SKF AWAIV-12, Руководство пользователя

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DC testing principles and theory
High-voltage DC testing of electric motors determines the integrity of the ground wall
insulation system of a motor’s winding. The ground wall insulation system consists of the
wire’s insulation, slot liner insulation, wedges, and varnish.
Before going further, we need to discuss the meaning of a “HiPot test.” The label “HiPot test”
describes the general idea of high-voltage testing and describes a specific type of high-
voltage insulation stress test. One must differentiate between the concept of HiPot testing
and the specific HiPot test based on the discussion’s context.
To perform any of the high-voltage DC tests the red test leads from the tester connect to the
motor’s three-phase coils and the black test lead connects to the motor’s steel core/frame.
The voltage on the red test leads raises to a predetermined test voltage. The leakage current
flowing from the motor’s coils through the ground wall insulation to the motor frame is
measured. The tester then calculates the resulting insulation resistance (IR) using Ohm’s law.
Megohm test
The megohm test applies a DC voltage to the windings of a motor after first isolating the
winding from ground. Usually, you choose the test voltage to be at or near the motor’s
operating voltage (see IEEE 43).
The purpose of the megohm test is to accurately measure the insulation resistance of the
ground wall insulation. The insulation resistance (IR) is a function of many variables: the
physical properties of the insulating material, temperature, humidity, contaminants, and so
on.
We calculate the IR value using Ohm’s law, dividing the applied voltage by the measured
leakage current:
This leakage current is the current that is actually able to pass from the winding through the
ground wall insulation to the motor’s steel core plus any surface leakage currents that flow
through moisture or contaminants on the insulation’s surface. To accurately determine the
insulation resistance, you must reduce the surface leakage to an inconsequential level. The
winding might need to be cleaned or heated to evaporate any moisture on its surface.
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, and so on. We can compensate for the effects of temperature by converting the IR
value to a standard temperature of 40 °C (104 °F), as shown later in this chapter. The effects
of humidity and contaminants cannot be readily taken into account. You must use good
judgment when analyzing IR values from motors that may be wet, dirty, loaded with carbon
dust, and so on.
A suggested test voltage for the megohm test is 1.7 times the applied/operating line voltage
for the motor. For example, a 480 V motor would be tested at 480 V × 1.7 = 816 V DC. You
can also find recommended test voltages in IEEE 43-2000, NEMA MG-1-1993, and EASA
technical manuals. Test voltages near the line-to-line operating voltages are often used.
For example, 480 V class motors would use 500 V; 2300 V class motors would use 2300–
2500 V; 4160 V class motors would use 4000–5000 V.
When first applying the voltage to a motor or when increasing the voltage, you will observe
an unusually high current. This high current is not a leakage current, but the charging
current of the “capacitor” formed by the motor’s copper coils, the ground wall insulation, and
the motor’s steel core. We usually call this capacitor the “machine capacitance.”
120 SKF Static Motor Analyzer—Baker AWA-IV User Manual
Motor testing theory and reference