The first order of the day with any measurement instrument is, of course, the reading. That’s the primary raison d’être. But electrical test equipment that gives accurate and reliable readings has existed for over a century now. Yes, there have been improvements and what was good to begin has gotten even better. Microprocessors and Liquid Crystal Displays have narrowed the percentage accuracy and provided precise measurements free of the parallax guesswork of analog pointers, as a prime example. But there’s also the Law of Diminishing Returns to consider. When selecting test equipment, high precision and accuracy may be indispensable, as in R&D work. But inexpensive, less well engineered testers also have their place, as may be the case for pass/fail or go/no go testing and for troubleshooting and repair
So once the range and accuracy have been decided, is that all there is? No. A cornucopia of modern technological advances, some not available until this century, have notably added to the selection process and the flexibility in acquiring an instrument and applying it precisely to the job at hand. This article focuses on the broad field of electric motor testing, but the features and functions here described have even broader application to testing in general and can be looked for in other instruments as well.
One of the most sweeping advances has been in something as simple as the size of the instrument. Yes, large, bulky testers do give a sense of steadiness and reliability, but they also can slow down the work. And speed is a paramount consideration in the modern world, which no longer favors artisanship but demands results. A complete electric motor tester can now be designed into a single handheld unit. One of the most practical axioms is that time is $$. Older instruments focused on doing a single job and doing it as well as possible. Nothing wrong with that. When economy became a prime concern, other instruments came on the market that instead of doing one thing well, they did multiple tasks passably. That may be acceptable and maybe not. But now, miniaturization of high quality circuitry makes the choice no longer necessary. A handheld unit can do all the functions the operator may want and all done well. The saving in time can be noteworthy and cost effective. No need to search out another instrument or set it up on a platform base. Hold with one hand and operate with the other through possibly every test you need to perform. You can move on to the next job quickly, or go home to dinner.
But wait. What about all the switching of leads, which can be time consuming and prone to error? A new capability permits three leads of a typical insulation/ motor tester to be connected to a 3-phase motor once and then manipulated from key presses on the tester rather than by physical reconnection. The guard terminal, a ubiquitous but often misunderstood or unused function, is of key importance in motor testing as it acts as a shunt circuit, removing whatever element is connected to it from the measurement. It permits the testing to be sectionalized, as in leakage from stator to rotor with ground guarded out. These functions can be smoothly operated from the panel without the degree of manipulation associated with older, cruder designs.
A note on safety: handheld instruments should be protected against arc flash/ blast in the event of a fault on the line during testing. And they are, by proper conformance to IEC61010, which prescribes creepage and clearance distances for instrument circuitry. Always note the IEC rating of the instrument and apply it correctly to the operating environment and voltage of the test item.
The most fundamental test of an electric motor, its insulation resistance to ground and between windings, is performed in this manner; leads connected across the insulation barrier that is under test, test voltage applied, leakage current drawn through the insulation to the degree that it will allow, and a measurement calculated and displayed (Fig. 1). But it is now much refined. Traditionally, insulation testers had three to five voltage selections. The 500 V test became the standard for virtually all motors running from building wiring (120, 240, 480 V). Modern testers now offer almost infinite voltage selection in 1V increments across the whole range. If desired, a 600V motor can be tested at 600V. Troubleshooting focuses on the low end of the insulation range, where breakdown has occurred or is imminent. For this, look for a fast rise time in test voltage so that critical measurements as the insulation approaches breakdown can be reliably measured at full selected voltage, not loaded down to a deceptive reading at a lower voltage. A load graph will account for this. Then at the other end of the scale, a high measurement range well into Gigohms (109 Ω) facilitates longterm graphing of insulation decline so as to head off breakdown with preventive maintenance.
But to be reliable, an insulation graph needs to be corrected to a common temperature base. Temperature has a profound effect on readings (about a 50% reduction with a 10°C rise) because the molecular agitation aids leakage current. Separate measurements with a thermometer and hand calculation are no longer necessary. Temperature can be entered at the operator’s convenience, by a thermocouple or manually. The reading will be automatically compensated to the desired temperature and stored properly. This takes a lot of potential error out of interpretation of trends.
The opposite of insulation is continuity, and this also needs to be effectively tested. The motor won’t run if it isn’t properly connected and that’s where continuity comes in. A quick test will expose wiring errors where a continuous circuit is anticipated but not indicated. Sophisticated testers will now show lead hookup and allow range selection, uni- and bi-directional testing and diode test, plus buzzer mode and failure selection all from touch selections. The time-saving bi-directional mode saves the operator from switching leads.
But while the indispensable continuity function has been improved, the addition of two more terminals in modern testers creates a whole new testing possibility seen previously only in separate testers…a Kelvin bridge and a low resistance ohmmeter function. As with continuity, the display can show you graphically how to connect the leads. If something goes wrong, like a poor connection, the display indicates it so that the operator does not lose time with failed tests. Lead continuity is constantly monitored (Fig. 2). The test can be run uni- or bi-direction, or in a continuous auto mode (Fig. 3). Again, no reconnecting leads with bi directional mode. Both test current and measured resistance are displayed while on a large load, the active progress of the test can be viewed. Test currents up to 0.2A with measurement down to 1 mΩ at 0.01 mΩ resolution are possible.