Page 122 - Intro Predictive Maintenance

Predictive Maintenance Techniques 113

Megger Testing

In order to measure high resistances, a device known as a mega-ohmmeter can be
used. This instrument differs from a normal ohmmeter in that instead of measuring
current to determine resistance, it measures voltage. This mode of testing involves
applying relatively high voltage (500 to 2,500 volts, depending on the unit) to the
circuit and verifying that no breakdown is present. Generally, this is considered a non-
destructive test, depending on the applied voltage and the rating of the insulation. This
method of testing is used primarily to test the integrity of insulation. It will not detect
shorts between windings, but it can detect higher-voltage–related problems with
respect to ground.

HiPot Testing

HiPot (high potential) testing is a potentially destructive test used to determine the
integrity of insulation. Voltage levels employed in this type of test are twice the rated
voltage plus 1,000 volts. This method is used primarily by some equipment manu-
facturers and rebuilding facilities as a quality assurance tool. It is important to note
that HiPot testing does some damage to insulation every time it is performed. HiPot
testing can destroy insulation that is still serviceable, so this test is generally not
recommended for field use.

Impedance Testing

Impedance has two components: a real (or resistive) component and a reactive (induc-
tive or capacitive) component. This method of testing is useful because it can detect
significant shorting in coils, either between turns or to ground. No other nonintrusive
method exists to detect a coil that is shorted between turns.

Other Techniques
Other techniques that can support predictive maintenance include acoustic emissions,
eddy-current, magnetic particle, residual stress, and most of the traditional nonde-
structive methods. If you need specific information on the techniques that are avail-
able, the American Society of Nondestructive Testing (ANST) has published a
complete set of handbooks that provide a comprehensive database for most nonde-
structive testing techniques.