Vanguard WRM-10 - Manual (Page 5)

REV 2 WRM-10 USER’S MANUAL
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1.0 INTRODUCTION
1.1 General Description and Features
The WRM-10 is designed to accurately measure the winding resistance of highly inductive
power transformers. The unit’s dual resistance-reading input channels can measure two
winding resistances simultaneously. Four-wire (Kelvin) connections provide high accuracy
voltage reading and require no lead compensation. The WRM-10 provides stable resistance
readings of very large transformers by utilizing a 36 Vdc power supply capable of outputting up
to 10 Amperes. The resistance reading of a 100MVA transformer can be achieved in 5 minutes
or less. The unit’s power supply is cooled by heavy-duty fans designed for continuous
operation.
Since the WRM-10 can accurately measure resistances ranging from 1 micro-ohm to 2,000
ohms, it can also be used to measure EHV circuit-breaker contact resistance, motor winding
resistance, or any low resistance. In addition to measuring the resistance value, the WRM-10
also checks the “make-before-break” tap-switching sequences of voltage regulators and load
tap changers.
The WRM-10 is furnished with three 50-foot test cables, a ground cable, a power cord, and a
cable carrying bag. Each test cable lead is terminated with a quick-disconnect test clip.
1.2 Functional Description
The WRM-10’s operation is based on the electrical relationship described by Ohm’s law: R=V/I,
where I is a known current and V is the DC voltage measured across the unknown resistance.
The value of the unknown resistance is calculated by dividing the measured voltage by the
current which is calculated by the microprocessor. Calculated resistance readings are then
displayed on the unit’s back-lit LCD screen.
A special current source allows the WRM-10 to output up to 10 Amperes of test current. The
test current is automatically adjusted depending on the load resistance. For added safety, the
current source circuit is thermally protected. Also, a built-in discharge circuit automatically
discharges the stored energy in the transformer at the end of each test.