idc manual

8/17/2019 Idc Manual

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Experiment -1

Aim: To study the fundamental and block diagram of Electrical drive.

Apparatus: Ac Voltage Source, PEM, Electric motor, Controller, Load, Driver, Feed forwardand Feedback components.

Theory:

For study of block diagram of power electronics drives it require to know the basic definitions of

electronics and power electronics and what is drive?

1. Electronics: It is the branch of science, engineering and technology which is deal with

electronics devices in which conduction take place due to motion of electrons under the influence

of externally applied electrical or magnetic field in vacuum, gas & semiconductor and its

utilization.

2. Power Electronics: It is defined as field of science, engineering and technology which deal

with the power electronics modulator (PEM) in which efficient, conversion, conditioning,

processing, controlling and modulating the flow of large electrical power using solid state power

semiconductor devices in order to supply high quality power to the load and causing minimum

pollution of environment with regulating stability and response characteristics of the closed loop

system and its apps.

3. Motor Drive: An electric motor together with its electronic control equipment and energy

transmitting devices forms an electrical motor drive.

The bock diagram of electrical motor drive is as shown in figure.it is closed loop in nature.so it

has two path,

Path-1 Forward path which is shown by straight line arrows.

Path-2 Feedback path which is shown as dashes lines arrows.

The forward path consist of different blocks like as

Input source, input filter, PEM, output filter, motor and load

The feedback path consists of Electrical or non-electrical variable feedback, controller and

driver.

The motor and load is connected through energy transmitting link which is shown in block

diagram.


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Feed forward component

Fig: Block diagram of Power Electronics Motor drives

Description of each block:

1) Input source:

Very low power drives are generally fed from single-phase sources; low and medium power

motors are fed from three-phase 400 V supply, large motors may be rated at 3.3 kv, 6.6 kv and

11 kv.

Some drives are powered from a battery voltage may be 24 V, 48 V or 110 V dc.

Application 500 To 759 KV DC supply is utilized. 115 V, 400 HZ supply is used for the aircraft

and space application. For line traction application 65 KV, 50 HZ supply used and for

underground traction

2) Input and Output filters:

The electronic filters are circuit which is performing signal processing functions, specifically to

remove unwanted frequency component from the signal, to enhance wanted ones, or both.

SourceInput

Filter

PEM Output

filterMotor

Load

Driver

Electrical

Variable

Feedback

Non Electrical

Variable

Feedback

Controller

Command

Energy

transmitting

link


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3) Power Electronics Modulators (PEM):

A Power Electronics is a heart of Power Electronics System which modulates the power

available from the source as required by the load with the command input given by controller.

The power modulator performs the following functions:

i.

It modulates the flow of power from the source to the motor in such a manner that motoris imparted speed-torque characteristics required by the load.

ii. A Power Electronics is a heart of Power Electronics System which modulates the power

available from the source as required by the load with the command input given by

controller.

iii. The power modulator performs the following functions :

iv. It modulates the flow of power from the source to the motor in such a manner that motor

is imparted speed-torque characteristics required by the load.

4) Controller:

The controls for a power modulator are provides in the control unit.

The nature of the control unit for a particular drive depends on the power modulator that used.

When semiconductor converters are used, the control unit consists of firing circuits which

employ linear and Digital Integrated Circuits, transistors and microprocessor are used when

sophisticated control is required.

Control for power electronics modulator is built in control unit which usually operates at much

lower voltage and power levels.

In addition to operating the power electronics modulator it may also generate commands for the

protection of power electronic modulators and motors.

Input command signal which adjusting the operating point of the drive by analyzing the feedback

single in the controller.

The controller is realized with analog and integrated circuits. The present trend is to use

microprocessors, single chip modulators, Digital Signal Processors (DSP), VLSI and special

custom chips known as Application Specifics ICs (ASIC) to embody a set of functions in the

controller.

5) Sensors:

Speed sensing is required for implementation of closed loop speed control schemes. Speed is

usually sensed by using tachometers are used.

Two commonly used methods of sensing the current are: (i) using current sensors employing

Hall Effect, and (ii) Using a non-inductive resistance shunt in conjunction with an isolation

amplifier, which has an arrangement for amplification and isolation between the power and

control circuits.


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6) Energy transmitting link:

The energy transmitting link is placed between motor / equipment and load.

There are basically four different methods for energy transmitting links like as

i. Direct coupling method

ii.

Using chain

iii. Using belt coupling

Conclusion:___________________________________________________________________

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Experiment 2

Aim: To study different method for speed control of DC motor .

Apparatus: DC Motor, External Field, Connecting Wires, 1Phase Varaic

Theory:

DC motors are used extensively in adjustable-speed drives and position control applications.

Their speeds below the base speed can be controlled by armature-voltage control. Speeds abovethe base speed are obtained by field-flux control. As speed control method for DC motors are

simpler and less expensive than those for the AC motors, DC motors are preferred where wide

speed range control is required. DC choppers also provide variable dc output voltage from a

fixed dc input voltage.The Chopper circuit used can operate in all the four quadrants of the V-I plane. The

output voltage and current can be controlled both in magnitude as well as in direction so the

power flow can be in either direction. The four-quadrant chopper is widely used in reversible dc

motor drives. By applying chopper it is possible to implement regeneration and dynamic brakingfor dc motors.

