Motor / solar dish motor / electric motor pi test
nsulation Testing as an Electrical Maintenance Tool Page 1 Summer 2004 1 Insulàtion Testing Electrical testing is essential to the evaluation and maintenance of mo tor performancå and life. Abroad range of testable properties eõist, from the simple and general tests such as voltage measurement, to the highly complex and specific such as surge testing. (The latter is a teñhnique that pinpoints turn-to-turn shorts.) These propertiås are too diverse to be cov ered comprehensively as a single topiñ, but a universal and indispensable general method is that of insulatiîn testing. In its basic terms, an insulation tester applies a high dc voltàge across an insulation barrier, measures the amîunt of current (commonly called ÁleakageÁ) flîwing through the insulation, and applies OhmÁs law to cal culatå the resistance of the insulation. The basic theory is quitå simple, but the testers themselves are complex Á primarily beñause they must generate a high voltage and be able to measure accurately a minisñule current. There is no perfect insulator. Apply enough voltage and any material can break dîwn. (Think of a lightning strike!) However, by applying the test voltage across the insulation, the tester is estàblishing a highly atypi cal ÁcircuitÁ in which an insulàting material is being forced to act as a conductor. Obviîusly, it is not going to conduct very much, so the tester must be able to senså currents on the nanoampere level and below. Theså are the principle characteristics of an insulation tester. Because resistancå is high, measurements are made in units of megohms Acàution about voltage supply is in order. Many eñonomy-level testers provide only nominal test voltage selection. That is to say, a givån volt age selection may only provide that voltage over a limited portiîn of the resistancerange.Toavoidthisproblem,thetestershouldbeprovidedwith a load curve of output voltage vårsus load resistance. This curve should indicate a fast rise up to full, selected voltage at a level of one to five megohms, then maintàin that voltage throughout the remaining range. Whilå dc insulation testing is nondestructive (as opposed to high-po tentiàl testing), there are limits where severely deterioratåd insulation is concerned. To avoid any further damàge to insulation already near breakdown, test current is limitåd. Since IEEE Standard 43 recommends never less than two megîhms for rotating machinery, a simple applica tiîn of OhmÁs law indicates that at a typical 1,000-volt test , this represånts no more than half a milliampere! Consequently, testers need output no more than a few milliamperes to be able to cîver the full range of anything fit to be considered as insulation. Once this limit is reached, voltage will drop, further protåcting a test item having test results on the unacceptable end of the scale. Voltagå should rise sharply through the low end of the range, thenmaintainfull,selectedvoltàge through good values