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CHOPPER FED DC DRIVES
AIM:
To study and the different types of chopper fed dc drives in open loop and closed loop analyze the
performance and their waveforms using MALAB9.0
THEORY:
A chopper is a static power electronic device that converts fixed dc input voltage to a variable dc output
voltage. The power semiconductor devices used for a chopper circuit can be forced commutated thyristor,
power BJT, MOSFET, IGBT and GTO. These devices are generally represented by a switch. The power
semiconductor devices have on state voltage drop of 0.5v-2.5v across them also requires a commutation
circuit. For simplicity, an ideal switch is used for neglecting the above shortcomings.
DC drives are less complex and less expensive than AC drives for low HP rating and is used as adjustable
speed machines. DC drives can be either chopper or rectifier controlled. As chopper involves one stage
conversion these are more efficient than rectifiers to feed DC motors. With chopper control it is is possible
to obtain regenerative braking down to nearly zero speed.
CONTROL STRATEGIES:
The average value of output voltage Vo can be controlled through duty cycle by opening and Closing the
semiconductor switch periodically.
Time ratio control (TRC):
In this control scheme, time ratio Ton/T (duty ratio) is varied. This is realized by two different ways as
Constant Frequency control and Variable Frequency control as described below
Constant frequency control:
In this scheme, on-time is varied but chopping frequency f is kept constant. Variation of Ton means
adjustment of pulse width, as such this scheme is also called pulse-width-modulation scheme.
Variable frequency control:
In this technique, the chopping frequency f is varied and either (i) on-time Ton is kept constant or
(ii) off-time Toff is kept constant. This method is also called Frequency-modulation scheme.
CHOPPER FED DC DRIVES:
A chopper fed DC drive can be operated under open loop as well as a closed loop.In open loop operation
the speed of the DC drive can not be controlled but it can be adjusted on adjusting the input voltage,
parameters of the motor or the chopper its duty ratio.
Duty rario δ = (Ton/ Ton+Toff) or (Ton/T)
Where, Ton- The on period of switch (pulse width)
Toff-the off period of switch
T=Ton+Toff (total time period)
By varying the duty ratio of the chopper we can control the speed of the DC drive.
GATE PULSE DC INPUT
Fig 1: Open loop control of chopper fed DC drive
The five different classes of chopper whose input controlled by the gate pulse, generated by the firing
pulse generation scheme (Ramp or cosine wave firing scheme) is made to provide electrical dc input to the
motor. The output armature voltage, armature current, field current,electrical torque and speed waveforms
are illustraed using simulink in MATLAB 7.9.
PROCEDURE:
1.Make / model the circuit by placing all its blocks from its corresponding Library/ Tool box, which is
clearly shown in table 1.
2.To change the circuit parameters applicable to the block by double clicking on the block/element and
type the values. Keep the values default for some blocks like thyristor, diodes etc.
3.To measure the voltage across or current through device, connect voltage measurement or current
measurement blocks
4.To observe the waveform in figure window, scope block is connected with voltage measurement and
current measurement blocks.
5.Make the connections as per the circuit diagram
FIRING PULSE
GENERATOR CHOPPER DC MOTOR
6.Set the reference speed (for closed loop-class E)
7.Click on simulation configuration parameters and make sure that solver option is „ode23tb‟, it is
essential when circuit contains power systems or power electronics tools
8.To run the simulation, select simulation and then start
9.Vary the torque of the Dc machine and run the simulation
10.Record the values in tabular column and repeat the steps for various values of reference speed.