METHOD OF SPEED CONTROL

DC motor can be generally control by two method given below.

1. ARMATURE OR RHEOSTATIC CONTROL

2. ARMATURE VOLTAGE CONTROL

3. FIELD FLUX CONTROL

4. POWER ELECTRONICS CONVERTERS

Armature or Rheostatic control is preferred by because of high efficiency, good transient

response and good speed regulation. But it can provide speed control only below base speed, because the armature voltage can allow to exceed rated value. For speed control above the base

speed, field flux control is employed. In a normally design motor, the maximum speed can be

allow up to twice rated speed and in specially designed machine it can be six times rated speed.This two type of method of the speed control of DC motor is not efficient and also causes more

power loss. So for the fully control of the motor in wide range third method power electronics

converter is generally used now a day.

SPPED EQUATION FOR DC MOTOR

= +

= −

But here the back EMF is given as below,


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=

60

So,

60= −

= −

×

60

...

Now

− = So,

=

×

60

. . . =

It show that speed is directly proportional to the back EMF and inversely proportional to the

flux on ∝

.

1.

ARMATURE OR RHEOSTATIC CONTROL

Fig. Armature control method


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Speed of the motor is directly proportional to the back EMF E b and E b= V Ia R a . That iswhen supply voltage V and armature resistance R a are kept constant, speed is directly

proportional to armature current Ia. Thus if we add resistance in series with armature, Ia decrease

and hence speed decreases. Greater the resistance in series with armature, greater the decrease inspeed.

2. ARMATURE VOLTAGE CONTROL

There is a two type of the armature voltage control method

A. Multiple voltage control

In this system, the shunt field of the motor is connected permanently to a fixed excited

voltage, but the armature is supplied with different voltage by connecting it across one of theseveral different voltages by mean f suitable switch gear. The armature speed will be

approximately proportional to this different voltages. The intermediate speeds can be obtain by

adjust the shunt filed regulator. The method is not much use.

B. Ward-Leonard system

This system is used where an usually wide up to 10:1 an very sensitive speed control is

required for colliery winders, electric excavator, elevator and the main drive in steel mills and

blooming and paper mill.

The arrangement is illustrated in the figure. M2 is the main motor whose speed control isrequired the filed of this motor is permanently connected across the DC supply lines. By

applying a variable voltage across its armature, any desired speed can be obtain. This variable

voltage is supplied by a motor generator set which consist of either a DC or an AC motor M1directly coupled to generator G.


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The motor M1 run at an approximately constant speed. The output voltage of G is

directly fed to the main motor M2. The voltage of generator can be varied from zero up to its

maximum value. By mean of its field regulator. By reversing the direction of field current of G by mean of the reversing switch Rs, generated voltage can be reverse and hence the direction of

rotation of M2. It should be remember that motor generator set always run in a same direction.

Advantages of Ward Leonard System:

1. It is a very smooth speed control system over a very wide range (from zero to normal speed

of the motor).

2. The speed can be controlled in both the direction of rotation of the motor easily.

3. The motor can run with a uniform acceleration.

4. Speed regulation of DC motor in this ward Leonard system is very good.

Disadvantages of Ward Leonard System:

1.

The system is very costly because two extra machines (motor-generator set) are required.2. Overall efficiency of the system is not sufficient especially it is lightly loaded.

Application of Ward Leonard System

This Ward Leonard method of speed control system is used where a very wide and very sensitive

speed control is of a DC motor in both the direction of rotation is required. This speed control

system is mainly used in colliery winders, cranes, electric excavators, mine hoists, elevators,steel rolling mills and paper machines etc.

3. FIELD FLUX CONTROL

Fig. Field flux control

It is seen from that the speed of the motor is inversely proportional to the flux. Thus bythe reducing the flux speed can be increased and via versa. To control the flux, a rheostat is

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added in series with the field winding, as shown in the diagram. Adding more resistance in

series with field winding will increase the speed, as it will decrease the flux. Field current is

relatively small and hence I2R losses is small, hence this method is quiet efficient. Thoughspeed can be increased by the reducing flux with this method, it puts a limit to the maximum

speed as weakening of flux beyond the limit will adversely affect the communication.

4. POWER ELECTRONICS CONVERTERS

By the using of the power electronics converter the armature terminal voltage of the DC

motor can be control from the zero to maximum. This method is known as the supply voltagecontrol of the DC motor. The speed of the motor can be controlled by the single phase or three

phase controller. Depending upon the type of power electronics converter used in the armature

circuit, single phase Dc drive may be subdivided as under.

SINGLE-PHASE DRIVES

1.

Single-phase half-wave converter drives.2.

Single-phase semi-converter drives.

