0278-0046 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIE.2017.2652359, IEEE Transactions on Industrial Electronics IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS Abstract—Electromagnetic induction coils are widely used in a variety of applications, many of them safety- critical. The insulation around the wire in an electromagnetic coil accounts for a significant portion of the failures in solenoid-operated valves and in electric motors. This paper presents a method of detecting the degradation of insulation used in low-voltage applications by assessing changes in impedance responses. The results indicate that coil impedance, resolved into resistance and reactance, evolves differently when the coil is subjected to different loading conditions, which reflects insulation degradation signatures due to different failure mechanisms. This method can be used to assess the insulation life of an electromagnetic coil, allowing replacement prior to the formation of harmful shorts or critical coil opens. Index Terms—Condition monitoring, diagnostics, electromagnetic actuator, electromagnetic coil, impedance response, insulation degradation, magnet wire insulation, prognostics, reliability, solenoid-operated valves. I. INTRODUCTION LECTROMAGNETIC coils are fundamental energy conversion and transformation components of many systems, widely used in motors, transformers, and solenoids, which are subsequently used in intelligent control of electro- pneumatic braking systems in motor vehicles, diesel fuel injection control systems, automobile transmission control, process control, and in critical safety-instrumented functions. A study conducted by Oak Ridge National Laboratory (ORNL) [1]–[3] showed that over 50% of solenoid-operated valve (SOV) failures in U.S. nuclear power plants were attributed to electromagnetic coil faults (e.g., coil opens and shorts). The IEEE Motor Reliability Working Group [4], [5] found that electromagnetic coil insulation problems contribute Manuscript received June 19, 2016; revised October 16, 2016; accepted December 3, 2016. The authors are with the Center for Advanced Life Cycle Engineering, University of Maryland, College Park, MD 20742 USA (e- mail: jjameson@umd.edu; mazarian@calce.umd.edu; pecht@calce. umd.edu). to one-quarter of motor failures, while Thorsen and Dalva found the contribution to be approximately one-third [6]. Downtime from these unplanned outages can be quite costly [7], hence identifying insulation degradation at an early stage of progress can reduce operations and maintenance costs by enabling condition-based maintenance or replacement. Angadi et al. [8], [9] conducted accelerated testing of SOVs and concluded that SOVs are susceptible to “coupled electrical-thermal-mechanical failure mechanisms.” The stresses driving these mechanisms were described as follows: Joule heating of the conductor wire subjects the insulation to thermal stresses from the temperature rise and mechanical stresses from the expansion of the conductor. One challenge is to monitor the health of the electromagnetic coil insulation in-situ. SOVs are often applied in safety-instrumented functions (SIFs), with between 2-4% of all solenoid valves used in chemical process plants being devoted to use in SIFs [10]. SIFs require high reliability, but are generally in-line with the process, rendering SIF functional tests disruptive and costly. A method that can assess the in-situ health of SOV electromagnetic coil insulation can improve the reliability of SIFs without affecting the process. Previous studies have addressed failure and condition monitoring of insulation in machinery. Chow et al. [11], [12] proposed a method of detecting turn-to-turn insulation faults (that is, a short formed between two adjacent turns of an electromagnetic coil) using a neural network with input torque and angular speed measurements. However, a method for detecting the degradation of the insulation, which is more desirable since this allows the replacement of the insulation prior to the formation of a harmful turn-to-turn short, was not addressed. Moreover, this method cannot be applied to devices that do not use or produce torque or angular speed. Kaufhold et al. [13] discussed the failure mechanisms of motor winding insulation due to electrical stresses and made recommendations on the limiting value of permissible terminal voltages in motors. However, low-voltage machines rarely experience the level of electrical stress necessary to elicit the failure mechanisms mentioned in the article, such as insulation erosion due to partial discharges. Stone et al. [14] discussed the available methods of diagnostics for insulation. These methods include the measurement of electrical parameters Impedance-Based Condition Monitoring for Insulation Systems Used in Low-Voltage Electromagnetic Coils N. Jordan Jameson, Student Member, IEEE, Michael H. Azarian, Member, IEEE Michael Pecht, Fellow Member, IEEE E