3. Electric Motors and Hybrids

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3. Electric Motors and Hybrids

by Dr. James Wang

For students to use in the 2022 eVTOL Design Short Course at SNU, please do not reproduce or distribute

[email protected]

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CT58(S-61) T64-GE3(CH-53A)

T700-GE700(UH-60A)

T700-GE701C(UH-60L)

CT7-8A(S-92) CT7-8B5(UH-60M SOF)

US Army AATE

PT6C-67C (AW139)

RR300 (R66)

O-540-F1B5 (R44)

HIO-360-D1A (Schweizer 300CB)

HIO-360-F1AD (Enstrom F28F)

O-360-J2A (R22)

HIO-360-G1A (Schweizer 300CBi) IO-540-AE1A5

(R44)

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

1950 1960 1970 1980 1990 2000 2010 2020 2030

Year

Mazda Wankel Gasoline Engine

Electric Motors

Aero-Piston Engines

Diesel Engines

Williams F-1 Racing Engine

Allison 250-C20J(Bell 206B3)

PT6B-37A (AW119Ke)

DeltaHawk Aero Diesel engine, best prototype diesel engine Mercedez

300SD

Axial Flux Brushless motor

Ferrite Core Brush Motor

Motor with Rare-earth Magnet

Propulsion Specific Power Trend

Specific Power (Watts/kg)

Note, 746 watts = 1 horsepower

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0 1000 2000 3000 4000 5000 6000

1940 1960 1980 2000 2020

Year

Axial flux

brushless motor

Ferrite core brush motor

Motor with rare-earth magnet Brushless DC

outrunner motor Coreless motor Brushless DC

in-runner motor

Electric Motor Specific Power Trend

5 kw/kg is 2020 standard

Specific Power (Watts/kg)

Note, 746 watts = 1 horsepower

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• 92-95% efficiency continuously

• 30% overpower for few min.

• No gearbox needed

• Easy to vary rotor rpm

• Instantaneous response

• Few moving parts

• Low noise

• No polluting exhaust

• Low thermal signature

• No oxygen combustion, altitude independent

Direct Electric Drive Has Many Benefits

Siemens 5kw/kg motor

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Unlike turbine helicopters, electric power requires no warm up, on instantly. Great for UAM,

emergency rescue, police and special operations.

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1. DC Brushed Motors (19^th century) 2. DC Brushless Motors (1960s)

3. AC Brushless Motors 4. Direct Drive

5. Linear Motors 6. Servo Motors 7. Stepper Motors

Different Types of Motors

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1. DC Brushed Motors

Carbon brush

2 carbon brushes

-

+

• Use DC voltage. Simple 2 wires

• Controls rpm by voltage

• Reverse direction by reverse voltage

• Carbon brushes wear with usage

+

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Disadvantages of DC Brush Motors

• Friction of the brushes sliding along the rotating commutator segments causes power losses

• Soft brush material wears down creating dust Difficult to replace brushes for sealed or maintenance-free applications.

• Electrical resistance of the sliding brush contact causes a voltage drop which consumes energy.

• Repeated abrupt switching of the current through the inductance of the windings causes sparks at the commutator contacts, which is a fire hazard in explosive atmospheres and a source of

electronic noise.

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2. DC Brushless Motors

Made possible by the development of solid state electronics in the 1960s.

Also known as Electronically Commutated Motor (ECM or EC motor) and synchronous DC motors, are synchronous motors powered by direct current (DC) electricity via an inverter or

switching power supply which produces electricity in the form of alternating current (AC) to drive each phase of the motor via a closed loop controller. The controller provides pulses of current to the motor windings that control the speed and torque of the motor.

This control system replaces the commutator (brushes) used in many conventional electric motors.

The construction of a brushless motor system is typically similar to a Permanent Magnet Synchronous Motor (PMSM), but can also be a switched reluctance motor, or an induction (asynchronous) motor.

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DC Brushless Motors

The spinning rotor is the outer casing with magnet.

The armature (stator and wire winding) is stationary at the center.

In-runner type Outrunner type

The spinning rotor with the magnet is at the center.

The armature (stator and wire winding) is stationary on the circumference.

