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AWAIV Surge/HiPot Resistance Tester

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The effect of temperature on both copper resistance and ground wall resistance can be substantial. Knowledge of temperature is especially important if test data is to be compared or trended to previous measurements. The temperature is entered manually into the test instrument from acquired temperature reading of a third party device. Theinstrument will correct the coil resistance tests to 25°C (per IEEE 118) and the IR (Insulation Resisatnce/Megohm) readings to 40°C per IEEE 43. The effects of temperature will be discussed further in each section below as it applied to each test.

The Resistance Test is a very simple test to perform and gives a first order indication of the health of the conductors in a winding. The test consists of injecting a known constant current through the winding, measuring the voltage drop across the winding, and calculating the coil resistance using Ohm’s law. If a coil is shorted somewhere in the interior of the winding, the resistance will be lower than normal. This lower coil resistance can be compared to previous measurements of the same coil, compared measurements of identical coils, or compared to the motor nameplate value to identify a “bad” coil. Measured values can also be higher than normal such as in the case of loose or corroded connections.

As mentioned above, the measured resistance is affected by the variation of copper conductivity with temperature. Therefore, the measured resistance value should be corrected to a common temperature, usually 25oC per IEEE 118, before comparing two different measurements.

Performing Resistance Tests on the same motor over a period of time provides early warning signs of motor problems.  Motors operated in conditions that allow corrosion, contamination, or other physical damage may show initial warning signs of failure through the Resistance Test.

Since the windings found in many motors have very low resistances, the injected current might have to be many amps to accurately measure the voltage drop across the coil. One of the difficulties encountered measuring the voltage drop across the coil itself are the affects of the contact resistance of internal relays and the contact resistance of the clip leads and used to connect to the motor’s winding. Contact resistances can be comparable or even greater than the resistance of some coils.

There is a practical lower limit to the coil resistance that can be successfully used to evaluate the condition of the copper conductor in a winding. An instrument must be able to resolve the change in copper resistance caused by a short in the winding before conclusions can be made regarding the coil resistance. The AWA IV is capable of a 1 milli-ohm resistance measurement with a separate, 4-wire resistance test cable.

The Analyzer Resistance Test compares the percentage difference in resistance between leads with the calculation of “Max Delta R”.  You define the acceptable Delta R tolerances for each motor, thereby giving the Analyzer pass/fail limits. 

When the Resistance Test results are displayed by the Analyzer, measured resistance values are listed, along with resistances corrected for temperature and the Delta Resistance percentage.  When Delta Resistance is high, a problem with the motor under test may be indicated.

When the Analyzer detects Delta Resistance values not within the prescribed limits, the motor fails the test. No further testing is necessary until the reason for the faulty resistance measurements is resolved.  Continuing the test sequence after a test failure is also possible.

The Analyzer can also compare measured resistances to expected or “Target” values. You specify the Target value. At the end of the Resistance Test, the Target values will be compared to measured values. If measured values are out of tolerance, the Analyzer will fail the motor.

The instrument does a Lead to Lead measurement with separate, 4-wire type leads (Low Voltage Measurement).  Different from the “Balance Test” which can still be performed with the High Voltage leads if desired.

Target Corrected Resistance

The Target Corrected Resistance is the expected resistance the Analzyer should measure for the stator. This value must be a temperature corrected resistance value. This expected resistance will be compared to the measured temperature corrected values, therefore, it is necessary to know more about how the Analyzer measures resistance. The Analyzer will make a resistance measurement of Lead 1 by injecting a known current into Lead 1 while holding Leads 2 and 3 at ground. The injected current creates a voltage drop between Lead 1 and ground which is measured. This measured voltage drop and the known current are used in Ohm’s Law to calculate the resistance for Lead 1.

Likewise, Lead 2 resistance measurements will be made by injecting current into Lead 2 while holding Leads 1 and 3 at ground. Same for Lead 3: current will be injected into Lead 3 while holding Leads 1 and 2 at ground.

This type of measurement is called a Balance measurement. The results show how well balanced the stator lead resistances are. These measurements are not the same measurements listed on the motor’s nameplate which are lead to lead measurements. Consider a 6 lead motor using the NEMA domenclature:

L - L1 = 1 – 4 in series with 5 – 2                        L – L1 = 1 – 4 in series with 2 -5
L – L2 = 2 – 5 in series with 6 – 3                       L – L2 = 2 – 5 in series with 3 - 6
L – L3 = 3 – 6 in series with 4 – 1                       L – L3 = 3 – 6 in series with 1 - 4

These measurements are very different than the Lead to Lead (or Phase to Phase) measurements that are shown on nameplates. When entering Target Corrected Resistance values on the Resistance Test screen, the Analyzer’s Balance values should be used – not lead to lead / phase to phase values.

The Megohm test consists of applying a DC voltage to the windings of a machine after first isolating the winding from ground.  Internal relays inside the Analyzer automatically isolate with windings – no operator action is required. The test voltage is usually chosen to be at or near the operating voltage of the machine (see IEEE 43). Recommended test voltages can be found in the previous chapter titled “Recommended Test Sequence, Voltages and Applicable Standards”.

