Transcript 幻灯片 1

4 Electrical actuation systems
4.1 Electrical systems
4.1.1 Switches
Types of switches:
Mechanical switch, Electromechanical switch,
Electronical solid-state device.
Mechanical switch—
Switch bounce.
4.1.2 Mechanical switches
4.1.3 Relays
4.1.4 Solid-state switches
Diode Thyristors, Triacs, Transisters.
4.1.5 Solenoids
solenoid can be used to provide electrically operated actuators.
IGBT – Insulated or Isolated Gate Bipolar Transistor
IGBT combines the positive attributes of BJTs and MOSFETs.
BJTs have lower conduction losses in the on-state, especially in devices with
larger blocking voltages, but have longer switching times, especially at turn-off while.
MOSFETs can be turned on and off much faster, but their on-state conduction losses
are
larger, especially in devices rated for higher blocking voltages.
IGBTs have lower on-state voltage drop with high blocking voltage capabilities in
addition to
fast switching speeds.
NPN IGBT
PNP IGBT
NPN IGBT
PNP IGBT
(+) Collector
(+) Collector
(+) Base



(-) Collector

(-) Base
(-) Emitter





(+) Emitter

(+) Base



(-) Collector

(-) Base
(-) Emitter





(+) Emitter

4.2 Stepping motors
Producing rotation through equal angles, the so-called steps,
for each digital pulse supplied to its input.
Solid-state electronics is used to switch the d.c. supply
between the pairs of stator windings.
Terminology: Holding torque Pull-in torque pull-out torque
Pull-in rate pull-out rate
Slew range.
Driving circuit of step motor
Homework: page 128, problem 1,2,3,5
4.3 Motors
Electromagnetic force F  BIL
(EMF)
1. Where L is the length of conductor in a magnetic field, I is
the current of the conductor, and B is flux density of the
magnetic field.
2. e  dΦ / dt is the back e.m.f, Φ is the magnetic flux.
Electric Motors
•
Basic types
 DC Motors: speed and rotational direction control
via voltage
=
=
=
=
Easy to control torque via current
low voltage
linear torque-speed relations
Quick response
=
=
=
speed fixed by AC frequency
low torque at low speed
difficult to start
 AC Motors: smaller, reliable, and cheaper
DC Motors: Principles of Operation
A
wire carrying current experiences a
force in a magnetic field.
F  i ( l  B)
Induced force
Current
(amp)
Magnetic Flux Density (Tesla)
Length of wire in the direction
of i (m)
IB
I  B  I B sin q
I
q
B
Electromagnetic Force
A
wire carrying current in a magnetic
field.
B
i
F = ilB
(l = wire length)
Electromagnetic Force
• The force is perpendicular to both the
magnetic field and current
Electromagnetic Force

A voltage is induced in a wire moved in a
magnetic field
 generators
eind  ( v  B)  l
Induced
voltage (volt)
Velocity of wire (m/s)
+++
eind
l
eind is also called electromotive
force (EMF感应电动势)
B
v
---
eind =vBl
(l = the wire length)
Principle of Electric Motors
• Fundamental principle behind electric motors
Current running through coil in magnetic field
experiences forces that cause it to rotate
Fundamental characteristics of DC Motors
Stator
S
N
N
S
N
N
End view
Time 0
N
S
Stator
Coils
S
N
N
S
Rotor
S
N
S
Stator
Coils
S
S N
Stator
Rotor
N
S
S
N
S
N
End view
Time 0+
Shifting magnetic field in rotor causes rotor to be forced to turn
Nature of commutation



Power is applied to armature
windings
 From V+
 Through the +brush
 Through the commutator
contacts
Rotor
 Through the armature (rotor)
winding
 Through the – brush
 To VRotation of the armature moves the
commutator, switching the armature
winding connections
Stator may be permanent or
electromagnet
V+
Stator
V+
Brush
Assembly
N
S
N
S
Comutator
N
V-
Stator
V-
Diagram of a Simple DC Motor
Commutator
Armature Armature conductors Field coil Field pole
Commutator Brush Brush wear Brushless
Excitation of motors:
(a) Series: highest starting torque and greatest no-load speed.
(b) shunt: lowest starting torque, a much lower no-load speed
and has good speed regulation.
(c) compound: high starting torque
and good speed regulation.
(d) separate:a special case
of the shunt wound motor and
ease to revert the direction.
The speed of such d.c motors can be
changed by either changing the
armature current or the field current.
The variable voltage is often obtained
by an electronic circuit.
Shunt Field
DC motor wiring topologies
100
Sh u n
Series Field
t
80
60
Se
40
Co
rie
s
Series
mp
o
un
d
Series Field
20
0
0
100
200
300
Percent of Rated Torque
400
Shunt Field
Percent of rated Speed
120
Shunt
Compound
Permanent magnet DC motors
Percent of rated Speed
120
100
Pe
80
rm
an
en
tM
ag
Permanent
Magnet
ne
t
60
40
20
0
0
100
200
300
Percent of Rated Torque
400
Permanent
magnet
poles
Permanent Magnet DC Motors
 Have permanent magnets rather than field windings but with
conventional armatures. Power only to armature.
 Short response time
 Linear Torque/Speed characteristics similar to shunt wound motors.
Field magnetic flux is constant
 Current varies linearly with torque.
 Self-braking upon disconnection of electrical power
 Need to short + to – supply, May need resistance to dissipate heat.
 Magnets lose strength over time and are sensitive to heating.
 Lower than rated torque.
 Not suitable for continuous duty
 May have windings built into field magnets to re-magnetize.
 Best applications for high torque at low speed intermittent duty.
 Servos, power seats, windows, and windshield wipers.
4.4 A.C. motors
Single phase (low power) and Poly phase (high power)
Induction (Cheaper) and Synchronous motors
Single-phase squirrel-cage induction motor: not self-starting, the rotor
rotates at a speed determined by the frequency of the alternating
current applied to the stator (synchronous speed), there is difference
between the rotor speed and synchronous speed (slip).
Three-phase induction motor: there is rotating magnetic field
which completes one rotation in one full cycle of the current, selfstarting, the direction of rotation is reversed by interchanging any
two of the line connections.
Synchronous motors: its rotor is a permanent magnet, not
self-starting, used in the case when the precise speed is
required.
Inducing magnetism in the rotor