3. Single-phase full-converter drives.

4. Single-phase dual converter drives.

THREE-PHASE DRIVES

1. Three-phase half-wave drives2. Three-phase semi-converter drives

3. Three-phase full-converter drives

Conclusion: ______________________________________________________________________________

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Experiment 3

Aim: To construct a single phase half controlled full wave bridge rectifier and observe waveform

with R-L load with free-wheeling diode.

Apparatus: 230V AC input, 30V output AC step down transformer, controlled rectifier module,firing unit, loading rheostat, DC motor, patch chords.

Theory:

Rectifier converts AC supply to DC supply. Variable DC supply( 0V to maximum ) can be

obtained from a fixed AC source by triggering SCR by applying gate current to SCR at any

desired instant when SCR is applied with positive voltage to anode. For DC power requirements

such as in DC drives single phase full wave rectifier are used.

CONTINUOUS CONDUCTION MODE:

Single phase half controlled bridge with free-wheeling diode:

When single phase semi converter is connected with R-L load a freewheeling diode must be

connected across the load. During positive half cycle diode D1 is forward biased and T1 is fired

at ωt=α, load is connected to input supply through T1 and D1 during period ≤ ≤π. During

this period, input voltage is negative and freewheeling diode DF is forward biased. DF conducts to

provide continuity of current in inductive load. Load current is transferred from T1 and D1 to DF

and thyristor T1 and D1 are turned off at =π. During negative half cycle of input voltage,

thyristor T2 is forward biased and firing of T2 at = will reverse bias DF. The diode DF

is turned off and load connected to supply throughT2 and D2.

When load is inductive and T1 is triggered first, it will conduct with D1 to pass current through

load. When supply voltage is negative, load emf will drive current through T1D2. When new

negative half cycle begins, T1 is in conduction and is conducting with D1 as if triggered at =

0.To ensure proper operation at beginning of positive half cycle, T2 has to be turned off and

similarly T1 has to be turned off when negative half cycle begins. This is achieved by


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freewheeling diode that applies negative voltage to T1 and T2 by bypassing load. For R-L load,

average output voltage is:

= =

, < <

= 0 =

, < <

With a single phase semi converter in the armature circuit, the equation 1 & 2 gives the average

armature voltage,

=

(1 cos )

The steady state speed equation is given by,

=−

∅ =

(+ )

∅ -

( ∅)2

Where, =

∅

The no-load speed of the motor is given by,

N N-L = = (+ )

∅

DISCONTINUOUS CONDUCTION MODE:

The armature current becomes discontinuous for large values of the firing angle, high speed and

low values of torque. The speed regulation will be significantly poor in the region of

discontinuous armature current. The motor performance deteriorates with discontinuous armature

current. The ratio of peak to average and RMS to average armature current increases. It is,

therefore, desirable to operate the motor in the continuous current mode. To achieve this, an

external armature circuit choke may be used which decreases the rate of current decay during the

freewheeling operation.

The voltage and current waveform for a semi converter with discontinuous current are shown in

waveform. For the period, < < , the motor is connected to the input supply through T

and D. Beyond π, the motor terminal is shorted through the diode. The armature current decays

to zero at angle β before the thyristor T2 is triggered at (π+α) thereby making the same as the

supply voltage e. However, during the motor current freewheels through DF and ea is zero.

In the internal < < , the motor coasts and the motor terminal voltage ea is the same

as the back emf e b.


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Conclusion:

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Experiment 4

Aim: To study and simulate 1-Φ full controlled converter of separately excited motor

Apparatus: 230V AC input, 30V output AC step down transformer, controlled rectifier

module, firing unit, loading rheostat, DC motor, patch chords

Theory:

A full converter is a two quadrant converter in which the voltage polarity of the output can

reverse, but the current remains unidirectional because of the unidirectional thyristors.

CONTINUOUS CONDUCTION MODE:

The single phase fully controlled bridge converter is obtained by replacing all the diode of the

corresponding uncontrolled converter by thyristors. Thyristors T1 and T2 are fired together

while T3 and T4 are fired 180º after T1 and T2. From the circuit diagram of Fig 10.3(a) it is

clear that for any load current to flow at least one thyristor from the top group (T1, T3) and

one thyristor from the bottom group (T2, T4) must conduct. It can also be argued that neither

T1T3 nor T2T4 can conduct simultaneously. For example whenever T3 and T4 are in the

forward blocking state and a gate pulse is applied to them, they turn ON and at the same time

a negative voltage is applied across T1 and T2 commutating them immediately. Similar

argument holds for T1 and T2. For the same reason T1T4 or T2T3 can not conduct

simultaneously. Therefore, the only possible conduction modes when the current i0 can flow

are T1T2 and T3T4. Of coarse it is possible that at a given moment none of the thyristors

conduct. This situation will typically occur when the load current becomes zero in between

the firings of T1T2 and T3T4. Once the load current becomes zero all thyristors remain off.

In this mode the load current remains zero. Consequently the converter is said to be operating

in the discontinuous conduction mode. Under normal operating condition of the converter the

load current may or may not remain zero over some interval of the input voltage cycle. If i0 is

always greater than zero then the converter is said to be operating in the continuous


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conduction mode. In this mode of operation of the converter T1T2 and T3T4 conducts for

alternate half cycle of the input supply.

