There is no shortage of acronyms in the modern English language. Here at The Snell Group, one of the acronyms we use quite often is EMT. To the average person on the street, Emergency Medical Technician probably comes to mind when they hear EMT, but for maintenance and reliability professionals EMT means Electric Motor Testing.

EMT can be performed either on a motor that is in use (energized) or on a motor that is not in use at the time (de-energized). Regardless of the type of EMT, the goal is to determine the health or reliability of the motor being tested. As one of the many reliability methods in use today, EMT can be a powerful addition to your PdM program if it’s not already.

Energized motor testing is the least disruptive to operations. De-energized testing requires that the motor being tested be removed from operational status LOTO (Locked Out Tagged Out) before testing can begin. Energized testing (sometimes referred to as on-line testing) carries some inherent safety risks or hazards that can be mitigated by using skilled, properly trained personnel and quality SOP’s (Standard Operating Procedures).

Energized motor testing collects data on a normally loaded, operating motor. This data is presented in visual representations that, in a manner of speaking, let us “see” how the motor is drawing power from the system. Current samples are taken to determine if the motor is running properly, balanced, and operating within the rating of the motor using spectral views, time domains, in-rush, and demodulated signals.

NOTE! Proper PPE and safety procedures are a must for energized EMT!

Spectral views look at the frequencies at which the motor is running from 0 Hz to 300 Hz (depending on the test equipment being used). In the US, 60 Hz is the dominant frequency but other peaks, such as slip frequency, show up along with some mechanical induced frequencies. All of these peaks can be measured and tracked for increases. These peak increases are an indication that something has changed in the operation of the motor.

Time domains let us view sine waves. The demodulated spectral view removes the line frequency so that the smaller mechanical peaks can be viewed more easily and tracked for increases. Power samples are also taken. These power samples are a combination of both current and voltage with views similar to what we would see in the current samples. The main focus of this type of sample is the balance between the phases and those phases to ground.

Care, caution and procedures must utilized to mitigate safety concerns with energized testing while making the connections to a live, active circuit As the intent is to not disrupt operations of the facility, the voltage probes are connected to live 480V systems or to PTs (potential transformers) on higher voltage (600V and above). The current clamps are also attached to the system while it’s “hot.” On these higher voltage systems, the current clamps are used in conjunction with the CTs (instrument current transformers).  Manufactures have developed optional permanently installed remote connections to minimize these issues.

De-energized, or Offline, EMT is conducted on an idle motor. This type of testing is more of a one-shot test in which we are looking for turn to turn, coil to coil, phase to phase or phase to ground insulation defects. De-energized EMT can also be used for tracking the health of motors over a long period of time. Test voltage is applied by the tester and can damage electronic drives (VFDs and soft starts). These pieces of equipment must be removed or electrically isolated from the tested circuit prior to testing.

Both the Energized and De-energized EMT methods have strengths and weaknesses. Both types of testing are attempting to determine the health of a motor, but in different ways. Both methods can stand alone, but when used together, they provide the best assessment of the health of a motor. This has been a brief explanation of what EMT is, much more information is available through Snell training. If you’re not already using EMT, hopefully this brief introduction has shown you what’s missing from your PdM or CM toolbox.