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“Harmony” Texas A&M University, USA

Brushless outrunner motor

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Inside of DC Brushless, 3-Phase,

Radial, Out-runner Electric Motors

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• Stator

• Winding

• Rotor

• Magnetic poles

• Brushless and brushed

• Kv (rpm/volt)

• Timing

• Idle current and impedence

Electric Motor Buzz Words

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Stator is Made From Iron

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Zefhir Parachute System from Italy

Winding around the stator

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Different Sizes

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Brushless 3-phase motor

components

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Winding around the stator

Winding determines

rpm/volt

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Example of an 8-pole motor

8 neodymium magnets

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i

Ampere’s Law - Current flow through a wire will Induce a magnetic field B at distance r

Current I

B r

Explains Ampere’s Law https://www.youtube.com/watch?v=S4uNHue5aNM The direction of the magnetic field lines obeys the Biot-Savart Law, right hand rule

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Source: https://www.youtube.com/watch?v=59HBoIXzX_c

Current Flow Will Induce a Magnetic Field

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3 Phase AC Induction Motors

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c

Coiling the Wires Will Produce a Magnetic

Flux Through the Center of the Coil and Make it an Electro Magnet

S N

Current flow direction

Magnetic field

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Coiling the Wires Will Produce a Magnetic

Flux Through the Center of the Coil and Make it an Electro Magnet

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3 Phase DC Brushless Motors

Source: LearnEngineering.com

Rotor

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Once the battery voltage is fixed, the frequency of the 3 phases controls the rpm, the current determines the power (P=V^xI)

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Source: LearnEngineering.com This explains how Tesla permanent magnet motor works: https://www.youtube.com/watch?v=esUb7Zy5Oio

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3-phase wires

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A Stator is made of many thin layers laminated together to minimize eddy current loss

Stator stack

Stator sheets

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Eddy Current

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Thicker winding wires

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“Harmony” Texas A&M University, USAThinner winding wires

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3 phase wires

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Bell housing with magnets

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Finished motor

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DC Brushless 3-Phase In-runner Motors

Wire winding is on the outside

Magnets are on the rotating rotor 3-phase power

supply and control wires

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Burned out winding on an in-runner motor

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Motor Burned in a 200^o C Fire Incident, Neodymium Magnet Became De-magnetized

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Sensor Type 3-Phase Brushless In-runner Motor

3-phase power wire Five wires for

rotor azimuth position sensors signal. Sensor type motor

controls rpm more precisely.

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Siemens’ Radial Motor on CityAirbus

Top motor

Bottom motor

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Bigger bearings

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Motor / Rotorshaft Bearings

Source: https://insideevs.com/news/322358/tesla-model-s-makes-use-of-skf-ceramic-motor-bearings-video/

Source: https://link.springer.com/article/10.1007/s00170-020-06069-0

Motor bearings can experience high loads and heat, especially at thousands of RPM or when rotor is mounted directly to motor shaft.

Use radial bearings to take side loads and thrust bearings to handle pushing/pulling on the shaft. Example, Tesla Model S’ induction motor uses SKF ceramic bearings to handle heat and prevent expansion

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Air Cool vs Liquid Cool

MAGicALL air cool motors on Airbus Vahana and on Boeing PAV demonstrators

Liquid cool on Project Zero

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Air Cooled Motor on Beta Technologies Alia

Cooling vanes

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Example: a Circular Radiator Design

Source: www.reactionengines.co.uk/technology/our-work/advanced-radiator-hybrid-electric-aircraft

Ampaire aircraft company in California commissioned Reactengine in UK to design this compact annual shaped radiator for cooling the coolant

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3. AC Brushless Induction Motors

The AC induction motor was invented by Nikola Tesla (1856‐1943). Three‐phase AC induction motors are the most frequently encountered in most industries. They are simple, rugged, easy to maintain and are less expensive than DC motors of equal power and speed ratings.

Induction motors inside can get very hot and requires cooling by a fan and fins.

Electric cars, like Tesla Model S, use 3-phase AC induction motors. They are self starting. They do not require

permanent magnets.

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3Phase AC Induction Motors are most

popular and they have 3 sets of windings

Source: https://www.youtube.com/watch?v=AQqyGNOP_3o

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Tesla Uses Induction Motors (IM) and Permanent Magnet (IM) Motors

Source: https://youtu.be/MQV3D8F6gvwand https://youtu.be/FVVT3FD30eY

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Axial Flux Motor

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Inside of an Axial Flux Motor

Permanent magnet

The rotor spins

Copper wire winding

Source: The Dawn of an Advanced Transportation Mode

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A Direct Drive with Axial Flux Motor

Permanent magnet

Stator with wire winding to generate electro-

magnetic flux

Ball bearing

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Axial Flux Motor Controls RPM by the 3 Phases

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Axial Flux Motor

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Axial Flux Motor

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Finished Axial Flux Motor

A B

C

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Torque = r x F

Advantages of Axial Flux Motor is More Torque at Lower RPM

A B C

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Three Methods to Increase Redundancy

1. Many motors/rotors

2. Stack multiple motors on one rotor shaft

3. Multiple windings inside each motor

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On a stacked motor, how do we achieve the 30% overpower mentioned earlier?

