Electric Motor “Bootcamp” for NVH EngineersHBM PRODUCT PHYSICS CONFERENCE 2020, DAY 1

Ed Green, Ph.D.HBK Sound and Vibration Engineering Services

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Ed Green, Ph.D.

• Ph.D. Purdue University - Ray W. Herrick Laboratories (1995)

• Noise and Vibration Engineer in the Detroit area for the last 26 years

• Principal Staff Engineer at HBK Sound and Vibration Engineering Services for last 9 years

• Three years as High-Voltage Product Engineer

Electric Motor “Bootcamp” NVH Engineers - Background

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• With the transition from ICE to electric motor propulsion, NVH engineers need to learn the basics of electric motor technology

• According to BloombergNEF, “By 2040, over half of new passenger vehicles sold will be electric.” (https://about.bnef.com/electric-vehicle-outlook/)

This Photo by Unknown Author is licensed under CC BY-SA-NC

Electric Motor “Bootcamp” NVH Engineers - Outline

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• Physics of electric machines

• Difference between Synchronous, Induction, and Reluctance motors

• Control circuitry and algorithms used to power electric motors

• Trade-offs of different types of traction motors and their impacts on N&V

• Value of measuring current, voltage, and torque ripple along with mic and accel measurements

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The physics of most traction motors is the same

Physics

𝐹𝐹 = 𝐵𝐵𝐵𝐵𝐵𝐵

Fleming’s Left Hand Rule

First Finger – FieldMiddle Finger – CurrentThumb - Motion

Maximize force (torque) by increasing flux density and current

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Shown is the magnetic circuit for a permanent magnet motor

The rotor is a magnet, and the stator is a magnetic material (iron/steel)

Magnet flux lines go from the north poles to the south poles

Want gap between rotor and stator to be as small as possible

Magnetic circuit

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Permanent magnet synchronous motor (PMSM) is shown

Conductors are in slots in the stator Force is exerted on conductors as shown earlier,

but equal and opposite force is also exerted on the rotor to produce torque

Alternating current is applied to the conductors (usually with multiple phases) to produce torque

To produce torque, the conductors must be energized at the same rate as the rotor spins -synchronous

Synchronous motor

GM Stator and Rotor

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Induction motor (IM) is shown Current from the stator coils induces

current in the cage (rotor conductors) which produces a magnetic field in the rotor – like a transformer!

Different from PMSM because rotor turns more slowly than the current stator coils are energized – slip

Slip is the percent difference between rotor speed and energizing speed

Invented by Nikola Tesla in 1887 Unlike a synchronous motor, the rotor

speed must be slower than the excitation of the stator. Higher slip produces more current demand and more torque

Induction motor

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Physics of a reluctance motor is different than IM and PMSM A bar of magnetic material (laminated steel/iron) wants to align with the magnetic flux

lines – like iron filings Hybrid Reluctance/PMSM have been used to increase the efficiency of latest Tesla and

other vehicles

Reluctance motor

Note that the polarity of magnetic excitation is not important to the direction of rotation

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Synchronous motor/induction motor tradeoffs

Induction Motor Synchronous Motor Switched Reluctance Motor

GM EV-1 Tesla Model 3 2WDUsed as Hybrid SM/SR for

newer TeslasTesla Roadster Tesla Model 3 4WD (rear)Tesla Model S Tesla Model Y 4WD (rear)Tesla Model X Nissan Leaf

Tesla Model 3 4WD (front) Chevrolet VoltTesla Model Y 4WD (front) Chevrolet Bolt

AC Propulsion Toyota PriusMahindra e2o RivianAudi e-Tron Hyundai Kona Electric

Ford Focus EVPorsche Mission E

Jaguar i-Pace

Induction Motor Synchronous Motor Switched Reluctance MotorCost Low Higher Low

Starting Torque Low Better PoorLow Speed Torque Better Good Good

Power Density Good Better GoodEfficiency Very Good Better Very Good

Torque Degradation None Slight over Time NoneControl Easy Not as Easy Not as Easy

Longevity/Durability Excellent Excellent ExcellentNVH Good Better Fair

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Electric vehicle batteries are DC devices, and traction motors are AC devices A high-current, high-voltage inverter is necessary to convert the DC battery voltage into AC

motor supply voltage It would be ideal for the inverter to produce a sine wave output, but solid-state-transistor

devices in the inverter prefer to be in an “on” or “off” state to manage heat generation -transistors want to produce square waves!

