**1. Introduction**

Polyimide tapes are used to manufacture rotating machines. Due to their relatively large cost, these tapes are mostly in special machines exposed to high temperatures [1, 2]. As an example, machines used for drilling oil wells are insulated using PI enamels or tapes.

The situation is changing, however. Transport electrification is demanding more compact machines, characterized by higher power densities. This implies that electrical machines will be designed having higher frequencies, temperatures, and voltages compared with the current standards. This evolution places PI insulation in the forelight. As a matter of fact, aircraft turbine-mounted generators and train machines are already experiencing high temperatures, and many manufacturers already used PI-based commercial products (e.g., Kapton). Indeed, the use of PI wires has been challenged for aircraft wiring due to its sensitivity to hydrolysis, wet and dry arcing (it is speculated that PI exposed to arcing is prone to catch fire prior than reaching a failure [3, 4]).

PI tapes can be used as slot liners (generally in form of laminates as NKN), phase separators, as well us around conductors to realize the phase-to-ground, phaseto-phase, and turn-to-turn insulation. In the last case, adhesive tapes are usually wrapped around the conductor. Complete adhesion is achieved by microwave heating of the conductor first and then infrared heating of the outer part. Eventually, the conductor is cooled down rapidly by immersion.

To date, most electrical machines are inserted in power electronic drives and are fed by a power electronic converter (inverter). The waveform provided by inverters is not the standard sinusoidal waveform, but a train of voltage impulses whose fundamental frequency is equal to the frequency of the reference sinusoidal voltage waveform. This type of voltage is sketched in **Figure 1**.

The advantages of PWM inverters are so many that nowadays only very few motors are still connected to the AC grid directly. As an example, the frequency of the drive is regulated in a straightforward way by changing the frequency of the reference voltage waveform. Furthermore, the maximum frequency of the drive can be much higher than that of the power grid, being the latter selected based on the limits of large rotating machines and with transmission losses and limits as a target. Going to higher frequencies ensures the possibility of shrinking the magnetic circuit of the motor, reducing the weight and the volume of the machine. Both advantages (controllability, high power densities) are vital for transport electrification to ensure that the drive works smoothly using a light actuator.

The "dark side" of PWM waveforms, however, is the electrical stress they impose on the insulation [5–8]. The voltage at the motor terminals can increase due to wave reflections when the voltage impulses reach the machine. This is because the connecting cables have a characteristic impedance of 20–30 Ω, whereas the machine characteristic impedance can be up to 400 Ω. Furthermore, a phenomenon known as "double pulse," which happens when two phases commutate simultaneously, can further raise the applied voltage. The theoretical limits of these phenomena are (a) the voltage can be doubled when only reflections occur and (b) the voltage can be four times larger if both reflection and double pulse occur (**Figure 2**).

Furthermore, the impulses must propagate within the winding of the machine, which are complex inductive capacitive networks. Without going into details, it is important to understand that, during the rising and falling flanks of the voltage impulses, the first coil of the machine will withstand most of the inverter voltage, whereas if Nc is the number of coils in the machine, all coils will be subjected to the same voltage V/Nc when subjected to AC voltage waveforms.

This change in the electrical stress levels brought about a radical change in the failure mode of low-voltage rotating machines. Prior to the advent of power electronics, insulation mostly failed due to thermal aging. The eventual failure mode was the opening of crack where a large leakage current, able to melt the dielectric,

**Figure 1.** *PWM voltage waveform.*

*Electrical Endurance of Corona-Resistant Polyimide for Electrical Traction DOI: http://dx.doi.org/10.5772/intechopen.93253*

**Figure 2.**

*Potential at a five-coil low-voltage motor terminal (U) and at the interconnection between adjacent coils (Ua, Ub, Uc, Ud). In gray, the voltage drops across the first coil.*

could flow. Under power electronics, if the electrical stress exceeds a critical threshold, partial discharges are incepted inside the winding. These discharges can occur with a very large repetition frequency, comparable with the switching frequency of the power converter.

The insulation that is most vulnerable to partial discharges is the turn insulation [9, 10], as often enamels or tapes having limited thickness are used (thus, it is easier to puncture the insulation, and, moreover, the electric fields are the highest in the machine when inverters are used). Dielectrics made of purely organic polymers degrade quickly under PD bombardment as the C▬C and C▬H bond energies are 3–4 eV, and a large percentage of electrons in the discharge have sufficient energy to cause dissociative electron attachment (DEA) leading to bond breaking and free radical formation [11, 12]. After long enough, this process leads to the puncture of the insulation.

Due to these considerations, dielectrics used for the turn insulation of rotating machines have been nanostructured, meaning that inorganic particles of nanometric size were added to the base polymer [13, 14]. Nanostructured insulation for rotating machines is often indicated as corona-resistant insulation (e.g., CR wires, CR tapes, CR laminates, etc.).These particles tend to form a barrier to the progressive degradation induced by partial discharge activity (as an example, the formation of ceramic-like layers has been observed on the outer surface of CR winding wires). CR PI tapes have been proposed as a solution to improve the reliability of inverter-fed machines [15, 16]. Besides these tapes can be of use also in mediumvoltage machines, where the large fields in the phase-to-ground insulation can lead to partial discharges in proximity of the conductors if the insulation detaches from the conductor due to the combined effect of high temperatures and thermal cycling (in that case, an air pocket can form in proximity of the conductor, leading to the inception of partial discharges that, in the long term, can cause a failure of the turn insulation).

The superior performance of CR insulation compared to standard insulation has been proven in literature [13, 16]. However, all the tests were conducted on pristine CR insulation. As this book is being written, activities are under way to test CR winding wires using repetitive voltage impulses and temperatures of 155°C. The limit of this approach is that CR insulation systems are compared in the absence of thermal aging. As a matter of fact, the partial discharge endurance tests are
