Post on 07-Apr-2018
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The Basics of Permanent MagnetMotor Operations
By George Holling
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Introduction
Introduction The physics of permanent magnets
Basic PM motor operating principles Basic motor parameters
Motor construction
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The Physics of Permament Magnets
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The Physics of Permament Magnets
a typical magnetization curve for a PM magnet is shown
here
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The Physics of Permament Magnets
the outmost outline of the magnetization curve is
for a single cycle of a fully magnetized magnet
as the operating point of the magnetic circuits
permeance PP line intersects with thedemagnetization curve the magnet weakens
the magnetization is not a single curve, but afamily of curves
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The Physics of Permament Magnets
the permeance
PPof the magnetic circuit determines theoperating point of the permanent magnet
inchesinfluxofdirectiontheparallelthicknessgapair:
1.5-1.1typically-factorreluctancemagnetic:
inchesinfluxofdirectiontheparallelcknessmagnet thi:
leakagefluxmagnetic:
g
m
F
g
mFP
=
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The Physics of Permament Magnets
plotting the permeance into the magnetization curve yield
the operating point of the magnetic circuit
idealistic assumption
no saturated steel
r>>1
ignores the effects of demagnatization
internal field from windings
demagnetization curve
magnet temperature
ignores the effect of the magnets temperature
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The Physics of Permament Magnets
the magnets flux changes with temperature reversible
irreversible
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The Physics of Permament Magnets
the magnets linear, reversible flux change as a function o
temperature is:
C)(peraturemagnet tem:
ationdemagnetizoftcoefficienlinear:tureat temperadensityfluxgapair:)(
tureat temperadensityfluxgapair:)(
)](1)[()(
00
00
=
T
TTB
TTB
TTTBTB
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The Physics of Permament Magnets
irreversible changes can occur in the magnet well below
its Curie temperature
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The Physics of Permament Magnets
the motors conductors can cause irreversible damage to
its magnet
the flux generated by an inductor in the magnet is:
inchesinfluxofdirectiontheparallelthicknessgapair:
inchesinfluxofdirectiontheparallelcknessmagnet thi:
circuitmagnetictheofpermeancethe:P
currentwindingthe:iconductorsofnumbertotal:
)(3
g
m
Z
gmP
iZH+
=
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The Physics of Permament Magnets
each PM motor therefore has
a thermal current rating due to wire constraints
an absolute peak current rating due to magnet constraints
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The Physics of Permament Magnets
most PM motors use one of the following PM
magnet materials
Alnico: Aluminum Nickel Cobalt
Fe3O4: Ceramic/Ferrite SmCo: Samarium Cobalt
NeFeBo: Neodymium Iron Boron
none is generally better or worse
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The Physics of Permament Magnets
a comparison of different magnet materials
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Basic PM motor operating principles
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Basic PM motor operating principles
PM motors operate on the principle that a force is
generated when current flows in an inductor that is placed
in a magnetic field
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Basic PM motor operating principles
force generated in a conductor in a magnetic field
rflux vectomagnetic:
flux)magneticular to(perpendicconductortheoflength:
conductortheinflowingcurrent:
vectorforcemechanical:
l
i
F
liF
m
m =
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Basic PM motor operating principles
this force generates torque in a rotary motor
conductortheinflowingcurrent:
conductorsofnumber:
length)(stacklengthconductoreffective:
radiuswindingaverage:
fluxmagnetic:
uerotor torq:
i
Z
l
r
B
T
iZlrBT =
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Basic PM motor operating principles
a conductor that moves in a magnetic field generates a
voltage
rflux vectomagnetic:
fieldmagneticthelar toperpendicuinductorofyvelocit:
voltageinduced:
0
x
y
l
xy
B
v
V
vlBdzBvV ==
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Basic PM motor operating principles
a rotary motor produces the back-EMF (Lenzs Law)
locityangular ve:
condutorsofnumber:
length)(stacklengthconductoreffective:
radius:
rflux vectomagnetic:
voltageinduced:
Z
l
r
B
V
ZlrBV =
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Basic PM motor operating principles
inductor loop in a magnetic field
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Basic PM motor operating principles
adding a mechanical commutator yields a brush PMDC
motor
adding two or more mechanically offset windings yields a
PM DLDC or PM BLAC motor
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Basic motor parameters
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Basic motor parameters
the torque constantK
t
A)/m(N
m)(N
=
=
ZlrBK
KT
t
t
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Basic motor parameters
the back-EMF (voltage) constantK
e
sec)(volt/rad/
(volt)
ZlrBK
KV
E
E
=
=
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Basic motor parameters
example of a typical speed/torque curve of a PM motor
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Basic motor parameters
the Safe Continuous Operating Area SCOA
the motor can safely be operated continuouslyanywhere in this region
the Safe Intermittent Operating Area SIOA
the motor may be operated in this region for shortperiods of time (typically < 1 min)
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Basic motor parameters
the electrical equation for the PM DC machine is:
++= Eaa KIRdt
dILV
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Basic motor parameters
the electrical equation for a single phase of the
PM BLDC/BLAC machine with sinusoidal phasecurrent is:
( ) KIRLV Eaa
+
+
= )360
2sin(
360
2)
360
2sin()
360
2cos(
360
2
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Basic motor parameters
the power balance for the PM DC machine is:
++++
++
=
++
=
)()()2
()2
(
)2
(
22
22
22
LmLM
E
TTDDJ
dt
dIR
IL
dt
dP
IKIRIL
dt
dP
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Basic motor parameters
the power analysis reveals that:
lossesfriction:)(
lossesdamping:)(
energymechanical:)2
(
heat-windingstheinlossescopperelectrical:
energyionmagnetizat:)2
(
2
2
2
2
+
+
>
Lm
LM
TT
DD
J
dt
d
IR
IL
dt
d
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Basic motor parameters
the power supplied to the load is:
-2
= mMEmech TDIKP
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Basic motor parameters
the maximum continuous torque allowed is:
(A))(
25maxcont
=
RTR
TTI
rise
Crise
the maximum continuous current allowed is:
m)(N)(
)( 25maxcont
=
RTR
TTTKT
rise
Criseriset
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Motor construction
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Motor construction
the field in the stator poles changes
continuously, thus we must use laminated steel
to reduce eddy current losses
the backiron to support the PM flux is relatively
constant and solid steel can be used
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Motor construction
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Motor construction
motor steel choices
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Motor construction
motor windings are generally copper wire with
insulation (magnet wire)
magnet wire comes in different grades temperature
B, F, H, C
insulation
double, triple
voltage rating
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Motor construction
the resistance of the magnet wire changes with (the
wires) temperature
C0.0039/isvaluetheecopper wirfortcoefficienthermal:
C)(retemperatuwinding:
(ohm)C25typicallyre,temperatureferenceatresistance:
]1[
0
00
+=
T
)R(T
)(T - T)R(TR(T)