AAM Ecosystem Aircraft Working Group: Electric Propulsion ...

AAM Ecosystem Aircraft Working Group: Electric Propulsion for Urban Air Mobility

AgendaSeptember 30, 2021

Topic Speaker Time (EDT)

Welcome and Introductions Carl Russell 3:00 - 3:05PM

RVLT eVTOL Propulsion overview Peggy Cornell 3:05-3:15PM

Power Quality research and

experimental capability Pat Hanlon and Dave Sadey 3:15-3:35PM

Motor Reliability research Tom Tallerico 3:35-3:50 PM

Working group open discussion 3:50-4:25PM

Wrap-up Carl Russell 4:25 – 4:30PM

Upcoming Meetings

The Aircraft Working Group will hold their meeting on the last Thursday of every month from 3:00PM -4:30PM EDT (12:00PM -1:30PM PDT).

• October 28, 2021: TBD

• November 2021: TBD

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Platform and Discussion

• Active Participants– Platform: MS Teams– Discussion: MS Teams microphone and chat functions

• Leave your cameras/webcams off to preserve WiFi bandwidth • Enter comments/questions in the chat function on the right side of the screen • Use your mute/unmute button • Use the “Raise Hand” function in MS Teams to let the emcee know you would like to make a

comment or ask a question • Say your name and affiliation before you begin speaking • Speak loudly and clearly

• Listen Only Participants– Platform: YouTube Live Stream (go to https://nari.arc.nasa.gov/aam-portal/ for the link!)

• All Participants– Polling and anonymous questions: Conferences.io

• Enter https://arc.cnf.io/ into your browser• Select the Aircraft Working Group: Electric Propulsion for Urban Air Mobility• Questions will be addressed at the facilitator’s discretion

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https://nari.arc.nasa.gov/aam-portal/

https://arc.cnf.io/

www.nasa.gov

Revolutionary Vertical Lift Technology (RVLT) Overview

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AAM Ecosystem Aircraft Working Group Meeting

Electric Propulsion for Urban Air Mobility

Peggy Cornell, eVTOL Propulsion Subproject ManagerSept 30, 2021

Summary

NASA RVLT is focused on

• Vertical lift supporting Urban Air Mobility

• Three On-going Tech Challenges

o Electric propulsion reliability and

performance

o Tools to compute vehicle source noise and

performance

o Fleet noise

• Approving new Tech Challenges

o Ride quality and passenger acceptance

o Crash safety

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Our vision is to create a future where

VTOL configurations operate quietly,

safely, efficiently, affordably and routinely

as an integral part of everyday life.

AAM and UAM

NASA Focus is on Advanced Air

Mobility (AAM) Missions

– AAM missions characterized by < 300

nm range

– Vehicles require increased automation

and are likely electric or hybrid-electric

– Rural and urban operations and cargo

delivery are included

– Urban Air Mobility (UAM) is a subset of

AAM and is the segment that is

projected to have the most economic

benefit and be the most difficult to

develop

o UAM requires an advanced urban-

capable vehicle

o UAM requires an airspace system

to handle high-density operations7

https://www.nasa.gov/aam-studies-reports/

NASA Concept Vehicles – Generic Geometries that Capture Many UAM Features

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NASA reference vehicles Widely shared Fully documented Realistic performance Realistic set of

compromises No plans to build or fly these concepts

• Vehicles contain relevant UAM features and

technologies

– Battery, hybrid, diesel propulsion

– Distributed electric propulsion

– High efficiency rotors

– Quieter rotors

– Wake interactions

• Provide configurations for

– Communication of NASA’s Urban Air Mobility research

– Design and analysis tool development

– Technology trade studies and sizing excursions

– Modeling operational scenarios

– Common configurations for studies in acoustics, flight

dynamics, propulsion reliability, etc.