The intended purpose of the Megohm test is to make an accurate measurement of the insulation resistance of the ground wall insulation. The insulation resistance, abbreviated IR, is a function of many variables: the physical properties of the insulating material, temperature, humidity, contaminants etc.  The IR value is calculated using Ohm’s law – the applied voltage is divided by the measured leakage current. This leakage current is that current which is actually able to pass from the winding through the ground wall insulation to the motor’s steel core plus any surface leakage currents. The surface leakage currents flow through moisture or contaminants on the surface of the insulation. To accurately determine the insulation resistance, the surface leakage must be reduced to an inconsequential level.

Principles of Dielectric Absorption (DA) Testing

The Dielectric Absorption (DA) Test is essentially a short-duration PI test and is usually intended for smaller motors. Larger motors whose insulation does not easily polarize are also good candidates for the DA Test. Other than the shorter test time, all other principles are the same as the PI test, explained in the next section.

While the PI test is recommended only for motors 200 horsepower or greater, the DA test is useful for motors in approximately the 50 to 200 horsepower range. The DA value is the ratio of the ground wall insulation resistance (IR) at 3 minutes to the IR value at 30 seconds.

The Polarization Index Test (PI test) is the most confusing HVDC test in use due to the subtleties in the interpretation of the results. The PI test is performed in order to quantitatively measure the ability of an insulator to polarize. When an insulator polarizes, the electric dipoles distributed throughout the insulator align themselves with an applied electric field. As the molecules polarize, a “polarization current”, also called an absorption current, is developed that adds to the insulation leakage current. This additional polarization current decreases over time and drops to zero when the insulation is completely polarized.

The PI result becomes confusing when attempting to attribute variations in the PI value to the polarizability of the insulator or other affects such as humidity or moisture, surface leakage or instrument error. The result is even more confusing when attempting to reconcile a PI of 1 when one is expecting some other PI.

The PI test is typically performed at 500, 1000, 2500 or 5000 volts, depending on the operating voltage of the motors being tested and takes 10 minutes to complete. The PI value is calculated by dividing the insulation resistance at 10 minutes by the resistance at 1 minute as shown below:

In general, insulators that are in good condition will show a “high” polarization index while insulators that are damaged will not.  IEEE 43-2000 recommends minimum acceptable values for the various thermal classes of motor insulation.

Unfortunately, most the insulating materials developed recently (last 20 years) do not easily polarize. For example the newer inverter grade wires and epoxy resins do not readily polarize. As recommended in IEEE 43-2000, if the one-minute insulation resistance is greater than 5000Mohms, the PI measurement may not be meaningful.

To address the situation where the PI may not be meaningful, the Dielectric Absorption (DA) is widely used instead. The DA is the IR value at 3 minutes divided by the IR value at 30 seconds. The motivation for even doing the DA test is to reduce the test time to 3 minutes instead of 10 minutes for the PI test when the PI test may not be worthwhile. To date there are no accepted values for the DA. However, some usefulness can be obtained by trending the DA value over time.

The HIPOT test consists of applying a DC voltage to the windings of the machine, same as a Megohm/PI test, but at a higher voltage – usually more than twice the voltage of the machine’s operating voltage.  Once again, the chapter titled “Recommended Test Sequence, Voltages and Applicable Standards” has information regarding the proper test voltage for the HiPot test. The intended purpose of the HiPot test is to prove that the ground wall insulation system can withstand a high applied voltage without exhibiting an extraordinarily high leakage current. Therefore, the HiPot test is often called a “Proof” test.  The observed insulation resistance or leakage current is recorded and compared to acceptable limits. If the insulation fails the HiPot test, the insulation to ground is determined to be unreliable.

Knowledge of the real behavior of insulators/resistors, not just ideal resistors, will help the operator to test the winding insulation to a point before the insulation breaks down.

For an ideal resistor, good or poor, as the voltage is increased, the leakage current will increase proportionately (Fig 2-2).  However, insulation resistance in the real world rarely behaves in this manner.  Instead, the current in a typical resistor will increase proportionately with voltage until the voltage is within as little as 5% of the breakdown voltage.  Just before insulation breakdown, the current will rise faster than the voltage.  At still higher voltage, the insulation will completely breakdown and the current will rise extremely fast.

The key to DC HiPot testing is to look for leakage current that is rising faster than the increase in voltage that is applied to the winding.  The test can then be stopped before the insulation is damaged.

The HiPot test is considered the mainstay of motor testing.  HiPot tests can be performed in one of two ways, AC or DC. The HiPot test brings the entire motor winding up to the same potential.  Since all the windings are at the same potential, there is no turn-to-turn, or phase-to-phase insulation stress.  There is uniform voltage stress applied between the winding insulation and the ground wall, throughout the entire winding. Although the Surge test can also test for grounds, it does not uniformly test all the ground wall insulation as does the HiPot test.

This DC Test is performed to a voltage of what the motor typically sees during starting and stopping. The test voltages are governed by IEEE and are posted below for reference.