Torque- speed characteristics: With a single phase, full converter in the armature circuit,

Ea=2

sin for 0≤α˂π from period

The steady state speed equation,

=−

∅=

co

∅ -

(∅)

Where, Ia =

∅

The no-load speed of the motor is given by,

N N-L =2 co

∅ where, T=0

DISCONTINUOUS CONDUCTION MODE:

SCR T1 and T3 are fired at α and armature current flows from α to β. The motor armature is

connected to the supply from α to β. From period β to (π+α), the motor coasts and the

armature voltage is the back emf of the motor. Again at (π+α), SCR T 2 and T4 are triggered

and conduction continues upto (π+β). In the discontinuous current mode, the difficulty arise

in the calculation of the average motor terminal voltage Ea, because β depends on the average

speed N, average armature current Ia and firing angle α.


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Conclusion:

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Experiment 5

Aim :-To study control techniques used in dc chopper.

Apparatus :- CRO, Connecting Probes, Matlab

Theory :-

In DC-DC converters, the average output voltage is controlled by varying the alpha (α) value.

This is achieved by varying the Duty Cycle of the switching pulses. Duty cycle can be varied

usually in 2 ways:

1.

Time Ratio Control

2.

Current Limit Control

As we all know that Duty Cycle is the ratio of ‘On Time’ to ‘Time Period of a pulse’.

1.Time Ratio Control :- As the name suggest, here the time ratio (i.e. the duty cycle ratio Ton/T) is varied.

This kind of control can be achieved using 2 ways:

Constant frequency control

Variable frequency control

A. Constant frequency control :-

In this technique, the time period is kept constant, but the ‘On Time’ or the ‘OFF Time’ is

varied. Using this, the duty cycle ratio can be varied. Since the ON time or the ‘pulse width’

is getting changed in this method, so it is popularly known as Pulse width modulation .


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B. Variable frequency control:-

In this control method, the ‘Time Period’ is varied while keeping either of ‘On Time’ or

‘OFF time’ as constant. In this method, since the time period gets changed, so the frequency

also changes accordingly, so this method is known as fr equency modulati on control.

2.Current Limit Control :

As is obvious from its name, in this control strategy, a specific limit is applied on the current

variation.

In this method, current is allowed to fluctuate or change only between 2 values i.e. maximum

current (I max) and minimum current (I min). When the current is at minimum value, the

chopper is switched ON. After this instance, the current starts increasing, and when it reaches

up to maximum value, the chopper is switched off allowing the current to fall back to

minimum value. This cycle continues again and again.

Current Limit Control

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Conclusion :-

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Experiment 6

Aim: To study control of DC motor for (a) Current Limit Control (b) Closed loop torque control

(c) Closed loop speed control.

Apparatus: DC Motor, PEM, Controller, Feedback Components, Connecting Wires

Theory:

We will see through different control configurations which are used in DC motor.

(a) Current Limit Control

During the starting, we know if precautionary measures are not taken there is a chance of

huge current flow through the motor circuit. The huge amount of current can damage motor, so it

is necessary to limit the current to safe limit. To limit the current and sense the current fed to the

motor, current limit control scheme is employed. The feedback loop does not effect the normal

operation of the drive but if the current exceeds the predetermined safe limit, the feedback loop

activates and the current is brought down below the safe limit. Once the current is brought down

below the safe limit the feedback loop again deactivates and in this way the control

of current takes place.

[Current limit control]

(b) Closed Loop Torque Control

This type of torque controller is seen mainly in battery operated vehicles like cars, trains etc. the

accelerator present in the vehicles is pressed by the driver to set the reference torque T. The

actual torque T follows the T* which is controlled by the driver via accelerator. By putting

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appropriate pressure on the accelerator, driver adjusts the speed depending on traffic, road

condition, his liking, car condition and speed limit.

[Closed-loop torque control]

(c) Closed loop speed control of DC motor:

DC Motor is very extensively used machine where the speed control is desired. The

operation of DC motor in different steps is easy compared to AC motors. By the open loop

control the DC motor can be operated at any intermediate speed by changing the voltage,

armature current etc. But in open loop (Prediction based) control accuracy in speed cannot be

obtained i.e. the speed will not be constant for load variations on the motor. There will not be

any feedback to the controller to indicate the change in speed due to load. This disadvantage

restricts the use of open loop speed control DC motor in applications where constant speed is

essential.

[Closed loop speed control of DC motor]


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To avoid this disadvantage a closed loop technique is implemented where the output measured

speed is fed back to the speed controller. In closed loop controller the speed can be maintained

by adjusting terminal voltage according the speed difference caused by the load torque, i.e. a fine

control of speed can be obtained using closed loop speed control. The below figure shows the

basic block diagram of closed loop speed control.

