Atec 830280 User Manual
Transformer Ohmmeter
Transformer Winding Resistance
and Tap-Changer Test Set
■
Direct digital reading
■
Built-in discharge circuitry
■
Two independent measuring channels
■
Tests operation of on-load tap-changers
Transformer Ohmmeter
Transformer Winding Resistance
and Tap-Changer Test Set
DESCRIPTION
The Transformer Ohmmeter is a line-operated, fieldportable instrument designed specifically to measure the
dc resistance of all types of magnetic windings safely and
accurately.
Its predominant use is the measurement of the dc
resistance of all types of transformer windings within the
defined ranges of current and resistance.
It can also test rotating machine windings and perform
low-current resistance measurements on connections,
contacts and control circuits.
Three features combine to make this instrument unique:
dual measurement, load tap-changer testing and safety
shutdown.
The dual set of potential inputs measure the resistance of
the primary and secondary windings of a single- or threephase transformer simultaneously. The dual reading
characteristic will speed up the measurement when it is
used to test windings on delta-delta connected windings
on three-phase transformers.
Due to circulating currents induced when the test current
is applied to the primary winding, this type of
measurement is countered by the same current on the
secondary winding. This action attenuates the circulating
current and the reading time is improved tenfold.
The Transformer Ohmmeter is extremely useful when
testing the windings and contact resistance on tapchangers with make-before-break contacts and voltage
regulators.
The internal shutdown circuit will be triggered by a
voltage kickback of a few microseconds if the tap-changer
contacts are opened when the tap-changer circuit is
operated through all of the tap positions.
This action will check for pitted or misaligned contacts as
the instrument will shut down if either condition occurs.
Users are protected by the shutdown circuit safety feature:
any inadvertent disconnection of a test lead or loss of
power to the instrument will safely discharge the energy
stored in the test sample.
APPLICATIONS
The Transformer Ohmmeter is used:
■
To verify factory test readings
■
As part of a regular maintenance program
■
To help locate the presence of defects in transformers,
such as loose connections
■
To check the make-before-break operation of on-load
tap-changers
A regular maintenance program that includes winding
resistance measurements is the most effective way to use
this instrument. Once a benchmark is established,
subsequent test results can be compared to determine if
changes are occurring in the transformers, instrument
transformers and associated control wiring, voltage
regulators, motors, generators, breaker contacts, all types
of connections (bolted, soldered, crimped, etc.).
One-Winding
Measurement
Transformer Ohmmeter
Transformer Winding Resistance
and Tap-Changer Test Set
Tap-changers are mechanical devices and the most
vulnerable part of a transformer. Tap-changers result in
more failures and outages than any other component and
so require frequent testing and attention to ensure proper,
reliable operation.
The Transformer Ohmmeter can be used to check the
make-before-break operation of on-load tap-changers and
also to measure the contact resistance of each tap position.
FEATURES AND BENEFITS
■
Direct digital reading saves time, no balancing is
required.
■
Built-in discharge circuit safely discharges the specimen
when test is completed, if lead accidentally disconnects
or if power is lost.
■
Electromechanical safety indicator gives a visual
indication of a charged or discharged specimen, even if
power to the instrument is lost.
■
Two independent measuring channels allow
simultaneous testing of primary and secondary windings
or measurement of two phases at a time.
■
The sensitive surge detection circuitry monitors the
contact operation of on-load tap-changers for the proper
make-before-break sequence. If an open circuit
condition exists, the instrument shuts down immediately.
■
High-accuracy, four-terminal bridge: no lead
compensation required.
■
Electronically generated and regulated current supply
overcomes high-inductance transformers quickly,
allowing fast measurements to be taken. Display of
measurement occurs only after test current stabilizes.
■
Wide resistance range allows heat-run testing of many
transformer types.
■
Lightweight and portable, the Transformer Ohmmeter is
ideal for use in shop or substation environments.
■
Overtemperature protection provides automatic current
shutdown and LED indication to prevent instrument
damage.
■
Instrument and accessories come in a foam-lined
transport case.
Schematic comparison of
one- and two-winding
measurements
Low-Voltage
Winding
High-Voltage
Winding
Simultaneous
Two-Winding
Measurement
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