Overcoming eVTOL’s High-Power Connectivity Challenges

October 1, 2022

There’s no question that significant amounts of power are needed for electric-powered vertical takeoff and landing (eVTOL) aircraft to become airborne and maintain flight. But designers of rotorcraft and personal air vehicles (PAVs) have many questions about what kinds of electrical interconnects can handle the required voltages and kW peak output for electric propulsion motors, inverters, controllers, batteries, infotainment, and sensors. To make eVTOL a reality, designers must identify the proper connectivity solution and implement a “follow-the-wire” design approach to overcome the following challenges:

Electrical Challenges

(1) Preventing partial electrical discharge — Designers need to minimize corona effects, voids and cavities that may cause insulation breakdown and electrical treeing.

Managing high voltage (HV) in the air is more complicated than on the ground. HV can ionize surrounding air, which then becomes conductive to produce a corona discharge. The corona effect is responsible for electrical power losses through voids, cavities, and electrical treeing. A self-sustaining ionizing discharge can initiate electrical arcing, potentially igniting a fire.

Selecting materials with dielectric properties suited to HV conditions — such as corona-resistant polytetrafluoroethylene (PTFE) — can minimize risks of corona discharge due to insulation breakdown.

(2) Encountering critical voltage stresses — Designers need to use appropriate techniques that protect against different voltage gradients.

Different techniques are used to protect conductor parts depending on the characteristics and strength (voltage gradient) of the electrical field around conductors (Figure 1). With relatively low-energy corona effects, connectors that use conductors with large radii and designs that avoid sharp points and edges help minimize air ionization.

To protect against air ionization breaking down dielectric between pins, each pair of conductor pins must maintain an adequate clearance distance as measured through air. These values will change as altitude and temperatures change. Electric discharges can also occur on or close to the insulation surface. A localized, partially conductive path on the insulation surface is termed “arc tracking.”

To minimize electric discharges along the insulation’s exterior, each pair of conductive parts (including the binding surface of equipment) must maintain adequate creepage distance as measured on the insulation surface. Creepage distance is typically equal to or larger than clearance distance. Minimum creepage distance can be determined by the insulation’s Comparative Tracking Index (CTI).

(3) Avoiding arc tracking damage — Designers need to minimize breaches in insulation to avoid electrical arcing between wires.

Current on the outer surface of polymeric insulators has the ability to create carbon tracks that damage insulation. Carbon tracks can cause the insulator to lose its dielectric properties and become an electric transmitter instead. Electrical arcing can then occur across the conductive path, resulting in power loss with a high probability of ignition. As noted above, maintaining proper creepage distance and using insulation materials that maintain their dielectric properties in HV conditions are critical.

(4) Handling high network operating voltages (>3kV DC) — Designers need to select relays and contactor components that can withstand extreme electrical stress.

Higher network operating voltages impose significant demands on relays and contactors used for propulsion motor power switching, battery charging management, comfort heating for passengers, and other auxiliary functions. HV relays and contactors are available to meet demanding peak load capacity, operating temperature, coil efficiency, short-circuit protection, breaking capacity, and other critical requirements, such as robust shock and vibration performance.

(5) Dealing with skin effect — Designers need to minimize opposing eddy currents to reduce the effective resistance of conductors at higher frequencies.

When determining proper shielding and filtering for electromagnetic compatibility (EMC), designers must account for skin effects — the tendency of AC to flow close to the surface of a conductor. Skin effect is the result of eddy currents induced by the changing magnetic fields of alternating current and, therefore, is a factor in nearly every AC design. Printed circuit board (PCB) traces and other aspects of AC power circuits can be designed to negate skin effect, but expert planning is required.

(6) Managing size and weight constraints — Designers need to select smaller components to trim aircraft loads.

Components employed in high-power electrical energy storage and management in industrial applications tend to be hefty. Fortunately for aircraft, high-efficiency relays and contactors are available that can handle higher voltage and amperage within a compact footprint. For example, well-known KILOVAC high voltage relays and contactors from TE Connectivity (TE) offer voltage ratings up to 70kV DC and current ratings up to 1,000 Amps (A) while providing a useful size-to-power ratio. Compact cables, terminations, and connectors are also available to meet demanding size, weight, and power (SWaP) requirements.