CLASS A CHOPPER:
CIRCUIT DIAGRAM:
Fig 2:Circuit diagram and quadrent operation of class A chopper
OPERATION: (Motoring)
During the duty interval(0≤t≤ δT) the SW is closed and a part of the total energy supplied by the source is
absorbed by the armature and converted into mechanical energy (motoring), a part is converted into heat in
resistance and switch.Remaining energy is stored in the inductance L this energy is responsible for
armature current to freeewheel through the diode D during(δT≤t≤T). Both mechanical and heat energy is
to be supplied from this stored energy . Under the conditions the inductance L of low value, back emf is
large and the off time is long the armature current is small(low motor torque) and is discontinous. The
output waveforms with a duty ratio(δ=0.5) is shown in the Fig 3
Fig 3: Outputwaveforms of class A chopper fed DC drive
CLASS B CHOPPER:
CIRCUIT DIAGRAM
Fig 4: Circuit diagram and quadrent operation of class B chopper
OPERATION:(Regenerative braking)
The switch SW is closed for the interval (0≤t≤ δT) and open for (δT≤t≤T). At on period the motor terminal
voltage is zero and the back emf E the armature current increases. The mechanical energy supplied by the
load is now converted into electrical energy. This energy is partly used to increase the stored magnetic
energy in the inductance L and the remaining is dissipated in the resistance and the switch SW. During off
period the energy generated by the machineand the stored energy in the inductor L is partly dissipated in
the diode D and remaining is fed to the source giving regenerative braking. The output waveforms with a
duty ratio(δ=0.5) is shown in the Fig 5
Fig 5: Outputwaveforms of class B chopper fed DC drive
CLASS C CHOPPER:
CIRCUIT DIAGRAM:
Fig 6: Circuit diagram and quadrent operation of class C chopper
OPERATION:
The switch SW1 and D2 constitute one chopper (motoring) and swith SW2 and D1 constitue to
another chopper (regeneration). The switches are closed alternatively. In the chopping period
T,SW1 is kept on for δT and SW2 is kept on from δT to T.
Discontinous conduction is absent i.e the armature current wil be flowing all time without tending to
zero for a finite interval. Freewheeling will occur when SW1 is off and the current is flowing
through D1, this occurs at the interval (δT≤t≤T) which is also the interval for SW2 receving control
signal. If iarmature current falls to zero in freewheeling interval the back emf will drive the switch
SW2 in reverse direction preventing discontinous current.
During the energy transfer interval (0≤t≤ δT) SW2 is off and D2 is conducting and if the armature
current falls to zero SW1 will conduct due to the reason that its subjected to control signal G1.Thus
the armature current is continous.
Va= δV
Ia= (δV-E)/Ra
Where,
V- source voltage
E-back emf, Ia-Armature current
Ra-armature resistance , δ –duty ratio
From the above equation that the motoring operation(+Ia) takes place when δ>(E/V) and regenerative
braking(-Ia) takes place when δ<(E/V). The output waveforms with a duty ratio(δ=0.5) is shown in the
Fig 7
Fig 7: Outputwaveforms of class C chopper fed DC drive
CLASS D CHOPPER:
CIRCUIT DIAGRAM:
Fig 8: Circuit diagram and quadrent operation of class D chopper
Two –quadrant operation consisting of forward motoring and reverse regenerative braking requires
a chopper capable of giving positive current and voltage in either direction. The switches SW1 and
SW2 are turned with a phase diffrence of T sec. SW1 is on for an interval of (0≤t≤2δT) and Sw2 is
on for interval (T ≤ t≤ T+2δT). The period of operation of each switch is 2T. The pattern of control
signal suggest that under continous conduction the drive will operate in four different modes. These
modes of opration and corresponding values of instantaneous output voltage of the chopper.
Mode Switch Ia Va
Mode1
Sw1on,
sw2 on
Source-
sw1-
load-sw2
Va=
V
Mode2 Sw1-on,
sw2-off D2-sw1 Va=0
Mode3 Sw1off
Sw2off Sw2-D1 Va=0
Mode4 Sw1off
Sw2off
Load-
D2-
source-
D1
Va=-
V
When instantaneous value of Va is –V or 0 the average output voltage of chopper is –ve and machine
operates in the fourth quadrent and during Va being +V and 0 the average output of chopper is +ve and
machine operates in the first quadrent. The output waveforms with a duty ratio(δ=0.75) is shown in the
Fig
9
Fig 9: Outputwaveforms of class D chopper fed DC drive
CLASS E CHOPPER:
CIRCUIT DIAGRAM:
Fig 10: Outputwaveforms of class C chopper fed DC drive
Class E chopper is a combination of of two class d chopper. It can be operated in any one of the
four quadrants. This chopper is also considered to be a four quadrant inverter.
OPERATION:
The switch SW4 is kept closedand SW1 and SW2 are controlled, one gets a two quadrent
chopperwhich provides a variable positive terminal voltage and armature current in either direction
giving the motor in quadrent Iand II.
