Baker D30R User Manual - page 25

12/3/2009 | 71-022 V6 

D30R 

25 

Current

Leakage

Measured

Voltage

Applied

IR

=

4  

Principles and theory of DC testing 

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. The test lead selector switch makes all test lead 

connections. 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 motor’s steel 

core 

plus

 any surface leakage currents. The surface leakage currents flow through moisture 

or contaminants on the surface of the insulation. To accurately determine the insulation 

resistance, the surface leakage must be reduced to an inconsequential level. 

The insulation resistance is a function of many variables: the physical properties of the 

insulating material, temperature, humidity, contaminants on the surface of the winding’s 

insulation, etc. The effects of temperature can be compensated for by converting the IR value 

to a standard temperature 40

o

C as shown later in this chapter. The effects of humidity and 

contaminants can not be readily taken into account. Good judgment must be used when 

analyzing IR values from motors that may be wet, dirty, loaded with carbon dust, etc.