Thermal Challenges

(7) Managing thermal issues — Designers need to dissipate heat in composite structures to create a stable electrical/electronic environment.

Dissipating heat in composite structures is difficult. Pick-up voltage (VPI) and coil resistance (RC) change as the temperature of wires and relays change. To ensure electrical stability, the designer must determine the steady-state characteristics for the temperature and voltage combination of a DC relay’s operating conditions. The same is true for AC applications, although their VPI varies less with temperature than DC relays. This evaluation ensures proper product selection.

(8) Battery charge cycles — Designers need to enable higher energy transfer rates by optimizing heat distribution and balance.

High Power Charging (HPC) for direct current (DC) presents a challenging electric load profile in eVTOL and UAM applications. Individual components are subjected to temperature extremes at resistance points along the HV path. For HPC system safety, a simulation can apply dynamic load profiles along the complete HV path to identify potential thermal bottlenecks. Every microohm (μΩ) of resistance counts. Areas to reduce resistance include: cable attachment (termination technology), contact interfaces (crimps and contact types), and contact materials — as well as applying optimized high-voltage contactors and relays. Thermal sensing, thermal system protection, and thermal modeling can also be used to avoid hot spots and design an HV path that can carry short-time dynamic loads.

Power-Management Challenges

(9) Handling power management — Designers need to use high-frequency switching to enable rapid bus transfer in the event of power loss.

Variable frequency AC power is used for typical aircraft loads. But fixed-frequency 400 Hz AC power enables smaller and lighter transformers and motors — and faster transfer of bus power if power is lost — all of which is desirable for eVTOL applications. Used throughout the aviation and aerospace industry, HARTMAN power switching technology from TE offers 400 Hz AC contactors. They are rated up to 500 A, and offer lightweight high-performance DC contactors, up to 1,000A. Hermetically sealed enclosures provide protection in severe environments. Multiple main contact configurations and auxiliary contact configurations are available. Additionally, modular power distribution units or backplane-type panels can be customized for fixed-wing aircraft and rotorcraft applications.

(10) Using hybrid electromechanical and solid-state power switching technologies — Designers need to evaluate advantages and disadvantages of both.

Solid-state relays (SSRs) offer silent operation, low electrical interference, functionality over a wide range of input voltages, low power consumption, and no electrical arcing. Additionally, zero-voltage crossover minimizes surge currents. Essentially an electronic circuit, an SSR employs no moving parts and resists physical shock, vibration, or changes in altitude. But as with any electronic circuit, SSRs are sensitive to ambient heat and require a heat sink. They are also vulnerable to power surges.

Electromechanical relays (EMRs) are far more tolerant of surges and overloads and can switch any AC or DC load up to their maximum rating. EMRs will operate at full load over a wide temperature range without requiring a heat sink. However, EMRs arc when contacts open and close, which may affect nearby equipment sensitive to radio frequency interference (RFI).

Consequently, EMRs are suitable in applications with heavy surge currents or spike voltages that tolerate RFI. Relay manufacturers should be consulted for SSR and EMR selections that fit particular applications.

“Follow-the-Wire” Methodology

To uncover hidden factors compromising power and signal reliability, designers can employ a “follow-the-wire” methodology. Success depends on viewing interconnects holistically as part of the system rather than a last-minute afterthought. Industry standards groups also provide insights to solve technology problems at the subsystem and component level.

Solution providers with extensive high-power expertise and a broad product portfolio can help designers strike the right balance between performance, cost, and time to market. Designers who need to iterate prototypes and products rapidly benefit by working with a manufacturer who can design, customize, manufacture, and implement all the components along the wire. A connectivity partner with broad experience can also offer innovative solutions that help eVTOL and UAM projects soar over high-voltage, high-current hurdles.

This article was written by Karl Kitts, Senior Manager R&D and Product Development Engineering, TE Connectivity (Berwyn, PA). For more information, go here  .

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