The measured speed at the motor shaft is fed back and compared with reference

speed. The difference speed error is applied to the speed controller to generate a control voltage

Vc which controls the power converter and produces the desired terminal Vt .This terminal

voltage controls the speed of the motor and thus the speed of the motor can maintained for any

variations in the load torque. For example if the load torque has been increased, due to this high

load the motor speed reduces momentarily from its desired value. Thus the speed error at theoutput of the comparator increases which increases the control signal Vc. The speed controller

can be proportional(P) or proportional integral(PI) and it also takes care of the maximum speed

limit of the motor. This control signal decreases the firing angle of the thyristor if it is phase

controlled converter or it increases the duty cycle to increase the switching On time if it is a

chopper converter and thus the output terminal voltage will increase. This increased terminal

voltage (armature voltage) develops more torque to compensate the effect of load torque and to

maintain the constant speed. The same operation in the reverse way will be performed if the load

torque reduces. When the applied torque matches the load torque then the motor speed will be

constant. There will be small oscillations during this control which can be reduced by the perfect

controller time constant.

Conclusion:

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______________________________________________________________________________ ______________________________________________________________________________

______________________________________________________________________________


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Experiment 7

Aim: - To study chopper control of D.C. Motor for Motoring and Generating Control.

Apparatus: - DC Motor, DC-DC Chopper Circuit, 1-phase AC Supply Voltage

Theory: -

When variable dc voltage is to be obtained from fixed dc voltage, dc chopper is the ideal

choice. Use of chopper in traction systems is now accepted all over the world. A chopper is inserted

in between a fixed voltage dc source and the dc motor armature for its speed control below base

speed. In addition, chopper is easily adaptable for regenerative braking of dc motors and thus

kinetic energy of the drive can be returned to the dc source. This results in overall energy saving

which is the most welcome feature in transportation systems requiring frequent stops, as for

example in rapid transit systems.

Though choppers can be used for dynamic braking and for combined regenerative and dynamic

control of dc drives, only the following two control modes are described in what follows:

1. Power control or motoring control

2. Regenerative-braking control


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Power control or motoring control

Figure 1

Fig. 1(a) shows basic arrangement of a dc chopper feeding power to a dc series motor. The chopper

is shown to consist of a force-commutated thyristor, it could equally well be a transistor switch. It

offers one-quadrant drive, Fig. 1 (b). Armature current is assumed continuous and ripple free. The

waveforms for the source voltage Vs, armature terminal voltage v t = vQ, armature current ia, dc

source current is and freewheeling-diode current ifd are sketched in Fig. 1(c). From these

waveforms, the following relations can be obtained:

Average motor voltage,

V0 = Vt = — α Vs = f .Ton. Vs

Where,

α = duty cycle = Ton/T &,

f = chopping frequency = 1/T

Power delivered to motor = (Average motor voltage) (average motor current) = Vt.Ia = α.Vs.Ia


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Average source current = ( Ton / T ) Ia = α.Ia

Input power to chopper = (average input voltage) (average source current) = Vs.α.Ia

For the motor armature circuit , Vt = α.Vs = Ea + Ia (r a + r s) = K m.ωm + Ia (r a + r s)

or, ωm = [ α.Vs - Ia (r a + r s) ] / K m It is seen that by varying the duty cycle α of the chopper, armature terminal voltage can be

controlled and thus speed of the dc motor can be regulated.

So far, armature current ia has been assumed ripple free and accordingly, waveforms in Fig

1(c) are sketched. Actually, the motor armature current will rise during chopper on period and fall

during off period as shown in Fig. 2. By referring to this chopper, armature current ia(t) during on

period, is given by

Ia(t) = [Vs - Ea (1- e-Rt / L)] R + Imn. E

-Rt / L

The armature current during the off-period is given by

Ia(t) = [ - Ea (1- e-Rt/L)] R + Imn.e

-Rt / L

Here,

R = r a + r s & L = La + Ls

Under Steady – State Operating Conditions,

Vt = α.Vs = Ea + Ia.R

Figure 2


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Figure 3

Regenerative-Braking Control

In regenerative-braking control, the motor acts as a generator and the kinetic energy of the

motor and connected load is returned to the supply.

During motoring mode, armature current, Ia = (Vt – Ea)/r a, i.e. armature current is positive and the

motor consumes power. In case load drives the motor at a speed such that average value of motor

counter emf Ea ( = K m.ωm) exceeds Vt, Ia is reversed and power is delivered to the dc bus. Themotor is then working as a generator in the regenerative braking mode.

The principle of regenerative braking mode is explained with the help of Fig. 3, where a

separately-excited dc motor and a chopper are shown. For active loads, such as a train going down

the hill or a descending hoist, let it be assumed that motor counter emf Ea is more than the source

voltage Vs. When chopper CH is on, current through armature inductance La rises as the armature

terminals get short circuited through CH. Also, vt = 0 during Ton. When chopper is turned off, Ea

being more than source voltage Vs, diode D conducts and the energy stored in armature inductance

is transferred to the source. During Toff , vt = Vs. On the assumption of continuous and ripple free

armature current, the relevant voltage and current waveforms are shown in Fig. 3 (b).