Tech Challenge: UAM Operational Fleet Noise Assessment

Tech Challenge: Tools to Explore the Noise and Performance of Multi-Rotor UAM Vehicles

RVLT Near Term Focus for ResearchFY21-FY23

Tech Challenge: Reliable and Efficient Propulsion Components for UAMVehicle Propulsion Reliability

UAM Fleet Noise

Noise and Performance

• Re-configure laboratories for electric

propulsion testing

• Conduct initial single string tests

• Develop tools to assess motor reliability

• Develop high reliability conceptual

motor design

• Generate Noise Power Distance (NPD) database for several UAM

reference configurations and trajectories

• Conduct fleet noise assessments

• Initiate psychoacoustic testing to assess human response to UAM vehicles

• Plan and conduct validation experiments

• Improve efficiency and accuracy of conceptual design tools

• Conduct high-fidelity configuration CFD for validation and reference

• Improve community transition and training for analysis tools

Safety and Acceptability • UAM crashworthiness and occupant protection

• Acceptable handling and ride qualities for UAM vehicles

• Ice accretion and shedding for UAM

Targeted Research in These Areas for Future Tech Challenges

9

RVLT.TC.UAM.Electric.1

Reliable and Efficient Propulsion Components for UAM

E-Drives Rig

Objective

• Develop design and test guidelines, acquire data, and explore new

concepts that improve propulsion system component reliability by

several orders of magnitude over state-of the-art technology for

UAM electric and hybrid-electric VTOL vehicles.

Approach

• Iterative design, model, test and analyze

– Apply vehicle level analysis

– Develop experimental / analysis capabilities

– Conduct tests (reliability of components, tool validation)

– Provide validated models

– Develop design guidelines & test procedures

Benefit/Pay-off

• Validated power/propulsion/thermal models

• High reliability motor design – feasibility of 2-4 orders of magnitude

reliability improvement

• Design guidelines for eVTOL propulsion and thermal components

• Test guidelines for propulsion and thermal components

• Candidate HVDC Power Quality and Permanent Magnet Machine

Standards10

Apply Vehicle Level AnalysisDevelop Experimental/ Analysis

Capabilities

Conduct Tests

Provide Validated Models Design Guidelines and Test

Procedures

eVTOL Power / Powertrain Testbeds

• Scaled Power Electrified Drivetrain (SPEED) -Low-Power Testbed

– Low power (up to 9 kW) motor & controls

– Low voltage test platform for high power testbed hardware

• Advanced Reconfigurable Electrical Aircraft Lab (AREAL) -High Power Testbed

– Emulated, reconfigurable system (single-string, multi-string)

– 1kVDC Peak, nominal 200kW source

• E-Drives Rig

– Test motors, gearboxes & power electronics

– Emulation of rotor loads

– Mechanical, electrical, vibration, & thermal measurements E-Drives Rig

RVLT Motor Design Efforts – Goal: Improved Motor

Reliability

• External Efforts1: 2 Contract Funded Design

1) University of Wisconsin via OSU ULI

• Integrated, fault-tolerant motor/drive design for RVLT

quadcopter

2) Balcones Technologies vis Phase III NASA STTR

• Developing Brushless Doubly-Fed Machine (BFDM)

design for RVLT-class vehicle (100-200 kW)

• Internal Efforts2:

1) Magnetically Geared Motors and Novel Designs

• Exploring trade space of reliable motor topologies for

UAM applications using in-house codes.

Example: Outer Stator Magnetically Geared Motor

2) Winding Reliability Model Development

• Developing modeling and experimental capability to

explore/predict winding reliability

1. J. Swanke, T. Jahns, “Reliability Analysis of a Fault-Tolerant Integrated Modular Motor Drive (IMMD) for an Urban Air Mobility (UAM)

Aircraft Using Markov Chains.” 2021 AIAA/IEEE Electric Aircraft Technologies Symposium (EATS).

