Mike Howell, PE
EASA Technical Support Specialist 

The insulation system chosen for any rewind should be suitable for the application, the voltage class, and the winding process capability of the service center. In most cases, adherence to “equal to or better than” selection is a good practice.

Insulation systems are classified according to their thermal rating in applicable standards (e.g. IEEE Std 1 and IEC 60085), and examples of such classifications applied are provided in Table 1. Manufacturers of large machines often design for a temperature rise significantly below the thermal class temperature. For example, a machine with a class F (155°C) insulation system might be designed for an operating temperature of 130°C.

It is important to separate the insulation system’s thermal class rating from the temperature rise. If the example machine above with an original operating temperature of 130°C is rewound with a class F insulation system having lower thermal conductivity than the original, the temperature rise will go up. 

Typical OEM Practice
For a variety of reasons, almost all form wound stators manufactured today are processed by global vacuum pressure impregnation (VPI). However, when machines are too large for the manufacturer’s processing equipment, the most common insulation systems include resin-rich (RR), thermosetting epoxy-based mica tapes and a combination of press and oven curing. 

It would be extremely unusual for a manufacturer to use the type of system typically referred to colloquially as dip and bake (DB), where fully cured, flexible mica tapes are used, voids are inherent, and adjacent layers are not bonded. 

Depending on the slot geometry, conductor section, and rated voltage, different insulation systems may also be more advantageous from a dimensioning perspective. Figure 1 provides typical design ground stress levels for common insulation systems across a range of common motor rated voltages. Note that while dip and bake type insulation systems are shown for 6.6 kV machines, they are not recommended for that voltage class, and generally wouldn’t be used by a manufacturer for any medium voltage.

Typical Repair Practice
Most medium voltage rewinds today are also processed by global VPI, but there are situational exceptions with the most common being the service center does not have a suitable VPI system or the stator is being wound on-site. 

Let’s consider situations where the service center does not have a suitable VPI system for the project. One option is to subcontract the VPI process, in which case consideration should be given to subcontracting the entire rewind to a qualified service center. This will mitigate risks associated with transport of an unprocessed stator and the mix-and-match application of labor, materials and process that can be encountered when one service center is winding and a second is VPI processing. 

If VPI is not an option, a resin-rich hard coil insulation system may be a suitable alternative, especially for machines rated above 5 kV. Machines with a large number of poles can generally be wound with most coils fully cured due to the minimum coil side deflection required for installation. Where the number of poles is lower and a greater deflection is required, coils with flexible end turns are often used. Though there are exceptions, the maximum deflection for fully cured coils usually should be limited to about 0.188 inches (5 mm). For machines rated below 5 kV, the permissible ground insulation thickness is often thinner than desired for a resin-rich hard coil system. For most coil manufacturers, the minimum single-sided ground insulation thickness is around 0.06 inches (1.5 mm), as shown in Figure 2. Dip and bake type coils are sometimes used in 6.6 kV rewinds, and if the ground insulation thickness is conservative, the end user might realize a reasonable winding life. However, the end user should also expect an elevated temperature rise and a winding much more susceptible to damage from partial discharge. 

When VPI is not an option and the rated voltage is below 5 kV, the best alternative may be a dip and bake type coil. Examples include motors where there just isn’t adequate space for a resin-rich hard coil or mid-20th century hydrogenators being uprated where an increase in copper area may be more desirable than conservative insulation builds. Whatever the voltage rating, motors originally built with global VPI stator windings will experience higher temperature rise with dip and bake type replacement windings.