Qualification of Hairpin Motors Insulation for Automotive Applications

Abstract

This paper is focused on the reliability evaluation of an electric vehicle (EV) motor based on hairpin technology. Besides bearings (not discussed here), the weak point of an electric motor is its insulating system. For inverter-fed motors, the inception of partial discharges might lead to failure in a matter of days, and thus, deserves particular attention. Qualification and lifetime evaluation of inverter-fed machines are described in IEC 60034-18-41, which specifies accelerated aging procedures and considers partial discharge inception as the end-of-life criterion. This standard was used as a reference for this paper. The only exception is that insulation models were subjected to the mechanical stress profile reported in the ISO 16750-3-2012 standard since an automotive motor is subject to significant vibrations during its operation. From the tests performed, we observed that, with the hairpin technology, turn-to-turn is the weakest link in the insulation system. All the insulation models were partial discharge (PD) free from the beginning of the tests to breakdown and the root cause for breakdown was, in all cases, traced back to cracks on the surface of the insulation. This suggests that, depending on insulating enamel thickness and conductor geometry, some insulation systems are intrinsically PD free by design, despite the effect of aging. Considering the severe vibration profiles typical of EVs, and the principal breakdown mechanism (cracking of the insulation), mechanical stress coupled with thermal stress appears as the main aging driver. Therefore, this paper spotlights the lack of proper standards for the qualification of automotive electric motors and hints at the possibility that IEC 60034-18-41 considers dealing with motors that might be intrinsically partial discharge free even after long-term exposure to operational stresses.

Modeling of Insulation Paper Damage in the Assembly of a Solid Slot Winding

“…Reference [54] studies the lifetime of electric vehicle motors. When the supply voltage is below the PDIV, the degradation rate associated with partial discharge electrical stress can be neglected.…”

“…Temperature cycling, high temperature and mechanical stress are then the primary causes of aging. Reference [54] suggests that the inverter fed machines should be separated in relevant standards to those that are partial discharge free by design and others that would be liable to become sites of partial discharge activity after a sufficiently long operation time. Such a separation could reduce end user cost in inverter-fed machine qualification.…”

“…At some stage of damage development the inception of partial discharges takes over the deterioration process. Partial discharges intensify the damage process and bring it to a point of insulation breakdown in a short time [54]. The literature does not report on the mechanisms of damage development in a mechanically pre-damaged insulation paper.…”

The Role of Neural Networks in Predicting the Thermal Life of Electrical Machines

“…To determine the thermal class (or temperature index) of the insulation system, technical standards are available and suggest methodologies for the thermal qualification of the insulation system. This allows for quite accurate estimations of the thermal lifetime at various operating temperatures [16], however, these qualification strategies require extensive testing, which is long-lasting and can endure up to thousands of hours [17]. This paper proposes a neural network to predict the trend of the insulation resistance under thermal aging that estimates the time-to-failure of each specimen at considered aging temperatures.…”

The impact of impulsive voltage waveforms on the electrical insulation of actuators for more electrical aircraft (MEA)

“…• during the surge flanks, the overvoltage can reach values between 2-3 times the DC bus voltage [2]- [4]; • the large slew rates support high turn voltages, even using stators manufactured through the hairpin technology [5], [6];…”