Now SW3 is closed continously and SW1 and SW4 are controled again a two quadrent chopper is
obtainedwhich can supply a variable negative voltageand the armature current in either direction
giving the motor in quadrent III and IV.
Change over from forward motoring to reverse motoring the following steps are involved,
In first quadrent SW4 is on continously and SW1 and SW2 are being controlled. For changeover δ
reducd to itds minimum value. The motor current reverses and and reaches the maximum value.
Fig 11: Outputwaveforms of class E chopper fed DC drive
Type Chopper configuration
V-I
charact
eristics
Function
CLASS A
CHOPPER
(step
down)
Va = V0 for S1 on
Va = 0 for S1 off and D1 on
CLASS B
CHOPPER
(step - up)
Va = 0 for S2 on
Va = V0 for S2 off and
D2 on
CLASS C
CHOPPER
ea = V0 for S1 or D2 on
(I quadrant)
ea = V0 for S2 or D1 on
(II quadrant)
ia>0 for S1 or D1 on
ia<0 for S2 or D2 on
CLASS D
CHOPPER
Va = +V0 for S1&S2 on
(I quadrant)
Va = -V0 for S1&S2 off and
D1&D2 on (IV quadrant)
CLASS E
CHOPPER
Four
quadrant
chopper
S4 on &S3 off S1&S2
operated
Va>0 ia - reversible
S2 on &S1 off S3&S4
operated
Va<0 ia - reversible
CLOSED LOOP OPERATION:
The speed controlled dc motor chopper drives is very similar to the phase controlled dc- motor in its
outer speed-control loop. The inner current loop and its controlare distinctly different from those of
the phasee controlled dc motor drives. This difference is due to the particular characteristics of the
chopper power stage. The current loop and the speed loop uses a controller .The controller used may
be a P or aPI controller. High performance drives usually uses a PI controller rather than a simple P
controller because PI controller provides a zero steady state current error, whereas the proportional
controller will have a steady state error.
G1
G2
G3
G4
Ia N(act)
Vc
I N(ref)
CURRENT CONTROLLER
SPEED CONTROLLER
Fig 12 : Closed loop operation of class E chopper fed DC drive
COSINE
FIRING
SCHEME
CLASS E
CHOPPER
<= DESIRED
SPEED
<=
DC
MOTOR
COSINE FIRING SCHEME:
Fig 12 : Block diagram of cosine firing scheme
Fig 13 : Output waveforms of cosine firing scheme
COSINE
WAVE
CONTROL
VOLTAGE
COMPARATOR
PULSE 1, 4
180˚ SHIFT PULSE 2, 3
SINE WAVE ZERO CROSSING
DEDECTOR
AND
OPERATION:
The closed loop operation involves the control of DC drive speed. The voltage from chopper(class
E) is fed to the DC drive as in the case of open loop and the output voltage current speed torque of
dc motor the dc motor is measured. The dc motor produces a speed which is not desirable hence the
speed has to be controlled so that the desired speed is obatained. The actual speed of the motor is
controlled with the desired or the reference speed. The error output corresponds to the actual
current. The PI speed controller has to be designed such that it prodces a current output. This current
has to be compared with the actual current output(armature current) of the dc motor. The error
output is passed again passed through a PI controller which is a current controller. The output of the
current controller is a constant may be negative or positive. It is the control voltage which has to be
compared with the reference cosine wave. We use a cosine firing scheme for generating pulse for
the chopper circuit used(class E chopper). Other firing schemes can also be used for producing
pulse.
The new pulses generated in accordance will make the input to the dc drive such that it produces the
desired speed.
TORQUE RIPPLE:
Torque ripple is the difference between the maximum and minimum value of torque in one
complete cycle.
CHOPPER
TYPE
TORQUE
RIPPLE
QUADRENT
OF
OPERATION
CLASS A 0.65 I
CLASS B 1.45 II
CLASS C 0.01 I,III
CLASS D I,IV
CLASS
E(open
loop)
0.01 I,II,III,IV
CLASS
E(Closed
loop)
0.01 I,II,III,IV
CONCLUSION:
The speed of a dc motor has been efficiently controlled by using Chopper along with PI type Speed and
Current controller. when compared to that of a phase controlled rectifier the regenerative braking is
made effective in a chopper fed dc drive and requires small filters at high ripple frequencies. Ultimately
simulation is done for the open loop and closed loop chopper fed dc drive is done.