*Note: IEEE references Nema MG 1-2006 Part 12, Page 2 and states in no case should the test voltage be less than 1500 volts.

The DC voltage is applied to all three phases of the winding and raised slowly to a preprogrammed voltage step level and held for a predetermined time period.  It is then raised to the next voltage step and held for the appropriate time period. This is continued until the target test voltage is reached. Typical steps for a 4160 volt motor are 1000 volt increments, holding at minute intervals.  For motors less than 4160, the step voltages should be 500 volts. See example below.

 Data is logged at the end of each step.  This is to ensure the capacitive charge and polarization current is removed and only real leakage current remains, thus providing a true indication of the groundwall insulation condition.  If at this point the leakage current (IµA) doubles, there is an indication of insulation weaknesses and the test should be stopped.  If the leakage current (IµA) raises consistently less than double, the motor insulation is in good standing.

The Step Voltage test is necessary to insure the ground wall insulation and cable can with stand the normal day to day voltage spikes the motor typically sees during operation. If a DC Step Voltage test is not performed, the operator can not be assured the motor will start and operate with out failing in service.

Whereas the Megohm/PI/ HIPOT tests are used to detect ground wall insulation weakness, the surge test is used to find turn-to-turn insulation weakness.  Motor winding insulation failures often start as turn-to-turn failures, which eventually damage the ground wall insulation and lead to catastrophic failure.  Surge testing can detect the early stages of a problem before the problem becomes much more severe.

As discussed above, surge testing is performed in order to detect insulation damage between turns within a giving winding. This type of insulation problem can be found no other way than with the surge test. The surge test consists of applying a fast rise time, high current impulse to a winding. This high rise time impulse will induce a voltage difference between adjacent loops of wire within the winding. If the insulation between the two loops of wire is damaged or somehow weakened, and if the voltage difference between the wires is high enough there will be an arc between the wires.  This arc shows up as a change in the surge waveform.

The wave pattern observed during a Surge Test is directly related to the coils inductance.  (There are other factors influencing the wave pattern but inductance is the primary one.) The coil becomes one of two elements in what is known as a tank circuit – an LC-type circuit made up of the coils inductance (L) and the surge tester’s internal capacitance (C).

The inductance (L) of a coil is determined by its geometry, number of turns of wire and the type of iron core it rests in.  The frequency of the wave pattern is approximately determined by the formula:

This formula implies that when the inductance decreases, the frequency will increase.

A surge test can detect a fault between turns by observing a jump in the resonant frequency of this LC tank circuit.  If the voltage potential is greater than the weakened dielectric strength of the turn insulation, one or more turns may be shorted out of the circuit.  In effect, the number of turns in the coil is reduced.  Fewer working turns reduce the inductance of the coil and increased the frequency of the ringing pattern from the surge.

The voltage or amplitude of the surge wave pattern is also reduced due to the decrease in inductance of a coil with a fault between turns.  It is determined by the formula:

When the insulation between turns is weak, the result is a low energy arc and a change in inductance.  When this happens the wave pattern becomes unstable – it may shift rapidly to the left and right, and back to the original position.

The Error Area Ratio

When testing three phase motors, the waveforms of the three phases can be compared to each other. They should all be very much the same: same shape, same zero crossings, and same amplitude. In practice, however, the three waveforms will never be exactly the same. There will be difference due to slight differences in the physical windings themselves as one phase is wound over another. The question is “How different should two waveforms be to identify a bad coil?” The Error Area Ratio (EAR) was invented to answer that question. The EAR values gives a quantitative number to how different two waveforms are. EAR is defined as:

Where:
F(1) = Data points representing waveform 1
F(2) = Data points representing waveform 2.
EAR 1-2 = error area ratio of waveform 2 with respect to waveform 1          

If two waveforms are exactly the same, the EAR value will be zero. Two waveforms that are “almost exactly the same” will have EAR values of 3-4%. Waveforms with obvious separation will have EAR values greater than 10%. This application of comparing one phase of a winding to another is called a Line-to-Line EAR.

The application of the Error Area Ratio above is used to compare two waveforms from two leads or two phases of a motor. A second application is to use the EAR formula as a way to compare the surge waveforms from a single lead or phase to itself. This application of the EAR is called the Pulse-to-Pulse EAR (abbreviated ppEAR).

To explain the ppEAR, recall that the arcing turn-to-turn short is Identified by a shift to the left of the surge waveform as the test voltage is slowly increased. On a good coil, the waveforms from consecutive pulses would look almost the same – the only difference being the amplitude increases as the test voltage increases. On a bad coil, the consecutive pulses would look nearly the same until the arcing short occurred. At this voltage, the whole waveform shifts to the left and possibly drops in amplitude.

Consider what an EAR calculation of two consecutive pulses would look like as voltage increases. Since the amplitudes of the two waveforms are different, there would be some EAR value calculated, possibly around 4-7%. Now consider doing the EAR calculation on the pulse just before breakdown and the pulse just after breakdown. The EAR value would jump to a significantly higher value.