The Magical Magnetizing Coil

Magnetic nondestructive testing methods are widely used in the world of OCTG and drill pipe inspection. Magnetic inspection is a fast and effective way to detect a myriad of defects within ferrous pipe. Electromagnetic inspection (EMI) methods are some of the most common testing methods found in our segment of the industry. To maximize EMI’s potential, the field strength of the electromagnet used for EMI should be properly designed for the application. However, a good method to qualify the performance of magnetizing coils is not common. Industry specifications, end users and manufacturers often improperly qualify a magnetizing coil’s “strength” using “ampere•turns” (amp•turns) instead of the more appropriate “gauss” field strength unit of measurement. Amp•turns, simply put, is the amount of DC amperes flowing through the magnetizing coil multiplied by the number of turns of magnet wire on the magnetizing coil. Ten thousand (10,000) amp•turns has been a common “magic rating” for magnetizing coils within this industry for many years. Also, a specific quantity of amp•turns per inch of pipe O.D or connection O.D. is also common in the industry.

Are all magnetizing coils created equal? How should magnetizing coils be rated? Is a magnetizing coil best suited for an application? These are all important questions which directly relate to the quality of the finished inspection job.

Ideally, magnetic saturation of the pipe body wall or connection is desired but may not be achieved when using amp•turns as the criterion for determining appropriate “magnetic field strength”. Saturation of the pipe body wall and connections requires a tremendous amount of field strength, especially when the magnetizing coil moves in relationship to the pipe being tested.

Below is an example which demonstrates limitations of the typical amp•turn specifications. An ideal magnetizing coil would have a maximized magnetic field strength while reducing or minimizing the heat load as much as possible.

Well – Head Tubing Inspection System Magnetizing Coil

Specifications of the original magnetizing coil:

14.5” I.D.3.0 amps applied current, 1860 turns of wire
Actual measured field strength144 Gauss
Actual wattage (cold)304 Watts

TABLE 1:    Comparison of Original Configuration vs. Three Alternate Configurations

Calculated Field
Strength (Gauss)

Notice the “ORIGINAL” calculated field strength value in Table-1 is higher than the actual original measured field strength. This is typical and usually a result of the ideal condition of the engineering calculations. In application however, other variables affect the end result not considered by the calculations. None-the-less, results can be compared and conclusions drawn.

Varying the parameters of the magnetizing coil such as applied amperes, wire size and number of wire turns can affect the field strength of the magnetizing coil. The summaries below are each compared to the performance calculations of the original coil configuration.

MODIFICATION “A”:  Changed AMPS, WIRE SIZE and TURNS from original
RESULT:  The field strength remained the same as the original calculation even though amp•turns increased by more than 3.2%.
— Should we care about amp•turns?
MODIFICATION “B”:  Increased AMPS, other parameters same as MOD-“A”.
RESULT:  The field strength increased by approximately 9.3%. Amp•turns increased by approximately 11.8%.
— Do we care about amp•turns?
MODIFICATION “C”:  Changed TURNS, other parameters same as MOD-“A”
RESULT:  The field strength increased by more than 17.4%. Amp•turns increased by approximately 21.3%.
— We really don’t care about amp•turns!

* Calculations for these comparisons were performed using software, written by myself, which implements formulas and equations from physics text and other references. This program is used regularly to target magnetizing coil performance prior to construction.

** Special Note: If it is possible to change the mechanical design of a magnetizing coil such as decreasing the inner diameter (I.D.), another large increase in magnetizing field strength could be achieved. For example, if the original magnetizing coil I.D. is reduced to 13” while maintaining the same AMPS, TURNS and WIRE SIZE as MOD-“C”, the result is a 30% magnetic field strength increase over the original calculation. Meanwhile, the amp•turns are still the same as MOD-“C”.

Table 1 reveals the short comings in the amp•turn methodology. In review, if 5700 amp•turns are required for an application, MOD-“A” clearly meets this requirement. But why can’t the original magnetizing coil configuration be used for the same job? Notice the calculated field strength for both the original magnetizing coil and the MOD-“A” magnetizing coil are the same. Even though MOD-“A” has more amp•turns, the actual field strength of MOD-“A” is not any stronger than the original design. Also obvious is the fact that as the various calculations were performed, the amp-turns did not change proportionally with respect to the change in the actual magnetic field strength in gauss.

1. Industry specifications which rely on amp•turns alone to rate the strength of a magnetizing coil are incorrect! The gauss unit of measure should be specified instead.
2. As shown from the numerical results, the amp•turn ratings are not directly related to the actual field strength in gauss. Yet in many cases, inspection specifications are printed with amp•turn ratings alone.
3. Digital gauss meters should be used to determine the actual field strength at the center of a magnetizing coil. Only then do you know the true “strength” of the device.
4. The original magnetizing coil design is deficient with respect to its field strength. It is not optimized for maximum field strength output.
5. Measuring the actual field strength of a magnetizing coil can be used as a conformance test to make certain the magnetizing coil is always generating the prescribed gauss field strength. This testing could be performed at given intervals throughout the year when other equipment is typically “calibrated”.

HOWEVER, just knowing the amp•turn rating of a coil can not help you find an internal defect such as a short circuit between some of the windings of a magnetizing coil. This type of internal damage can happen as the coil age increases during normal use. If the coil is powered by a constant current power source, the inspector would never know there was a problem! Simply calibrating a current meter for accuracy WILL NOT ALERT YOU TO THIS TYPE OF PROBLEM.