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?
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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. By monitoring a wide range of parameters, including the current and current spectra, the NetEP can pick up many issues that may be developing — from broken rotor bars to saturating supply transformers. On-Line testing — that is, testing while the motor is running — can also be carried out on an as-needed basis with a portable on-line monitor like the Baker EXP These on-line techniques, of course, cannot tell you about all types of incipient faults.
Indeed, when warnings are given, a full diagnosis may require complementary off-line testing. In addition, off-line tests are often used on their own in a planned maintenance regime that takes motors off-line for testing and other maintenance activities at appropriate intervals.
Off-line testing operates by applying a stimulus of some kind to the motor and measuring the response. This is similar to a doctor asking you to cross your legs and then tapping you on the knee to see how your leg kicks up. Options include insulation resistance, leakage current, polarisation index and step voltage testing, and there are few who would argue against the benefits of these core electrical tests.
However, these tests will not reveal one of the most common early initiators omotor faults: inter-turn insulation breakdown. To detect such a breakdown, it is necessary to use a surge test. In a surge test, a short-duration test current with a fast rise time — typically around ns — is generated and applied to the motor coil. If the motor coil were in air, the pulse would be travelling at almost the speed of light and the voltage would be evenly distributed across the coils.
But in a motor, the coil is not in air, it is wrapped around a steel core, and so the pulse travels much more slowly. In fact, it will typically take the pulse ns to travel from one coil-turn to the next, which is equal to the rise time of the pulse. The result is that the pulse produces a significant voltage difference between adjacent turns of the coil, something that it is impossible to achieve with any other test technique.
The surge test will, therefore, reveal turn-to-turn insulation weaknesses. FIGURE 1: Surge test detection of inter-turn insulation problems Figure 1 provides examples of surge traces for a good winding and a winding that has weak insulation or a turn-to-turn short. There are other effects due to filtering of the higher frequency components of the pulse-wave as a result of attenuation by the steel and the coils own inductance.
The speed of propagation of the pulse wave is, however, the key aspect that allows the surge test to detect bad turns. But what overall voltage is necessary to show up these turn-to-turn faults, and will this voltage be harmful if applied to a fault-free motor? How do the test voltages relate to the dielectric strength of the windings? When a V motor is assembled, the insulation applied to the wire used to wind the stator has a dielectric strength of approximately V.
During its lifetime, this insulation will degrade primarily due to heat in the motor, but also as a result of environmental conditions and the coil movements which arise from starting, stopping and load changes.
The graph in Figure 4 illustrates the voltages involved relative to the dielectric strength of the motor winding insulation. It applies a voltage and rise-time to enable you to see the inter-coil response, but with a signal controlled in voltage, time and energy, so that the impact on the motor is similar to the spikes that the motor receives as a result of typical power-system variations during everyday operation.
In conclusion, a surge test should form part of the diagnostic toolkit of every motor maintenance and facilities management professional, who can be confident that a fault found by doing this test has avoided a probable unplanned failure and plant shut-down. Search for: Tags.
Electrical testing solutions built for your application needs
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.
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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.
Electrical testing solutions built for your application needs
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.
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