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8/11/2019 Inverter Driven Synchronous
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Inverter-D riven Synchronous
Motors for Constant Power
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nverter-driven synchronous motors (SMs) are
widely used in high-performance variable-speed
I
rive systems. The typical schematic of an SM
drive system is shown in Fig. 1. The armature current
vector of an SM is decided according to the command
(torque or speed) and the current-regulated voltage-
source pulse width modulated (PWM) inverter
drivesSMso that the instantaneous armature currents
follow their commanded values. The stator and wind-
ing of SMs are the same asa standard induction motor,
but the rotor configuration is different depending on
the type of machine. Therefore, SMs can be modeled
by the same d-q reference frame circuit and generally
analyzed, where the machine parameters in the d-q
equivalent circuit depend on the rotor configuration.
Several rotor configurations of SMs have been
reported for high-performance drives
[
1-61, Fig. 2
shows the typical rotor cross-sections of SMs. SMs
can be classified according to the torque production
(excitation torque and/or reluctance torque) and
the machine parameters (per unit open circuit volt-
age
Eo,
per unit d and q-axis reactances Xd, Xq):
SPM ( E o 0, Xd = X,): the surface projecting
permanent magnet synchronous motor
(SPM) shown in Fig. 2(a), in which the mag-
nets are projected from the surface of the
rotor, is a non-salient pole machine (Xd= X,),
and as a result only the magnet torque is
produced.
SynRM ( E o
=
0, Xd
Xd
(Motor
2,
# 3 ,
# 4
and 7), the
operating limits exist, but the maximum torque
Ti
derived by substituting
(7)
and (8) nto ( 3 ) ,and the
output power at low speeds are larger than that in
the case of
Eo =
Xd. Therefore, SMs have to be
designed
so
that t he machine parameters satisfy the
condition ofEo2
Xd ,
which corresponds to the area
below the curve (c) in Fig.
3.
If the large constant
power speed region is desired, the motor design
with
Eo
= X d
is required. This
is
possible in PM
motors , for example the axially-laminated IPM
motor with
Eo
Xd is reported in
147.
In case of an
ideal SynRM, in which a saliency ratio
p ( =
X,/Xd)
is infinity and the machine parameters are
E o
= Xd
=
0 and X, = & as derived from
( 2 5 ) ,
he maxi-
mum torque is l i h n the constant torque region,
and the maximum output power
is 1 O
at infinity
speed. But, such design is impossible, and as
a
result the power capability of SynRM cannot ex-
ceeds that of the optimal designed PM motor.
The maximum operating speed 0 n the PM
1
o5 1 ~ , i , , , , i , ~ , , i , , , , , , , , ~ ~ ~ i r j
\ '
Maximum operating speed wc pu)
Fzg
5 Operatzng charactertsttcs as a unction of maxzmum
operurzng speed W
E Industry Applications Magozine Novernher/Deremher 1996
motor with
E o
2Xddepends only on Eo-Xdas given
by
(15).
From Fig.
4,
t can be found that Motors
2, 3
and
#4,
in which
Eo
- X d
is
the same
(We
is
the same) have almost the same output power
versus speed characteristics even if they have differ-
ent machine parameters. Therefore, it can be con-
cluded that the output power versus speed
characteristics depend only on the parameter of
E o
-
Xd( =
1/0,
in per uni t form. It is very interesting
that the several combinations of machine parame-
ters such as
E o ,
Xd>and X,, which depend on the
rotor configuration, can be selected to achieve the
desired output characteristics.
Fig. 5 shows the operating Characteristics as a
function of the maximum operating speed
(=
l I ( E o - X d ) ) ,
where
amp
s the speed producing the
maximum output power. The power factor be-
comes unity and the output power becomes
1.0
when
w
=
amp.
he constant power speed region
K,pr
and
amp
ncrease almost linearly as ncreases.
On the other hand, the maximum torque
Ti
de-
creases extremely in the range of 0 10 and
reaches
0.71-0.72
when
Ci),
becomes infinity
(Eo-
X d
= 0),
which
is
almost the same value as the ideal
SynRM with an infinity saliency ratio. From Fig.
5 ,
Kcpr
can be expressed by (26) as a function of
We.
Kip?
