Unit-4 Elecrical Drives and Controls II Part

8/22/2019 Unit-4 Elecrical Drives and Controls II Part

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ME 2205Electrical Drives and Controls Compiled by Mr. V.Suresh AP/EEE

1. Time Ratio Control method (TRC)

2. Current Limit Control method (CLC)

By changing the duty cycle, the motor speed also changes.

5.3. Time Ratio Control Method

In the Time Ratio Control, the value of is varied. This is effected in

two ways.

1. Constant Frequency System

2. Variable Frequency System.

1. Constant Frequency System

In this control method the on time Ton is varied but chopping frequency f (or

chopping period T) is kept constant. The width of the pulse is varied and this type

of control is known as pulse width modulation (PWM).

Figure: Principle of pulsewidth modulation (Constant T)

Figure shows illustrate the principle of pulse width modulation. Here

chopping period T is constant. In figure Ton = 1/4T, so that = 25%. In figure, Ton

= 3/4 T, so that = 75% or 0.75. Ideally can be varied from Zero to unity.


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Therefore output voltage Vo can be varied between zero to source voltage Vs

The constant frequency control gives low ripple and requires smaller size of filter

and has fast response. Thus this scheme is preferred for chopper drives.

2. Variable Frequency System

In this control method, the chopping frequency f (or chopping period T) is

Varied and either

1) On - time Ton is kept constant

2) Off- time toff is kept constant


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This method of controlling duty cycle is called frequency modulation

scheme (FM).Figure illustrates the principle of frequency modulation. In figure (a)

Ton is kept constant but T is varied.

In this upper diagram of figure Ton = l/4T, so that = 0.25. In lower diagram

of figure. Ton = 3/4T, so that = 0.75. In figure (b), Toff is kept constant and T is

varied. In the upper diagram of this figure (b). Ton = 1/4 T, so that = 0.75.

Frequency modulation control method has some disadvantages as compared

to pulse width modulation scheme.

The chopping frequency has to be varied over a wide range for the control ofoutput voltage in frequency modulation. The filter design is very difficult for

wide range of frequency variation.

This type of control would generate harmonics at unpredictable frequenciesWhich would produce interference with signaling and telephone lines.

The large of time in FM scheme may make the load current discontinuousWhich is undesirable.

5.4. Current Limit Control (CLC)

In this strategy, the chopper is switched ON and OFF so that the current in

the load is maintained between two limits Iomin to Iomax). The chopper is switched

ON when the output current equal to Iomin. (Lower limit). The chopper is kept

closed till load current increases upper limit (Iomax ). When the current exceeds

upper limit, the chopper is switched OFF. During off period, the load current

freewheels and decreases exponential. When the load current reaches the lower

limit (Iomin), the chopper is switched ON. Current limit control method is possible

either constant frequency and variable frequency system. The current limit control

is used only when the load has energy storage elements. Figure. Shows the

principle of current limit control.


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Figure: Current limit control method

5.5. First Quadrant or Type-A Chopper or Motoring Chopper

Figure. Shows circuit diagram of chopper circuit feeding power to DC motor.


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In this chopper circuit diagram consists of chopping device (SCR, Power

MOSFET, and BJT etc), freewheeling diode and motor.

Input DC supply is fed to the chopper circuit. Then the chopper CH1 is

turned on by applying trigger pulse. Now the input voltage is fed to the motor.

During the ON time (Ton) of the chopper, output voltage is equal to input voltage

i.e., V0 = Vs.

After Ton period, chopper CH1 is turned off. Now the load is disconnected

from the supply but the motor current flows through the freewheeling diode. In the

turn-off period (Toff), output voltage is zero but load current flow through the

freewheeling diode. (FD - motor - FD). i.e., V0

= 0 Again the chopper CH1, is

turned on and this cycle is repeated. Here assume armature current is constant.

Figure. Shows input and output wave forms of DC chopper.

Average output voltage is given by


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During the on time period Vo = Vs

During the off time period Vo = 0

Where = duty ratio or duty cycle of the chopper

Ton = Turn on time of chopper

Toff= Turn off time of chopper

T = Total time or chopping period (Ton + Toff)

Vs = Supply voltage

f = Chopping frequency

By varying the on time or off time of the chopper, the duty cycle a is change.

