* Innerturn Dielectric Test
The "Surge" portion of this tester is the only known method of determining the inner-turn insulation (dielectric) strength in a wound coil. This device can detect weaknesses or fractures in the thin insulation coating on the conductor (magnet wire). Each turn of wire must be electrically separated from each adjacent turn, or a turn-to-turn short will develop (see comparison test).
The surge tester applies a very brief high voltage surge (generated by capacitor discharge) across the winding to confirm the insulation integrity. If the applied voltage is higher than the actual insulation strength between turns, coils, and phases the waveform on the oscilloscope will flicker - indicating a very brief current flashover. When performed as directed by the equipment manufacturer, this test will not damage healthy insulation, but can further weaken damaged or degraded insulation.
* Impedance Balance Test
The Surge Comparison tester will also accurately perform a “Comparison” of the winding impedance of a multi-phase motor. We know that all three phases of 3-phase motor must be electrically balanced. That is, the impedance of each phase must be identical. A simple comparison of the resistance or continuity of each phase will only indicate the presence of an open winding, or a dead short, it will not detect the presence of a few shorted turns – the resistance will not change appreciably. Also, the continuity test has no potential (voltage). The comparison function of the surge comparison test will compare the impedance of two phases simultaneously while applying the full (dc equilalent) operating voltage of the motor. The standard multimeter will not come close to applying the operating voltage of the motor. In other words, a motor winding could check out normal with a multimeter, yet fail on start-up due to shorted turns or weak insulation that went undetected by other test methods.
More about applications, test standards, test procedures and analysis can be found in the High Potential Seminars training material. Contact us for details.
AC HIPOT TESTER
* Proof Test
Commonly known as a "proof test", the ac hipot (hypot or high potential) test is used primarily in the manufacture of consumer products where a failure of the line voltage power circuit is distinctly destructive. Prevention of a faulty appliance in the hands of a consumer is the highest priority, so destruction of the insulation upon failure is completely acceptable. Although the ac hipot has some limited uses in the motor repair shop to pinpoint faulty ground insulation for possible repair, it has no place in the maintenance environment. The high current associated with a flashover is a result of the collapsing field of the step-up transformer.
DC HIPOT TESTER
* Ground Wall Dielectric Test
The DC Hipot test is very common in all areas of motor manufacturing, maintenance, and repair to safely perform a ground wall dielectric test. The dc hipot test is safe to use due to the current-limiting feature incorporated into most dc hipots. If a flashover does occur, the very low current - usually only a few milliamps, will not further degrade the insulation. This will allow the motor to still operate temporarily (in most circumstances) until a replacement or repair is immediately scheduled.
One known instrument manufacturer incorporates a circuit that will trip the unit on a sigle flashover. This is significant because other dc hipots may flash-over many times before the current rises high enough to open the internal tripout circuit. It's that repetitive "arcing" that creates excessive heat and carbon tracking and can leads to complete failure upon energising the motor in service.
* Step Voltage Test
The secret to a successful dc hipot test on medium and high voltage motors is to incorporate a DC HIGH VOLTAGE LEAKAGE TEST. This test is easy to use and has produced tremendous results as opposed to an ordinary high voltage “breakdown” test.
More about applications, test standards, test procedures and analysis can be found in the High Potential Seminars training material. Contact us for details.
MEGOHMMETER
* Insulation Resistance Measurement
Dirt, moisture, and other contaminants will often build up on the insulation of motors and generators. These contaminants are usually conductive and will create a low resistance path for current to flow from the winding to ground. Using this description, “insulaton resistance” (I.R.) may be incorrect - “ground resistance” is a more accurate term for the test.
A low ground resistance is considered to be around 1 million ohms (1 megohm) or less. A Megohmmeter with an output voltage of 500 or 1000 volts is required to measure these very high resistances of 1 megohm to 100 megohms, or even up to 1000 megohms. Many companies have test standards for minimum resistance values for their motors so that wet or dirty motors can be removed and reconditioned before more serious insulation damage occurs.
* Polarization Index Measurement
Some users will perform a Polarization Index (P.I.) test on their larger motors. This 1 minute/10 minute ratio has shown limited success on the newer VPI insulation systems. However, this new insulation also allows the motors to operate with much lower ground resistance values than the older insulations.
More about applications, test standards, test procedures and analysis can be found in the High Potential Seminars training material. Contact us for details.
OHMMETER
* Phase Resistance/Continuity
Although phase resistance is not a quality indicator of the condition of a winding, it can be very useful as an initial test to detect a complete open or short. If either of these two conditions exist, it may not be necessary to proceed any further.
More about applications, test standards, test procedures and analysis can be found in the High Potential Seminars training material. Contact us for details.
MICRO-OHMMETER
* Low Resistance/Continuity
Motor windings or individual coils with very few turns of large wire can be quite difficult to test because the very low impedance appears as a “dead short” on most instruments. A digital low resistance ohmmeter (DLRO) can be very useful in these applications.
In the motor shop environment, it is important to confirm that all form-would coils have the same resistance. In the maintenance environment, the DLRO can be used to measure contact resistance to prevent unbalanced phase current, or dropping out a phase completely (single-phasing).
More about applications, test standards, test procedures and analysis can be found in the High Potential Seminars training material. Contact us for details.
ROTOR BAR ANALYSIS
* Current Signature Deflection
This is a test that is distinctive to a particular instrument manufacturer and can be discussed during the High Potential Seminars workshop.