Understanding Electric Motors and
Loss Mechanisms
Dr. Bulent Sarlioglu
Assistant Professor
Associate Director of WEMPEC
sarlioglu@wisc.edu
WEMPEC
Wisconsin Electric Machines and Power Electronics Consortium
Copyright@ Bulent Sarlioglu 2016
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Outline
1.  Classification of Electric Motors
2.  American Wire Gauge
3.  Resistive Loss
4.  Core Loss
5.  Windage loss
6.  Efficiency of NEMA Class Induction Motors
7.  Efficiency of PM Motors
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Classification of Machines by
Excitation Source
Squirrel cage
DC or AC
Excitation
DC
Switched Reluctance machine (SRM)
Wound Field
Induction
Without PM
AC
……
Brushless
With PM
Synchronous
Synchronous Reluctance machine (SynRM)
Wound Field
Brushless DC (BLDC)
Surface Permanent Magnet Machine (SPM)
Interior Permanent Magnet Machine (IPM)
PM-assisted Synchronous Reluctance machine
(PMaSynRM)
High-speed
compressor motor
Courtesy of Dyson
Flux Switching Permanent Magnet Machine (FSPM)
Doubly Salient Permanent Magnet Machine (DSPM)
Flux Reversal Permanent Magnet Machine (FRPM)
Choose appropriate topology for the specified
application. More topologies are under
development.
Vernier Machine
……
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Evaluation of Common Machine Topologies
(1)
(2)
Machine Type
(3)
(4)
Advantages
(5)
Disadvantages
(1) Induction machine
1.  Most widely used.
2.  Simple and robust structure
1.  Relatively low efficiency
(2) Surface PM Machine
1.  High torque density
2.  Easy control
(3) Interior PM Machine
1.  High torque density due to
flux concentration effect
2.  Good flux weakening
capabilities
(4) Switched Reluctance
1.  Robust rotor good for high
speed operation
1.  Large torque ripple
2.  High acoustic noise
(5) Synchronous Reluctance
1.  Salient machine without
magnets
2.  Simple control algorithm
1.  Low power factor
2.  Hard to design rotor flux
barriers and flux carriers
1.  Large equivalent airgap and
low motor inductance
2.  Magnets retention issue
1.  Not very amenable to very
high-speed operation
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Evaluation of Common Machine Topologies
(6)
Machine Type
(7)
(8)
Advantages
(6) Brushless DC Machine
1.  Responsiveness & quick
acceleration
2.  High speed operation
3.  High reliability
(7) Flux Switching Machine
1.  Robust rotor structure
2.  High torque density
3.  PM on the stator and easy to
cool
(8) PM-Assisted SynRM
Machine
1.  Improved power factor
2.  Higher power density
3.  Wider constant power speed
ratio (CPSR)
Disadvantages
1.  Need to use electronic
commutation instead of
mechanical commutation
1.  Difficult to manufacture
2.  Requires more PM materials
1.  Complicated rotor structure
2.  Not suitable for high-speed
operation due to rotor
mechanical integrity
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Induction Motor
Stator
Slots
Rotor bars
Rotor
Shaft
Induction machine is one of the most mature and used electric
machine topology
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Induction Motor
Source: GM
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Surface PM Motor
Components of a four pole SPM motor
Stator
Slots
Magnets
Rotor
Shaft
Surface PM Machines are used in many applications including fans,
compressor – They are typically more efficient than Induction motors
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Internal PM Motor
Source: GM
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Interior PM/PM-Assisted Synchronous
Reluctance Machine
Source: N. Bianchi, “Rotor flux-barrier design for torque ripple reduction in synchronous reluctance and PMassisted synchronous reluctance motors,” IEEE Trans. on Industry Applications, vol. 45, no. 3, 2009
IPM and PM-assisted synchronous machines both have magnets in the
rotor. Both machines produce magnet torque and reluctance torque
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P vs. p – Number of Poles
Commonly used notation
P – Number of poles
p – Number of pole pairs
Concentrated Winding Surface PM Machine
P=2
p=1
P=4
p=2
P=8
p=4
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AWG
AWG – American Wire Gauge
Standard specification for
nominal diameters and crosssectional areas of solid round
wires.
