# Fall transformer

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AC/DC MACHINES LAB                            Name: _______________________________________
Lab Section: _______ Group: _______ Date: _________
TRANSFORMERS
Lab 7                                         Partners: _______________________________________
_______________________________________
_______________________________________

DETERMINING NO LOAD AND FULL LOAD LOSSES OF TRANSFORMERS

PURPOSE:

To utilize open circuit tests and short circuit tests to determine the resistive and reactive components of
a practical transformer.

APPARATUS REQUIRED:

TYPE                   QUANTITY               DESCRIPTION

Meters                     1                  0-300 Volt AC Voltmeter
1                  0-150 Volt AC Voltmeter
2                  0-5 Amp AC Ammeter
1                  Wattmeter

Power Supply                                  0-125 Volt Variable AC

Oscilloscope               1                  2 Trace Minimum

Transformers               1                  Staco, 115/230 – 24/12

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INTRODUCTION:

No Load Losses

1. No load losses are found by doing an Open Circuit test on a transformer. By recording the
voltage, current and power being drawn, the components of the magnetizing circuit for a
transformer can be found by the following:

Where the magnetizing circuit looks like the following:

Note that the primary and secondary winding resistances and leakage fluxes are ignored for this
test.

Again, remember that the no load losses are very important to utilities, it is power that must be
supplied to this transformer whenever it is energized.

Full Load Losses

2. Another important test performed is the Short circuit test or Full Load test to determine
transformer losses at full load. This done by energizing the transformer with the secondary
terminals short circuited. The incoming voltage is increased until rated current flows in the
shorted secondary.

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3. By measuring the current, voltage and power drawn under these conditions, the equivalent total
resistance and inductive reactance of the transformer can be calculated from

4. Remember that REQ,HS = RHS + a2RLS and XEQ, HS = XHS + a2XLS, where we say that the low
side resistance and reactance is being “reflected through the transformer by the square of the
turns ratio”. Also, XEQ is the quantity used to determine the %X stamped on the transformer
nameplate and is usually considered the only mechanism that limits short circuit current
through transformer.

Procedure

Open Circuit Test(No Load Losses)

1. Connect the transformer for 115 volt to 24 volt operation. Make the test connections as
shown:

HS                LS

2. Using the variable AC supply on the Hampden console apply 115 volts and make the following
recordings:

POC ____________              IOC ____________             VOC _____115_____

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3. Using the equations given in the introduction for No Load Losses Calculate the following:

Ife __________      Rfe __________     IM __________ XM __________

4. Draw the magnetizing circuit for this transformer.

Short Circuit Test(Full Load Losses)

5. Reduce the applied voltage to zero! And place an ammeter across the secondary terminals of
the transformer. This creates a short circuit.

6. Slowly increase the applied voltage until rated current flows in the secondary. In this case that
is 4 amps.

7. Record the following:

PSC ____________              ISC ____________              VSC ____________

8. Using the equations given in the introduction for Full Load Losses Calculate the following:

ZEQ, HS __________       REQ, HS __________      XEQ, HS ________

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9. Draw the entire transformer as it would be seen from the High Voltage Side, if you need some
help refer to figure 2.11(a):

Per Unit Impedance:

10. On the nameplate of every transformer is usually stamped a %X or %Z, if %Z is shown is it
usually considered to be all inductive reactance. The method commonly employed to
determine this number is simply (VSC/VOC) x 100.

Calculate %X ___________ from the above.

11. Another way to determine %X is to use (XEQ, HS/ XBASE, HS)x100, where XBASE, HS =
(VBASE)2/SBASE. For this transformer SBASE = (0.835amps)x(115 volts) = 96 VA and VBASE =
115.

Determine:

XBASE, HS__________ and %X ___________

12. Compare %X found in 11 to that found in 10.

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