Lube-Tech
Electrical properties Prominent among the new requirements for EV fluids are suitable electrical properties. As fluids are exposed to electric fields within drive units, it is vital to understand the effects of these fields on their performance. The chemical makeup of a fluid (both base stock and additive package) will determine how it performs in an electric field and how the effects of the field may change the fluid as it ages.
Unfortunately, the standard tests that are currently available to assess a fluid’s electrical properties have limitations when applied to EV fluids. Many are constrained by limited ranges of operating frequency, temperature, and electrode spacing. Among these are tests for electrical conductivity (ASTM D1169), dielectric breakdown (ASTM D1816), dielectric constant (relative permittivity) and power factor / dissipation factor (ASTM D924).
If electrical conductivity is too high, current may ‘leak’ from the system, which will decrease its efficiency and, in extreme cases, cause a shock hazard. If electrical conductivity is too low, a static charge may develop, discharge of which may cause damage to bearings and other components. Fluids with lower dielectric constant will demonstrate a reduced ability to store an electric charge. Low power factor fluids are less efficient and may lose electric power to heat. If a fluid is not able to withstand sufficient applied voltage, dielectric breakdown may result. Electro-rheology (fluid flow in the presence of an electric field) and electro- tribology (wear in the presence of an electric field) are also important areas of study when assessing EV fluid performance.
In electric drive units, there is a tendency of electric currents to seek a ground path through the bearings. This can lead to fluting damage at the bearing race. The Flucon E-Lub tester replicates electrical discharge machining (EDM) using test bearings and either oils or
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.141 page 2
greases. In this test, grounding of current induced by the rotating motor occurs through the test bearing. This phenomenon is also under investigation by the AFEV Consortium.
Heat transfer
As advances in electric drive units continue, the need exists for increased power levels in smaller motor packages. Designers of wet motors look to the EV fluid to cool the drive units, making good heat transfer properties a must. The fluid’s ability to store and transfer heat are defined by thermal conductivity, thermal diffusivity, and specific heat capacity.
Thermal conductivity refers to the rate of heat transfer in a fluid via conduction through a unit area. Thermal diffusivity refers to the measurement of the rate of heat transfer through a material, expressed as the ratio of thermal conductivity to specific heat and density. Specific heat capacity is the amount of heat required to raise the temperature of a unit mass of material by 1°C. Existing tests that can be used to quantify a fluid’s fundamental heat transfer properties include ASTM D7896 (thermal conductivity, diffusivity, and heat capacity), ASTM E1269 (specific heat capacity by differential scanning calorimetry), and ASTM E2716 (specific heat capacity by sinusoidal modulated temperature differential scanning calorimetry).
Material compatibility Electrified vehicle fluids must be compatible with a variety of materials, including copper at connection points, coatings on motor windings, structural plastics, and wire insulation. Fluids must guard against copper corrosion, both for submerged components and in the vapour space. A copper wire corrosion test has been developed for this reason. The wire is partially submerged and is also exposed to the vapour space above the fluid. A small current is passed through the wire to monitor its condition. As the copper corrodes, the diameter of the wire decreases, causing its resistance to increase.
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