Download Chopper-Fed DC Motor Drive (Discrete)

MAXIM INSTITUTE OF TECHNOLOGY, BHOPAL
Session: 2012-2013
Student Name :
Enrollment No.:
Subject :Lab-II (Software and simulation)
Semester: M.Tech I Sem (Power Electronics)_
Subject Code : MEPE 107
List of Experiments
S.
NO
QUESTIONS
PAGE NO.
1. Simulation of single phase Half wave
converter circuit using MATLAB.
\ 2.
3.
4.
5.
6.
Simulation
MATLAB.
of
chopper
circuit
using
Simulation of speed torque characteristics of
DC motor using MATLAB.
Simulation of single phase Half wave
Inverter Circuit using MATLAB.
Simulation of 3Φ Inverter in 180º mode of
conduction using MATLAB.
Find the output response of single phase
uncontrolled & controlled bridge rectifier
Using MATLAB.
7.
Simulation of transfer
operational amplifier.
function
using
8.
Simulink model of armature voltage speed
control.
.
SIGNATURE
REMARKS
Experiment 01
single-Phase Rectifier
Description
This system contains two identical circuits showing the operation of a single phase
rectifier.
The rectifiers are fed by a 120 V/ 24 V linear transformer. The rectified voltage is filtered
by a 100 mH / 200 uF filter and applied to a 5 ohm resistive load. The load voltages are
measured by two Voltage Measurement blocks Vd1 and Vd2.
The top circuit (circuit 1) uses four individual diodes connected in a bridge configuration.
The currents of diodes 2 and 4 are obtained at the measurement 'm' output of the diode
blocks and sent to input 1 of Scope 2 through Selector and Multiplex blocks.
The bottom circuit (circuit 2) is functionally identical to circuit 1, but the circuit assembly
is considerably simplified by the use of the Universal Bridge.
Demonstration
Open the Universal Bridge dialog box and notice that in order to obtain a four-diode
bridge configuration, the Power Electronic device field has been set to Diodes and that
the number of arms has been set to 2. Also, observe that the Measurements field has
been set to Device currents, thus allowing measurement of diode currents through the
Multimeter block.
Now, double click on the Multimeter block. The four diode currents are listed in the left
column. Notice that only currents in diodes 2 and 4 have been selected and transferred in
the right column. The number of available signals at the multimeter output (2) is
displayed on the block icon.
Open the two scopes and start the simulation. Compare the two load voltages Vd1 and
Vd2 which are superimposed on Scope1. Also, compare on Scope2 the currents flowing
in diodes 2 and 4 for circuit 1 and circuit 2.
Notice that the circuit has been discretized by means of the Powergui block. The sample
time (50e-6 s) appears on the block icon. Now, open the Powergui block and select the
simulation type Continuous. The continuous ode23tb solver specified in the Simulation
Parameters will now be used. Restart the simulation and compare the continuous
simulation results with the previous simulation results obtained with a discretized system.
Continuous
powergui
m
k
g
a
Thyristor
+
i
-
dc current
Current Measurement 1
AC Voltage Source
+
-
v
Voltage Measurement 1
R Load
t
Pulse
Generator
Thyristor 1
g
a
m
k
+
-
v
+
ac volt
Voltage Measurement
t
Pulse
Generator 1
Simulink Model
Scope 1
v
dc volt
Voltage Measurement
2
Experiment 02
SIMULATION AND PERFORMANCE OF POWER ELECTRONIC
CIRCUITS
The simplest phase controlled rectifier circuit connected to inductive load is shown in
Fig. 2. The presence of a commutating diode which prevents the load voltage reversing
beyond the diode volt-drop value, resulting in the waveformsis shown in Fig. 3.
Fig. 2 Single phase half wave controlled rectifying circuit with commutating diode
Fig. 3a Waveform for signal phase half wave controlled circuit with commutating diode
During the “thyristor on” period the load current is dictated by equation (1), but once the
voltage reverses VL is effectively zero and the load current follows an exponential decay.
