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JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
ELECTRONICS AND COMMUNICATION ENGINEERING
DESIGNING SPACE VECTOR CURRENT
CONTROLLERS FOR PMSM DRIVES
1 PROF.
VINOD J. RUPAPARA. 2 PROF. M.V.MAKWANA
1 Assistant
2 Assistant
Professor & P.G. Student, Power Electronics Dept, L. E. College, Morbi
Professor, Power Electronics Dept, Om Shanti Engineering College, Rajkot
[email protected], [email protected]
ABSTRACT : AC induction motor has lower efficiency and poor performance. PM
synchronous motors (PMSMs) can achieve the servo performance characteristics of dc
machines. This requires the PMSM to be vector controlled with additional tight current
control. An experimental evaluation of hysteresis, delta, and ramp comparison and space
vector current controllers is made, or which circuits are given.
Keywords—PMSM, hysteresis, delta, space vector, PSIM
I.
INTRODUCTION
Permanent magnet ac motors have the advantage of
not requiring any magnetizing current, and hence can
operate at a higher power factor and efficiency than
an induction motor in the fractional to 30 kW
regions. Inverter-fed PM machines do not need slip
rings or brushes; hence the reliability of the motor is
higher than that of a wound rotor synchronous motor.
The use of the permanent magnets tends to reduce the
weight compared to other motors of equivalent power
output. This leads to an increased torque to inertia
ratio and power density [1], which make PM ac
motors suitable for a variety of applications including
robotics and aerospace [1]
Figure 1basic block diagram of vector controlled
PMSM drive.
A current controlled inverter is required to provide
the dc machine servo characteristics, however the
different current control strategies available require
investigation as their performance differs over a
range of operating conditions. An analytical study of
hysteresis controllers has been performed [5]
While the space vector controller is described in [6].
The ramp comparison controller is modeled on a
computer in [3] and the delta controller discussed in
[4]. This paper uses two criteria of current controllers
to evaluate them as a function of operating
conditions. The inverter transistor average switching
frequency and the rms current error have been chosen
as readily accessible parameters by which the current
controllers can be evaluated and compared. To
compare the controllers with each other, the motor is
run at a constant speed and the controller parameters
adjusted to produce the same rms current error. Due
to the complexity of the controllers, only the constant
torque operating condition is considered in this paper.
II. VECTOR CONTROL OF A PMSM
Vector control enables ac motors to obtain
performance characteristics similar to dc machines.
This control technique uses the instantaneous rotor
position to calculate the required stator currents
which are oriented at some specified angle with
respect to the rotor. Current controllers are used to
force the actual current to follow the commanded
current.
Under usual operating conditions, the magnetic flux
of the permanent magnets remains constant.
Therefore by controlling only the stator currents, the
magnet flux in the air gap and stator can be
controlled. The rotor position is used to orientate the
stator currents at any required phase angle with
respect to the rotor flux, and hence with respect to the
back-emf [9]. For a non-salient motor (as used in this
paper), the most efficient operation occurs when the
stator flux is perpendicular to the rotor flux, that is
when it is in phase with the back-emf.
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JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
ELECTRONICS AND COMMUNICATION ENGINEERING
III.
TYPES OF CURRENT CONTROLLERS
A permanent magnet synchronous machine has a
sinusoidal back-emf, and hence for smooth torque
generation, the stator currents must also be
sinusoidal. Of the various methods of non-linear
current control, the hysteresis method is one of the
simplest Conceptually. It also has the advantage of a
fast transient response compared to other methods
such as delta or ramp comparison. In the discussion
to follow, the current error is defined as:
AI - I - I*, where I - actual current
I* = commanded current
III.I HYSTERESIS CURRENT CONTROL
The hysteresis controller compares the actual phase
current with the commanded, and if the magnitude of
the error is greater than a preset level, the inverter leg
is switched appropriately [10], although the actual
current also depends on the currents in the other
phases. This is shown for a single phase in figure 2.
Figure 2 single phase commanded current with
hysteresis band.
For a given phase, the transistor that requires to be
switched is dependent on both the error sign and the
sign of the commanded current (I*). For a hysteresis
bandwidth of h, the algorithm used for the inverter
(figure 3) leg of T1/T4 and individual current control
in each phase is:
if I*>=O and I>(I* + h/2) then turn off T1
if I*bO and I<(I* - h/2) then turn on T1
if I*<O and I>(I* + h/2) then turn on T4
if I*<O and I<(I* - h/2) then turn off T4
This is non-complementary switching, which has
been used for all the controllers. The disadvantage of
the hysteresis current controller is that the switching
frequency is dependent on the motor parameters,
speed and dc bus voltage.
Figure 3 inverter circuit
III.II DELTA CURRENT CONTROL
The delta controller [6] uses a fixed frequency
sampling of the current with zero hysteresis
bandwidth logic, as shown in figure 4, and control is
only applied at those sampling times. This has the
advantage of limiting the inverter switching
frequency, but the disadvantage of a poorer transient
response and current error when compared to the
hysteresis controller.