Rules of Thumb
•  When the diameter of a wire
is doubled, the AWG will
decrease by 6
•  When the cross-sectional
area of a wire is doubled,
the AWG will decrease by 3
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Winding Resistance
coil
conductor
Resistance of a single coil
Rcoil = Tcoil
Rcoil
Tcoil
ρ
Acond
L
ρL
Acond
Single layer
Double layer
- Resistance of a coil
- Number of turns per coil
- Resistivity of the conductor (usually is copper)
- Cross-sectional area of a single conductor in the coil
- Length of a single conductor in the coil
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Winding Resistance (cont’d)
Temperature effect
Given the resistance of R1 at temperature T1 (usually at room temperature 20
°C), the resistance R2 at temperature T2 can be calculated by the equation
R2 234.5 + T2
=
R1 234.5 + T1
R2 390 + T2
=
R1 390 + T1
[C]
Resistance should be calculated
at working temperature
[F]
Resistivity of copper
T0 °C
51 %
Increase
from 20 °C
to 150 °C
Resistivity of Copper
Ohm-m
Ohm-in
0
1.589e-8
0.625e-6
20
1.724e-8
0.679e-6
150
2.605e-8
1.026e-6
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B-H Curve
[Electronics-tutorials]
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Core Loss
Core loss
Core loss in high speed electric machines is hard to be estimated, due to [W. Choi, et.al.,
2013],
Ø  Higher excitation frequency
Ø  Harmonics due to Pulse Width Modulation (PWM)
Ø  Influence of machine geometry on flux density distribution
Conventional core loss estimation
Conventionally, the core loss is estimated using Steinmetz Equation with fixed
coefficients, under the assumption of sinusoidal excitation
Pc = Ph + Pe + Pex = kh f Bα + ke f 2 B 2 + kex f 1.5 B1.5
where kh is hysteresis loss coefficient, ke is eddy current loss coefficient, kex is excess
loss coefficient, and α is the Steinmetz coefficient. These coefficients are usually
provided by manufacturers of magnetic materials and are fixed values
Conventional core loss estimation method is inaccurate for high speed
electric machine
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Core Loss Relation to Frequency
For 0.15 mm Hiperco Alloy 50
For 0.25 mm Hiperco Alloy 50
Source:
Carpenter
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Windage Loss
Windage loss-Smooth cylinder
Ø  Under high speed rotation, windage loss in the rotor becomes significant
Ø  Theoretical equation for windage loss estimation of a smooth cylinder rotating within a
concentric cylinder [J. Vrancik, 1968]
Pw = π Cd ρ r 4ω 3 L
where ρ is the density of the fluid (kg/m3), r is the radius of rotor (m), ω is the angular
velocity (rad/s), L is the rotor length (m), and Cd is the skin friction coefficient
Ø  Skin friction coefficient Cd [J. Vrancik, 1968]
1
= 2.04 + 1.768ln Re Cd
Cd
(
)
Ø  Reynolds number Re [J. Vrancik, 1968]
Re = ω r
ρ
ϕ
µ
where µ is the dynamic viscosity of the cooling fluid (Pa•s or N•s/m2 or kg/(m•s)) and φ is
the airgap length (m)
Theoretical equations for windage loss estimation of smooth rotating
cylinder exists, with limited accuracy
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NEMA Premium Efficiency Motor
Examples
Copper loss
(stator winding)
Copper loss
(rotor bar)
Iron loss
(stator core)
Source: Baldor
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NEMA Premium Efficiency Motor
•  The National Electrical Manufacturers Association
(NEMA) participated in meeting the requirements of
Energy Independence and Security Act (EISA) of
2007 to promote premium efficiency motors
•  Reasons to establish NEMA Premium program
•  Electric motors have significant impact on total energy cost
•  NEMA premium program will assist purchasers identify higher
efficient motors, and optimize motor systems efficiency
•  NEMA premium motors will reduce electrical consumption and
pollution associated with electrical power generation
