1.0 OBJECTIVES

The main objectives are:

1. Determining how a permanent magnet DC motor may be controlled by varying the

magnitude and direction of its armature current.

2. Obtaining the characteristics of a DC motor by plotting torque and speed.

3. Establishing the scaling factor of the tachogenerator.

EQUIPMENTS

Below are the equipments and components used in the experiment.

1. Attenuator Unit (AU150B)

2. Servo amplifier (SA155BD)

3. Power supply (PS150E)

4. DC Motor (DCM150F)

5. Loading Unit (LU150L)

6. Reduction Gear Tacho Unit (GT150L)

7. Base Plate

8. Digital Multi-meter

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2.0 THEORY

Servo Motor is actually a combination of four things which is a normal DC motor,a

gear reduction, a position sensing device and a control circuit.The function of the servo is to

receive a control signal that represents a desired output position of the servo shaft and apply

power to its DC motor until its shaft turns to that position.It uses the position sensing device

to determine the rotational position of the shaft.So it knows which way the motor must turn to

move the shaft to the commanded position.The shaft typically does not rotate freely round

and round like a DC motor,but rather can only turn 200 degress or so back and forth.The

servo has 3 wire connection which is power ,ground and control.The power supply must be

constantly applied so the servo has its own drive electronics that draw current from the

power lead to drive the motor.

To recap, there are two important differences between the control pulse of the servo

motor versus the DC motor. First, on the servo motor duty cycle is on-time vs. off-time which

has no meaning of the matters in the absolute duration of the positive going pulse, which

corresponds to a commanded output position of the servo shaft. Second, the servo has its

own power electronics, so very little power flows over the control signal. All power is draw

from its power lead, which must be simply hooked up to a high-current source of 5 volts.

The motor is permanent magnet type and has a single armature winding.Current flow

through the armature is controlled by power amplifier as shown in Figure 5.1.As a motor

accelerate the armature generated an increasing “back electromagneticforce”,Va tending to

oppose the driving voltage.The armature current is roughly propotional to Voltage input,Vin

toward the Va. If the speed drop because of the attached load,,Va reduces and the current

and motor torque will increase.Figure 5.2 shows how the speed Varies with the load torque.

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3.0 PROCEDURE

1) The instrumentation was set up according to the connection diagram as shows in

Figure 5.3. The magnetic bases was use to organize the various apparatus.The

system was provided a tachogenerator couple to the motor and the generator

was necessary calibrate by finding the calibration factor,Kg,which is the volt

generated per thousand rev/min of motor shaft.The switch on top of the

GTX150X was used to display the tacho volt or speed as required.The

potentiometer was turned until the tachogenerator voltage,Vg is 1V on the volt

meter.The input voltage was recorded and output speed of DC servo motor.The

result in Table 5.1 was completed by increase the one volt step until 5V.The

Graph of speed against voltage of the tabulated result in Table 5.1 was

plotted.The calibration factor was determine by measuring the slope of the plotted

graph and the relationship between the input voltage and the output speed was

discussed.

2) For the No Load characteristic experiment,the potentiometer was varied until the

speed of DC servo motor shows a reading of 2000rpm.The loading unit(LU150L)

was set to scale 0, the loading unit(LU150L) was inserted into a moving disc.The

readings of the input voltage and the speed of corresponding tachogenerator

output was record.The result in Table 5.2 was completed by increase the scale of

the loading one step until to 10.

3) For Full Load characteristic experiment,the loading unit(LU150L) was set at scale

10. the potentiometer was varied until the speed of DC servo motor shows a

reading of approximate 200rpm . The readings of the input voltage and the speed

of corresponding tachogenerator output was record.The result in Table 5.3 was

completed by decreasing the scale of the loading one step until to 0.From Table

5.2 and Table 5.3,the speed against torque was ploted and from the plotted

graph was conclude

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4.0 RESULT

Table 5.1

Input voltage ,tachogenerator voltage and motor speed

Tachogenerator Voltage,Vg

(volts)

Input Voltage,Vin

(volts)

Speed

(r/min)

1.27 15 460

2.0 15 740

3.0 15 1080

4.0 15 1440

5.0 15 1880

Table 5.2

Brake Position,tachogenerator voltage and motor speed(Test No Load Characteristics)

Brake Position

(Magnetic Scale)