Figure 4 delta current controller
III.III SPACE VECTOR CURRENT CONTROL
The space vector [4] is defined as:
I = 2/3(Ia+aIb+a21c), a=e(j2n/3)……………(1)
The two axes used are the real axis, which is
coincident with the phase a-axis, and an axis
orthogonal to the a-axis. The effect is the same as
vector ally adding the phases. The resultant can be
used to determine the inverter gating signals, taking
into account all the current errors simultaneously.
The space vector current error is defined as:
- AI = 1. -I*………………………………….. (2)
A space vector current controller compares the space
vector current error with hysteresis boundaries, as
shown in figure 5, and then applies the voltage vector
nearest to the current error vector. It is shown in the
appendix that due to the absence of a neutral, the
dependency between the phases causes the space
current error vector to exceed the hysteresis boundary
whenever the largest individual phase error exceeds
2/3
of
the
hysteresis
boundary.
Figure 5 space vector diagram of the current error,
showing the vector hysteresis bands
III. DESIGN AND IMPLEMENTATION OF
THE
PMSM
VECTOR
CURRENT
CONTROLLERS
III.I
SPACE
VECTOR
CONTROLLER
IMPLEMENTATION
Implementing space vector control is a matter of
using the standard hysteresis controller (for zero
hysteresis bandwidth) with a comparator to select the
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JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
ELECTRONICS AND COMMUNICATION ENGINEERING
largest error and compare it to the hysteresis
bandwidth. This can then operate a latch. The circuit
used is given in figure 7. The voltage drops across the
diodes are taken into account by the provision of
biasing voltages.
Figure 6 space vector current controller
The diodes are used to compare the current errors,
such that the most positive error will appear at F and
the most negative error at G. The negative error is
inverted and is then compared to the positive error by
another pair of diodes. The larger of the two is now
compared to the hysteresis band. Should either of the
two exceed the hysteresis band, the latch is enabled
to update the inverter gating signals from three other
phase current error comparators with zero hysteresis.
The advantage of the space vector controller over the
hysteresis is that no zero voltage vectors can be
applied.
IV. CURRENT CONTROLLER RESULTS
To find the result of current controller, a single speed
and commanded current was chosen. The controller
parameters were adjusted to produce the same rms
current error. The speed of the motor affects the
controller characteristics due to the back-emf.
The average inverter transistor switching frequency
was then measured. This is relevant to inverter
efficiency and also to rating the transistors
adequately. The overall ability of the current
controller to track the current can be described by the
rms current error, which is defined 2s the rms of the
difference between the actual and commanded
currents
IV.I SPACE VECTOR CURRENT CONTROL
The commanded current was set at 1.10 A (rms), with
the bandwidth set at 0.58 A. The bandwidth is
defined from zero.
Motor speed
1250
1500
avg. switching
frequency
(kHz)
0.726
3.32
2.80
5.00
IV.II CONSTANT SPEED WITH VARIABLE
COMMANDED CURRENT
Motor speed of 1500 rpm was chosen, which was
maintained for different commanded currents by
adjusting the load on the motor. The commanded
current was increased in all cases from approximately
1.4 A (rms) to 4.3 A (rms).
Both the hysteresis and the space vector controller
showed no clear trend in the average switching
frequency and the current error as a function of the
commanded current, both remaining approximately
constant.
The delta controller shows a pronounced effect as the
commanded current increases, with the-switching
frequency decreasing and the current error increasing.
The reason for this can be seen in figures 7 & 8.
Figure 7 Commanded and actual delta controlled
current with I* = 1.73 A (rms), 1490 rpm
Figure 6 shows that the current often reaches zero
before increasing. At larger current amplitudes
(figure 7, note the change in scale), there is further
for the current to fall and therefore the current error
will increase.
As the commanded phase current increases from 0.9
A (rms) to 4.24 A (rms), the ramp comparison
controller average switching frequency decreases
from 6.5 kHz to 4.5 kHz and the rms current error
increases from 0.25 A to 0.33 A.
rms ∆i
(A)
0.486
0.652
0.609
0.750
Figure 8 Commanded and actual delta controlled
current with I* = 4.28 A (rms), 1500 rpm.
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JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
ELECTRONICS AND COMMUNICATION ENGINEERING
V. CONCLUSION
This paper has presented the study of three different
current controllers suitable for vector controlled
drives. They include the hysteresis, delta and space
vector controllers. An evaluation based on average
switching frequency and rms current error for space
vector type has also been included.
The hysteresis controller has a reduced average
switching frequency near the waveform peaks due to
the increase in the magnitude of the back-emf, and
for the same reason, as the motor speed increases, the
average switching frequency over the entire
waveform decreases. Negotiable effect is produced
by changes in the magnitude of the commanded
current waveform. The space vector controller is
similar, but produces no zero voltage vectors. At a
motor speed of 1500 rpm, with an rms current error
of 0.49 A, the vector control controller demands the
suitable switching frequency from the inverter.
REFERENCES
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[11] Shenzhen - sunfar electric technology company
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