W3007 1B Exercises

1

DC Chopper Regulators W3007-1BConfidential. Copyright WUEKRO GmbH

Training & Didactic Systems

Experimental Manual

Power Electronics

DC Chopper Regulators

Order No. W3007-1B Edition 1. 2002

© WUEKRO GmbH

All rights reserved. Reprint not permitted.

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Technical data, order no. and quantities are a subject of alterations

All rights reservedCopyright by WUEKRO GmbH Würzburg, 2002

Contact address:

WUEKRO GmbH Hafenstr. 5D - 97424 SchweinfurtGermany

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Contents Page

1. DC Chopper regulators 4

1.1 Introduction 4

1.2 Principle of operation of DC chopper regulators 5

1.3 Functional description of the thyristor DC chopper regulator 9

1.4 Functional description of the Darlington DC chopper regulator 13

2. Putting DC chopper regulators into service 17

2.1 Procedures 17

2.2 Thyristor DC chopper regulators: list of equipment 19

2.3 Measuring setup: Thyristor DC chopper regulator 20

2.4 Thyristor DC chopper regulators: list of equipment 22

2.5 Measuring setup: Darlington DC chopper regulator 23

3. Measurements on the thyristor chopper 25

3.1 Establishing the power characteristics of a thyristor DC chopper regulator 25

3.2 Measurements at various points on the thyristor DC chopper regulator 29

4. Measurements on the Darlington DC chopper regulator 33

4.1 Power characteristics of a Darlington DC chopper regulator 33

4.2 Measurements at various points on the Darlington DC chopper regulator 37

5. Solutions to the questions and exercises 1. – 8.

Re. 1. DC chopper regulators

5.1 Answers to the thyristor chopper questions 3

Re. 1.3 Functional description of the thyristor DC chopper regulator

5.2 Answers to Darlington chopper regulator questions 6

Re. 1.4 Functional description of the Darlington DC chopper regulator

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DC chopper regulators

1.1 Introduction

DC chopper regulators are primarily used for controlling the speed of DC machines.

The best-known type of DC chopper regulator is the Ward-Leonard set, where a mechanically drivenDC generator is controlled via its field winding. The disadvantages of the Ward-Leonard set are itsrelatively poor efficiency, high initial costs and a limited correcting rate due to the exciter inductance's.

In the case of modern DC chopper regulators, the power from the DC system is supplied to the load inchopped or switched via suitable switching devices. Thyristors and power transistors are can be usedfor this purpose.

Unlike the switching of AC current by means of semiconductor valves, where the AC current, whichchanges direction periodically, passes through zero after each half-wave, the DC circuit can beinterrupted only by a reverse voltage. which has to be generated by a switching device. For thispurpose, the semiconductor switching device requires a turn-off facility. A semiconductor switchingdevice in the DC circuit is therefore a feature of self-commutated converters.

The commutation voltage is supplied by an energy store (turn-off capacitor) of the converter or isgenerated within the circuit by increasing the resistance of the converter to be turned off (powertransistor or GTO). The DC chopper regulator therefore belongs to the family of forced-commutatedconverters.

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1.2 Principle of operation of DC chopper regulators

A semiconductor switching device for DC circuits can not only be used for opening and closing thecircuit at any desired point in time. If it is turned on and off periodically at a defined switchingfrequency, the power input of a load can be controlled or set from the DC voltage source.

Period T= constant, "on" time te = variable:

This control method is referred to as pulse width modulation.

Another method is pulse frequency control, where the "on" time te is constant, and the period T isvariable.

The following chapters refer exclusively to pulse-width modulation.

Ideal current and voltage characteristic of the DC chopper regulator

t

iD

L

D

o

t1

U1

Si 1 I 2

u 2

M

Figure 1.2.1

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to t1

U2AV

0

u2

U1

t

t

T

te a

I1AV

iD iD iD iDi1 i1 i1 i1

I 2

0

i

t

Figure 1.2.2

If switch S is closed periodically at the instant to and opened again at the instant t1, the current and

voltage characteristic will be as shown in Figure 1.2.2.

