Transcript Microelectromechanical Devices

ECE 8830 - Electric Drives
Topic 17: Wound-Field Synchronous
Machine Drives
Spring 2004
Introduction
For high power (multi-MW) applications,
the high efficiency of synchronous motors
makes them more appealing than induction
motors. Indeed, most of today’s electrical
power generators are 3 synchronous
generators.
Brushless dc Excitation
Wound-field synchronous motors require dc
current excitation in the rotor winding. This
excitation is traditionally done through the
use of slip rings and brushes. However,
these have several disadvantages such as
requiring maintenance, arcing (which
means they cannot be used in hazardous
environments), etc. An alternative approach
is to use brushless excitation which is
illustrated on the next slide.
Brushless dc Excitation (cont’d)
Brushless dc Excitation (cont’d)
A wound-rotor induction motor (WRIM) is
mounted on the same shaft as the woundfield synchronous motor. This is acting as a
rotating transformer with the rotor as the
primary and the stator as the secondary.
The stator of the WRIM is fed by a 60Hz
supply and the rotor of the WRIM rotates at
a speed set by the supply frequency. The
slip voltage in the rotor winding of the
WRIM is rectified to provide the current
feed to the rotor windings of the
synchronous motor.
Load Commutated Current-Fed
Inverters
Thyristor current-fed, load commutated
inverters (LCI’s) are very popular for
high power (multi-MW) wound-field
synchronous motor drives.
We will briefly review current-fed
thyristor inverters and then discuss
load commutated inverters in some
detail. We will then see how to apply
them to wound-field synchronous motor
drives.
Review of Current-Fed Thyristor
Inverter
Let us first briefly review the operation of
the current-fed thyristor inverter.
Review of Current-Fed Thyristor
Inverter (cont’d)
Initially, ignore commutation considerations.
Induction motor load is modeled by back emf
generator and leakage inductance in each
phase of the winding.
The constant dc current Id is switched
through the thyristors to create a 3 6-step
symmetrical line current waves as shown on
the next slide.
Review of Current-Fed Thyristor
Inverter (cont’d)
Review of Current-Fed Thyristor
Inverter (cont’d)
The load or line current may be expressed
by a Fourier series as:
1
1


ia 
I d cos t  cos5t  cos 7t  ...

5
7


2 3
where the peak value of the fundamental
component is2
given
/ 3 2 3Id /  . Each thyristor
conducts for
radians. At any instant
one upper thyristor and one lower thyristor
conduct.
Review of Current-Fed Thyristor
Inverter (cont’d)
The dc link is considered harmonic-free
and the commutation effect between
thyristors is ignored.
At steady state the voltage output from
the rectifier block = input voltage of
inverter.
For a variable speed drive the inverter
can be operated at variable frequency
and variable dc current Id.
Review of Current-Fed Thyristor
Inverter (cont’d)
If thyristor firing angle  > 0, inverter
behavior.
If thyristor firing angle =0, rectifier
behavior.
Max. power transfer occurs when =.
Inverter Operation Modes
Two inverter operation modes are
established depending on the thyristor
firing angle:
1) Load-commutated inverter
Applies when /2<<.
2) Force-commutated inverter
Applies when <<3/2.
Load-Commutated Inverter Mode
Consider =3/4. In this case vca < 0 =>
thyristor Q5 is turned off by the load. This
requires load to operate at leading power
factor => motoring mode of a synchronous
machine operating in over-excitation.
Vd=-Vd0cos
Load Commutated Inverters
Let us initially consider a single-phase
inverter before discussing the 3 case. A
single-phase, current-fed, parallel
resonant inverter with load commutation
is shown below:
Load Commutated Inverters (cont’d)
A phase-controlled rectifier provides the dc
input and a large capacitor C provides the
load commutation of the thyristors. Assuming
perfect filtering of harmonics by the capacitor
and the dc link inductor, the inverter load
voltage and current waves are shown below:
Load Commutated Inverters (cont’d)
The thyristor pairs Q1Q2 and Q3Q4 are
switched alternately for  angle to
produce the square wave output. The
fundamental of the current wave leads
the sinusoidal voltage wave by . Thus,
when Q1Q2 turn on, the Q3Q4 pair has a
negative voltage for duration  which
provides the load commutation. Since
=tq, the time tq must be sufficiently
long for the thyristors to turn off.
Load Commutated Inverters (cont’d)
The equations for the inverter circuit are:
diL
vL  iL R  L
dt
dvL
ic  C
dt
iL'  iL  iC  i1  i2
id  i1  i2
did
vd  vL  id Rd  Ld
dt
where Rd is the resistance of the inductor Ld.
Load Commutated Inverters (cont’d)
These equations can be expressed in statevariable form and solved to model the
steady state and dynamic performance of
the inverter.
We will now consider an approximate
steady state analysis assuming that Ld is of
infinite size and is lossless. We will also
assume that the load is highly inductive,
i.e. L>>R.
Load Commutated Inverters (cont’d)
Consider the series R-L load to be resolved
into parallel R1 and L1 components in which
real current IP flows through R1 and
reactive current IQ flows through L1. The
load impedance ZL can be written as:
R1 j L1
Z L  R  j L 
R1  j L1
R1 2 L12
R12 L1
 2
j 2
2 2
R1  j L1
R1  j 2 L12
Load Commutated Inverters (cont’d)
If the load is highly inductive (as we had
assumed) R1>>L1 and
R
 2 L12
R1
and L  L1.
The fundamental component of the current
is given by:
I 
'
L
2 2