2. T. F. Tallerico, Z. A. Cameron, J. J. Scheidler and H. Hasseeb, "Outer Stator Magnetically-Geared Motors for Electrified Urban Air Mobility

Vehicles," 2020 AIAA/IEEE Electric Aircraft Technologies Symposium (EATS), New Orleans, LA, USA, 2020, pp. 1-25.

Standards & Tools

• Leading two SAE standards

– AS7499 - Aircraft High Voltage Power Quality Standard (D. Sadey)

– AS8441 – Minimum Performance Standard for Permanent Magnet Propulsion Motors and Associated Drives

(Pat Hanlon)

• Participate in AAM (UAM) Aircraft Design

& Development Working Group

• Developing Analysis Tools:

– NPSS

• Developed high level electrical power system models & electrical port for architecture trades

• Demonstrated electrified propulsion system models

• Accepted into next release of NPSS

– EPS-SAT

• Electrical power system sizing

• Utilized for trade studies and sensitivity analyses

– Toolset for Motor Reliability

• Reliability Modeling of Electric Motor

RVLT Experimental Capabilities and Power

Quality Investigation

Pat Hanlon & Dave Sadey

NASA GRC

RVLT E-Drives Rig (POC: Justin Scheidler/GRC)

Scope

• Test motors, gearboxes, & power electronics

• Mechanical, electrical, vibration, & thermal measurements

• After upgrades completed, capable up to 21,500 rpm input & 7,400 rpm output

Key capabilities for this tech challenge

• Validation-quality data (very high precision – e.g., gearbox efficiency up to < ±0.2% with 95% confidence)

• Directly measure temperature of energized motor windings (up to 1000 V RMS continuous)

• Emulate rotor loads with new generator dynamometer

• Year-round operation (no reliance on cooling tower water)

• Provides platform for magnetic gear and motor evaluation

Specific Experiments

• Motor efficiency, output torque, and temperatures under steady-stateand transient conditions

• Performance mapping of magnetic gears (& possibly magnetically-gearedmotors) for motor design code validation & TRL advancement of new concepts

New Generator

E-Drives Rig (pre-upgrades)

Motor

(input)

Dynamometer

(output)

Generator capabilities:

• Controlled via ~500 Hz control loop

• Continuous

– 0-2400 rpm 238 Nm

– 2400-3840 rpm 60kW constant

– 3840-7400 rpm decreasing power to 31kW at 7400 rpm

• Short duration (60 seconds)

– 160% overload

Generator capability overlaid with eVTOL

motor operating points (known or

approximated)

RVLT E-Drives Rig Capabilities

RVLT Scaled Power ElectrifiEd Drivetrain (SPEED)

• Capabilities

– Low power (up to 9 kW) motor & controls

– Low voltage test platform for high power testbed

hardware

– Allows team to investigate controls and start

validation/refinement of tools (NPSS and

MATLAB)

– Single-string (source-load) analysis

• Status

– Assembly complete

– Initial integrated motor/inverter tests and

characterization begun

– Integration and EMI lessons learned will be of

benefit to AREAL

RVLT Advanced Reconfigurable Electrified Aircraft Lab (AREAL) -

Overview

• Scope

– Investigate Power Quality & Integration Issues

– Feed Standards (AS7499, AS8441)

• Capabilities

– Emulated, Reconfigurable System

• Single-string, Multi-String

– 1kVdc Peak, 600-700Vdc Nominal

– 200kW Nominal Source Capacity

– Ability to test Faults

• Experiments

– Nominal, Transient, Fault Operation

– Characterization and Response

AREAL Hardware

• DC Emulators

– Reconfigurable to emulate most DC sources

from a small-signal and transient level

• Physical Motor Stand

– State-of-the-Art Inverter/Motor, Back-to-Back

– Can replace as needed

• Motor Emulator

– Ability to Emulate Physical Motor

– Ability to Introduce Faults

• Fault Protection

– Initially Contactors/Fuses

– In the process of procuring advanced fault protection devices

AREAL DC Emulator Capabilities

• High Bandwidth DC Power Supply– 20kHz Large Signal Bandwidth

– 40kHz+ Small Signal Bandwidth

• 500V, 100kW Bidirectional (2Q) per unit

• Multiple Applications – Source Emulator

– Power Sink

– Limited Capability as Amplifier for Impedance

Sweeps

– Constant Power Load with programmable

bandwidth

• Currently Developing Modifications – Developed dynamic model of 650V Wound Field