Pulse-shaped voltage blocks U2 are generated on the load side. Their height corresponds to voltageU1, and their width to turn-on time te. During turn-off time ta, the current flows over free-wheelingdiode D.

Let us define the pulse control factor of the switch as

T

t

tt

t e

ae

e =+

=λ .

The average DC voltage U2aV can be calculated from the pulse control factor and DC voltage U1

ae

e1av2 tt

tUU

+⋅=

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Assuming that current I2 is completely smoothed, this results in the rectangular current blocks shown inFigure 1.2.2. Current is drawn from the DC voltage source during turn-on time te only. The average DCcurrent I1aV can be calculated from the pulse control factor and load current I2.

ae

e2av1 tt

tII

+⋅=

It can therefore be seen that there is a constant DC voltage with "current blocks" on the DC voltagesource side, but voltage blocks and a constant DC current on the load side.

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Questions and exercises

1. What do we need DC chopper regulators for?

2. Which kind of switching devices are suitable for DC chopper regulators?

3. Give a brief description of the basic principle of a DC chopper regulator with switch.

4. How do we define the pulse control factor λ?

5. What is the difference between the voltage and current characteristics on the DC voltagesource side and load side?

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1.3 Functional description of the thyristor DC chopper regulator

In this type of DC chopper regulator, a thyristor (V1) is used as the switching device. However, athyristor can only be turned on via its gate, and can only be brought into the non-conducting state bybriefly interrupting the current flow.

This is done by the turn-off circuit, consisting of turn-off capacitor C, turn-off thyristor V2, ring-aroundinductance Lv and diode V4.

The timing and physical phenomena occurring in the main and the auxiliary (turn-off) thyristors areexplained by the following diagrams.

i

L

U1

i 1 i 2

U2

C

t

t1

o

v1

v3v2

v4v3

M

L v

u v1uC

Figure 1.3.1

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u 2

0 t

t0 t1 t 2 t c

U 1

U 1u 2 u 2U 2AV

0

i

t

0

u c

t

0

u v1

t

0

iv1

t

0

ic

t

2Ii 1 i 1v3i

1AVI

Figure 1.3.2

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After applying the DC voltage source, the turn-off thyristor V2 must be triggered before the mainthyristor V1 is triggered for the first time. As a result, turn-off capacitor C is charged to voltage U1 viathe load.

If main thyristor V1 is triggered at instant t0, the energy stored in the turn-off capacitor C is loaded intothe ring-around inductance Lv, which, in turn, reverses the capacitor charge when C is discharged.

The turn-off capacitor and ring-around inductance thus form a resonant circuit in which diode V4

prevents the capacitor from discharging again.

Turn-off capacitor C now has the necessary polarity to apply a negative voltage to main thyristor V1

following firing of turn-off thyristor V2 at the instant t1. Current i1 commutates from V1 into the turn-offcircuit until the current in V1 is chopped; the main thyristor is then again in the non-conducting state.

The time the capacitor voltage takes to reach zero is referred to as the hold-off interval tc. This timemust be longer than the main thyristor's critical hold-off interval, otherwise the thyristor would fire as theanode voltage becomes positive even without receiving a firing pulse.

After the thyristor is turned off, the load current, which is maintained by inductance L, continues to flowthrough the turn-off arm and reverses the charge of the turn-off capacitor.

At the instant t2, the capacitor voltage becomes greater than U1, and the freewheeling diode becomes

conducting. The load current commutates from the turn-off arm into the freewheeling arm. The wholeprocess repeats when main thyristor V1 is retriggered.

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1. What is diode V4 used for in the turn-off circuit?

2. Why must the turn-off thyristor be fired before switching the thyristor DC chopper regulator on?

3. What is the hold-off interval tc and what is the connection between it and the critical hold-offinterval tq of the main thyristor?

