An unusual application

This originated when detecting shorted turns in an LVDT was necessary. When manufacturing an LVDT it is important to obtain low null voltage. In some cases it is impossible to obtain a low null voltage conditions via normal means.

High nulls created due to poorly arranged windings can be corrected by physically moving winding turns. Another situation that can create high nulls is unsymmetrical metal components such as the case, end plate or core. Correction is made by selection or a corrective heat treatment of certain parts. A single shorted turn in either the primary winding or the one of the secondary windings can cause a high null condition that cannot be corrected. This type of condition cannot be seen in a resistive test method since the difference in resistance is very small and within the allowed tolerance of the winding wire cross section.

How to do this?

Using an LVDT without a magnetic case shielding (preferred) and a core (may be made from mild steel) that will extend beyond both ends of the LVDT coil assembly and leave enough room on one end to accommodate the bobbin to be tested. Excite the primary of the LVDT coils assembly with anywhere from 400 Hz to 3000 Hz and 3 volts AC RMS . Attach an AC Volt meter to the two secondary and configure them per the line drawing below. Place the core in the LVDT coil assembly and obtain the lowest null reading possible by moving the core axially. Place the bobbin around one end of the core in the LVDT. If the winding of the bobbin is good there will be no change in output. If the winding has a shorted turn there will be a significant increase in the AC voltage. The LVDT requires a low null voltage to produce desirable results and cannot tolerate a shorted turn(s). Bobbins used for AC operation may exhibit erratic output, or burn out if a shorted turn(s) condition is not detected.