0.7Wc
-1 ( 2 6 )
If the machine parameters of
E o
and Xd are
given, the maximum torque in t he constant torque
region Ti ) nd the constant power speed region
can be found according to Fig.
5
and
(26).
In order to produce the largest torque
Ti,
the
condition
of = 1.0
is
opt imum , where the ideal
machine parameters are
Eo =
1 O
Xd
= X,
= 0,
and
Ti
s
1.0;
however, this
is
impossible. Therefore,
the machine design with large
E o
and small Xdand
X, is desired
for
a
high-torque machine. On the
other hand, the optimum value
of E o
- Xd has to be
designed for
a
wide constant power operation.
From ( 2 6 ) , he optimum value of EO
xd
can be
found according to the desired constant power
speed
region.
0.7
KP+ 1
Eo-Xdf-
(27)
As shown in the previous chapter, there are
many combinations of machine parameters even if
the value of
Eo -
Xd and the output power versus
speed characteristics are the same. Fig. 6shows the
combinations of machine parameters and the con-
tent rat io of excitation torque
Te
to the total torque
Ti
for
0 = 5 ,
10,
20,
and infinity. The relation-
ships between Xd and
Eo
is linear, which is given
8/11/2019 Inverter Driven Synchronous
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by
Xd
= Eo -l /oc. , as well as the saliency ratio
increases as E o decreases and the relationship be-
tween
X,
and Eo
is
almost independent of the value
of
0
except for large
Eo
region. The reluctance
torque becomes dominant
as
compared with the
excitation torque as
E o
decreases
( E o < 0 .5 ) .
The
right end of the curves in this figure corresponds
to
SPM
(p = 1) and the other corresponds to
Hybrids (p > 1). The motor with
p
=
2-5
corre-
sponds to the configuration shown in Fig. 2(b)-(d),
and the motor with saliency ratio over
5
corre-
sponds to Fig.
2
(e)-(g).
Now we turn to design of PM motors for wide
constant power operation. In order to obtain the
large constant power speed region,
Xd
has to be
increased as both
Eo
and
CO,
increase. In this case, if
the permanent magnet material
is
the same, the
large volume and thickness of permanent magnet
is required for the large magnet flux-linkage; as a
result the equivalent air-gap length in the d-axis,
including the permanent magnet thickness, in-
creases because the permanent magnet permeabil-
ity is very close to
Po;
thus
l i d
decreases. This is
contrary to the requirement of the opt imum design
for a large constant power speed region. Therefore,
the design with large
Eo
and large
Xd
s difficult.
If high remanence permanent magnets such as
rare-earth cobalt and neodymium-iron-boron are
used, the thickness of the permanent magnet can
be reduced, and as a result Xdniay increase. I t seems
that this design can apply to the
IPM
motor with
the rotor shown in Fig. 2(c), and the constant power
speed region from
2
to
4
may be obtained
E7-91.
If
d s
designed as low as possible, a high X,
and a large saliency ratio are rrquired. This may be
possible if the rotor is axially laminated, in which
case a saliency ratio up to 10 may be obtainable
11-61.In
this case, the low
E o is also
required; as
a
result, the low cost and low remanence permanent
magnets such as ferrite magnets are available,
where the irreversible demagnetization has to be
minded, and the constant power speed region over
5
is obtainable {3,
41.
The PRl motors with small
E o
have an advantage that the overexcitation
threshold speed
(=
l /Eo)
becomes large. The opera-
tion over the overexcitation threshold speed, in
which the motor back-EMF exceeds the dc source
voltage, may cause problems at the unexpected
situation (wrong switching
of
inverter, interrup-
tion)
[7}
The design of PM motors including the design
of rotor configuration, the selection of permanent
magnet materials, and their dimensions has to be
carried out
so
as to satisfy the desired output char-
acteristics such as the rated torque, the rated power,
the base speed, the constant power speed region,
and
so
on, according t o t he applications. The results
of examinations in th is article
as
shown in Figs.
5 ,
6,
and
(27)
will be useful for this procedure.
Fig. 6 .
Combinations
of
machine parameters an d content
rutio of
excitation torque.
U)
D- and q-axir reactances.
h i
Saliency
ratio and content ratio of excitation torque
t o
total torque
in
constant torque
region.
