Chuck Yung
EASA Technical Support Specialist
When rewinding motors rated 6 kV and above, there are certain steps beyond the normal rewind procedures used for 2.3 kV/4 kV machines. Whether a machine is to be VPI processed makes a difference in how the winding should be treated.
Aside from the obvious issues of insulation and higher voltages, there is also the possibility of partial discharge (PD), which brings its own unique set of problems. Air is an electrical insulator, albeit one of inconsistent quality. Increased humidity lowers the dielectric breakdown voltage of air, so an air gap that might be adequate under dry conditions may prove inadequate when the humidity is high (Table 1). Even though a form coil is fully taped, sealed and processed, the presence of air in voids within the coils, or between the coil and ground (i.e., in the slot) can cause problems with PD.
Note: A void only 0.040” diameter (1 mm) is sufficient for partial discharge to occur. A larger void increases the possibility of PD.
What is PD?
When we apply voltage across any insulation, and raise the voltage, at some point that insulation will break down. The voltage at which that happens is the ultimate dielectric strength. Manufacturers of insulating materials apply a safety factor (a factor of 5 is common) to arrive at the published “dielectric rating” of insulation (usually described in volts per mil or kilovolts per mm). That keeps us out of trouble, most of the time.
Air is an insulator, so it also breaks down if we apply too much voltage through it; one example of this is an arc across a spark plug gap.
As voltage is impressed on a conductor (in this case, the turns of a form coil), the ground insulation polarizes to resist the passage of leakage current. Air molecules start to break down if exposed to too high a voltage. When that happens, the oxygen (O2) releases a molecule (O), which joins another oxygen molecule to become ozone (O3). The ozone chemically attacks nearby insulation.
At the same time, there is arcing associated with the insulation breakdown—just as when groundwall insulation burns if we perform a hipot test at too high a voltage. The combination of electrical arcing and chemical deterioration eventually destroys the insulation. Sometimes, you can smell the ozone when a motor is operating with severe PD activity. There are also health concerns with breathing ozone.
Fitted coil is recommended
In operation, it only takes about a 1 mm void (0.040”) to permit PD to occur. To prevent this, “high-voltage” machines must be wound with coils that fit snugly in the slots. The coils must be isolated from one another at the extensions and well insulated— especially the first and last coil in adjacent groups, where adjacent coils are at phase voltage potential. The connections must also be securely insulated with as little trapped air as possible. The VPI process is preferred for this reason.
Large machines may not fit in a VPI tank, in which case B-stage coils are used. These are true B-stage coils, with the straight sections taped with resin-rich tapes, then hot-pressed using a platen to form them to 0.010” to 0.020” (1/4 to 1/2 mm) under the slot size. Ideally, the straight sections are hot-pressed to the desired size and are void-free, with the coil extensions left semi-cured and flexible to facilitate coil insertion. Such coils should not be baked before insertion; to do so would cure the coil extensions and result in cracking of the insulation during insertion.
Side-packing
In past years, side-packing was often used to keep the coil forced firmly against one side of the slot. The side-packing material is a corrugated (wavy) conductive material. An assortment of thicknesses was needed so that each coil could be tightly fitted. One problem with the side-packing was that the full coil length could rarely be fitted with a single piece of side-packing material.
Consequently, gaps between the abutted segments were likely. As long as all the “joints” were made in the vent ducts, that presented no problem. But if two pieces of side-packing are butted between the coil and slot side, a void in excess of that 0.040” (1 mm) maximum is inevitable. Partial discharge would likely result in insulation damage at those gaps. See Figure 1.
Fitting of the side-packing was time-consuming and required the winder to fit the material as tightly as possible. Hot-pressed coils can be produced to fit the slot, assuring that no excessive voids occur. Uneven slot widths, coupled with too tight a coil fit, may require the winder to use a bore press (aka, a coil press) to force each coil into the slot. That increases the chance of cracking the hard coil sides. Depending on the irregularity of the slot width, the preferred coil fit is usually 0.010” – 0.020” (1/4 to 1/2 mm) smaller than the slot.
Loop (“horseshoe”) preferred for 7 kV and higher
The series may be connected in “stub” (straight side-by-side joint) or “horseshoe” (U-shaped series joint) manner, with most repairers and manufacturers using the horseshoe for machines rated 7 kV and higher (Figure 2). Regardless of the method used, the series connections and the jumpers should be well-insulated.
Good practice is to half-lap wrap the series connections with B-stage (resin-rich) felt, then to tape them with mica tape. The B-stage felt will produce fewer voids than multiple layers of tape.
The leads should be connected to the group ends with a loop / straight-in arrangement, rather than a stub configuration. This is to prevent work hardening of the copper (as the windings vibrate at twice line frequency, and the leads flex considerably during starting or sudden changes in load), and to reduce the possibility of point discharge damage through the insulation.