Difference between
angular velocity of
rotor and angular
velocity of the field
magnetism causes
squirrel cage bars to
cut the field magnetic
field inducing current
into squirrel cage
bars.
This current in turn
magnetizes the rotor
Rotor
N
S
Difference in
rotation of field
magnetism and
rotor rotation
% of Full-Load Torque
Torque/speed curve
250
Pull-up Torque
Locked rotor
torque
Breakdown
Torque
200
150
Full-Load Torque
100
Slip (Full load)
50
0
0
20
40
60
80
% of Synchronous Speed
100
 A.C. motor is cheaper, more rugged, reliable and maintenance free.
 Speed control of A.C. motor is more complex than with d.c. motors.
4.4.1 Brushless permanent magnet d.c. motors
High performance, reliability
and low maintenance.
Current-carrying conductors
are fixed and the magnet moves.
The current to the stator coils is
electronically switched, the
switching being controlled by the
position of the rotor so that there
are always forces acting on the
magnet causing it to rotate in
same direction.
4.4.2 Speed and position control of D.C motors
PWM modulation：
Position servo system of D.C. motor
4.4.3 A.C servo system
D.C./A.C. Inverter
PWM Variable Frequency Drives
AC to DC converter and a DC to AC converter (inverter)
 Inverter frequency and voltage output can be varied to allow
motor speed to be varied.
 Very efficient and cost effective variable speed
650 V
Control Logic