Two strategies to produce approx. sine wave current are pulse width modulation (PWM) and multi-level step

Motor inverter/controller

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Voltage –Current –Torque –

Inverter voltage influence on mechanical torque

Torque has frequency component• AC excitation• Slotting effects

Control type effects torque• PWM excitation on the left• 6 step excitation on the right

These effects will result in N&V at the machine and down stream

Voltage, current, and torque for a control change in a 3 phase machine highlighting the

dependence of torque on excitation

Slide from “eDrive NVH HBM eDrive Testing” webinar, Oct 2019, Mitch Marks

Pulse Width Modulation Multi-Level Step

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Diagram shows PWM voltage and resulting current in a coil

Voltage ‘duty cycle’ is varied to modulate current waveform (pseudo-sine-wave)

Wider pulses for higher current

Pulse width modulation

𝑖𝑖 = −1𝐵𝐵�𝑣𝑣𝑣𝑣𝑣𝑣

PWM Circuitry with IGBT Transistors

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Plot shows multi-level step control

Control can switch from PWM at low load to multi-level at high load

Multi-level step

V

Time

Mitch Marks slide

Multi-Level Circuitry with IGBT Transistors

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Noise and vibration – basic mechanisms

Consider the simple synchronous motor. The rotor is a powerful magnet, the stator is iron, and they are separated by as small of a gap as possible

Thus, huge radial forces (Maxwell forces) are exerted on the stator, and these forces move as the rotor moves

In general, these radial forces are much larger than the tangential forces that produce motor torque and increase as load increases

The fundamental excitation frequency is:

𝑓𝑓𝑒𝑒𝑒𝑒(𝐻𝐻𝐻𝐻) =𝑝𝑝𝑝𝑝60

p is number of polesN is the rpm

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Noise and vibration – basic mechanisms – cont.

In addition there are harmonics and subharmonics that arise from normal operation and also quality issues

Induction motors have induced poles which move at the speed of rotor, but there are also alternating forces at the frequency that is applied to the motor – two sets of frequencies

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Noise and vibration – basic mechanisms – cont. The stator is approximately a cylindrical

shell that has natural frequencies These are excited by the radial forces Degree of excitation depends on

participation factor (spatial and temporal matching of force and mode shape)

Generally requires software (such as EOMYS Manatee) to determine the level of excitation

Best designs preferentially excite higher modes, have more poles, have many slots, and are multi-phase

Skewing and pole shaping to smooth excitation from on to off – analogous to using helical gears rather than spur gears

Breathing Mode

First Cylindrical Bending Mode

Second Cylindrical Bending Mode

Third Cylindrical Bending Mode, etc.

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Noise and vibration – basic mechanisms – cont.

Our engineering partner, EOMYS, provides an in-depth webinar series on NVH

https://eomys.com/e-nvh/webinaires/?lang=en

EOMYS Manatee Software can be used to evaluate:• Current amplitude unbalance• Demagnetization• Stator ovality• Pole misplacement• Parallel static eccentricity• Parallel dynamic eccentricity

M.S. Islam, R. Islam, and T. Sebastian, “Noise and Vibration Characteristics of Permanent-Magnetic Synchronous Motors Using Electromagnetic and Structural Analysis”, IEE Transactions on Industry Applications, Vol. 50, No. 5, Sept./Oct. 2014 is a very good reference on basic noise mechanisms

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Noise and vibration – basic mechanisms – cont. The PWM also produces high

frequency noise which can be efficiently radiated by the stator or invertor housing

Simultaneous acquisition of N&V signals and high voltage motor excitation signals is useful to understanding response

HBM Genesis system has capability• High frequency• High voltage• High current• CCLD (ICP) inputs for accels and mics

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Voltage –Current –Torque –

Noise and vibration – basic mechanisms – cont.

Torque ripple also produces N&V input to the vehicle through the mounts and suspension

Influenced by motor design and control strategy

Important to note that actual control does not always match intention

Voltage, current, and torque for a control change in a 3 phase machine highlighting the

dependence of torque on excitation

Mitch Marks slide

Pulse Width Modulation Multi-Level Step

DuT

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Electric powertrain and NVH testing

VoltageCurrent

ANDAcceleration

ALL synchronously and

continuouslyrecorded with eDrive

Time domain and FFT display in Perception

Post process analysis in BKconnect

P U B L I C

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Traction motors are IM, PMSM, and new PMSM/SRM hybrids Many tradeoffs between IM, PMSM, and SRM! Controllers do not produce sine-wave excitation - PWM and multi-level step are common

control strategies Control produces dynamic torque fluctuations and high-frequency noise and vibration EM forces produce strong radial forces that excite the stator at a frequency of the number of

poles times the motor speed

Summary

Thank You

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