4. Why can't a thyristor DC chopper regulator have a pulse control factor of λ = 1?

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1.4 Functional description of the Darlington DC chopper regulator

A Darlington transistor is used as the switching device in this type of DC chopper regulator. Unlike thethyristor, the transistor can be turned on and off via its gate electrode. It is turned off by continuouslyincreasing the value of the switching resistor.

As far as the basic principles are concerned, an energy store is not needed to make the switchingdevice conducting. However, we use one here in order to limit the switching losses, which wouldotherwise cause an unnecessarily temperature rise in the switch.

The timing and physical phenomena of the turn-on and turn-off cycles are explained by the followingdiagrams, assuming a large smoothing inductance L to guarantee a constant DC current on the loadside.

L

U1

i 1

ic

Ct1

v1

v2

i v1

RB

t0

i 2

T

U2

M

uT uC

Figure 1.4.1

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t

u 2

0

0

i

t

0

u T

t

0

u c

t

0

i c

t

U 2 U 2

t 2t 1t 0

U2AV

I 1AV

I 2i v1 i 1

Figure 1.4.2

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At the instant t0, switch T closes and applies input voltage U1 to the series connection consisting of theload and inductance L.

Up to the instant t1, U2 = U1. At the same time, capacitor C, which was charged to voltage U1 via diodeV2 prior to instant t0, is discharged via resistor RB.

The energy stored in capacitor C is therefore converted into heat in resistor RB.

At the instant t1, the resistance of switch T continuously increases via its gate electrode from theconducting to the non-conducting state.

However, since the smoothing inductance maintains the current flow, switch T would have to convertconsiderable losses into heat during the turn-off process

The instantaneous heat losses in switch T are, as we all know, i2 • R. From the instant t1, at which theswitching resistance is still approximately 0, to the instant t2, at which the load current commutates intothe freewheeling path V1, these losses increase to the value U1 • i1.

In order to lessen the load on the transistor during this time, energy store C is used. When switch Tincreases its resistance, load current i now commutates into capacitor C via diode V2 and charges thecapacitor until freewheeling diode V1 becomes conducting and accepts the load current

Assuming a constant load current (large smoothing reactor L), the voltage across capacitor C thereforerises linearly, with the following relationship applying to the voltage gradient versus time of capacitor Cand load current i:

C

i =

t

Uc

Capacitor C is therefore rated to suit the maximum permissible load current i and the maximum turn-offtime of the transistor.

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1. What are the functions of capacitor C?

2. How high would the instantaneous losses in switch T be without the RB/C circuitry just prior tothe instant at which the load current commutates into the freewheeling diode V1?

Maximum load current 5 A.

Maximum input voltage 320 V.

3. How high is the instantaneous current when switch T closes if the current in the freewheelingpath is 3 A and the resistance RB = 100 Ω?

Base your calculation on U1 = 320 V.

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2. Putting DC chopper regulators into service

2.1 Procedures

A number of special safety measures and conditions have to be observed when taking DC chopperregulators into service:

Motors :

The DC motors used should be designed for a maximum armature voltage of 300 V.

Motors with thermal monitors should be used since a motor could be overloaded or even destroyed bymaloperation of the regulator.

Measuring instruments :

Since it is the average value over time that is of interest when measuring the current and voltage, onlyintegrating instruments should be used for this purposes, and preferably slow-response moving-coilinstruments or commercial digital instruments for DC voltages and currents.

Power supply of DC chopper regulators :

The DC chopper regulators require a DC voltage of up to 320 V and a maximum load current of 5 Afor the power section

The output of the DC voltage source must be free from leakage inductances and, if necessary, shouldbe provided with a smoothing capacitor.

The control section of the DC chopper regulator requires a DC voltage of ±15V and up to 100 mA. Thetype W3644-4B power supply unit meets these requirements.