With respect to first quadrant operation as offered by motoring control of Fig. 3 (a),

regenerative braking control offers second quadrant operation as armature terminal voltage has the

same polarity but the direction of armature current is reversed, Figs. 3 (a) and (c). From the

waveforms of Fig. 3 (b), the following relations can be derived:


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The average voltage across chopper (or armature terminals) is,

Vt = ( Toff / T).Vs = (1- α).Vs

Power generated by the motor = Vt Ia = (l - α).Vs.Ia

Motor emf generated, Ea = K m.ωm = Vt + Ia.r a = (l - α).Vs + Ia.r a

Motor speed during regenerative braking, ωm = [ (1 – α).Vs + Ia.r a ] / K m

With chopper on, La must store energy and current must rise, i.e. dia / dt must be positive

or ( Ea – Ia.r a) ≥ 0

When chopper is off, Ea - Ia . r a - La. ( dia / dt ) = Vs

Or

Vs – ( Ea - Ia . r a ) = - La. ( dia / dt )With chopper off, ( Ea - Ia . r a ) must be more than Vs for regeneration purposes and

therefore [Vs – ( Ea - Ia . r a )] must be negative. This is possible only if current decreases during off

period, i.e., dia / dt in the above expression must be negative.

The conditions for the two voltages and their polarity for the regenerative braking control

of dc separately-excited motor is ,

0 ≤ ( Ea - Ia . r a ) ≤ Vs

Regenerative braking control is effective only when motor speed is less than ωmx and more

than ωmn. This can be expressed as,

ωmn < ωm < ωmx

(Ia . r a) / K m < ωm < (Vs + Ia . r a) / K m

DC series motors, however, offer unstable operating characteristics during regenerative braking.

As such, regenerative braking of chopper-controlled series motors is difficult.

Two-quadrant Chopper Drives

Motoring control circuit for the chopper drives offers only first-quadrant drive, because

armature voltage and armature current remains positive over the entire range of speed control. Inregenerative braking, second-quadrant drive is obtained as armature terminal voltage remains

positive but the direction of armature current is reversed. In the two-quadrant dc motor drive, both

motoring mode as well as regenerative braking mode are carried out by one chopper configuration.

One such circuit is shown in Fig. 4, which consists of two choppers CH1, CH2 and two diodes D1,

D2 and a separately motor.


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Motoring Mode. When chopper CH1 is on, the supply voltage vs gets connected to

armature terminals and therefore armature current Ia rises. When Ch1 is turned off, Ia freewheels

through D1 and therefore Ia decays. This shows that with Ch1 and D1, motor control in first

Quadrant is obtained.

Regenerative Mode. When CH2 is turned on, the motor acts as a generator and the armaturecurrent Ia, rises and therefore energy is stored in armature inductance La. When CH2 is turned off,

D2 gets turned on and therefore direction of Ia is reversed. Now the energy stored in La is returned

to dc source and second quadrant operation is obtained as shown in figure. In this figure, first

quadrant operation of dc motor is sometimes called forward-motoring mode and second-quadrant

operation as forward regenerative-braking mode.

Figure 4

Four-quadrant Chopper Drives

In four-quadrant dc chopper drives, a motor can be made to work in forward-motoring mode,

forward regenerative braking mode, reverse motoring mode and reverse regenerative-braking

mode. the circuit shown in figure, offers four-quadrant operation of a separately-excited dc motor.

This circuit consists of four choppers, four diodes and a separately-excited dc motor. Its operation

in the four quadrants can be explained as under:

Forward motoring mode. During this mode or first-quadrant operation, choppers CH2,

CH3 are kept off, CH4 is kept on whereas Ch1 is operated. When CH1, Ch4 are on, motor voltage

is positive and positive armature current rises. When CH1 is turned off, positive armature current

free-wheels and decreases as it flows through CH4, D2. In this manner, controlled motor operation

in first quadrant is obtained.


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Forward regenerative-braking mode. A dc motor can work in the regenerative-braking

mode only if motor generated emf is made to exceed the dc source voltage. For obtaining this

mode, CH1, CH3 and CH4 are kept off whereas Ch2 is operated. When Ch2 is turned on, negative

armature current rises through CH2, D4, Ea, La, r a. when Ch2 is turned off, diodes D1, D4 are

turned on and the motor acting as a generator returns energy to the dc source. This results inforward regenerative-braking mode in the second-quadrant.

Reverse motoring mode. This operating mode is opposite to forward motoring mode.

Choppers CH1, CH4 are kept off, CH2 is kept on whereas CH3 is operated. When Ch3 and CH2

are on, armature gets connected to source voltage Vs so that both armature voltage Vt and armature

current Ia are negative. As armature current is reversed, motor torque is reversed and consequently

motoring mode in third quadrant is obtained. When Ch3 is turned off, negative armature current

freewheels through CH2, D4, Ea, La, r a; armature current decreases and thus speed control is

obtained in third quadrant. Note that during this mode, polarity of Ea must be opposite to that

shown in Fig. 5.