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Energy Saving by using Premium
Efficiency Motors
Source:
Nidec
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Induction Motor Torque and Slip Relation
Source: Engineering Toolbox
Source: NEMA standard
Different NEMA designs of induction machine have different torquespeed profiles
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Motor Efficiency vs Power Rating
NEMA Efficiency
Classification
European Efficiency
Classification
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Efficiency of IM – NEMA B
(Influence of Power and Speed)
Analytical Equation
0.03
η = 0.712 Pmech
P −0.025 f
Mechanical
speed
−0.01
Number of
poles
Approximate efficiencies of NEMA Clase B three-phase 60 Hz induction motors
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Efficiency of IM – NEMA B
(Influence of number of poles)
Efficiencies of standard Class B squirrel cage induction machines of one manufacturer
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Efficiency of IM – High Efficiency Motor
Efficiencies of high efficiency squirrel cage induction machines of one manufacturer
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Efficiency of PM Machine - Example
PM machines typically higher efficiency than Induction
motors because of magnet excitation
Source: Marathon Electric
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References
[P. B. Reddy, et.al., 2011], T. M. Jahns, “Scalability investigation of proximity losses in fractionalslot concentrated winding surface PM machines during high-speed operation,” in Energy
Conversion Congress and Exposition, 2011, pp. 1670–1675.
[P. B. Reddy, et.al., 2009], T. M. Jahns, and T. P. Bohn, “Transposition effects on bundle proximity
losses in high-speed PM machines,” in IEEE Energy Conversion Congress and Exposition, 2009,
pp. 1919–1926.
[K. Yamazaki, et.al., 2013], and Y. Kato, “Optimization of high-speed motors considering
centrifugal force and core loss using combination of stress and electromagnetic field analyses,”
Magn. IEEE Trans., vol. 49, no. 5, pp. 2181–2184, May 2013.
[M. Dems, et.al., 2014], and K. Komeza, “Performance Characteristics of a High-Speed EnergySaving Induction Motor With an Amorphous Stator Core,” IEEE Trans. Ind. Electron., vol. 61, pp.
3046–3055, 2014.
[J. Vrancik, 1968], “Prediction of windage power loss in alternators,” NASA Technical Note, 1968.
[W. Li, et.al., 2014], H. Qiu, X. Zhang, J. Cao, and R. Yi, “Analyses on Electromagnetic and
Temperature Fields of Superhigh-Speed Permanent-Magnet Generator With Different Sleeve
Materials,” IEEE Trans. Ind. Electron., vol. 61, no. 6, pp. 3056–3063, Jun. 2014.
[N. Bianchi, et.al., 2006], S. Bolognani, and F. Luise, “High Speed Drive Using a Slotless PM
Motor,” Power Electron. IEEE Trans., vol. 21, no. 4, pp. 1083–1090, Jul. 2006. H. Mizumoto and
Copyright@ Bulent Sarlioglu 2016
References
[A. Boglietti, et.al., 1992], P. Ferraris, M. Lazzari, and F. Profumo, “About the design of very high
frequency induction motors for spindle applications,” in Industry Applications Society Annual
Meeting, vol. 1, pp. 25-32, 1992.
[H. Mizumoto, et.al., 2010], and S. Arii, “Vibration control of a high-speed air-bearing spindle using
an active aerodynamic bearing,” in International Conference on Control, Automation and Systems,
2010, vol. 2, pp. 2261–2264.
[J. Asama, et.al,. 2013], Y. Hamasaki, T. Oiwa, and A. Chiba, “Proposal and Analysis of a Novel
Single-Drive Bearingless Motor,” Ind. Electron. IEEE Trans., vol. 60, no. 1, pp. 129–138, Jan.
2013.
[A. Looser, et.al., 2011], and J. W. Kolar, “A hybrid bearing concept for high-speed applications
employing aerodynamic gas-bearings and a self-sensing active magnetic damper,” in 37th Annual
Conference of the IEEE Industrial Electronics Society, 2011, pp. 1686–1691.