If the current level decays below the diode holding level then the load current is
discontinuous as shown Fig. 3b. Fig. 3
shows a continuous load current condition where the decaying load current is still
following when a thyristor is fired in next cycle. The load voltage waveform has a mean
value for voltage given by:
Inspection of the waveforms clearly shows that the greater the firing delay angle σ the
lower is the mean load voltage, equation 2 confirmation that it falls to zero when σ=180o.
The thyristor voltage waveform Vt (Fig. 4) shows a positivevoltage during the delay
period and also both the peak forward and peak reverse voltage are equal to Vmax of the
supply. Inspection of the waveforms in Fig. 2 and Fig. 3 clearly shows the two roles of
the commutating diode, to prevent negative load voltage and to allow the thyristor to
regain its blocking state at the voltage zero by transferring (or commutating) the load
current away from the thyristor.
Fig. 3b Thyristor voltage waveforms for single phase half wavecontrol circuit with
commutating diode with σ=30o, σ=120o
A. Fully Controlled Single Phase Converter
The main elements of a fully controlled single phase converter are illustrated in Fig. 4,
comprise four thyristors connected in a bridge formation [5]. The positive load terminal
can be connected (via Thy1) to terminal A or (via Thy2) to terminal B of the input source
and likewise the negative load terminal can be connected either to A or to B via Thy3 or
Thy4 respectively. The interest is to find the model operation applicability of the output
voltage waveform on the d.c. side and in particular to discover how it can be controlled
by varying the firing delay angle σ. The voltage waveform for a given σ will depend up
on the nature of the load and to explore the basic mechanism of phase control the case
where the load is a resistive is considered.
..
..
Fg. 5a Output voltage waveforms of signal phase fully controlledrectifier with resistive
load for firing angle delays of 60o
Thyristors Thy1 and Thy4 are fired together when terminal A of the supply is
positive while on the other half cycle when B is positive Thy2 and Thy3 are fired
simultaneously. The output voltage waveforms are illustrated in Fig. 5. At every
instant the load is either connected to the mains by the pair of switches Thy1 and
Thy4 or it is connected by the pair of switches Thy2 and Thy3 or it is disconnected.
It is much smoother than in the single pulse circuit, although again it is
far from pure d.c. The waveform shown in Fig. 5a corresponds to σ=60o, while Fig.
5b is for σ=120o. It is clear that the larger the delay angle the lower the output
voltage. The maximum output voltage (Vdo) is obtained with σ=0,and given by:
Where Vmax is the r.m.s voltage of the incoming mains. Obviously when σ is zero
the output voltage is the same as it would be for an uncontrolled diode bridge
rectifier, since the thyristors conduct for the whole of the half cycle for which they
are forward biased. The variation of the mean d.c. voltage with σ is given by:
From which it can be seen that with a resistive load the d.c voltage can be varied
from a maximum of Vdo down to zero by varying σ from 0 to 180O.
B. Inductive Load
Motor loads are inductive and it is well known that thecurrent cannot change
instantaneously in an inductive load. Therefore the behaviour of the converter with
an inductive load differs from that of a converter with a resistive load. With a fixed
σ, the mean output voltage with a resistiveinductive load is not the same as with a
purely resistive load and therefore it is difficult to give a simple general formula for
the mean output voltage in terms of σ.
However, fortunately it transpires that the output voltage for a given σ does become
independent of the load inductance once there is sufficient inductance to prevent the
load current from falling to zero. This condition is known as continuous current and
many motor circuits have sufficient self inductance to ensure that this condition is
achieved. Under continuous current conditions the output voltage waveform only
depends on the firing angle, and not on the actual value of the inductance present in
the circuit. This makes matters straightforward and typical output voltage
waveforms for this continuous current condition are illustrated in Fig. 6.
