Basic Motor Magnetics and Principles

8/6/2019 Basic Motor Magnetics and Principles

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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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a typical magnetization curve for a PM magnet is shown

here


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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 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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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 magnets flux changes with temperature reversible

irreversible


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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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irreversible changes can occur in the magnet well below

its Curie temperature


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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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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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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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a comparison of different magnet materials


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Basic PM motor operating principles


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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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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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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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a conductor that moves in a magnetic field generates a

voltage


fieldmagneticthelar toperpendicuinductorofyvelocit:

voltageinduced:

0

x

y

l

xy

B

v

V

vlBdzBvV ==


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a rotary motor produces the back-EMF (Lenzs Law)

locityangular ve:

condutorsofnumber:

length)(stacklengthconductoreffective:

radius:


voltageinduced:

Z

l

r

B

V

ZlrBV =


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inductor loop in a magnetic field


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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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the torque constantK

t

A)/m(N

m)(N

=

=

ZlrBK

KT

t

t


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the back-EMF (voltage) constantK

e

sec)(volt/rad/

(volt)

ZlrBK

KV

E

E

=

=


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example of a typical speed/torque curve of a PM motor


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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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the electrical equation for the PM DC machine is:

++= Eaa KIRdt

dILV


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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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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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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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the power supplied to the load is:

-2

= mMEmech TDIKP


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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)

Basic Motor Magnetics and Principles

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