Tom Bishop, P.E.
EASA Technical Support Specialist
 
Prior to rewinding it is advisable to assess the condition of the core iron of stators, armatures and wound rotors. The assessment is performed by a core test, which magnetizes the core to a pre­scribed magnetic flux density. The predominant tests used to determine core condition are the hot spot test and the core loss watts test. The hot spot test compares the hottest spot in the core to either ambient temperature or core average temperature. The watts loss test compares the core loss test watts prior to winding removal to the same test af­ter the windings have been removed and the core prepared for rewind. 

Core testing traditionally was performed by the use of the loop (ring) test. That required multiple turns of wire to be passed through a core in order to magnetize the core and test for shorted laminations. Mag­netic strength is related to the ampere-turns (amperes x turns) of the magnetizing coil. Mod­ern core testers make it possible to test a core with a single turn of wire, by using high current. Thus the core tester uses one turn and many amperes, whereas the loop test typically uses many turns and a relatively low current. 

The loop test primarily probed for hot spots, indicating shorted laminations. The core tester can do more than just probe for hot spots; it can give an indication of the overall core condition in terms of watts per kilowatts (or watts), and induced voltage. Larger core testers may provide for parallel conductors to carry the test current. Our definition of a core tester includes commer­cial core testers and those that could be built by or for a service center for its own use. 

Core testing requires that the core be magne­tized to a prescribed level. Good practice dictates that the core be tested before the winding re­moval process begins, and then again after the windings have been removed. The before-re­moval testing should be done prior to any heating or mechanical pulling. 

A significant benefit of the core test is to be pound (or watts per kilogram) of loss. A detailed explanation of stator core testing is given in Tech Note 17, so that won’t be repeated here. Our ob­jective is to explain the proper uses and interpretation of core testing using a core tester. 

Core tester definition 
Let’s begin by explaining what is meant by a core tester. It’s a device that supplies high current, up to thousands of amperes, such that a single turn is all that is required to magnetize a core to be tested. The core tester will also have built-in in­strumentation to indicate test current and able to compare the “after” to the “before” to confirm that no core damage has been done in the winding removal process. The core test can also provide an in­dication of shorted laminations by revealing hot spots. 

Properly used, the core tester has certain advantages over the loop test in providing before and after winding removal core con­dition data, and for assessing hot spots. The core tester gives an accurate indication of the watt­age required to magnetize the core to a prescribed level. 

Although the wattage indi­cates a core loss value, the core is not magnetized the same as when a winding energizes it. The flux path in the core test circles the core, like a ring, thus the name “ring test.” The winding flux magnetizes sections of the core, e.g., poles, not complete loops or rings. 

The inherent accuracy of the core test relies on the precision measurement of power, and also by magnetizing the core to the same level for both the before and after tests. The magnetic flux level is directly related to the induced voltage in the core. The induced voltage is measured by passing a relatively small gauge insulated wire through the core from the high current turn, e.g. core test cable on bottom and induced voltage sensing coil on top. The single turn induced voltage wire is termed a search coil. It should pass close to the core as it goes around the stator or rotor/armature, for best sensitivity. 

General guidelines to follow 
The proper application of a core tester fol­lows some general guidelines. One is that the core test is not limited to stators. Armatures and rotor cores can be tested. Armatures need to be tested because although we think of them as a di­rect current device, their windings are AC. The commutator changes DC to AC in a motor and AC to DC in a generator as power enters or leaves the armature windings. Rotor cores can be core tested; however, it must be remembered that rotors usually operate at very low frequency, proportional to slip. 

Thus a typical 60 Hz motor rotor, with about 2% slip at full load, is exposed to only about 1.2 Hz, and a comparable 50 Hz motor rotor would be subject to about 1 Hz. Testing at 60 Hz may indi­cate hot spots that do not appear when magnetized at only 1 or 2 Hz. The point is that a hot spot test of a rotor can give a false indication of a problem. If the rotor is a wound rotor that is used for vari­able speed operation, then the hot spot test is more relevant, as the rotor will be subject to higher slip frequencies inversely proportional to speed. For example, at 25% speed the rotor frequency will be 75% of line frequency. 

The core tester ring flux does not couple to the windings; therefore the core can be tested with the windings installed, removed, in satisfactory con­dition or failed. It is possible for a hot spot to be located beneath the test cable; therefore it is good practice to move the cable after the remainder of the core has been inspected for hot spots, to check the core area that had been masked by the cable. An alternative is to space the loop turns a few slots apart to allow access to the entire core. 

Use instrument to test for hot spots 
Testing for hot spots should always be done with an instrument, and not with your hand (to avoid a serious burn). Thermocouple type ther­mometers are normally very accurate, and rapidly detect temperature. However, the leads can induce flux from the core test and indicate an incorrect temperature. Thermocouples should be used to measure core temperature with the core de-ener­gized. An infrared camera or thermometer can be used to detect core temperature at a distance (at least 3 feet or 1 meter) from the core, even with the core energized. The key is not to place a de­tection instrument into the bore of a stator or close to an armature or rotor while energized. The mag­netic field can cause erroneous readings and could damage certain instruments. 

Interpreting core test results 
Now that we have addressed some of the key points about how to perform a core test, the next critical issue is how to interpret the results. The two primary functions of core testing are to assess the core loss and check for hot spots. The core loss assessment is not based so much on a limiting value of watts per unit weight (pounds or kilo­grams) as it is on the comparison of loss before and after the winding removal process. 