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To enable you to carry out the measurements described below with the necessary expertiseand apply the knowledge you have gained so far without endangering yourselves or others,carefully read the following page with the ten most important safety measures to be observedwhen working with DC chopper regulators, and always keep them in mind in all your practicalwork in this connection:

WARNING!

1. Before taking the regulator into service, always read the accompanying operating instructions

2. Use only safety laboratory cables providing adequate protection against accidental contact!

3. Make sure all measuring equipment you use is fully functional and protected against accidentalcontact.

4. Accidental contact with DC voltages is much more dangerous than with AC voltages. DCvoltages cause electrolytic decomposition phenomena in the human body whose products arehighly poisonous and, in addition to the electric shock, can cause serious impairment to health.

5. The following terminals can still carry dangerous voltages even if the motor is at standstill: Thepower supply sockets, the input sockets and the actual motor terminals (armature and fieldterminals).

6. The power supply unit's output is still live for up to 5 minutes after being disconnected from themains power system!

7. Make sure all equipment is dead before carrying out any circuit assembly work or making anychanges to the setup.!

8. Define a two-pole power switch S1 and turn it off after every measurement series.

9. The regulator controls rotating machines. Make sure the shaft and other moving parts are freeto rotate and are protected against accidental contact.

10. Always observe the general installation and safety regulations applying to the working withlaboratory equipment (VDE, DIN etc.).

The non-observance of these safety measures can result in death, severe physical injury orserious damage to equipment or property!

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2.2 Thyristor DC chopper regulators: list of equipment

Item Qty. Description Order No. Remarks

1 1 Power supply unit W3644-4B U2 1 Thyristor DC chopper regulator W3644-4C Q13 1 DC motor W3365-5C M14 1 Magnetic powder brake W3360-1E B15 1 Mounting rack W3360-8A R16 3 Voltage indicator W3414-4E P1/P2/P37 2 Current indicator W3411-4A P4/P58 1 2-pole OFF switch W3211-4B S19 1 Field exciter W3360-1M E110 Connecting cables W390411 1 Oscilloscope with differential inputs

Options:

11a 1 Oscilloscope with grounded measuring inputs11b 1 Isolating transformer ≥ 1.5 kVA W3642-4S

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2.3 Measuring setup: Thyristor DC chopper regulator

This chapter will help you to gain practical experience with the thyristor DC chopper regulator.

Figure 2.3.1 shows the basic input and load connections, together with a front view of the thyristor DCchopper regulator.

Figure 2.3.1 Front view of the thyristor DC chopper regulator

You require a number of devices for the measurement setup; these are listed on page 18.

Assemble the equipment as shown in the schematic circuit diagram, Figure 2.3.2 on page 21.

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Figure 2.3.2Schematic circuit diagram of the experimental setup - Thyristor DC chopper regulator

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2.4 Thyristor DC chopper regulators: List of equipment

Item Qty. Description Order No. Remarks

1 1 Power supply unit W3644-4B U2 1 Darlington DC chopper reg. W3644-4D Q13 1 DC motor W3365-5C M14 1 Magnetic powder brake W3360-1E B15 1 Mounting rack W3360-8A R16 3 Voltage indicator W3414-4E P1/P2/P37 2 Current indicator W3411-4A P4/P58 1 2-pole OFF switch W3211-4B S19 1 Field exciter W3360-1M E110 Connecting cables W390411 1 Oscilloscope with differential inputs

Options:

11a 1 Oscilloscope with grounded measuring inputs

11b 1 Isolating transformer ≥ 1.5 kVA W3642-4S

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2.5 Measuring setup: Darlington DC chopper regulator

This chapter will help you to gain practical experience with the Darlington DC chopper regulator.

Fig 2.5.1 shows the basic input and load connections, together with a front view of the Darlington DCchopper regulator.

Figure 2.5.1 Front view of the Darlington DC chopper regulator

You require a number of devices for the measurement setup; these are listed on page 21.

