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