Post on 01-Apr-2021
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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