Motor insulation test voltages: how high is too high? This article looks at similar issues, but deals with the testing of motors rather than cables. As mentioned in the previous article, there are many options for testing insulation, and Megger supplies an extensive range of testers for such applications, from 50 V to 15 kV insulation testers, through VLF and AC Tan Delta test sets to diagnostic Dielectric Frequency Response instruments and, under the Baker brand, specialist motor testing equipment up to 40 kV. A test regime designed for a particular facility must first consider how maintenance efforts can be optimized for the various motors in use. Permanent continuous monitoring using, for example, the Baker NetEP system may be selected for the most critical motors.
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As you know good insulation has high resistance and poor insulation relatively low resistance. The actual resistance values can be higher or lower, depending upon such factors as the temperature or moisture content of the insulation resistance decreases in temperature or moisture. Remember that good insulation has high resistance; poor insulation, relatively low resistance.
With a little record-keeping and common sense, however, you can get a good picture of the insulation condition from values that are only relative. The Megger insulation tester is a small, portable instrument that gives you a direct reading of insulation resistance in ohms or megohms. For good insulation, the resistance usually reads in the megohm range.
The Megger insulation tester is essentially a high-range resistance meter ohmmeter with a built-in direct-current generator. This meter is of special construction with both current and voltage coils, enabling true ohms to be read directly, independent of the actual voltage applied. This method is non-destructive; that is, it does not cause deterioration of the insulation. Figure 2 — Typical Megger test instrument hook-up to measure insulation resistance.
The generator can be hand-cranked or line-operated to develop a high DC voltage which causes a small current through and over surfaces of the insulation being tested Fig. This current usually at an applied voltage of volts or more is measured by the ohmmeter, which has an indicating scale. The wire itself is usually copper or aluminum, which is known to be a good conductor of the electric current that powers your equipment.
The insulation must be just the opposite from a conductor: it should resist current and keep the current in its path along the conductor. In Figure 1 we show this comparison. Pressure on water from a pump causes flow along the pipe Fig. With electricity, voltage is like the pump pressure, causing electricity to flow along the copper wire Fig.
As in a water pipe, there is some resistance to flow, but it is much less along the wire than it is through the insulation. Also, the lower the resistance of the wire, the more current for the same voltage.
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As you know good insulation has high resistance and poor insulation relatively low resistance. The actual resistance values can be higher or lower, depending upon such factors as the temperature or moisture content of the insulation resistance decreases in temperature or moisture. Remember that good insulation has high resistance; poor insulation, relatively low resistance. With a little record-keeping and common sense, however, you can get a good picture of the insulation condition from values that are only relative.
Motor insulation test voltages: how high is too high?
Data sheets still available for some older models. In , the company decided to change its name to Megger, which is what many people have called it all along. Today, Megger enjoys an outstanding reputation in the areas of insulation testing, low resistance and ground resistance testing, and a wide variety of electrical contractor maintenance tools such as multimeters, portable appliance testers and clamp-on meters. For many years, the James G. Biddle Company in Pennsylvania, was the U. A number of years ago, Megger became the sole owner of James G.
The next Sydney Evershed is an English brewer and politician. Again, weird combination, but apparently a popular profession in the late s! And there it is — the third listing down — Sydney Evershed, English electrical engineer, inventor, and co-founder of Evershed and Vignoles. Evershed, not a brewer nor a cricket player nor a politician, but the father of insulation test equipment invented the insulation tester. Big thanks to this man, right? Regardless, he was excited by electricity, and particularly in-awe of incandescent electric lamps used for street lighting.
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