Example: Voltage is fixed at 800 volts, electric motor kv is fixed at 2, so it will want to spin at 1600 rpm. Motor will pull current to spin at that rpm. Power= v ^x i , if one motor failed, the blade pitch remains the same, the

second motor will pull more current. The second motor will start to heat up, it could take a couple minutes

before motor temperature reach 180^oC.

That’s why motors and inverters have a continuous rating and a peak rating for current.

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Electric Motors Have Another Advantage

With an internal combustion engine, if the air/fuel

mixture is not changed, but suddenly there is a loading change (example, suddenly reducing propeller’s pitch angle), the engine will overspeed because the air/fuel mixing has not changed. While an electric motor will automatically draw less current when external load is reduced.

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RR Spirit of Innovation Clocks 387 MPH

January 2022

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RR Spirit of Innovation

Source: The world’s fastest electric airplane https://youtu.be/GsXGJ1O3ccQ

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3 x 250kw Axial Flux Motors on One Shaft

In flight only a total of 210kw used to keep motors cool

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Three Motors on One Shaft

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Inside Axial Flux Motors

Source: The world’s fastest electric airplane https://youtu.be/GsXGJ1O3ccQ

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Wires in Winding Used in Automotive are Flat

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Each Motor Develops 580 ft-lbs of Torque

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Motor Testing

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Indoor Motor Test Cell

350 kw 340 volts

-10 °C to +80°C

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A Megawatt Power Test Grid

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Small Scale Motor Test Stand

RCBenchmark.com makes small test stands and software

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• Temperature of motor and controller

• Temperature of coolant

• Voltage

• Current

• RPM

• Command from the flight computer to the motor controller

• Command received and output at the motor controller (check for noise)

Always Record and Monitor Motor Conditions in Realtime or Datalog

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diameter: 1.6 meter rpm: 2000

weight: 5.6 kg

optional payload: 5 kg

Can Collect Power Data on Model Flight Test

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Data Log Result for Model Tested on Previous Slide

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Model Rotor Test Stand

Outside wind tunnel

Inside wind tunnel

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Indoor Motor/Rotor Test Stand

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Then Test on Actual Aircraft

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Full-Scale Whirl Tower

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Collecting Rotor Data

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NASA Tiltrotor Test Rig (TTR) at NASA 40’ x 80’

Wind Tunnel at Ames Research Center

Up to 26 ft diameter

90 degrees nacelle rotation 100 knots edgewise

300 knots for airplane mode

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NASA Tiltrotor Test Rig (TTR)

4 electric motors, total 5000 hp or 3.7 MW 126 – 630 rpm, 20,000 lbs thrust steady

30,000 lbs peak

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NASA Tiltrotor Test Rig (TTR)

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Iron Bird for Testing Electric Powertrain Only

Roll Royce Electric Airplane

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Iron Bird for Testing Complete Drivetrain

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Iron Bird for Testing Complete Drivetrain

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Electric Motor Trade Study with Design Tips for eVTOL Aircraft

(All data are estimated or obtained from public domain, do not assume they are all accurate)

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0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

YASA-P400 Remy HVH250 HT TP POWER TP100XL Emrax 348 Emrax 268 Rolls-Royce SP260D-A

Specific Power (kW/kg)

Specific Power for Electric Motors

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Continuous Specific Power vs Motor Weight

magni250

magni500

Safran EngineUS

MGM Compro REX90

MGM Compro RES120

Siemens SP200D

Siemens SP260D-A

YASA P400 R

YASA 750R Emrax 268

Emrax 348

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

0 20 40 60 80 100 120 140

Specific Power (kW/kg)

Motor Weight (kg)

At a given weight, the higher the specific power better

Compiled in 2020 by author

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Max Continuous Torque vs Motor Weight

magni250

magni500

Safran EngineUS MGM Compro REX90

MGM Compro RES120 Siemens SP200D

Siemens SP260D-A

YASA P400 R

YASA 750R Emrax 268

Emrax 348

0 500 1000 1500 2000 2500 3000

0 20 40 60 80 100 120 140

Max Continuous Torque (Nm)

Motor Weight (kg)

Typical range for electric car motors

Bigger motor = Higher torque

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magni250

magni500

Safran EngineUS

MGM Compro REX90

MGM Compro RES120 Siemens SP200D

Siemens SP260D-A

YASA P400 R YASA 750R

Emrax 268 Emrax 348

0 500 1000 1500 2000 2500 3000

0 100 200 300 400 500 600

Max Continuous Torque (Nm)

Max Continuous Power (kW)

Max Continuous Torque vs Max Continuous Power

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Max Continuous Power vs Voltage

magni250 magni500

Safran EngineUS

MGM Compro REX90 MGM Compro RES120

Siemens SP200D Siemens SP260D-A

YASA P400 R

YASA 750R Emrax 268 Emrax 348

0 100 200 300 400 500 600

350 450 550 650 750 850 950

Max Continuous Power (kW)