Assemble the equipment as shown in the schematic circuit diagram, Figure 2.5.2 on page 25.

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Figure 2.5.2Schematic circuit diagram of the experimental setup - Darlington DC chopper regulator

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3. Measurements on the thyristor chopper

3.1 Establishing the power characteristics of a thyristor DC chopper regulator

Once you have assembled the measuring setup as per the schematic circuit diagram (Figure 2.3.2),proceed as follows:

Set the field exciter E1 to its minimum value and switch it on.

Then set the field voltage to the value shown on the rating plate of the motor (e.g. 150 V).

Switch brake B1 on, put the mode selector to control (→ →) and set a torque of 1.2 Nm with thesetpoint potentiometer.

Put the switch on the thyristor chopper regulator to Internal and set the control potentiometer to MIN.

Connect the oscilloscope to the isolated measuring inputs V1´ and V2´ (vertical 0.5 V/cm with 10 : 1scaler, horizontal 0.5 ms/cm). Then close S1. Trigger the channel to V1´ and set the firing pulse V1´ tothe left side of the oscilloscope grid; set the next firing pulse V1´ to the right-hand side of the grid byturning the setting knob of the time base.

You now have 10 increments of the grid between two successive firing pulses of V1´ (≅ approx. 5 ms)

te ta

T

0 5 10

Channel v1´

Channell v2`

Figure 3.1.1

Now make a comparison between the calculated and measured currents and voltages by carrying outa number of measurements. To do this, set turn-off pulse V2´ to one grid division behind firing pulse V1´

with the control potentiometer on the regulator .(see Figure 3.1.1). This corresponds to a pulse control

factor ae

e

tt

t

+ of 0.1.

Now enter the current and voltage values of the input and output sides in the following Table 3.1.2. Theturn-off pulse is then always displaced by one grid division to the right and all values are measured andrecorded.

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Once you have entered all the measured values in Table 3.1.2, transfer the results to diagram 3.1.3.Plot curves (one each) for the input power (measured), output power (measured) and motor power.When you do this, you will notice that, in the case of small pulse control factors (= low motor voltage),there are considerable deviations between the input and output powers and that the two curvesapproach each other more and more with increasing pulse control factor.

You will also observe that the values in the measured value table also deviate from the calculatedvalues for input current and output voltage.

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Table 3.1.2

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Diagram 3.1.3

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3.2 Measurements at various points on the thyristor DC chopper regulator

Now use the oscilloscope to take measurements at various points on the thyristor DC chopperregulator and verify your measurements by comparing them with representative current and voltagewaveforms.

Before you start taking measurements with the oscilloscope, however, there are a few points youshould note:

1. In the measuring circuit shown on page 21, the power system AC voltage is rectified direct, i.e.there is no galvanic isolation from the power system.

2. You must therefore never carry out measurements in the power section using an oscilloscopewith grounded measuring inputs!

3. For taking measurements in the power section, it is best to use an oscilloscope with differentialinputs or instrument amplifiers (W3410-4B).

4. You can also operate power supply unit W3644-4B with an isolating transformer whose ratingcorresponds to the load connected to the thyristor DC chopper regulator.In this case, you can also take measurements using an oscilloscope with grounded measuringinputs, but you must make sure that the grounded measuring inputs are always at the samepotential

The measuring setup shown on page 24 can be used unchanged for taking measurements with theoscilloscope.

Set the brake again to 1.2 Nm with the control potentiometer and connect the two probes of youroscilloscope as shown in Figure 3.2.1.

Position the two zero lines of your measuring channels exactly one above the other.

Set the oscilloscope's time base to 1 ms/cm and the sensitivity of the Y channels to 200 mV/cm. Themeasuring resistors Rm have a value of 0.1Ω, and the effective value of the current is therefore

cm

A2

1.0cm

mV200=

Ω⋅

Set the control potentiometer on the DC chopper to Min and close switch S1.