Fig. 6 Output voltage waveforms for fully controlled rectifiersupplying an inductive
load for σ=120o
VI. CONCLUSION
The transient analysis of power electronic circuits presents a case where any small
amount of gain in speed in any aspect of the solution accumulates into a large saving in
costly computer time due to the number of computations done. Methods have been
suggested to increase the computational efficiency using the Matlab/Simulink package
version 4.1 and 3.1. This is achieved by representing the thyristors based converter
systems by a series of switches. The power of the simulation is evident in the clear
presentation of the models in this paper.
Model for single phase half wave controlled rectifying circuit with commutating diode
Model for fully controlled single phase converter
Experiment 03
Simulation of Chopper Circuit using MATLAB.
Circuit Description
The DC motor is fed by the DC source through a chopper which consists of GTO
thyristor and free-wheeling diode D1. The motor drives a mechanical load characterized
by inertia J, friction coeficient B, and load torque TL.
The hysteresis current controller compares the sensed current with the reference and
generates the trigger signal for the GTO thyristor to force the motor current to follow the
reference. The speed control loop uses a proportional-integral controller which produces
the reference for the current loop. Current and Voltage Measurement blocks provide
signals for visualization purpose.
Demonstration
Motor starting
Start the simulation. Observe the motor current, voltage, and speed during the starting on
the scope. At the end of the simulation time (1.5 s), the system has reached its steadystate.
Response to a change in reference speed and load torque
The initial conditions state vector 'xInitial' to start with wm = 120 rad/s and TL = 5 N.m
has been saved in the 'power_dcdrive_init.mat' file. This file is automatically loaded in
your workspace when you start the simulation (see Model Properties). In order to use
these initial conditions you have to enable them. Check the Simulation/Configuration
Parameters menu , then select "Data Import/Export" and check "Initial state".
Now, double click the two Manual Switch blocks to switch from the constant "Ref. Speed
(rad/s) " and "Torque (N.m)" blocks to the Step blocks. (Reference speed wref changed
from 120 to 160 rad/s at t = 0.4 s and load torque changed from 5 to 25 N.m at t= 1.2s).
Restart the simulation and observe the drive response to successive changes in speed
reference and load torque.
Chopper-Fed DC Motor Drive (Discrete)
Circuit Description
A DC motor is fed by a DC source through a chopper which consists of GTO thyristor
and a free-wheeling diode.
The motor drives a mechanical load characterized by inertia J, friction coeficient B, and
load torque TL. The motor uses the discrete DC machine provided in the Machines
library.
The hysteresis current controller compares the sensed current with the reference and
generates the trigger signal for the GTO thyristor to force the motor current to follow the
reference. The speed control loop uses a proportional-integral controller which produces
the reference for the current loop.
Demonstration
Start the simulation and observe the motor voltage (Va), current (Ia) and speed (wm) on
the scope. The following observations can be made:
0 < t < 0.8 s: Starting and Steady State Operation
During this period, the load torque is TL = 5.N.m and the motor reaches the reference
speed (wref = 120 rad/s) given to the speed controller . The initial values of reference
torque and speed are set in the two Step blocks connected to the TL torque input of the
motor. Notice that during the motor starting the current is maintained to 30 A, according
to the current limit set in the speed regulator. Zoom in the motor current Ia in steady
state. Observe the current triangular waveshape varying between 5 A and 7 A,
corresponding to the specified hysteresis of 2 A. The commutation frequency is
approximately 1.5 kHz.
t = 0.8 s: Reference Speed Step
The reference speed is increased from 120 rad/s to 160 rad/s. The speed controller
regulates the speed in approximately 0.25 s, and the average current stabilizes at 6.6 A.
During the transient period, current is still limited at 30 A.
t = 1.5 s: Load Torque Step
The load torque is suddenly increased from 5 N.m to 25 N.m. The current increases to 23
A, while speed is maintained at the 160 rad/s set point.
Experiment 04
Simulation of speed torque characteristics of permanent magnet
synchronous motor using MATLAB.