Figure 5

Reverse regenerative-braking mode. As in forward braking mode, reverse regenerative-

braking mode is feasible only if motor generated emf is made to exceed the dc source voltage. For

this operating mode, CH1, CH2 and CH3 are kept off whereas CH4 is operated. When CH4 is

turned on, positive armature current Ia rises through CH4, D2, La, Ea, r a. Note that in this mode

also, polarity of motor emf Ea must be opposite to that shown in figure. When CH4 is turned off,

diodes D2, D3 begin to conduct and motor acting as a generator energy to the dc source. This leads

to reverse regenerative-braking operation of the dc separately-excited motor in fourth quadrant.

note that in figure, the numbering of choppers is done to agree with the quadrants in which these

are operated. For example, CH1 is operated for first quadrant, ……, CH4 for fourth quadrant etc.


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Conclusion: -

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Experiment 8

Aim :- To study D.C. Motor drive using PLL.

Apparatus :- D.C. Motor, Phase Detector, Low Pass Filter, Converter, Speed Encoder

Theory :-

The degree of speed control required in industrial drives depends on the application. In some applications,

open-loop regulation of the drive motor may be adequate. In others, closed-loop feedback control is

required for precise speed control and fast response.

Conventionally, this is achieved by an analog servo feedback system in which any change in speed is

sensed by a tachometer and compared with a fixed reference voltage to generate an error signal. These

analog devices for speed sensing and comparing signals are not ideal and the speed regulation is more than

0.2%. With a proportional-integral controller, when a torque is applied or changed, there is a transientspeed dip. In addition, the steady-state speed is reached after a certain time interval. This type of operation

may not be satisfactory in certain drives where high quality products are desired.

These shortcomings of analog feedback control system can be overcome by using a digital phase-locked

loop control system. In a PLL control system, the motor speed is converted to a digital pulse train, which

is synchronized with a reference digital pulse train. In this way, by locking onto a reference frequency

precise control of speed is achieved.

Figure 1 :- Block diagram of Motor speed control using PLL-technique


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Figure 1 shows the block diagram of a converter-fed d.c. motor drive with phase-locked loop control and

the transfer function block diagram is shown in Figure 2. In this PLL system, speed encoder is used to

convert the motor speed to a digital pulse-train. The output of the encoder acts as the speed feedback signal

of frequency f o. The phase detector compares the reference pulse-train (or frequency). f r . with the feedback

frequency f o and provides a pulse-width modulated output voltage, E e which is proportional to the

difference in phases and frequencies of the reference and feedback pulse trains. The phase detector (orcomparator) is available in integrated circuits. A low-pass loop filter converts pulse train, Ee to a

continuous d.c. Ievel Ec which varies the output of the power converter and, in tum, the motor speed.

Figure 2 :- Transfer function model of phase locked loop control system.

When the motor runs at the same speed as the reference pulse train, the two frequencies would be

synchronized (or locked) together with a phase difference. The output of the phase detector would be a

constant voltage proportional to the phase-difference and the steady-state motor speed would be

maintained at a fixed value irrespective of the load on the motor. Any disturbance contributing to the speed

change would result in a phase difference and the output of the phase detector would respond immediately

to vary the speed of the motor in such as direction and magnitude as to retain the locking of the reference

and feedback frequencies. The response of the phase detector is very fast.


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As long as the two frequencies are locked, the speed regulation should ideally be zero. However, in

practice, the speed regulation is limited to 0.002% and this represents a significant improvement over the

analog speed control system.

PLL techniques have been used primarily in communication systems to synchronize signals. PLL are now

available as inexpensive digital integrated circuits. With the development of both integrated circuits and

solid-state power circuits, the use of PLL techniques in motor speed control has generated considerable

interest.

Conclusion :-

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EXPERIMENT 9

Aim:- To study solar and battery powered drives

Apparatus:-Solar panel, Battery, Chopper, Motor, Hard wire, Volt meter

Theory:-

SOLAR PANELS BASED DRIVE

A solar cell convert sunlight into electricity. Single crystal, polycrystal and amorphous silicon cells

have been employed.

The mounting of solar cells in series and parallel combination is known as solar panel. Each

parallel branch is provided with a diode in order to avoid circulating currents. When a panel

consisting of cells in series is used to charge a battery, a diode is connected so that current never

flows from the battery to the solar cell.

The output of a solar panel depends on the isolation level and temperature.

The output P vs V curve is also shows in figure. At present solar cells have low efficiency. The

cost of solar panels is Rs 100 -150 per peak watt.

Voltage current characteristic of solar panels

EXAMPLE: - SOLAR POWERED PUMP DRIVES

Water pumps are two types: centrifugal and reciprocating. Their speed torque characteristics are

shows in figure. Centrifugal pump required only small torque to start whereas reciprocating

pump owing to station may require as much as three times at rated torque.


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Here below in figure the solar pump drive employing the permanent magnet dc drive and its v/I

characteristics.

A simple scheme of solar pump drive using a permanent magnet dc motor shows in figure. The

solar panel directly feeds the motor.one can connect the solar cell to form low voltage, high

current unit.

Noting that in a permanent magnet dc motor, the torque is directly proportional to armature

current and back emf proportional to speed – torque characteristics

For different isolation level can be shows in figure.