Id
Load Commutated Inverters (cont’d)
The real and reactive components of the
load current are given by:
VL
I P  I cos  
R1
VL
'
'
I Q  I L sin   '
XC
'
L
and
X C'  j L1 ||1/ jC
where
. Through some
algebraic manipulation we get:
2
L
2 2V
I 
 RV
1 d
'
L
and VL 

8
Vd
R12  X C'2
X C'
Load Commutated Inverters (cont’d)
From the above equations we can
calculate the load voltage, currents, and
commutation angle .
Example:
Single-phase synchronous motor;
Vd=200V, f=60Hz, R=0.2, L=1.2mH,
Id=240A, C=150F. Find .
Load Commutated Inverters (cont’d)
There are basically two control variables
for the load commutated inverter - the
dc link current and the frequency. For a
variable load, a variable capacitance can
be used to provide desired margin of
commutation angle . However, a better
way to operate is to use a PLL to control
the inverter frequency to just above the
load resonance frequency.
Load Commutated Inverters (cont’d)
The single-phase inverter concepts can be
extended to 3 LCI’s. The figure below
shows a three-phase LCI with lagging
power factor load. Here load commutation
is achieved by using a leading VAR load
connected at the load terminal.
Load Commutated Inverters (cont’d)
In the case of a variable load, a fixed
capacitor bank is connected at the
terminals and the inverter frequency
adjusted so that the effective inverter
load has a leading PF so that
commutation occurs at a fixed angle .
Load Commutated Inverters (cont’d)
As mentioned earlier, thyristor current-fed,
load commutated inverters (LCI’s) are very
popular for high power (multi-MW) woundfield synchronous motor drives where it is
easy to maintain the required leading PF
angle by adjusting the field excitation.
Load Commutated Inverters (cont’d)
The fundamental frequency phasor diagram for
a salient pole synchronous machine under
motoring condition is shown below:
Note: The winding resistance and the
commutation overlap effect have been neglected.
Load Commutated Inverters (cont’d)
A flux linkage has been included in the
phasor diagram where f= field flux linkage,
a=armature reaction flux linkage and
S=resultant stator flux. We can write the
de and qe components of a as follows:
 ds  Lds Ids  2Lds I s sin(   )
 qs  Lqs I qs  2Lqs I s sin(   )
For a salient pole machine, LdLq the phasors
a and Is are not in phase.
Load Commutated Inverters (cont’d)
The motor phase voltage and current waves
are shown below including the commutation
overlap effect:
Load Commutated Inverters (cont’d)
The load commutated inverter with an
over-excited synchronous machine load
depends on sufficient back emf which is
not available at low speeds. The critical
speed required for load commutation to
work is about 5% of base speed. A forced
commutation approach is required below
these speeds and to start the motor. (see
Bose text pp. 284-285 for details).
Load-Commutated Inverter Drive
Having seen how a current-fed, thyristor
inverter can be load commutated with a
wound-field synchronous motor by
operating the machine at a leading power
factor, we can now consider how to design
a self-controlled drive system for a woundfield synchronous motor based on a loadcommutated inverter drive. As mentioned
earlier, this type of drive is popular for high
power (multi-MW) drives for compressors,
pumps, ship propulsion, etc.
Load-Commutated Inverter Drive
(cont’d)
A block diagram of a self-controlled loadcommutated, current-fed inverter drive for a
wound-field synchronous motor is shown below:
Load-Commutated Inverter Drive
(cont’d)
The phasor diagram for the LCI in
motoring mode driving a synchronous
motor is shown below:
Note: The saliency and stator resistance
have been neglected.
Load-Commutated Inverter Drive
(cont’d)
The field flux f is established by the field
current If and depends on the rotor position.
The armature flux a =IsLs is determined by
the stator current and stator winding
inductance. The delay angle command d*
sets the position of a relative to f since a
leads f by ’ given by:


 '          
2

where = torque angle.
Load-Commutated Inverter Drive
(cont’d)
Thyristors require a minimum turn-off
time toff for successful commutation. This
corresponds to a turn-off angle =toff. For
reliable operation of a LCI drive and
minimum reactive current loading to the
synchronous motor, turning off the
thyristors at a fixed time every cycle is a
good approach. A complete speed control
system for a LCI synchronous motor drive
incorporating constant turn-off angle
control is shown on the next slide.
Load-Commutated Inverter Drive
(cont’d)
Load-Commutated Inverter Drive
(cont’d)
This drive operates in the constant torque
region in motoring mode with stator flux
s maintained constant (open loop). There
are four control elements:
 Speed and dc link current control
 Field flux/field current control
 Generation of f*, d*, * and * command
signals (where  is the commutation
overlap angle)
 Delay angle control.
Load-Commutated Inverter Drive
(cont’d)
Speed and dc link current control:
r compared to r* and error goes
through P-I controller and absolute value
circuit -> Id*. Id and Id* compared and
controls thyristor firing angle  in rectifier
to control dc link current.
The generated motor torque  Id (see
Bose text pg. 499 for derivation).
3 6P
Te 
  s I d cos   K ' I d
 2
Load-Commutated Inverter Drive
(cont’d)
Field flux/field current control:
The command field flux f* is given by:

*
     2  cos    
2

*
f
*2
s
*2
a
*
s
*
a
where s*= constant, a*=LsIs=KaId* and
*= *+k*. To obtain  we need * which
can either be measured or calculated using
the expression:
*

 *
2
L
I
1
*
c d
  cos cos  

*
6 s 

Load-Commutated Inverter Drive
(cont’d)
The command flux current If* is then
generated from the command flux f* by
through a function generator that
corrects for saturation effects. A phasecontrolled rectifier can then be used to
control the flux current as shown in the
system block diagram.
Load-Commutated Inverter Drive
(cont’d)
Generation of f*, d*, * and * command
signals:
We have discussed how all of the
command signals can be obtained with
the exception of the * angle. This is
obtained from the equation:
 6 Ls cos  * 
  sin 
Id 
  f