By changing , the output voltage can be changed. This variable output voltage is

fed to the DC motor. Then the DC motor speed can be controlled.

In the type A chopper offers one-quadrant drive. It means, the average

output voltage and output current is always positive. i.e., power flows from sourceto load. It is shown in figure.

This type of chopper is also called step down chopper because average

output voltage is always less than the input DC voltage.


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5.6. SecondQuadrant, or Type - B Chopper

Second quadrant or type-B chopper is shown in figure (a). In this chopper,the load must contain the dc source E, like a dc motor (or a battery).

When CH2 is on, output voltage is equal to zero, i.e, Va = 0 but load voltage

E drives current through L and CH2. During on time of the chopper (Ton), inductor

L stores energy.

When CH2 is off, output voltage Va = [E + L di / dt] exceeds source voltage

Vs. As a result, diode D2 is forward biased and conducts, thus allowing power to

flow to the source. Chopper CH2 may be on or off, load current I0 flows out of

the load. Here load current I0 is treated as negative.


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The power flows from load to source because output voltage Va is always

positive and load current Ia is negative. As load voltage Va is greater than the

source voltage Vs, type B chopper is also called as step-up chopper or boost

converter. It is also known as regenerative chopper. Figure shows the second

quadrant of Va - Ia plane.

5.7. Two quadrant type - A chopper or type - C chopper

Type C chopper is obtained by connecting type-A and type-B chopper in

parallel as shown in figure (a). Here, the output voltage Va is always positive but

the load current Ia is positive as well as negative.

When chopper CHI or FD conduct, the output voltage and load current is

always positive. In other words, CH1 and FD operate together as type-A chopper

in first quadrant, when chopper CH2 or diode D2 conduct, the output voltage is

positive but the load current is negative. In other words CH2 and D2 together as

type-B chopper in second quadrant.


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Figure: Type C chopper

Average load voltage is always positive but average load current may bepositive or negative as explained above. Therefore, power flow may be from

source to load (first quadrant operation) or from load to source (second quadrant

operation)

Choppers CH1 and CH2 should not be on simultaneously otherwise direct

short circuit will occur. This type of chopper configuration is used for motoring in

and regenerative braking of dc motors. The operating region of this type chopper is

shown in figure (b). By hatched area in first and second quadrants.

5.8. Two quadrant type-B chopper or type-D chopper

The power circuit diagram for two quadrant type B chopper or type D

chopper is shown in figure (a). The output voltage is equal to supply i.e. V0 = Vs

when both CHI and CH2 are on and output voltage is equal to value of supply

voltage i,e.Va = -Vs when both choppers are off but both diodes and D2 conduct.

Average output voltage Va is positive when on time of the choppers (Ton) is

more than their turn off time (T0ff) shown in figure (c). Average output Va is

negative when choppers turned off time (Toff) is more than their turn on time (Ton)

as shown on figure (d).


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The direction of load current is always positive because choppers and diodes

can conduct current only in the direction arrows shown in figure (a). Here output

voltage Va is negative, the power flows from load to source (Power flow is

reversible).The operation of this type-D chopper is shown by hatched area in first

and fourth quadrants in figure (b).

Figure (a) Type D chopper Figure (b) Va - Ia Plane

Figure (c) V0 is Positive Figure (d) V0 is negative

TON > TOFF TOFF > TON

5.9. Four quadrant chopper or type - E chopper

Figure shows the power circuit diagram for a four quadrant chopper or type

E chopper. It consists of four semiconductor switches CH1 to CH4 and four


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power diodes D1 to D4 in antiparallel. Working of this chopper in the four

quadrants is explained as follows.

Figure (a): Four quadrant or type E chopper

Figure (b): Operation of conducting devices


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Forward motoring mode: For first quadrant operation of figure (a), CH4 is kept

on, CH3 is kept off and CH1 is operated. With CH1, CH4 on, load is equal to

supply voltage i.e. Va = Vs and load current Ia begins to flow. Here both output

voltage Va and load current Ia are positive giving first quadrant operation. When

CH1 is turned off, positive current freewheels through CH4, D2 in this way, both

output voltage Va, load current Ia can be controlled in the first quadrant. First

quadrant operation gives forward motoring mode.