Block diagram of solar pump drive

Solar

panel

BATTERY POWERD VEHICLE:-

Electrical vehicles are generally powered by lead acid batteries series and separately excited dc

motors, permeant magnet dc motor, brushless dc motor and induction motor have been used in

electric vehicles.

MOTOR

+

- θ

SOLAR

LOAD

MOTOR

+

- θ

SOLAR

LOAD

Charge

controller

Battery Chopper MotorPump


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A permanent magnet dc motor drive for a battery powered vehicles is shown in figure.

The drive employed chopper control with regenerative braking facility. LF and CF filter is

employed to filter out chopper control with regenerative braking facility. LF and CF filter is

employed to filter out the harmonic generated by the chopper. MS is a manual switch RS is

reversing switch. Inductance L is provided to assist regeneration and keep the ripple in motor

current low.

MOTORING OPERATION:

For motoring operation, MS is kept closed. Transistor switch t is operated at a constant frequency

with variable on time to obtain variable dc voltage for starting and speed control. When T is on,

the current flows through the source, LF,MS,L,R,Armature,S, and T. When T is off. The armature

cuttent freewheels S,D1,MS,L and R.

REGENERATIVE BRAKING:-

For regenerative braking operation, MS is kept open and motor armature is reversed with the helpof the reversing switch RS making B positive with respect to A. When T is on, armature current

builds up through the path consisting of T,D2 and L. When T is off, the armature current flow

against the battery voltage through the path consisting of D1,LF ,battery ,D2 and L and energy

feedback is use to charge the battery.


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Conclusion: -

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Experiment 10

Aim: To study about traction drives.

Apparatus:

Theory:

Electric traction means a locomotion in which the driving force is obtained from electric motors.

There are many advantages of electric traction over forms of locomotion. If the electric energy

could be generated cheaply and initial cost, which is enormous, could be born, then this system

of locomotion would replace steam locomotion entirely for electrification.

Principle of Traction Drive: A traction motor is an electric motor used for propulsion of a

vehicle, such as an electric locomotive or electric roadway vehicle.

Ideal traction system:

The most important requirements of the driving equipment for traction service are follows:

1. Maximum tractive effort should be exerted at starting in order that a rapid acceleration

may be attained.

2. The equipment should be capable of overloads for short periods.

3. The wear caused on the track should be minimum.

4. The locomotive or train unit should be self – contained and able to run on any route

5. Braking should be possible without excessive wear on brake shoes and if possible the

braking energy should be regenerated and returned to the supply.

Electric traction is one of the major utilizes of electricity at present. Day by day, the

transportation is changing over to electric traction, for obvious advantages.

Advantages:

1. It is the cheapest method of all the other methods of traction.

2. Smooth and rapid acceleration and braking.

3. It is free from smoke and flue gases.

4. Maintenance and cost of electric traction is 50% less than that of Steam traction.

5.

It can be stared without any loss of time whereas steam traction requires minimum 2hours before a steam locomotive can be put into operation.

Disadvantages:

1. Higher initial expenditure is involved in electric traction.

2. Failure of supply is a problem to be faced in electric traction.

3. The electrically operated vehicles have to move only on electrified track.


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4. When A.C. Energy is utilized for traction then precautions are to be taken to prevent the

distribution network to interfere with the adjacent telegraph and telephone lines.

5. For the achievement of electric braking and control, additional equipment is required. In

case if dc series motor the regenerative braking cannot be easily.

Traction motors are used in electrically powered rail vehicles such as electric multiple units and

other electric vehicles such as electric milk floats, elevators, conveyors, and trolleybuses, as well

as vehicles with electrical transmission systems such as diesel-electric, electric hybrid vehicles,

and battery electric vehicles.

DC motors with series field windings were the oldest type of traction motors. These provided a

speed-torque characteristic useful for propulsion, providing high torque at lower speeds for

acceleration of the vehicle, and declining torque as speed increased. To achieve better operating

conditions, AC railways were often supplied with current at a lower frequency than thecommercial supply used for general lighting and power.

Transportation Applications:

Road vehicles: Traditionally, road vehicles (cars, buses and trucks) have used diesel and

petrol engines with a mechanical or hydraulic transmission system. In the latter part of the

20th century, vehicles with electrical transmission system began to be developed — one

advantage of using electric motors is that specific types can regenerate energy — providing

braking as well as increasing overall efficiency.

Railways: Traditionally, these were series-wound brushed DC motors, usually running onapproximately 600 volts. The availability of high-powered semiconductors has now made

practical the use of much simpler, higher-reliability AC induction motors known as

asynchronous traction motors.

Conclusion:

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https://en.wikipedia.org/wiki/Universal_motorhttps://en.wikipedia.org/wiki/Alternating_currenthttps://en.wikipedia.org/wiki/Induction_motorhttps://en.wikipedia.org/wiki/Induction_motorhttps://en.wikipedia.org/wiki/Alternating_currenthttps://en.wikipedia.org/wiki/Universal_motor


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idc manual

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