
Impedance Amps - 1.88
Impedance Watts - 230
Impedance Volts - 460"
RE: Small Transformer Impedance
xnuke
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RE: Small Transformer Impedance
Apparent power in the test case: S=Isc*Vsc=1.88*460=864.8 VA.
If measured active power P=230 w then Q=SQRT(S^2-P^2)=SQRT(864.8^2-230^2)=833.654 VAR.[as xnuke said].
R=P/Isc^2=230/1.88^2=65.07 ohm. X=Q/Isc^2==833.654/1.88^2=235.87 ohm [as submonkey said].
Isc=Vsc/(R+j*X)=Vsc*(R-jX)/(R^2+X^2)=0.5-j*1.812 A.
VforR=Isc*R=65.07*(0.5-j*1.812)=32.535-j*117.926=|1.88*65.07|=122.33 V
check: R= VforR^2/P=122.33^2/230=65.07 ohm
VforX=Isc*X=j*235.87*(0.5-j*1.812)=427.465+j*117.935=|1.88*235.87|=443.4356 V
X=VforX^2/Q=443.4356^2/833.654=235.87 ohm.
VforR+VforX=32.535-j*117.926+427.465+j*117.935=32.535+427.465=460 V