*
1
Load-Commutated Inverter Drive
(cont’d)
Delay Angle Control:
For a six-step inverter we need six
discrete firing pulses at /3 intervals
apart within a cycle. A block diagram
showing how this can be achieved is
shown on the next slide.
Load-Commutated Inverter Drive
(cont’d)
Load-Commutated Inverter Drive
(cont’d)
Load-Commutated Inverter Drive
(cont’d)
The corresponding alignment of reference
signal S1 and the waveforms for phase a in
motoring mode are shown below. These
diagrams can be used to determine the
inverter firing angles.
Load-Commutated Inverter Drive
(cont’d)
The absolute position sensor can be
eliminated and the machine terminal
voltage signals can be used instead to
estimate the rotor position for inverter
firing angle determination. Details are
presented in the Bose textbook pp. 504507.
Cycloconverter Drive
High power, wound-field synchronous
motors can be operated at unity power
factor when excited by phase-controlled,
line-commutated, thyristor
cycloconverters. Drive control for such
drives can be both scalar and vector
control, similar to that of the voltage-fed
inverter drive.
The next slide shows a simple scalar
control method for a cycloconverter drive
for a wound-field synchronous motor.
Cycloconverter Drive (cont’d)
Cycloconverter Drive (cont’d)
There are three control variables in this
control system:
 The stator current amplitude, I s
 Phase angle, ’ (see phasor diagram below)
 The field current, If.
Cycloconverter Drive (cont’d)
The torque generated by the motor is
proportional to the in-phase stator current.
The command stator current Is* is generated
from the error in the speed control loop.
The angle ’ and the field current If can be
determined from Is as
shown in the figure. Thus,
Is* is used to generate ’*
and If* using function
generators.
Cycloconverter Drive (cont’d)
The position sensor and encoder generate
the cose and sine signals and the speed
signal, r. The 2-phase unit signals are
converted to 3-phase unit signals using the
following transformations:
Ua  cose
2

U a  cos  e 
3

1
3

sin  e
   cos e 
2
2

2

U a  cos  e 
3

1
3

sin  e
   cos e 
2
2

Cycloconverter Drive (cont’d)
Each of the 3 unit signals is then
multiplied by Is* and phase shifted by
angle ’* to produce the three phase
current command signals as follows:
*
s
i  I U a 
'*
*
s
'*
*
a
i  I U b 
*
b
*
s
i  I U c  '*
*
c
Cycloconverter Drive (cont’d)
The performance of the cycloconverter drive
can be enhanced if vector control is used
rather than scalar control. In the constant
torque region, the field current must be
increased if we want to increase the developed
torque at the constant rated stator flux.
However, the field current response is slow
and this leads to sluggish motor response. In
vector control we inject a transient
magnetizing current in the direction of the
stator flux to obtain a much faster response
than with scalar control. This current is set to
zero in steady state to maintain unity PF.
Cycloconverter Drive (cont’d)
A vector control implementation is shown:
Cycloconverter Drive (cont’d)
A phasor diagram for the vector control
approach is shown below:
Cycloconverter Drive (cont’d)
Notable points from phasor diagram:


The torque component of the stator current IT
is in phase with Vs and forms a triangle with
Is and the injected magnetizing current IM.
IM=0 at steady state and IT=Is.
The magnetizing current, Im, the field current
If, and the torque component of the stator
current IT form a right-angled triangle (which
is a scaled version of the flux triangle).
Cycloconverter Drive (cont’d)


There are three sets of d-q axes:
- de-qe in reference frame of rotor;
- ds-qs in reference frame of stator;
- de’-qe’ with qe’ aligned with Vs and de’ aligned
with s.
At steady state, s and a are at quadrature.
Also, Is is in phase with Vs which leads s by
90° => unity PF.
Cycloconverter Drive (cont’d)
From the phasor diagram, at steady state, we
can write:
I m  I f cos 
This equation gives the control equation for
If*. Under transient conditions, the command
injected transient magnetizing current, IM*, is
given by:
I  I  I f cos 
*
M
*
m
Under steady state conditions, IM=0 and the
above steady state equation is re-established.
Cycloconverter Drive (cont’d)



Control features of the vector control of a
wound-field synchronous motor drive:
Speed control error generates the torque
component of current through P-I controller.
Command currents IT* and IM* are compared
to feedback currents, IT and IM to generate
command voltages vT* and vM* through P-I
compensators.
A vector rotator uses unit control signals
cos and sin to transform the vT* and vM*
signals to phase command voltages va*, vb*,
and vc*.
Cycloconverter Drive (cont’d)

A transient change in the required torque
causes IM to be injected because of the
sluggish response of If, thereby maintaining
a constant flux s. As If builds up, IM drops
down to reach zero when If has reached its
new steady state value.
The complete vector control feedback
signal processing is shown on the next
slide.
Cycloconverter Drive (cont’d)