L1
480V L2
L3
Rectifier
Filter
M
Inverter
SPWM Modulation
改变脉冲宽度调节平均电压
Speed servo control of synchronous A.C. motor
Direct Vector Control
Introduction
Scalar control of ac drives produces good steady
state performance but poor dynamic response.
This manifests itself in the deviation of air gap flux
linkages from their set values. This variation
occurs in both magnitude and phase.
Vector control (or field oriented control) offers more
precise control of ac motors compared to scalar
control. They are therefore used in high
performance drives where oscillations in air gap
flux linkages are intolerable, e.g. robotic actuators,
centrifuges, servos, etc.
Introduction (cont’d)
Why does vector control provide superior
dynamic performance of ac motors compared
to scalar control ?
In scalar control there is an inherent coupling
effect because both torque and flux are
functions of voltage or current and frequency.
This results in sluggish response and is prone
to instability because of 5th order harmonics.
Vector control decouples these effects.
Torque Control of DC Motors
There is a close parallel between torque
control of a dc motor and vector control of an
ac motor. It is therefore useful to review
torque control of a dc motor before studying
vector control of an ac motor.
Torque Control of DC Motors (cont’d)
A dc motor has a stationary field structure
(windings or permanent magnets) and a
rotating armature winding supplied by a
commutator and brushes. The basic structure
and field flux and armature MMF are shown
below:
Torque Control of DC Motors (cont’d)
The field flux f (f) produced by field current
If is orthogonal to the armature flux a (a)
produced by the armature current Ia. The
developed torque Te can be written as:
Te  Kt' I a I f
Because the vectors are orthogonal, they
are decoupled, i.e. the field current only
controls the field flux and the armature
current only controls the armature flux.
Torque Control of DC Motors (cont’d)
DC motor-like performance can be achieved
with an induction motor if the motor control is
considered in the synchronously rotating
reference frame (de-qe) where the sinusoidal
variables appear as dc quantities in steady
state.
Two control inputs ids and iqs can be used for a
vector controlled inverter as shown on the
next slide.
Torque Control of DC Motors (cont’d)
With vector control:
ids (induction motor)  If (dc motor)
iqs (induction motor)  Ia (dc motor)
Thus torque is given by:
Te  K t r iqs  K t'ids iqs
where r   r is peak value of sinusoidal
space vector.
Torque Control of DC Motors (cont’d)
This dc motor-like performance is only
possible if iqs* only controls iqs and does not
affect the flux  r , i.e. iqs and ids are
orthogonal under all operating conditions
of the vector-controlled drive.
Thus, vector control should ensure the
correct orientation and equality of the
command and actual currents.
Equivalent Circuit of Induction Motor
The complex de-qe equivalent circuit of an
induction motor is shown in the below figure
(neglecting rotor leakage inductance).
Equivalent Circuit of Induction Motor
(cont’d)
Since the rotor leakage inductance has
been neglected, the rotor flux  r =  m , the
air gap flux.
The stator current vector Is is the sum of the
ids and iqs vectors. Thus,
the
stator
current
Is
magnitude, is related to ids and iqs by:
I s  ids2  iqs2
Phasor Diagrams for Induction Motor
The steady state phasor (or vector) diagrams
for an induction motor in the de-qe
(synchronously rotating) reference frame are
shown below:
Phasor Diagrams for Induction Motor
(cont’d)
The rotor flux vector  r (  m ) is aligned with
the de axis and the air gap voltage V m is
aligned with the qe axis. The terminal voltage
Vs slightly leads the air gap voltage because of
the voltage drop across the stator impedance.
iqs contributes real power across the air gap
but ids only contributes reactive power across
the air gap.
Phasor Diagrams for Induction Motor
(cont’d)
The first figure shows an increase in the torque
component of current iqs and the second figure
shows an increase in the flux component of current,
ids. Because of the orthogonal orientation of these
components, the torque and flux can be controlled
independently. However, it is necessary to maintain
these vector orientations under all operating
conditions.
How can we control the iqs and ids components of
the stator current Is independently with the desired
orientation ?
Principles of Vector Control
The basic conceptual implementation of
vector control is illustrated in the below block
diagram:
Note: The inverter is omitted from this diagram.
Principles of Vector Control (cont’d)
The motor phase currents, ia, ib and ic are
converted to idss and iqss in the stationary
reference frame. These are then converted
to the synchronously rotating reference
frame d-q currents, ids and iqs.
In the controller two inverse transforms are
performed:
1) From the synchronous d-q to the
stationary d-q reference frame;
2) From d*-q* to a*, b*, c*.
Principles of Vector Control (cont’d)
There are two approaches to vector control:
1) Direct field oriented current control
- here the rotation angle of the iqse vector
with respect to the stator flux qr’s is being
directly determined (e.g. by measuring air
gap flux)
2) Indirect field oriented current control
- here the rotor angle is being measured
indirectly, such as by measuring slip speed.
Direct Vector Control
In direct vector control the field angle is
calculated by using terminal voltages and
current or Hall sensors or flux sense windings.
A block diagram of a direct vector control
method using a PWM voltage-fed inverter is
shown on the next slide.
Direct Vector Control (cont’d)
Direct Vector Control (cont’d)
The principal vector control parameters, ids*
and iqs*, which are dc values in the
synchronously rotating reference frame, are
converted to the stationary reference frame
(using the vector rotation (VR) block) by
using the unit vector cosqe and sinqe. These
stationary reference frame control
parameters idss* and iqss* are then changed
to the phase current command signals, ia*,
ib*, and ic* which are fed to the PWM inverter.
Direct Vector Control (cont’d)
A flux control loop is used to precisely control
the flux. Torque control is achieved through
the current iqs* which is generated from the
speed control loop (which includes a bipolar
limiter that is not shown). The torque can be
negative which will result in a negative phase
orientation for iqs in the phasor diagram.
How do we maintain idsand iqs orthogonality?
This is explained in the next slide.
Direct Vector Control (cont’d)
Direct Vector Control (cont’d)
Here the de-qe frame is rotating at
synchronous speed e with respect to the
stationary reference frame ds-qs, and at
any point in time, the angular position of
the de axis with respect to the ds axis is qe
(=et).
From this phasor diagram we can write:
   r cosq e and
s
dr
 qrs   r sin q e
Direct Vector Control (cont’d)
Thus,

cosq e 
r
s
dr
, sin q e 
 qrs
r
, and  r  
   
s 2
dr
The cosqe and sinqe signals in correct
phase position are shown below:
s 2
qr
Direct Vector Control (cont’d)
These unit vector signals, when used in the
vector rotation block, cause ids to maintain
orientation along the de-axis and the iqs
orientation along the qe-axis.
Summary of Salient Features of
Vector Control
A few of the salient features of vector
control are:
 The frequency e of the drive is not
controlled (as in scalar control). The motor
is “self-controlled” by using the unit vector
to help control the frequency and phase.
 There is no concern about instability
because limiting I swithin the safe limit
automatically limits operation to the stable
region.
4.5 Design method
Transient response will be fast because
torque control by iqs does not affect flux.
 Vector control allows for speed control in all
four quadrants (without additional control
elements) since negative torque is directly
taken care of in vector control.