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Turn the control potentiometer approximately into the middle position. The two component currents,the one that passes through main thyristor V1 and the one that passes through freewheeling diode V3,appear on the screen.

Figure 3.2.1 Measuring points for the oscilloscope

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Trigger your oscilloscope until the following display appears.

iv 1iv

3

Figure 3.2.2 Oscillogram of the currents

1. The current waveforms measured deviate appreciably from the ideal waveform

described in the section entitled "Principle of Operation of DC Chopper Regulators". Therectangular current blocks you were introduced to in the introductory section assume a largesmoothing inductance. Although smoothing inductances have the effect of producing an almostconstant DC current, they have the disadvantage of becoming larger, heavier and moreexpensive the greater the inductance becomes. Add to this the fact that the possible correctingrate decreases with increasing inductance.

As rule, the smoothing inductance chosen is such that the current never becomes intermittentinside the working range of the motor

2. The two spikes in the oscillogram represent the current of the ring-around phenomenon in theturn-off circuit. We can clearly make out a current half-sine-wave.

The current maximum is reached when the turn-off capacitor voltage is zero. The energyoriginally stored in the turn-off capacitor is stored at this instant in the ring-around inductanceLv

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3. There is an intermittent gap to be observed in the current between iV1 and iV3. This is the timeduring which the turn-off arm conducts the load current. The latter does not cause a voltagedrop across either of the two measuring resistances, and an intermittent gap of about 0.4 mstherefore appears in the oscillogram.

During this time, however, the DC voltage source must supply the current for reversing thepolarity of the turn-off capacitor, which is tantamount to an effective increase in the on" time byexactly this time.

This is also the reason for the measured values deviating from those calculated.

4. The charge-reversal current of the turn-off capacitor flows both on the DC voltage source sideand the load side. The DC voltage on the DC voltage source side has the constant valueU1,while only the mean value of the voltage is on the load side. side. This means that themeasurement method applied here to determine the average value for current and voltage onthe load side may well be correct, but is not permissible for measuring the power by themultiplication of the two average values.

If we wanted to measure the power on the load side correctly, we would always have to multiply theinstantaneous values of current and voltage and display the result in an integrating measuringinstrument, i.e. we would have to use a genuine kW meter to measure the output power of the thyristorDC chopper regulator exactly.

It can be clearly seen from the diagram that the "deviations" of the input power and output powerdiminish the closer the output voltage gets to the input voltage.

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4. Measurements on the Darlington DC chopper regulator

4.1 Power characteristics of a Darlington DC chopper regulator

Once you have assembled the measuring setup as per the schematic circuit diagram (Figure 2.5.2),proceed as follows:

Set the field exciter E1 to its minimum value and switch it on.

Then set the field voltage to the value shown on the rating plate of the motor (e.g. 150 V).

Switch brake B1 on, put the mode selector to Control (→ →) and set a torque of 1.2 Nm with thesetpoint potentiometer.

Put the switch on the thyristor chopper regulator to Internal and set the control potentiometer to MIN.

Connect the oscilloscope to the isolated measuring output V (vertical 0.5 V/cm with 10:1 scaler,horizontal 50 µs/cm). Then close S1. Turn the control potentiometer on the Darlington DC chopperregulator clockwise until you can see a narrow pulse at the right-hand side of the oscilloscope screen.

Position the first positive-going edge of the pulse to the right-hand side of the oscilloscope grid.

Position the next positive-going edge with the knob of the time base to the right-hand side of the grid.You now have ten grid divisions between two positive-going edges of the measuring pulse(≅ ca. 500 µs).

Now make a comparison between the calculated and measured currents and voltages by carrying outa number of measurements.

To do this, position the negative-going (trailing) edge of the gate pulse one grid dimension behind thefirst positive-going edge, using the control potentiometer on the DC chopper. This corresponds to a

pulse control factor ae

e

tt

t

+ of 0.1.

te ta

T

0 5 10

Figure 4.1.1

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Now enter the current and voltage values of the input and output sides in the following Table 4.1.2.