Generator

under Phase III SBIR with PCKA for PQ Study

– Generator Model meets Lift+Cruise Power

System PQ Requirements in accordance with

AS7499 HVDC Power Quality

– Using DCE dynamic model from WPAFB to match

transient and frequency response of generator

with mods

– Also investigating low cost/ quick turnaround

emulation strategy w/ Speedgoat

AREAL Status

AREAL Testbed

• Lab layout being finalized

• Facility 480V Power Complete

• Cooling System being upgraded

• Arc Flash Study Completed for

Single String

• Safety Permit being updated

• What is Power Quality and why does it matter?

• Physical description of power, namely voltage (DC)

• Applicable during any operational period (Normal, Abnormal, Emergency)

• Voltage: Steady-state, transient, ripple

• Stability, fault conditions, & much more

• Improves reliability

• Reduces component failures by defining operational boundaries

• Ensures stable operation

• Defines/drives proper fault recovery

• Drives one towards ‘plug and play’ approach to design and integration

• Not completely obtainable, but moves one closer

• Helps guide lower level standards (e.g. components, connectors, etc.)

DC Power Quality

• Work done via contract with PCKA

• Lift-Plus-Cruise Vehicle Model (Conceptual NASA Design)

• 650V, <1MW Simulink® Power System Model for PQ Studies

• Generator, one cruise motor, four lift motors, and a power

distribution unit (PDU) for each bus, with two busses total.

• Each generator and motor has an integral rectifier and

inverter and are scaled based on validated models

• Utilizes directional overcurrent protection scheme

RVLT Example Study – Model Overview

• System iteratively designed and tuned to meet an

internal PQS specification

• Normal Operation• Steady-State, Transient Voltage

• Stability

• Abnormal Operation• Voltage response under short circuit conditions

Power Quality Analysis Overview

• Requirement: Steady-State Voltage must remain between 0.93-1.04 p.u.

• at UE Terminals• Covers No-Load to Full-Load

• 604.5 to 676 Vdc at UE Terminals

• No Load Voltage of 650Vdc

• Full Load Voltage 649.1Vdc

Steady State Voltage

• Requirement: EPS Transient Voltage must remain within the

defined window limits under 50% load steps

• Resistive Loads stepped 10 to 60%, 60 to 10%, 45 to 95%,

and 95 to 45% at UE Terminal Locations

• Worst-case voltage response was at Cruise Motor Terminals

• Spikes <10usec ignored

Load Step Transient Voltage

• Requirement: Ratio of Source to Load Impedance from 30Hz to

100kHz shall remain within the 60 degree, 3dB bounds shown• Required modifying controller gains & input/output filters

• Stayed within the required bounds for all loads & at main bus

|Zs/Zl| at Cruise Motor Terminals |Zs/Zl| at Lift Motor 8 Terminals

Small Signal Stability

• Source and Load Complex Impedance Plots

Cruise Motor Lift Motor 8

Small Signal Stability (cont.)

• Requirement: EPS Transient Voltage must stay within the over- and

under-voltage limits shown in event of a fault

• Introduced short circuit faults onto Lift Motor branch circuits

• Observed lift motor terminal voltages on unfaulted branch

circuits

• Spikes <10usec ignored

Abnormal Voltage Response

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Lift Motor 4 UE Terminal Voltage

Fault Initiated/Cleared on Branch Circuit 8

Lift Motor 8 UE Terminal Voltage

Fault Initiated/Cleared on Branch Circuit 4

679Vdc

384Vdc

Abnormal Voltage Response (cont.)