[A. Borisavljevic, 2013], “Limits, Modeling and Design of High-Speed Permanent Magnet
Machines”, no. ISBN-10: 3642334563. Berlin, Heidelberg: Springer Berlin Heidelberg, 2013.
[A. Borisavljevic, et.al., 2010], H. Polinder, and J. Ferreira, “On the speed limits of permanentmagnet machines,” Ind. Electron. IEEE Trans., vol. 57, no. 1, pp. 220–227, 2010.
[J. Ede, et.al., 2002], Z. Zhu, and D. Howe, “Rotor resonances of high-speed permanent-magnet
brushless machines,” Ind. Appl. IEEE Trans., vol. 38, no. 6, pp. 1542–1548, 2002.
[D. M. Ionel, et.al., 2006], M. Popescu, S. J. Dellinger, T. J. E. Miller, R. J. Heideman, and M. I.
McGlip, “On the variation with flux and frequency of the core loss coefficients in electrical
machines,” Industry Applications, IEEE Transactions on, vol. 42, no. 3, pp. 658–667, 2006.
Copyright@ Bulent Sarlioglu 2016
References
[C. Zwyssig, et.al., 2005], J. Kolar, W. Thaler, and M. Vohrer, “Design of a 100 W, 500000 rpm
permanent-magnet generator for mesoscale gas turbines,” Industry Applications Conference, IAS
Annual Meeting, vol. 1, pp. 253–260, 2005
[D. Gerada, et.al., 2009], A. Mebarki, R. P. Mokhadkar, N. L. Brown, and C. Gerada, “Design
issues of high-speed permanent magnet machines for high-temperature applications,” in
International Electric Machines and Drives Conference, 2009, pp. 1036–1042
[Z. Kolondzovski, et.al., 2010], P. Sallinen, and A. Arkkio, “Thermal analysis of a high-speed PM
machine using numerical and thermal-network method,” in International Conference on Electric
machines, 2010, pp. 1–6.
[W. Choi, et.al., 2013], S. Li, and B. Sarlioglu, “Core loss estimation of high speed electric
machines: An assessment,” in Industrial Electronics Society, IECON 2013 - 39th Annual
Conference of the IEEE, 2013, pp. 2691–2696.
[M. Albach, et.al., 1996], T. Durbaum, A. Brockmeyer, “Calculating core losses in transformers for
arbitrary magnetizing currents a comparison of different approaches”, in PESC 96 Record. 27th
Annual IEEE Power Electronics Specialists Conference, June 1996, vol. 2, pp. 1463-1468.
[J. Liu, et.al., 2002], T. G. Wilson, Jr., R. C. Wong, R. Wunderlich, F. C. Lee, “A method for
inductor core loss estimation in power factor correction applications”, in Proceedings of APEC
2002 – Applied Power Electronics Conference and Exposition, 2002, p. 439.
[Z. Gmyrek, et.al., 2008], A. Boglietti, and A. Cavagnino, “Iron loss prediction with PWM supply
using low-and high-frequency measurements: Analysis and results comparison,” Industrial
Electronics, IEEE Transactions on, vol. 55, no. 4, pp. 1722–1728, 2008.
Copyright@ Bulent Sarlioglu 2016
Appendix
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Additional Copper Losses
Copper loss-High frequency effect
Under high frequency operation, the conduction losses in electric machines
will increase significantly mainly because of two effects
Ø  Skin Effect
Skin effect is the tendency of an alternating electric current (AC) to become
distributed within a conductor such that the current density is largest near the
surface of the conductor, and decreases with greater depths in the conductor.
Ø  Proximity Effect
Proximity effect refers to the influence of alternating current in one conductor on
the current distribution in another, nearby conductor. The resulting current
crowding gives an increase in the effective resistance of the wire, and this
resistance increasing will be more significant at high frequency.