The negative-going edge of the gate pulse is then always displaced by one grid division to the right andall values are measured and recorded.

Once you have entered all the measured values in Table 4.1.2, transfer the results to diagram 4.1.3.

Plot curves (one each) for the input power (measured), output power (measured) and motor power.When you do this, you will notice that, in the case of small pulse control factors (= low motor voltage),there are considerable deviations between the values you have measured and the values calculatedfrom them.

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Table 4.1.2

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Diagram 4.1.3

0.1

0.2

0.3

0.4

0.6

0.5

0.7

0.8

0.9

1.0

tete+ta

λ=λ=

Power [VA]

25 50 75 100 125 150

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4.2 Measurements at various points on the Darlington DC chopper regulator

Now use the oscilloscope to take measurements at various points on the thyristor DC chopperregulator and verify your measurements by comparing with typical current and voltage curves.

Before you start taking measurements with the oscilloscope, however, there are a few points youshould note:

1. In the measuring circuit shown on page 24, the power system AC voltage is rectified direct, i.e.there is no galvanic isolation from the power system.

2. You must therefore never carry out measurements in the power section using an oscilloscopewith grounded measuring inputs!

3. For taking measurements in the power section, it is best to use an oscilloscope with differentialinputs or instrument amplifiers (W3410-4B).

4. You can also operate power supply unit W3644-4B with an isolating transformer whose ratingcorresponds to the load connected to the thyristor DC chopper regulator.In this case, you can also take measurements using an oscilloscope with grounded measuringinputs, but you must make sure that the grounded measuring inputs are always at the samepotential

The measuring setup shown on page 24 can be used unchanged for taking measurements with theoscilloscope.

Set the brake again to 1.2 Nm with the control potentiometer and connect the two probes of youroscilloscope as shown in Figure 4.2.1.

Position the two zero lines of your measuring channels exactly one above the other.

Set the oscilloscope's time base to 0.1 ms/cm and the sensitivity of the Y channels to 50 mV/cm. Themeasuring resistors Rm have a value of 0.1Ω, and the effective value of the current is therefore

cm

A5.0

1.0cm

mV50=

Ω⋅

Set the control potentiometer on the DC chopper to Min and close switch S1.

Turn the control potentiometer approximately into the middle position. The two component currents,the one that passes through main Darlington transistor T and the one that passes through freewheelingdiode V1, appear on the screen.

38


Figure 4.2.1 Measuring points for the oscilloscope

39


Trigger your oscilloscope until the following display appears.

i T i v1

i c

0

Diagram 4.2.2 Oscillogram of the currents

We can learn the following from this oscillogram:

1. The current waveform coincides relatively well with the ideal waveform described in the sectionentitled "Principle of Operation of DC Chopper Regulators".

2. We observe a slight knee in the curve at the transition from iT to iV1. This is the instant at whichthe load current commutates into capacitor C via diode V2.

Within this time (approx. 15 ), capacitor C is charged to input voltage U1; the input side mustsupply the necessary current for this. From the measurement point of view, this is tantamountto an increase in the "on" time, and also explains the deviation between the calculated valuesfor input current and output voltage and the values measured.

40


3. As long as the Darlington transistor T is turned on and off, there is always be a difference ofabout 4 W to be observed in the table and/or diagram between the input and output powersmeasured.As long as capacitor C is charged, the more or less constant load current, multiplied with theconstant input voltage U1, will flow. The constant load current causes linear charging of thecapacitor and the actual power output is the sum of the instantaneous values of load currentmultiplied with the difference between U1 and UC. Consequently, it is only half as high(approx. 2 W).The other half of this energy is converted into heat in resistor RB when T becomes conducting.This power (loss) is not taken into account in the measurements.

4. If transistor T is continuously turned on (λ = 1), the input power is practically equal to the outputpower since no more switching losses occur.

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