Generator Test Procedures

1. CORE DEFFECT DETECTOR “ELIN CM2”
General:
The „ELIN-CM2“ is a measuring device which is used for the detection of lamination short circuits in
the stator cores of large rotating machines (motors and generators)
Introduction
During the service-time of rotating machines it may become necessary to check the electrical
integrity of the laminated stator iron. Reasons for such inspections usually are suspected hot spots
which are caused by e.g. mechanical damages of the lamination by ground fault currents (due to
winding failures), by abrasion of lamination insulation due to lamination vibrations or due to improper
cleaning agents or by other reasons.
It is commonly known, that lamination short circuits cause short circuit currents IK whenever the
stator core is energized, i.e. when a back iron flux IR is forced (see figure 2). During machine
operation these short circuit currents heat up the areas of core damages and thus hot-spots are
created. If the temperature of such a hot-spot exceeds the maximum permissible value of the
lamination insulation, this insulation is damaged and thus leads to a continuous and rapid increase
of the core fault.
Detection of Core Lamination Short by the “ELIN-CM2”
The principle of operation of this measuring device can be explained by means of figure 3: The
CM2-apparatus feeds and exciter coil which creates a (comparatively small) back iron flux IR’.
Now this flux IR’ forces a (comparatively small) current IK’ through the area of core damage (i.e. the
latent hot-spot in case of machine operation) which itself is surrounded by a magnetic flux IK . Now
by means of an appropriate probe (and some electronics) this flux IK easily can be picked up.
By using high sensitive probes there is no necessity for a high energy exciting coil to establish a
back iron flux: A few ampere-turns are sufficient to force lamination short circuit currents. As only few
ampere-turns are necessary, thin wires may be used for exciter coil; such thin wires easily can be
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2. threaded through the air-gap of the machine under test and thus the CM2-test can be done without
removing the rotor (the probe is small enough, too. To be placed in the air-gap between stator and
rotor). As an example, figure 1 shows the practical application of the probe (here hand operated) in
the air-gap of a turbo-type generator.
Although the short circuit-current IK (which is picked up by the CM2-probe) is the reason for the hot
spot temperature, no simple law can be established to calculate an actual in-service hot spot
temperature from a given short circuit current (beside IK .Δ√ depends on the spot-resistance of the
lamination short as well as on the thermal conductivity of the core and on other cooling
circumstances). So the CM2 test device is able to detect a lamination damage as a fact but cannot
predict precisely the in-service over temperature of this damage point; nevertheless it is a test
experience that in most cases high CM2-readings coincide with high hot-shot-temperatures at
machine operation (and vice versa). In such a case it is recommended to verify the actual over
temperature either by a core induction test (core flux test) and/or by monitoring the hot spot in
service by means of e.g. a thermocouple.
The main benefit of the CM2-test is to enable the detection of hidden core faults, i.e. even if the
damaged area is covered by e.g. the stator winding: The CM2 picks up a flux (IK ) and thus is
independent to the thermal conductivity of the core. It is a test experience (and is visualized by figure
4), that a severe core fault at the bottom of a slot leads only to an unremarkable over temperature at
the core teeth.
General Requirements for the CM2-test
The CM2-apparatus needs a power supply 220 V/600 VA approximately.
To ensure operation of the flux probe as close to the core as possible, it is advisable to have the
core under test clean to certain extent (heavy dust at the bore surface would lead to irreproducible
test result, especially if the dust is somewhat conductive). The lamination sheets must be connected
conductivity to the stator frame.
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3. As the CM2-measurement needs some experience in interpreting the test results, a well trained test
engineer is recommended for operating the equipment, accompanied by an appropriate assistance
technician.
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