Circuit Description
This circuit uses the AC6 block of SimPowerSystems™ library. It models a permanent
magnet (PM) synchronous motor drive with a braking chopper for a 3HP motor.
The PM synchronous motor is fed by a PWM voltage source inverter, which is built using
a Universal Bridge Block. The speed control loop uses a PI regulator to produce the flux
and torque references for the vector control block. The vector control block computes the
three reference motor line currents corresponding to the flux and torque references and
then feeds the motor with these currents using a three-phase current regulator.
Motor current, speed, and torque signals are available at the output of the block.
Demonstration
Start the simulation. You can observe the motor stator current, the rotor speed, the
electromagnetic torque and the DC bus voltage on the scope. The speed set point and the
torque set point are also shown. At time t = 0 s, the speed set point is 300 rpm. Observe
that the speed follows precisely the acceleration ramp.
At t = 0.5 s, the full load torque is applied to the motor. You can observe a small
disturbance in the motor speed, which stabilizes very quickly.
At t = 1 s, the speed set point is changed to 0 rpm. The speed decreases down to 0 rpm
following precisely the deceleration ramp.
At t = 1.5 s., the mechanical load passes from 11 Nm to -11 Nm. The motor speed
stabilizes very quickly after a small overshoot.
Finally, note how well the DC bus voltage is regulated during the whole simulation
period.
1) The power system has been discretised with a 2 us time step. The speed controller uses
a 140 us sample and the vector controller uses a 20 us sample time in order to simulate a
microcontroller control device.
2) A simplified version of the model using average-value inverter can be used by
selecting 'Average' in the 'Model detail level' menu of the graphical user-interface. The
time step can then be increased up to 75 us. This can be done by typing 'Ts = 75e-6' in the
workspace, by setting Vector control sample time to 75e-6 and by setting Speed controler
sample time to 150e-6 in the case of this example. See also ac6_example_simplified.mdl.
Experiment 05
Simulation of 3Φ Inverter in 180º mode of conduction using MATLAB.
THEORY:A basic three phase inverter is a six-stepbridge inverter. This inverter
usesminimum of six switches and six diodes. This circuit is made of six MOSFET’s,
butpresently, the use of IGBT’s in place of switches is on rise.In 180-mode operation,
eachswitch conducts for 180 degree for everyfull cycle and for other half cycle theother
switch connected to the same leg isconnected
CIRCUIT DIAGRAM FOR PHASE VOLTAGES
WAVEFORMS FOR PHASE VOLTAGES: Van,Vbn, Vcn
CIRCUIT DIAGRAM FOR LINE VOLTAGES:
Waveform for phase voltages:
Experiment 06
SIMULINK MODEL OF ARMATURE VOLTAGE SPEED CONTROL
In this section, MATLAB/Simulink model of DC motor driven from single phase AC/DC
semi and full converters are presented and the performance of the DC motor drive is
analyzed. A 5-HP DC motor of 240-V rating 1220 rpm is used in the simulation model.
Fig. 3 shows the Simulink realization of the semi converter drive. The armature circuit is
supplied from a single phase semi converter in which a thyristor is used as an electronic
switch and a freewheeling diode is used to solve the stored inductive energy problem in
the circuit. The field circuit is separately excited from an ideal DC voltage source. A DC
motor block of SimPowerSystems toolbox is
used. Where, Fig. 4 shows the Simulink realization of the full converter drive.
Thearmature circuit is supplied from a single phase full converter in which a thyristor is
used as an electronic switch and no freewheeling diode is used. The field circuit is
separately excited from an ideal DC voltage source. A DC
motor block of SimPowerSystems toolbox is used.
Fig: Simulink model
Single Phase Semi converter Drive
In order to investigate the effect of armature voltage on the torque-speed characteristic,
Four different armature voltages with average values Vt = 240 V, 200 V, 160 V and 120
V are applied while the voltage applied to the field circuit is kept constant at its nominal
value 240 V. A constant 380 V, 60 HZ AC is applied to the input of single phase semi
converter. The average value of the converter output is changed by changing the firing
angle (α). A pulse generator is used to change the firing angle. The following firing
angles are used to obtain 120, 160, 200 and 240 V average output voltages applied to the
armature: α = 89 o, 70o, 47.5o and 0 o. The torque-speed characteristics are obtained for
these armature voltages.
