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4.8
Tips for Making Precise Ratio Measurements
NOTE: The DTR is designed to test step down transformers. It works
with low voltage on the primary, limited to 40V max. If the secondary
reaches 40V before the primary, a safety feature stops the mea-
surement. The recommended procedure to test step up transform-
ers is to reverse the low and high leads and read the reverse ratio.
• Always verify the integrity of test cable connections and reposition test
clips as necessary. Inspect transformer terminals for dielectric coat-
ings, fungus and corrosion.
• During VT/PT Mode operation, select Continuity Test when possible.
Besides being a useful way to check windings and connections, this
test has the added benefit of reducing magnetic remnants in the trans-
former under test. This is important primarily in transformers with high
series resistance, and in low-KVA transformers with extremely high pri-
mary-side inductance. It is not required in most tap-change and regula-
tor transformers.
• When testing polyphase transformers, keep in mind that in some cases,
measured ratios must be multiplied or divided by √3. Refer to § 4.10 for
polyphase connections diagrams and associated ratio equations.
4.9
Ratio Test - 1:1
A simple test can be conducted to test functionality of the DTR
connecting H
RED
transformer is simulated. Operation of the DTR
should yield a ratio very nearly equal to 1.000. If it does not, the DTR
require repair or calibration.
H
RED
H
BLACK
CONNECTIONS
Digital Transformer Ratiometer DTR
to X
, and (separately) H
RED
X
RED
X
BLACK
Model 8500
®
to X
BLACK
BLACK
in this configuration
®
1 : 1
EQUIVALENT
. By
®
, a 1:1
may
®
19
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