Small Transformer Impedance 2

[mesutphen]

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?

 

[submonkey]

Doesn't R=65 and X=235 for the measurements you quote?

 

[jghrist]

What is the transformer primary and secondary voltage? What is the grounding resistor rating? Did the impedance tests include the resistor?

 

[mesutphen]

submonkey: I used R=V^2/P and X=V^2/Q, S=460*1.88=864.8, Q=sqrt(S^2-P^2)=833.6 R=(460)^2/230 and X=(460)^2/833.6

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"

 

[xnuke]

You used the wrong formulas to calculate X and R for a short circuit test. The answers given by submonkey are correct. The 460 V shows up across the series pair of X and R, not across each individually. You don't know how the 460 V divides across the series X and R equivalents, so you can't use the equations based on voltage. Since the current is the same through both X and R, you need to use equations based on current: R = P/I² and X = Q/I², with the formula for Q you indicated.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[7anoter4]

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