Forward braking mode: Here CH2 is operated and CHl, CH3 and CH4 are kept

off. With CH2 on, reverse (or negative) current flows through L, CH2, D4 and

E. During the on time of CH2 the inductor L stores energy. When CH2 is turned

off, current is fed back to source through diodes D1, D4 note that there [E + L di /

dt] is greater than the source voltage Vs. As the load voltage Va is positive and

load current Ia is negative, it is second quadrant operation of chopper. Also power

is flows from load to source. Second quadrant operation gives forward braking

mode.

Reverse motoring mode: For third quadrant operation of figure (a), CH1 is kept

off, CH2 is kept on and CH3 is operated. Polarity of load emf E must be reversed

for this quadrant operation. With CH3 on, load gets connected to source V s, so that

both output voltage Va, load current Ia are negative. It gives third quadrant

operation. It is also known as reverse motoring mode. When CH3 is turned off,

negative current freewheels through CH2, D4. In this way, output voltage Va and

load current Ia can be controlled in the third quadrant.

Reverse braking mode: Here CH4 is operated and other devices are kept off.

load emf E must have its polarity reversed to that shown in figure (a). For

operation in the fourth quadrant. With CH4 on, positive current flows through

CH4, D2, L and E. During the on time of CH4 inductor L stores energy. When

CH4 is turned off, current is feedback to source through diodes D2, D3. Here load


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voltage is negative, but load current is positive leading to the choppers operation in

the fourth quadrant. Also power is flows from load to source. The fourth quadrant

operation gives reverse braking mode. The devices conducting in the four

quadrants are indicated in figure (b).

6. University Solved Problems

1. A 500V series motor having armature and field resistances of 0.2 and0.3 ohm respectively runs at 500rpm when taking 70amps. Assuming

unsaturated field, find out its speed when field divertor of 0.684 is

used constant torque load.

(AU/MechMay 2007)

Given data:

Supply voltage V = 500V

Armature resistance Ra = 0.2


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2. A 250V dc series motor takes 40 ampere of current when developing afull load torque at 1500 rpm. Its resistance is 0.5. If the load torque

varies as the square of the speed, determine the resistance to be

connected on series with the armature to reduce the speed to 1200 rpm.

Assume that the flux is proportional to the field current.

(AU/Mech - Dec 2004)

Given data:

Supply voltage V = 250V

Armature current Ia1 = 40A


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42. Write the applications of dc chopper.

1. Electric traction

2. Steel mills

3. Printing mills

4. Hoists

5. Paper mills

6. Hoists

7. Excavators and cranes

8. Machine tools


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(TWO MARKS QUESTION)

1. What are the fields of application of WardLeonard drive?2. Define Duty cycle of DC chopper.3. Field control is employed for getting speeds higher than rated and armature

voltage control is employed for getting speed less than rated why?

4. What is time ratio control?5. How can speed be controlled in a DC shunt motor?6. List the advantages of six pulse converter compared with three pulse converter.7. Write the applications of DC chopper.8. State the advantage of Ward-Leonard control system


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299. What is static Ward - Leonard drive?10.What is chopper?11.State the advantages of DC chopper drives.

(16 MARKS QUESTION)

1. Describe with the help of a connection diagram the Ward Leonard method ofspeed control. Explain its operation starting, stopping and reversing. State the

merits and demerits of this scheme.

2. Discuss in brief various conventional methods of speed control of DCmotors.(shunt, series)

3. Draw and explain the operation of a speed control of a DC shunt motor by asingle phase fully controlled converter for continuous motor current.

4. Explain in detail the operation of a speed control of a DC series motor fed by asingle phase semi converter for the continuous motor current. Draw relevant

circuit diagram and waveforms.

5. Describe the operation of a two quadrant type A chopper fed separately excitedDC motor drive.

6. Explain the multi quadrant operation of chopper and methods of controlling achopper with neat sketch.

7. With neat diagram, explain the operation of two quadrant DC chopper feddrive.

8. Classify the characteristics of class A, B, C, D and E chopper drives.9. Draw and explain some of the commonly used controlled rectifier circuits for

DC drive.

10.Draw and explain some of the commonly used chopper circuits for DC drive.

Unit-4 Elecrical Drives and Controls II Part

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