The characteristics of dc drive processing commutators and brushes make it uneconomical and
undesirable for high speeds. But the reduced complexity of chopper circuit makes it available with lesser
cost compared to rectifier controlled dc drive.
REFERENCE:
1. Muhammad H .Rashid “Power Electronics –Circuits, Devices & Applications” Prentice-Hall
of India Private Ltd, Second Edition 1988
2. M.D Singh, K.B Kanchandani,”Power Electronics” Tata McGraw Hill Publishing Company
Limited New Delhi 1998.
3. “Power Electronics”; P.C. Sen; Tata McGraw Hill Publishing Company Limited 1995.
4. “Power Electronics, circuits, devices and applications”; Second Edition; Muhammad H Rashid
Prentice-Hall of India: 1994
SN
O
COMPONENTS TOOL BOX BLOCK
NAME
PARAMETER
S
REMARKS
1 DC VOLTAGE
SOURCE
SIMPOWER
SYSTEMS-
ELECTRICAL
SOURCE
DC VOLTAGE
SOURCE
AMPLITUDE
(V) = 220
PROVIDES A
CONSTANT DC
SUPPLY
2
SWITCH
SIMPOWER
SYSTEMS–
POWER
ELECTRONICS
IDEAL SWITCH
DEFAULT
CONTROLABLE
SWITCH
3 DIODE
SIMPOWER
SYSTEMS–
POWER
ELECTRONICS
DIODE
DEFAULT
UNCONTROLLA
BLE SWITCH
4
CONSTANT
COMMONLY
USED BLOCK
CONSTANT
SUITABEL
VALUE
CONTROL
VOLTAGE(Vdc)
REFERENCE
SPEED
5
MEASUREMENT
SIMPOWER
SYSTEMS-
MEASUREMENT
S
CURRENT
MEASUREMEN
T
VOLTAGE
MEASUREMEN
T
DEFAULT
INPUT AND
OUTPUT
CURRENT
INPUT AND
OUTPUT
VOLTAGE
6
DC SEPARATELY
EXCITED MOTOR
SIMPOWER
SYSTEMS–
MACHINES
DC MACHINES
DEFAULT
LOAD
9
POWERGUI
SIMPOWER
SYSTEMS
POWERGUI
CONTINOUS
IT IS TO STORE
EQUIVALENT
SIMULINK
CIRCUIT
10
COMPARATOR
SIMULINK-
MATH
OPERATION
REPEATING
SEQUENCE
GREATER THAN
OR EQUAL TO
(>=)
COMPARES TWO
SIGNAL AND
PRODUCES
ERROR
11
LOGICAL
OPERATOR
SIMULINK-
COMMONLY
USED BLOCK
LOGICAL
OPERATOR
AND
IT PERFORMS
THE LOGICAL
AND OPERATION
OVER TWO
SIGNALS
12 AC VOLTAGE
SOURCE
SIMPOWER
SYSTEMS -
ELECTRICAL
SOURCE
AC VOLTAGE
SOURCE(2)
5 V, 50Hz, NO
PHASE SHIFT
5V, 50Hz, PHASE
SHIFT OF 90
USED IN COSINE
FIRING(CLOSED
LOOP
OPERATION)
13
TRANSPORT
DELAY
SIMULINK-
CONTINOUS
TRANSPORT
DELAY
TIME
DELAY=0.01
OTHER
PARAMETERS
DEFAULT
PULSE
GENERATION
FOR NEGATIVE
PAIR OF
SWITCHES
14
REPEATING
SEQUENCE
SIMPOWER
SYSTEMS-
ELECTRICAL
SOURCE
REPEATING
SEQUENCE
VALUES BASED
ON DUTY
RATIO
PULSE
GENERATION
15
SCOPE
SIMULINK-
COMMONLY
USED BLOCK
SCOPE DEFAULT
VIEW THE
SIGNALS AT
OUTPUT AS
WELL AS INPUT