Fig. 7 .
Outpu t power versur .speed characteristicsoJprototype P M
INOtOrJ .
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41
fEf Industry Applicutions Muguzine November December
1996
8/11/2019 Inverter Driven Synchronous
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as the speed increases; however, both charac-
teristics are almost the same in the practical
operating region (constant torque region and
constant power region). Fig.
7
shows the
ability of the examinations in this article.
References
111 T Sebastian and
G R
Slemon, Operat ing Limits of
Inverter-Driven Permanent Magnet Motor Drives,
IEEE Trans Ind
Appl,
ol 23,pp 327-333, Mar iApr
1987
121T.M Jahns,
G
B Kliman, and T W Neumann, interior
Permanent-Magnet Synchronous Motors for Adjustable-
Speed Drives,
IEEE
Trans Ind Appl, vol 22, pp
738-747,JulyiAug 1986
an IPM Suitable for Field-Weakened Operation, Proc
ICEM90, pp 1059-1065, 1990
E41 W
L
Soong, D
A
Staton, and T J
E
Mille r, Design of
a New Axially-Laminated Interior Permanent Magnet
Motor,
Proc
IEEE IAS Ann Meet pp 27-36, 1993
157A Fratta, G P Troglia, A Vagati and F Villata, Evalu-
ation of Torque Ripple in High-Performance Synchro-
nous Reluctance Machines, Proc IEEE
I A S
Ann Meet ,
167 T Matsuo and T A Lipo, Current Sensorless Field
Oriented Control of Synchronous Reluctance Motor,
Proc IEEE IAS Ann
Meet
pp 672-678, 1993
131A Fratta, A Vagati and F Villata, Design Criteria
of
RatedTorque
T Nmy
Rated
Power P W)+
*
a i
base
speed pp
163-170,
1993
jlj
Expe r imen ta l Resu l t s
177 T M
Jahns, Flux-Weakening Regime Operation
of
an
The parameters of prototype PM are Interi or Permanent-Magnet Synchronous Motor Drive,
an
SPM
motor with the rotor shown in Fig. 2(a),
Fig. 2(c). They are indicated by open circles in Fig
3
The experimental and simulated output power
listed in Table 3 . The prototype
PM
motor a 1s
ZEEE TrdnJ
Ind APPl, vel 23, PP 681-689 , JulyiAug
1987
181B K
Bose, A High-Performance Inverter-Fed Drive Sys-
tem
of
an interior Permanent Magnet Synchronous Ma-
chine,
IEEE
Truns
Ind Appl,
ol 2 4 , pp 987-997,
Nov iDec 1988
and
b
an IPM
motor
with the rotor shown
In
versus speed characteristics are shown in Fig.
7 .
The
solid curves show the maximum power capability,
and are strictly calculated considering the armature
resistance. The broken curves show the calculated
results based on the equations in this article, in
which the armature resistance is neglected and the
ceiling voltage given by subtracting the maximum
resistance drop RI,, SV) from the actual ceiling-
voltage (50V) is used instead of the actual
Vam.
he
differences between both simulated results with
and without consideration of the resistance appear
at high speeds, in which the power factor decreases
19) S.
Morimoto,
T.
Ueno, M. Sanada,
Y .
Takeda, T. Hirasa,
and
A.
Yamagiwa, Effects and Compensation of Magnetic
Saturation in Permanent Magnet Synchronous Motor
Drives,
Proc. IEEE IAS A n n . M e a . ,
pp. 59-64,
1993.
110) R.F. Schiferl and T.A. Lipo, Power Capability of Salient
Pole Permanent Magnet Synchronous Motors in Variable
Speed Drive Applications,
IEEE Tram.
Ind.
Appl ,
vol.
26,
pp. 115-123,Jan./Feb. 1990.
111) S.
Morimoto,
Y.
Takeda, T. Hirasa, and
K.
Taniguchi,
Expansion of Operating Limits for Permanent Magnet
Motor
by
Current Vector Control Considering inverter
Capacity, IE E E Trans. Ind Appl.,ol. 26,
pp.
866-871,
Sept./Oct. 1990.
I
Industry Appl icntionsMngnzine November December
1996