• Designed and Tuned 650Vdc, <1MW UAM Power System

• Met Internal PQS Requirements

• Normal and Abnormal Response Data to provide point design for

standards development

• Future Work• Currently analyzing soft faults (may require updates and re-evaluation)

• De-tuning filters to analyze marginal stability

• Analyze different fault strategies / responses

• Introduce cross-tie and analyze bus recovery & other PQ metrics

Conclusion and Future Work

National Aeronautics and Space Administration Urban Air Mobility Electric Motor Reliability 34

RVLT Motor Reliability Research

Thomas Tallerico

National Aeronautics and Space Administration

NASA Glenn Research Center

www.nasa.gov AAM Working Group Meeting 9/30/21

This material is a work of the U.S. Government and is not subject to copyright protection in the United States.


Winding Reliability White Paper

• Expect to publish in next couple

months

• Currently collecting feedback

from SME’s

• Copy available on request


Summary of 2019 RVLT-sponsored FMECA Study

36

• Likely that the propulsion system would need to have 10-10 failure rates per flight hour, or less, in order

to meet the EASA SCVTOL- 01 air vehicle requirement of 10-9 catastrophic failures per flight hour


Motivation and Study Goals

RVLT Technical Challenge

Because there is a lack of data for propulsion systems and thermal management systems for

UAM vehicles, NASA will

▪ develop design and test guidelines,

▪ acquire data,

▪ and explore new concepts

that improve propulsion system component reliability, culminating by demonstrating 2-4 orders of

magnitude improvement in 100kW-class electric motor reliability.


Motor Subcomponents

& Key Failure Modes

~ Prioritized ~

VTOL Electric Motor Unit Reliability Assessment

Framework for Failure Mode Prioritization & Reliability PredictionResearch Required to Improve Reliability for

VTOL

Critical Reliability Parameter Needs Refinement

and Engineering Guidelines for VTOL

Important Reliability Parameter with Available

Engineering Solutions for VTOL

Failure Modes &

Reliability Influencing Parameters & Variables

~ Prioritized ~

VTOL Electric Motor Reliability

Statistical Combination of Sub-Component Reliabilities

Motor Windings/Coils1. Degraded insulation2. Voltage / Current Overloading3. Short circuit

1. Insulation Material /

Grade

2. Temperature (Mean &

Peak)

3. Thermal Expansion

4. Electrical Stress

5. Vibration, Fretting

7. Cleanliness / Environment (humidity,

debris, …)

8.Manufacturing Quality

Motor Bearings1. Improper, dirty, or degraded

lubrication2. Overloading3. Corrosion4. Vibration

1. Bearing DN & Surface

Speed

2. Materials

3. Lubrication

4. Alignment & Dimensional

Precision

5. Gyroscopic Loads

(maneuver)

6. Rotor Lift & Moment

Loads

7. Cleanliness

8. Alternate Bearing

Technologies

9. Bearing Arcing

Motor Magnets1. Demagnetization2. Brittle fracture

1. Temperature

2. Magnetic Loading

3. Current Transients

4. Shock & Vibration

Motor Heat Extraction1. Reduced heat transfer rate2. Loss of flow3. Overheating

1. Thermal Cycles

2. Heat Transfer

3. Materials

4. Temperature Distribution

5. Coolant Temperature

6. Coolant Degradation

7. Altitude Effects

8. Cleanliness / Debris

9. Corrosion

Rotor & Structure1. Fatigue failure

1. Low Cycle Fatigue

2. High Cycle Fatigue

3. Gyroscopic Loads

(maneuver)