High excitation frequency resulting to high AC winding resistance
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Core Loss Estimation
Core loss estimation improvements
Ø  Variable Steinmetz equation coefficients [D. M. Ionel, et.al., 2006]
Pc = kh ( f , B) f Bα ( f , B ) + ke ( f , B) f 2 B 2 + kex ( f , B) f 1.5 B1.5
Ø  Modified Steinmetz equations to include harmonic contents
•
Improved Steinmetz Equation (ISE) [Z. Gmyrek, et.al., 2008]
•
Modified Steinmetz Equation (MSE) [M. Albach, et.al., 1996]
•
Generalized Steinmetz Equation (GSE) [J. Liu, et.al., 2002]
Ø  Finite element analysis (FEA)
Reduction of core loss
Ø  Design optimization [K. Yamazaki, et.al., 2013]
Ø  Thinner lamination
Ø  Better core materials [M. Dems, et.al., 2014]
Advanced techniques help estimating and reducing the core loss in high
speed electric machine
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Skin Effect
Skin effect
ρ
δ=
π fµ
Skin depth
resistivity of conductor [ohms*m]
permeability [H/m]
operating frequency [Hz]
where ρ is the resistivity of the
conductor, ω is the excitation angular
frequency, µ is the absolute magnetic
permeability of the conductor
Ø  For round wire, the relationship
between AC resistance and DC
resistance resulting from skin
effect is approximately
Rac ≈ Rdc (1 +
2r
δ
)
Where r is the radius of round wire
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Skin Effect (Cont’d)
Skin depth
Skin depth as a function of frequency for several materials
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Proximity Effect
•  Proximity effect refers to the influence of alternating current in one
conductor on the current distribution in another, nearby conductor.
•  The resulting current crowding gives an increase in the effective
resistance of the wire, and this resistance increasing will be more
significant at high frequency.
•  On the other hand, the proximity effect will be increased with the
number of wire layers
The right figure shows the AC to DC
resistance for a portion of a strip winding at
different frequencies (h/δ). It can be seen
that the ratio will be increased dramatically
with increasing number of layers and
frequencies. h is thickness of the
rectangular wire.
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Reduction of Proximity Effect
Alternative wire Transposition Configurations
Reduction of proximity effect
Ø  Litz wire
Ø  Transposed wire
1
Strand
transposition
0°
Bundle
transposition
0°
2
180°
180°
3
360°
180°
4
180°
360°
5
360°
360°
6
Single bundle twist by 180°
7
Single bundle twist by 360°
Type
Pac/Pdc losses ratio as a function of excitation frequency with
three different winding configurations (untransposed,
transposed, Litz wire)
[P.B. Reddy, et.al., 2011]
Losses in 16-strand cables for seven configurations
[P.B. Reddy, et.al., 2009]
Proximity loss, which becomes significant at high frequency, can be
reduce by using transposed wire or Litz wire
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Windage Loss Calculation
Windage loss-Salient pole
Ø  For electric machines which have salient pole rotor structure, an extra coefficient will
be added [J. Vrancik, 1968]
⎛H ⎞
⎟ + 2.2
⎝ r ⎠
K = 8.5 ⎜
Pw = π KCd ρ r 4ω 3 L
where H is the rotor pole height (m)
Ø  Unfortunately, this coefficient is valid only for the specific machine structure and
cannot be easily applied to other machines
Windage loss using CFD
Ø  Theoretical equations for windage loss estimation of machines which do not have
smooth rotor surface is not accurate, computational fluid dynamics (CFD) often
required
Windage loss estimation of salient pole rotor can hardly be done with
closed form solution, thus require CFD simulation
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Reduction of Proximity Effect
P. B. Reddy, T. M. Jahns, and T. P. Bohn, “Transposition effects on bundle proximity losses in high-speed PM
machines,” 2009 IEEE Energy Conversion Congress and Exposition, no. 1, pp. 1919–1926, Sep. 2009.
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Power Factor of IM – NEMA B
(Influence of Power and Speed)
Approximate power factors of NEMA Class B three-phase 60 Hz induction motors
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Power Factor of IM – NEMA B
(Influence of Number of Poles)
Approximate power factors of standard Class B induction motors
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