CURRENT STATE OF SURGE TESTING INDUCTION …

1 Iris Rotating machine Conference 1 of 13 June 2003, Santa Monica, CA CURRENT STATE OF SURGE TESTING INDUCTION MACHINES John Wilson Baker Instrument Company ABSTRACT SURGE TESTING of motor coils has been an industry practice since J. L. Rylander published A High Frequency Voltage Test for Insulation of Rotating Electrical Apparatus in Transactions of the AIEE, in February of 1926. Since that time the use of SURGE TESTING to diagnose failing or damaged insulation in motors has advanced greatly. In 1926, the indication of a turn-turn insulation failure was the drop of the coil voltage amplitude as determined by a vacuum tube rectifier circuit using an apparatus the size of a large work bench with rotating spark gaps for switches and large step up transformers to charge large high voltage capacitors. Today s instrumentation uses high-speed digitizers to observe the entire waveform. Instead of a work bench, a 50lb portable enclosure contains the entire instrument which includes small high voltage power supplies, power semiconductors for switching elements, small efficient high voltage capacitors and a complete Pentium class computer with color display.

2 The principles of SURGE TESTING , the strength of the turn-turn insulation being tested, the motivation to perform the SURGE test, the required test voltages necessary to succeed with the test, the industry standards surrounding the technology and the modern diagnostic algorithms will be presented. PRINCIPLE OF THE SURGE TEST If a rapidly increasing CURRENT is applied to a coil, a voltage will be generated across the coil by the principle of INDUCTION . The voltage across the coil is given by V=L*di/dt Where V is the terminal voltage across the coil, L is the coil s inductance, and The time rate of change of CURRENT pulse is di/dt. The terminal voltage V at the leads of the coil is actually a summation of the induced voltage created between individual loops in the coil. If the insulation separating adjacent coils is weak and if the induced voltage is higher than the dielectric strength of the weak insulation, an arc will form between the coils.

3 SURGE TESTING equipment is designed to create the induced voltage between adjacent coils and detect the arcing indicative of weak or failing insulation. Figure 1 below shows a block diagram typical of today s instrumentation. The internal capacitor is charged to a known voltage by the power supply. At a specific time, a high voltage switch closes which transfers the charge from the capacitor through the windings of the coil. If the resistances and loss of the entire circuit are such that the system is under damped, charge will be able to flow through the inductor and on to the other side of the capacitor resulting in an oscillation This process of ringing will repeat until the resistances and losses in the circuit completely absorb all of the energy that was originally on the capacitor. The measurement of the terminal voltage of the coil vs time gives the SURGE waveform, which shows the damped oscillation. Iris Rotating machine Conference 2 of 13 June 2003, Santa Monica, CA FIGURE 1.

4 Block Diagram of Typical SURGE Instrumentation The ringing frequency of the dampened sinusoidal waveform will be according to the following formula: If the turn-turn insulation fails with an arcing short between two turns in the coil, a fraction of the inductance will be shorted out of the circuit. From the equation above, the ringing frequency f will increase as the inductance decreases due to the short. An increase in the ringing frequency will show itself to be a jump to the left of the ringing pattern. To reiterate, it is this sudden increase in ringing frequency that is the indication of the arcing turn-turn fault. Depending on the coil and the location of an arcing short, the magnitude of the SURGE waveform may also slightly decrease. Today s instrumentation will slowly increase the test voltage and look for the increase in ringing frequency. TURN-TURN INSULATION STRENGTH Dielectric Strength of Round Wire Consider the illustration below showing two round, film covered wires.

5 The film is made of a number of different materials with varying abilities to withstand temperature, with different capabilities to withstand voltage and with different abilities to resist attack by solvents or chemicals. FIGURE 2. Round Wire Insulation The minimum dielectric strength as required by NEMA MW 1000 is 5700V for a twisted pair of wires. However, wire manufacturers exceed this value by a significant amount. A datasheet for wire typically used in motors will show a dielectric strength of 8,700V at its rated temperature of 200 C. The 8,700V dielectric strength is a high number, especially since this wire is most likely to go into a 480V motor. =22L4 RLC121f Iris Rotating machine Conference 3 of 13 June 2003, Santa Monica, CA Dielectric Strength of Rectangular Wire Rectangular wire is most often used in higher voltage machines (2300V and above) and higher power machines (500hp and above).