Fig. 5 shows the torque-speed curves for a single phase semi
converter drive.
It is clear that torque-speed curves contain both linear and nonlinear regions. The linear
region of operation for 240V approximately starts at TL = 30 N.m. But for 200V starts at
TL = 50 N.m. While for 160V approximately starts at TL = 60 N.m. Finally for 120V
approximately starts at TL = 75 N.m. The discontinuous armature current results in a
highly nonlinear torque-speed characteristic. Fig. 6 and Fig. 7 show the armature voltage
and current obtained at 25 Nm (in the nonlinear region) and 125 Nm (in the linear region)
for average value of 160 V. These figures clearly illustrate the discontinuous and
continuous operation of the single phase semi converter drive in non-linear and linear
regions, respectively.
Experiment 07
Simulation of 1Φ Half Wave Inverter Circuit using MATLAB.
This demonstration illustrates use of the IGBT/Diode block in voltage-sourced
converters. It also demonstrates harmonic analysis of PWM waveforms using the
Powergui/FFT tool.
Circuit Description
The system consists of two independent circuits illustrating single-phase PWM voltagesourced converters (VSC).
1. Half-bridge converter
2. Full-bridge converter
The converters are built with the IGBT/Diode block which is the basic building block of
all VSCs. The IGBT/Diode block is a simplified model of an IGBT (or GTO or
MOSFET)/Diode pair where the forward voltages of the forced-commutated device and
diode are ignored. You may replace these blocks by individual IGBT and diode blocks
for a more detailed representation. VSCs are controlled in open loop with the Discrete
PWM Generator block available in the Extras/Discrete Control Blocks library. The two
circuits use the same DC voltage (Vdc = 400V), carrier frequency (1080 Hz) and
modulation index (m = 0.8).
In order to allow further signal processing, signals displayed on the two Scope blocks
(sampled at simulation sampling rate of 3240 samples/cycle) are stored in two variables
named 'sps1phPWM1_str' and 'sps1phPWM2_str' (structures with time).
Demonstration
Run the simulation and observe the following two waveforms on the two Scope blocks:
Current into the load (trace 1), Voltage generated by the PWM inverter (trace 2).
Once the simulation is completed, open the Powergui and select "FFT Analysis" to
display the 0 - 5000 Hz frequency spectrum of signals saved in the three
"sps1phPWMx_str" structures. The FFT will be performed on a 2-cycle window starting
at t = 0.1 - 2/60 (last 2 cycles of recording). For each circuit, select Input labeled "V
inverter" . Click on "Display" and observe the frequency spectrum of last 2 cycles.
The fundamental component of V inverter is displayed above the spectrum window.
Compare the magnitude of the fundamental component of the inverter voltage with the
theoretical values given in the circuit. Compare also the harmonic contents in the inverter
voltage.
The half-bridge inverter generates a bipolar voltage (-200V or +200V) . Harmonics occur
around the carrier frequency (1080 Hz +- k*60 Hz), with a maximum of 103% at 1080
Hz.
The full-bridge inverter generates a monopolar voltage varying between 0 and+400V for
one half cycle and then between 0 and -400V for the next half cycle. For the same DC
voltage and modulation index, the fundamental component magnitude is twice the value
obtained with the half-bridge. Harmonics generated by the full-bridge are lower and they
appear at double of the carrier frequency (maximum of 40% at 2*1080+-60 Hz) As a
result, the current obtained with the full-bridge is smoother.
If you now perform a FFT on the signal "I load" you will notice that the THD of load
current is 7.3% for the half-bridge inverter as compared to only 2% for the full-bridge
inverter.
Fig: Half Bridge inverter