Jumpers between groups (intra-phase connections) are insulated in a similar manner, with B-stage felt and then multiple layers of mica tape. Given a popular tape that is 0.007” thick (0.18 mm), and rated 600 volts/mil (24 kV/ mm), one layer can withstand 4,200 volts. It is common practice—and strongly recommended—to use at least 1 layer, half-lapped, per 1 kV of rating. Many service centers double that for safety.
The phase connections (jumpers) between groups should be separated, as well as being adequately insulated, to minimize the possibility of PD.
Use conductive tape
For machines rated 7 kV and higher, the use of an outer layer of conductive tape on the straight sections of the coils is essential. This provides a path to ground for voltage that builds up on the outer surface of the coil. Without this conductive path, the voltage stress may reach a high enough level that it discharges and damages the insulation.
Conductive paints were once used, but had the following drawbacks:
- The need for constant stirring to ensure uniform distribution of the conductive material.
- Coating thickness was not consistent.
- Even a small gap or void in the paint could result in PD.
The conductive tape products solve those problems and reduce the labor involved while improving reliability. It is also critically important to use gradient tape (semiconductive) for a short distance beyond the coil extensions. There are two reasons for this: One is to provide a path to ground for the (somewhat lower) voltages that build up on the surface of the coil extensions. The second, more important, reason is to avoid the sharp radius that occurs at the abrupt end of the conductive material. Without the gradient tape, the electrical stress would be much higher at the end of the conductive tape. This gradient tape should slightly overlap the conductive tape, and must extend 2-4 inches (5 – 10 cm) past the conductive material.
The sharper the radius, the higher the localized voltage stress becomes. So a thin layer of conductive tape presents a flat plate to the slot side, but a sharp radius at the end of the tape. The electric field is greatly magnified by a small radius, so gradient tape serves to gradually discharge voltage stresses at the ends of the conductive material. (If you have ever pointed your finger at an operating Van der Graff generator, you may have learned this principle the hard way.) Silicon carbide is the preferred material for gradient tape, as its resistance decreases as voltage stresses across it increase (R= 1/E ). This means that, as the impressed voltage increases, the silicon carbide provides a better path back to the conductive tapes and thereby to ground.
Depending on the manufacturer and the winding treatment method (VPI, dip & bake or B-stage coils) the use of conductive tapes varies. Windings that are VPI processed are less likely to use conductive tapes below 7 kV. As Table 2 illustrates, the higher the voltage rating, the thicker the total groundwall insulation. So it follows that the VPI resin must be of low viscosity (thin) and that a longer VPI pressure cycle is required to penetrate all that tape.
Windings utilizing dip & bake coils, or in-field rewinds that will have very limited varnish treatment, are more likely to have a conductive coating for corona protection. Industry understanding of PD has progressed so that the threshold voltage rating for corona protective measures has gradually increased from 2.3 kV prior to 1940, to 7 kV today.
Avoid damage to conductive tape
Coil insertion is as with lower-voltage form coils (see Tech Note 35), with care taken to avoid damage to the conductive tapes. Whenever conductive tapes are used, any slot filler must also be of conductive material. Bottom sticks and separators can be cut from semi-conductive material available from sources such as the Gund Company or Port City Cabinet. When slot RTDs are used, each RTD must be inserted into a separator of semi-conductive material. The RTD leads—if spliced— must be insulated from the conductive material to prevent ground failure of an RTD. As with any RTD lead splice, the splices should be staggered to avoid possible shorting. Electromagnetic interference increases the advisability of shielding the RTD cable. Care should be taken to keep the leads for each RTD bundled. If separate leads are too widely spaced, voltage induced by the stators’ magnetic field into the RTD may be injurious to personnel or equipment.
Tightness of the coils in the slot is important in both directions. The wedges must be tight! One tip is to insert a thin strip of resin-saturated felt, topped by a thin piece of glastic, beneath the wedge. The compressed felt swells to fill space. The glastic is used so the wedge does not snag on the felt. Some winders use resin to “grease” the glastic strip so the wedge will slide in more easily.
Winding treatment
One question that arises is what to do with the finished winding, whether new or after it has been in operation. Is it okay to topcoat the coils? That is, can we dip / flood / spray an epoxy coating over those semi-conductive coil surfaces? The answer is yes. The resin coating will not interfere with the semi-conductive materials used to prevent partial discharge. It is desirable to coat the completed new winding, to seal it and saturate ties, etc. However, saturation will not be as effective as with a VPI process, so use B-stage felt blocks rather than dry felt.
Transpositions
One final caution about machines rated over 6 kV is that they are more likely to include a transposition. While uncommon below 13 kV, machines rated over 6 kV should be carefully inspected when collecting data. A transposition is a method by which each strand occupies every position in the coil cross-section. If a coil has multiple strands, a transposition switches the position of each strand from one turn to the next.
A Roebel transposition is made in the straight section of the coil and can only be revealed by stripping the coil insulation from both coil sides. Some transpositions are made in the diamond area after the coil exits the slot and can be as simple as a twist.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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