6 A three turn coil is illustrated in Figure 3 below and shows the major components that make up the turn-turn insulation. FIGURE 3. Rectangular Wire Insulation The rectangular wire has basically the same film insulation material as used in round wire, but often includes a glass wrap around the outside of the film. The purpose of this glass layer is to provide a porous material to physically separate two adjacent wires from each other in such a way that resin can fill the space separating the wires. A form coil using rectangular wire is constructed by coiling the wire in a loop with the required number of turns, mechanically forming the coil into the proper shape for the motor and then wrapping the outside of the shaped coil with a tape made of Dacron, mica, or other material. Finally, the coil is placed in the stator, electrical connections are made, and the whole stator and coil assembly is then vacuum impregnated with the resin.

7 The purpose of the tape is to provide an insulation barrier between the rectangular wire and the steel slot of the stator. The tape is made to be permeable to resin so the resin can get through the tape to the underlying wire and impregnate the glass wrap, if present. Resin also fills the porous spaces in the tape increasing its dielectric strength and also mechanically fixes the coil in the stator slot. To determine the turn-turn insulation capability of a form wound coil some assumptions will be made. 1. Rectangular wire with a glass wrap will be used in the calculation. 2. It will be assumed that the resin completely fills the space in the glass wrap. 3. The glass wrap is about thick on each wire. 4. The film is the same as round wire insulation from above. 5. The resin is a polyester base and has a dielectric strength of 4600V per 6. All materials are physically in contact with each other with no extra space between turns.

8 Using these assumptions, the turn-turn dielectric strength of a form wound coil is: Iris Rotating machine Conference 4 of 13 June 2003, Santa Monica, CA 8700V (film) + [4600 * (glass wrap filled with resin)] * 2 (two wires)= 54,700V. The assumptions made above can be considered academic . The point to be understood is that the turn-turn insulation in well made motors is really quite high. Indeed Gupta et al [1] STATE that motors can be manufactured with 10pu or higher turn-turn insulation strength (10pu = 38kV on a 4160V motor). Although we have plans for manufacturing coils with 40kV+ turn insulation, nature has different plans for us. Due to inconsistencies in the manufacturing process, the resin does not completely impregnate or fill the glass, the tape, and other void spaces. The result is the appearance of voids in the insulation. These void spaces are where PD can become a major mechanism of premature aging of the insulation.

9 Additionally, winding equipment can damage the wire film or glass wrap exposing bare copper even before the VPI process. MOTIVATION TO SURGE TEST A basic question to be answered is Why SURGE test? The most direct answer is the only way to detect turn-turn insulation weakness is with the SURGE test. For those who manufacture or rewind motors, the ability to manufacture coils free of insulation defects is paramount to reliability. Therefore, the SURGE test is used universally in the manufacture of motor coils for small and large motors alike. For motors in service, the dielectric strength of the turn insulation slowly decreases with time as the insulation ages. Some factors causing the insulation to age include: thermal cycling, vibration, mechanical movement of coils abrading insulation, chemical attack, partial discharge, exposure to damaging transients, exposure to radiation, VFD operation, etc.

10 For operators of electric motors, the knowledge that a motor s insulation is weakened is paramount to maintaining a productive and profitable process. Another related question is: How good does turn-turn insulation have to be? In the mid 80 s, Gupta et al [2] monitored 33 motors in 11 power plants over a period of 3 years. Their conclusion was that motors see ~3pu surges in normal operation. Some motors may see surges as high as (1pu = per unit = sqrt(2/3) * VLine = maximum voltage with respect to ground in a three phase system.) Observed rise times were as low as s to over 1 s. The conclusion to be drawn is that a motor must have a minimum turn-turn insulation dielectric strength in order to survive in its environment. If a motor is operated with a severely weakened dielectric strength, a failure of the turn-turn insulation quickly results in catastrophic failure of the motor and failure of the process the motor is turning.