Tachogenerator Voltage,Vg

(volts)

Speed

(r/min)

0 5.61 2000

1 5.60 2000

2 5.53 1990

3 4.95 1780

4 4.41 1580

5 3.83 1380

6 3.46 1240

7 3.26 1170

8 2.69 960

9 2.17 780

10 1.96 700

8Table 5.3

Brake position,tachogenerator voltage and motor speed(Test Full Load Characteristics)

Brake Position

(Magnetic Scale)

Tachogenerator Voltage,Vg

(volts)

Speed

(r/min)

10 0.564 200

9 0.614 220

8 0.684 240

7 0.762 270

6 0.822 290

5 0.884 310

4 1.011 360

3 1.162 420

2 1.277 460

1 1.290 460

0 1.290 460

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5.0 DISCUSSION

In this experiment we conducted three experiment which were to know the

relationship between the tachogenerator voltage and motor speed,relationship between the

tachogenerator and brake position or called as Test No Load characteristic and Full Load

Test.

For the first experiment we conducted it by adjusted the Tachogenerator voltage with

the same input voltage.We started it with 1.27 tachogenerator voltage,Vg due to the

limitation of instrument which can’t adjusted lower than 1.27V.For the others we used the

fixed point which are 2,3,4 and 5.The data of the experiment was tabulated in Table 5.1 and

from there we realise that as Tachogenerator voltage,Vg increasing ,the speed in revolution

per minutes also will increase.This can be proven as in Graph 5.1 Vg against

Speed(r/min).So we can conclude that Vg is directly propotional toward Speed.

As for 2nd experiment we start conducted it by set up the speed to 2000 r/min.Then

we will start to take reading of Tachogenerator voltage ,Vg and recorded it in Table 5.2 by

insert the loading unit toward the moving disc by one step from 0 to 10.Roughly from the

Table 5.2 ,we can see that increasing in the break position will lead to decreasing on the

Tachogenerator voltage,Vg and speed,r/min.This is because of the as the loading unit

entered more in the disc area ,it will create greater electromagnetic force which will become

the friction or the load.So we can conclude that Vg and speed are inversly toward brake

position.

Last experiment we conducted it same as the 2nd experiment,but with different

started speed which is 200 r/min.Actually the needed speed is approximate 60~70 rpm but

seem the limitation of insturement which can started with the 200rpm.We start the

experiment with the loading unit scale at 10 ,then when we removed it by 1 step from 10 to

0.From the table 5.3,we can see that decreasing in the break position will lead to increasing

on the Tachogenerator voltage,Vg and speed,r/min, This is because of the as the loading

unit will slowly leaving the disc area ,it will lower the electromagnetic force existed which will

which the friction.So we can conclude that Vg and speed are inversly toward brake position

same as the 2nd experiment.So that why graph 5.2 ,we can see the same pattern of curve.

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6.0 CONCLUSION

In this experiment, we learned that permanent magnet DC motor can be controlled by

varying the magnitude and direction of its armature current. We also have seen the graph of

torque/speed which is the characteristics of the DC motor. Also, we can now establish the

scaling factor of the tachogenerator. So now we can conclude that speed of motor can be

controlled easily using varies of ways.Such in this experiment ,we control the speed of motor

using the load.When we can control the speed of motor ,so we can do many things with the

motor,example by adjusting the speed of the motor ,we also can control the tachogenerator

voltage, Vg values that we want.

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7.0 REFERENCES

[1] P.C.Sen, Principe of Electric Machines and Power Electronics, 2nd Edition,John Wiley

& Sons, Inc. 1997

[2] Hughes, A., Electrical Motors and Drives, Hinesmann Newnes, Nov 2005

[3] Alexander, C.K., and Sadiku, M.N.O.,Fundamentals of Electric Circuits Fourth

Edition, New York: McGraw Hill. 2004

[4] H. Dadhwal .(2012 February 13). How Dc Servo Motors Can be Advantageous and

Disadvantageous [Online] .Available:http://www.sooperarticles.com/technology-

articles/gadgets-gizmos-articles/how-dc-servo-motors-can-advantageous-

disadvantageous-826555.html

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TABLE OF CONTENT

1.0 Objectives and Equipments 1

2.0 Theory 2

3.0 Procedure 4

4.0 Result 5

5.0 Discussion 9

6.0 Conclusion 10

7.0 References 11