4. Vibration

5. Materials

6. Rotordynamics

7. Thermal Cycling


Motor Winding Insulation Stresses

Electrical Thermochemical Mechanical Contamination


Electrical Stress

• Drives Final Failure

• Instantaneous Breakdown

Unlikely

• Electrical Aging-

Repetitive Partial Discharges

Creating Electrical Trees

Through Insulation


Electrical Aging in UAM Motors

• Onset of Electrical aging ~= End of life

• Voltage source inverter fed

• f = 20 -100 kHz

• Small voltage rise time

• Type 1 (Organic Insulation)

• Polymers (Polyimide - Polyethylene)

• Higher Design Electrical Field than Type 2 (In-

organic)

• Very susceptible to damage in presence of PD

• IEC 60034-18-41 – relevant standard

• Monitoring for PD important but difficult in inverter fed systems

[1] 3-parts: https://ieeexplore.ieee.org/document/9350653

[2] https://ieeexplore.ieee.org/document/9287278

https://ieeexplore.ieee.org/document/6232601





Thermal Chemical Aging

• Chemical degradation of

insulation at set temperature

• Oxidation most common

• ASTM D2307 (Magnet wire

thermal classification)

• Causes insulation to shrink and

increase capacitance [3]

• Insulation becomes more brittle

and more susceptible to

mechanical stresses [2][1] https://ieeexplore.ieee.org/abstract/document/8927464

[2] https://ieeexplore.ieee.org/document/8088680

[3] https://ieeexplore.ieee.org/abstract/document/8921657

https://ieeexplore.ieee.org/abstract/document/8927464


https://ieeexplore.ieee.org/abstract/document/8921657


UAM Motor Design for Thermochemical Aging

70

90

110

130

150

170

190

210

230

250

270

0 500 1000 1500 2000 2500 3000 3500

Win

din

g H

ot

Sp

ot T

emp

erat

ure

Mission Time(s)

Multirotor Vehicle Motor Thermal Profiles

50kW D1 100 kW D1 100 kW D2

• Paper published at EATs 2021

• Motor Design for UAM missions

constrained by thermochemical

aging

• Assuming Class 220 C

insulation

• Target 10,000 Missions

https://arc.aiaa.org/doi/10.2514/6.2021-3279

https://arc.aiaa.org/doi/10.2514/6.2021-3279


Mechanical Aging

• Mechanical Fatigue

• Thermo-Mechanical is dominant

Electromagnetic forces are weak

• Least studied stress

Only matters for starts and stops

• Likely most important for UAM

Starts and stops every 20 min


Coil Mechanical Stress Studies

• Example Designs Explored

for Mechanical Stress due to

Thermal Mission Profile

• Coil Mechanical Stress likely

more limiting than

thermochemical aging for

UAM motor design


Problems for UAM Motor Winding Design

• Data for PDIV as a function of mechanical

and thermal aging doesn’t exits in open

literature

• Most Standards specify comparative

accelerated aging to qualify/certify windings

• Requires reference system that has

known life in target operating condition

• IEC-60034 and IEEE 117

• Reference motors/systems for UAM

• Accelerated thermo-mechanical

aging not possible


NASA Plan Forward

• Develop fundamental insulation

system test method

• Collect PDIV as function of

combined thermochemical and

mechanical aging


NASA Plan Forward Continued

Upcoming Meetings

The Aircraft Working Group will hold their meeting on the last Thursday of every month from 3:00PM -4:30PM EDT (12:00PM -1:30PM PDT).

• October 28, 2021: TBD

• November 2021: TBD

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Aircraft Working Group Point of Contacts

• Coordinator: BreeAnn Stallsmith ([email protected])

• Technical POC: Carl Russell ([email protected])

Comments, suggestions for future topics, and other workgroup information:

• Visit the website: https://nari.arc.nasa.gov/aam-portal/ ; or

• Email us at: [email protected]

See you at the next meeting on October 28, 2021!

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mailto:[email protected]


https://nari.arc.nasa.gov/aam-portal/


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