The key criteria in judging core loss is that the watts per unit of weight after winding removal should not increase by more than 20% from the value obtained before the winding or core have been disturbed by the removal process. The 20% factor incorporates the degree of accuracy inher­ent in trying to duplicate the before and after tests. 

Maintaining flux level 
It is also critical that the core test be performed at the same magnetic flux value for both tests. Most core testers have a default value of 85 kilolines per square inch (1.32 Tesla) for the mag­netic flux value. The flux is measured indirectly, but accurately, by measuring the induced voltage with a search coil. The flux is directly propor­tional to the induced voltage. For example, if 4.65 volts were equivalent to 85 kilolines per square inch (kl/in²), then 3.72 volts would indicate the core was only magnetized to 68 kl/in². In this case, 3.72/4.65 = .8, and .8 x 85 = 68. When the core is physically measured prior to disturbing the winding, the back iron measurement is often diffi­cult to accurately measure. Consequently, the actual back iron value may differ from the value used in the “before” core test. 

It is important that the core be tested to the same flux level during the “after” test, even if that means that the actual core flux value will not pro­duce the default or desired flux level. That is, the before and after comparison of watts loss must be based on the same parameters, especially core di­mensions. The induced voltage must be the same on both tests in order to have comparable values in establishing that core loss has not been signifi­cantly increased. Tip: Bend a wire or paper clip into an “L” to use it to determine the exact slot depth. Lower the L between two coils, turn it and raise it until the hook of the L engages the bottom of the coil. Mark the wire flush with the bore, ro­tate and remove the wire, then measure the distance from the mark to the hook. Subtract that from the total bore-to-backiron distance, to get the backiron dimension. 

Test acceptance range 
The raw value of watts per unit of weight has some rela­tive value; however, it must be considered a “rule of thumb” at best. Core tester manufacturers often cite an acceptance range for watts per pound or kilo­gram. Unless there is a database of similar cores from the same motor manufacturer, the watts per unit weight ac­ceptance criteria may result in condemning of good cores or, even worse, acceptance of high loss cores. The main drawback to before and after testing is method is not comparable to the core test. Like it or not, the repair industry does not have a reliable reference source for acceptance criteria of core test wattage per unit weight. 

Hot spot check important 
The other key parameter in core testing is the hot spot check, which should also be performed both before and after winding removal, in con­junction with the wattage test. The hot spot temperature rise is an indicator of possible shorted laminations. The core is magnetized to a prescribed magnetic flux level and then scanned for local hot spots. 

There are two common methods of perform­ing the hot spot test. One is to leave the core magnetized for a relatively long period of time, and the other is to apply an overcurrent to the test lead for a brief period. In the long-term test, the magnetic flux is maintained at the same level as the core loss watts test. This level is usually that a core may have high losses as received. The comparison of before and after may not in­dicate a significant change. 

That is when the raw value of watts per unit weight becomes an important consideration. If the value exceeds the core tester acceptance range, that is a good indicator that the core is faulty.

However, there are no industry-accepted values for an acceptable core loss with a core tester. When manufacturers test motors for core loss, the 85kl/in2 (1.32 Tesla) when us­ing a core tester.

The time period for the test typically ranges from 15 minutes to 1 hour. The larger the core, and more deep-seated a fault, the longer it will take for the sur­face temperature to reflect a hot spot. 

General rule on temperature rise 
There is no industry-ac­cepted value for a temperature rise value that indicates a prob­lem hot spot. A general rule is that a temperature rise in excess of about 15º C (27º F) over a 15-20 minute period indicates a hot spot that should be cor­rected. The temperature rise is determined by the change in 
temperature between the core at the beginning of the test, and any hot spot at the end of the magne­tization period. Some end user specifications interpret temperature rise as the difference in tem­perature between the coolest and hottest parts of the core at the end of the magnetization period. If you are core testing to a specification, be certain that the correct interpretation of temperature rise is used in assessing the outcome of the core test. 

The other variation of the hot spot test is to raise the core test current to a higher level and probe for hot spots. Typically the higher level is 11/2 to 3 times the current used in the core loss wattage test.The difficulty with this technique is that it is sub­jective. There is no criteria for unacceptable temperature rise, nor a time period in which to make an assessment. The advantage of this test is that it can quickly identify significant hot spots. 

Tips about core testing 
We will conclude with some tips about core testing. To check for hot spots, an infrared cam­era or infrared thermometer work very well. The infrared camera is particularly useful in assuring that small hot spots are not missed. A simple but effective approach is to rub suspect areas, or the edges of tooth tops, with a paraffin stick. The wax melts at a relatively low temperature, identi­fying hot spots. 

When testing armatures or rotors it is often not possible to pass a search coil turn through the core, e.g., when the core is solid. The voltage from high current lead to high current lead is of­ten used as the induced voltage, but that results in an erroneous value. All these measures is the volt­age drop across the armature, not induced voltage or flux in the core. Since the real value of the core test is to compare before winding removal values to the after removal values, apply the same cur­rent for both before and after tests. A current density of at least 12 ampere-turns per inch (30ampere-turns per centimeter) of core mean periph­ery is suggested. If this density was achieved by an acurrent of 350 amps in the before test, apply 350amps again for the after test and compare core watts loss and temperature rise. 

Categories: AC motors, Service center equipment, Testing, Motor testing

Tags: Core loss testing

Rate this article:

4.0

Print