Baker D30R, User'S Manual & Operating Instructions

Page: 41 / 142

D30R Users Manual
41
Do Not Touch Test Leads while test is in Progress!
71-023 Rev C 5/8/2007
Principles of DC Testing
High voltage DC testing of electric motors is performed in order to determine
the integrity of the ground wall insulation system of a motor’s coil. The ground
wall insulation system consists of the wire’s insulation, slot liner insulation,
wedges, varnish, and sometimes, phase paper.
There are three types of DC tests performed by Baker test instruments:
Megohm
tests,
HiPot
tests, and
PI
tests. Each type of test is designed to answer
a specific question regarding the properties of or the integrity of the ground wall
insulation system. There is also a
Stepped HiPot
test that can be performed with
Baker testers. A brief discussion on each of these tests follows below.
Before going further the meaning of “HiPot test” needs to be discussed. The
phrase “HiPot test” is used to describe the general idea of high voltage testing as
well as to describe a specific type of high voltage insulation stress test. One
must differentiate between the concept HiPot testing and the specific HiPot test
based on the context of the discussion.
To perform any of the DC Tests, the motor’s windings are isolated from ground,
the red test leads from the Baker Tester are connected to the motor’s three phase
coils and the black test lead is connected to the motor’s steel core/frame. The
output voltage on the red test leads is raised to a predetermined test voltage and
the leakage current flowing from the motor’s coils, through the ground wall
insulation, to the motor frame is measured. The Digital Tester then calculates
the resulting insulation resistance (IR) using Ohm’s law.
The
Megohm Test
consists of applying a DC voltage to the windings of a
machine after first isolating the winding from ground. Internal relays inside the
Analyzer automatically isolate with windings – no operator action is required.
The test voltage is usually chosen to be at or near the operating voltage of the
machine (see IEEE 43). Recommended test voltages can be found in the
previous chapter titled “Recommended Test Sequence, Voltages and Applicable
Standards”.
The intended purpose of the Megohm test is to make an accurate measurement
of the insulation resistance of the ground wall insulation. The insulation
resistance, abbreviated IR, is a function of many variables: the physical
properties of the insulating material, temperature, humidity, contaminants etc.
The IR value is calculated using Ohm’s law – the applied voltage is divided by
the measured leakage current. This leakage current is that current which is
actually able to pass from the winding through the ground wall insulation to the