Voltage (V)

Power = voltage x current

400 Volt was popular, now the trend is 800 volts

Porsche Taycan saved 30 kg harness weight by going to 800V

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Continuous Specific Power vs Max Continuous Power

magni250

magni500

Safran EngineUS

MGM Compro REX90

MGM Compro RES120

Siemens SP200D

Siemens SP260D-A

YASA P400 R

YASA 750R Emrax 268

Emrax 348

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

0 100 200 300 400 500 600

Continuous Specific Power (kW/kg)

Max Continuous Power (kW)

This chart indicates bigger motors (higher continuous power) tends to have slightly higher specific power

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Specific Continuous Power vs Designed Motor RPM

magni250 magni500

Safran EngineUS MGM Compro REX90

MGM Compro RES120

Siemens SP200D

Siemens SP260D-A

YASA P400 R

YASA 750R Emrax 268

Emrax 348

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

0 1000 2000 3000 4000 5000 6000 7000

Specific Power (kW/kg)

Motor RPM

Motors designed for higher rpm usually give higher Specific Power The right rotor diameter is a combination of efficient specific power, direct drive vs gearbox, enough torque to spin the rotor, proper rpm for reasonable blade tip speed for acoustic reason, available inverter voltage, power wire weight, cooling, etc.

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Pure Electric vs Hybrid

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Flight Speed Power

Required For Flight

Available power from engine

+

0 250

Using Hybrid Design to Increase Endurance or Payload

Use battery to

supplement takeoff

Use surplus engine power during

cruise to recharge the battery

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Generic eVTOL Hybrid-Electric Schematic

Generator Generator

Electric Motor

Electric Motor Electric Motor

Rechargeable battery

Multiple motors provides redundancy

Needs redundancy

Light weight

Supplement energy, and for maneuvering

Inflight charging

Flight Computer Avionics Communication

Lighting Cabin environment Fossile fuel engine Usually < 30% efficiency

Electric Motor

Electric Motor Electric Motor

Example, a hexrotor copter Inverter and

Power Management

motor

> 93% efficiency

> 5 kw/kg

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All-electric vs Serial vs Parallel Hybrid-Electric

Generator Rechargeable

battery

Serial hybrid suitable for:

• <2 hours flight

• Medium size aircraft

• Reasonable payload

Light weight Fossile fuel

engine

Rechargeable battery

Inverter and Power Management

Electric Motor

Gearbox Generator

Fossile fuel engine

Electric Motor Electric

Motor

All-electric suitable for:

• <30 min flight (200 Wh/kg pack level)

• <60 min flight (400 Wh/kg pack level)

• Small to medium size aircraft

• Small payload percentage

Parallel hybrid suitable for:

• Long endurance

• Medium to large aircraft

• Larger payload Electric

Motor

Electric Motor Inverter and

Power Management

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Payload Structure

Equipment

Electric propulsion

Mechanical propulsion Battery

Fuel

All electric Hybrid-electric Single turbine engine

Twin turbine engine

Assume the MTGW are Identical, below results from Honeywell shows all-electric VTOL has the lowest percentage useful payload

From data presented by Honeywell at 2019 VFS Arizona eVTOL Conference

8% 18%

40% 35%

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Fossil fuel Engine vs. Electric Power

+ +

12,500 Wh/kg

200 Wh/kg

25% thermal efficient

95% efficient

=

^{190 Wh/kg}

4000 Wh/kg

A hybrid could be in between, or improve battery’s specific energy

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How Big Can Electric Get?

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Airbus Roadmap for Electric Flight

From www.newsatlas.com

200 hp 2000 hp 20,000 hp

( 746 Watts = 1 hp )

eVTOL

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2010 “Cri Cri” MTOW 175 kg, 30 min at 60 kts, 2x25 hp

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2015 “E-Fan” 2x40 hp, 240v, 1 hour Endurance

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Original Engine Powered Walter Extra 300

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2016 “Extra 330LE” 260 kw (348 hp), 50 kg motor Top speed 337.5 km/h, 20 min cruise at 1000 kg

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Airbus Roadmap for Electric Flight

200 hp 2000 hp 20,000 hp

( 746 Watts = 1 hp )

eVTOL

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Both have 260 kw total Both are 1000 kg MTGWg

CTOL gives 20 min versus

VTOL gives 10 min

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20 megawatts (26,700 hp)

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3. Electric Motors and Hybrids

by Dr. James Wang

For students to use in the 2022 eVTOL Design Short Course at SNU, please do not reproduce or distribute

[email protected]