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Interior Magnet Permanent Motors & Drive Technologies
· 17
June 2000
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*Toshiyuki Kaitani and Hiroki Matsubara are with the Nagoya Works.
by Toshiyuki Kaitani and Hiroki
Matsubara*
Interior Magnet
Permanent Motors
& Drive Technologies
T
his report introduces the magnetic and
mechanical design of interior permanent mag-
net (IPM) motors, and describes an original
sensorless drive control system. The motors
exhibit excellent torque-speed characteristics
over a 1:10 operating region while tests at
7,200rpm indicate good torque-load response.
Since a large portion of the electrical power
used in manufacturing is spent driving electric
motors, improvements in motor efficiency are
essential to reducing power use. There is also
a continuing demand for motor size and weight
reductions. The IPM motors described here offer
advances on both of these fronts.
Motor Design
Fig. 1 shows the rotor design of a typical IPM
motor. The torque T on the d-q axis, which is
fixed with respect to the rotor magnets, is
given by the following:
T = Pn.[
a.iq + (Ld Lq) id.iq] ................. Eq. 1
where Pn is the number of pole pairs;
a is the
magnetic-flux leakage; id and iq are the d- and
q-axis components of the armature current;
and Ld and Lq are the d- and q-axis compo-
nents of the armature self-inductance.
The first term expresses the magnetic torque,
the second term the reluctance torque caused
by the inductance differential. Our smaller,
lighter, more efficient design derives from
effective use of the reluctance torque compo-
nent.
IPM motors offer many advantages over in-
duction motors:
s Overall efficiency is higher, since the rotor
losses are close to zero.
s Effective use of the reluctance torque main-
tains high efficiency at both low- and high-
speed extremes.
Permanent magnets
Lq
q axis
d axis
Ld
Core
Fig. 1 Basic elements of an IPM motor.
s The smaller losses permit a reduction in
motor thermal capacity, leading to lower
size and weight.
s The smaller motor size reduces the moment
of inertia, raising frequency with which
speeds can be changed.
s Use of flux weakening control based on
salient pole behavior supports a wider range
of speeds at any given output level.
IPM motors are also superior to surface per-
manent magnet motors: Use of salient pole
behavior permits sensorless control with ben-
efits of reliability, tolerance of environmental
extremes, and simpler maintenance. Further,
embedding the magnets in the rotor core yields
a simpler, stronger mount that supports high
operating speeds.
Design Methodologies
Extensive magnetic flux analysis was per-
formed to optimize the motor’s magnetic cir-
cuit, while structural analysis was used to
improve the mechanical design. The stator
incorporates high-density winding and manu-
facturing technologies.
Leakage flux components appear between
the permanent magnets embedded in the mag-
netic core material. This has been minimized
by thinning the core material between the
magnets so that saturation occurs. Saturation
limits the flux magnitude.
The core material must serve as a secure
mechanical anchor for the magnets while also
providing the desired magnetic behavior. This
requires a tradeoff between the core’s struc-
tural and magnetic properties.
Fig. 2 shows the magnetic flux distribution
of a typical IPM motor. The motor shape was
designed to optimize magnetic characteristics
and thereby minimize use of permanent mag- R & D P
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Mitsubishi Electric ADVANCE
18 ·
net material—one of the motor’s more costly
components. The design also reduces the cog-
ging torque.
Fig. 3 shows the predicted and measured
back EMF waveforms, indicating a high degree
of analytical accuracy. The ability to predict
the EMF waveform accurately has made it
possible to achieve a sinusoidal waveform that
minimizes loss.
Sensorless Drive Technology
Synchronous motors generally employ a posi-
tion sensor to support position-based control
algorithms. The authors have introduced a
sensorless position sensing method for this
application. The system permits use of an
inverter control system sharing the variable-
speed operation and easy implementation of
inverter-controlled induction motor drive sys-
tems.
The voltage formula for IPM motors is given
by the following:
Vd = R+ pLd Lq id + 0 ... Eq.2
Vq Ld + pLq iq
a
where Vd and Vq are the d and q components
of the terminal voltage, is the electrical
angular velocity, R is the armature resistance
and p represents the differential operator d/dt.
The Vd and Vq values are the sum of one
term proportional to the current and another
term proportional to the motor speed. Sensor-
less operation is achieved by controlling Vd
and Vq as follows:
Vd = Kd.id + Vcd
.......................... Eq.3
Vq = Kq.iq + Kv. 1
+ Vcq
where Kd and Kq are the resistance compen-
sation gain for the d- and q-axis, Kv is the back
EMF voltage compensation gain, Vcd and Vcq
Fig. 2 Magnetic flux distribution.
are the stabilizing voltages for the two axes,
and
1
is the output frequency.
The speed compensation term
T
is calcu-
lated from the torque transients when a speed
command * is applied, so that the following
expression for
1
yields results in stable motor
operation: 1
= * +
T .........................................................................
Eq.4
The speed fluctuation ratio can be held to zero
without sensor-based control by synchronizing
the motor rotation speed with the output fre-
quency and zeroing
T
under constant torque
conditions.
Estimating Rotor Position at Standstill
Achieving smooth startup of synchronous IPM
motors requires a known initial rotor position.
A system was developed that can make this
determination over a brief interval immedi-
ately preceding motor startup. Fig. 4 shows its
accuracy.
Fig. 4 Motor position detection at standstill.
0
60
120
180
240
300
360
0
60
120
180
240
300
360
Estimated position (deg
rees)
Actual magnetic pole position (degrees)
Fig. 3 Back EMF waveform.
Measured
Calculated R & D P
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· 19
June 2000
Performance Evaluation
Fig. 5 shows the motor torque vs. speed char-
acteristics for a 7,200rpm motor. The motor
exhibits excellent torque availability from 720~
7,200rpm, the typical 1:10 operating region.
Fig. 6 shows the motor’s speed and torque
response when 100% step torque loads are
applied during 7,200rpm operation. The motor
exhibits excellent stability.
Work is continuing toward developing still
smaller, more powerful and more efficient
motors, and applying these technologies in
commercial drive products.
-200
-100
0
100
200
2,000
4,000
6,000
8,000
Motor speed (rpm)
T
orque (percent of r
ated output)
Fig. 5 Torque vs. speed characteristics.
Speed
7,200rpm
Torque 0%
100rpm
/division
100%/division
2s/division
Regenerating
Powering
Fig. 6 Motor response to torque loading.