Core loss testing is a way to determine core condition and a valuable tool to avoid rewinding unacceptably high core loss stators and rotors.

A core loss test is to be performed before burnout. In this way, the time and effort to burnout, strip, and clean the core is saved if the test shows an unacceptable core.

A core loss test is to also be performed after burnout, stripping, and cleaning to ensure the core has not been damaged during this process.

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LEXSECO Core Tester with Printer

Procedure

Refer to the figures at right during the following discussion. For the purpose of this instruction, the LEXSECO 1081D will be used.

Do NOT test transformer cores; the windings can attain dangerously high voltages.

Avoid placing the test object (core or rotor) on iron tables, on floor plates, or setting test objects directly on concrete floors with iron reinforcing bars. It is preferable that they be placed on wooden skids or non-magnetic tables.

Avoid placing power cables or voltage pickup leads too close to magnetic objects.

Avoid current conducting supports for armature and rotor shaft projections to prevent a short circuit path.

It is important that measurements be accurate due to the accuracy required by the metering and computing circuits of the LEXSECO 1081D. Back iron and yoke dimensions should be recorded to 1/64 inch. The total slot depth should also be to within 1/64 inch. The core length should be measured in an area free from excessive flair or spread. The core diameter is the least critical of all the measurements and should be to within 1/32 inch.

Measure and record the six required dimensions:
· core length
· core diameter
· back iron or yoke depth
· total depth of the coil slot
· width of ventilation ducts
· number of ventilation ducts.

It is assumed you know how to access a PC-style computer and operate a keyboard for input to a computer program.

Enter this data in the computer program where specified on the screen. Continue with the computer input and printout called for on the computer display panel. This is detailed in the instructions inside the instrument's cover.

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Rotor and Armature Data

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Stator Core Dimension Data

Stator

Place the heavy test cable loop through the bore of the stator and connect to the number 1 or number 2 tap as indicated by the CPU tape voltage printout.

Place the small flux meter cable through the stator bore and plug it into the panel jacks on the side of the tester.

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Core Tester Hookup: Stator

Armature or rotor

Plug the extra cable in the tap specified by the computer and connect it to one of the shaft clamps. Connect the lead coming out of the tester case to the other shaft clamp. Tighten the clamps. This allows the tester to pass current through the shaft to complete the test loop through the core. As a precaution, avoid clamping on bearing fits.

Place the flux measurement cable through spokes or a vent hole in larger machines when possible. If this is not possible, the induced flux can be accurately measured by affixing the cables to the shaft clamps.

Push the main circuit breaker handle to the upper, or ON, position to energize the tester.

The rheostat knob on the control panel must be turned counterclockwise toward the OFF position until it engages a stop.

Press the START button. Immediately, the fluxmeter, ammeter, and wattmeter should start to indicate. Also, the START button should light up, indicating power is being applied to the test loop.

Observe the digital flux meter while turning the rheostat knob clockwise until you attain the number of flux indicated on the CPU printout tape.

The inductance and losses of the core being tested may choke down the current, limiting the flux to a lower value than the CPU indicated value for 85,000 flux lines. If you are using the tap number 1, you may want to try tap number 2, limiting the applied current to about 1000 amps for not over 2 minutes.

If you are not able to reach the full CPU indicated flux , then input to the computer the highest value you read on the Flux meter. The printout will show the induced flux lines with all the other output test data at this lower flux density.

Check the core for hot spots or warm areas. If any are found, mark for possible correction by clearing up lamination shorts. Try running a knife blade between the laminations at hot spots, grinding, bumping, or, if necessary, restacking or replacing.

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Core Tester Hookup: Rotor