LK2686 - 27 07 15.pub
Contents
1. C on Picoscope and obtain another trace showing the outputs on channels A and D i e one input phase and the output e Save it for your records with an explanation of what it shows Copyright 2014 Matrix TSL Limited locktronics Page 21 Worksheet 7 Introduction to Half wave rectification Three phase Power So what The diagrams show typical signals The rectified output is on channel D shown in gold 160 16 Notice that it sits below the corresponding AC phase signal This is because of the 0 7V approx drop across a conducting silicon diode The trace is always positive and so it is a DC signal though not smooth DC In one AC cycle there are three peaks in the rectified signal one for each of the three phases This indicates that it is half wave rectification only the negative half cycle of each phase is ignored Compare this with the next worksheet The variation in the output voltage across the load is known as the ripple voltage Challenges e Connect a 33uF non electrolytic capacitor in parallel with the 1kQ load e Obtain new Picoscope traces to see the effect on the ripple voltage 2012 A typical output is shown opposite Notice the reduced ripple e What is the effect on ripple voltage of using different values of load resistor You could increase the load resistance by connecting several 1kQ resistors in series or reduce it by connecting them in parallel Two 1kQ resistors in parallel have2. Simplifying Electricity Three phase systems LK2686 sild 1 RIX www matrixtsl com Copyright 2014 Matrix Multimedia Limited locktronics Contents Worksheet 1 Worksheet 2 Worksheet 3 Worksheet 4 Worksheet 5 Worksheet 6 Worksheet 7 Worksheet 8 Worksheet 9 Page 2 Introduction to Three phase Power Introduction Single phase AC Three phase AC Phase basics And now the phasor It s a star On the delta Half wave rectification Full wave rectification Delivering power Instructor Guide Copyright 2014 Matrix TSL Limited 10 12 14 16 18 20 22 24 26 locktronics Page 3 Introduction to Introduction Three phase Power The hardware Digital oscilloscope Digital multimeter Three phase motor Three phase generator The hardware Three phase generator e converts single phase AC mains into three phases These can behave as three separate sources of AC power or can be linked as one source Three phase motor e contains three independent sets of coils each of which uses an AC supply to produce a rotating magnetic field inside the motor This creates torque which makes the motor s rotor rotate Digital oscilloscope e is used to study the AC signals involved in the three phase systems e can create image files that record these signals for later analysis or as records of the behaviour of the three phase systems Current
3. e what indicated that it was a full wave rectified output as opposed to half wave e Describe and explain the effect on ripple voltage of increasing the size of e the load resistor e the smoothing capacitor Copyright 2014 Matrix TSL Limited locktronics Page 24 Worksheet 9 Introduction to Delivering power Three phase Power In the end it is a matter of delivering power from transmitter to load Not surprisingly there is a complication Real power is the product of voltage and current in phase with the voltage As we have seen for many loads current and voltage are not in phase This worksheet examines how this situation is handled in three kWh circuits Two use a single phase supply in circuits which contain aS reactance The third uses a three phase supply to drive the three phase motor Over to you An R C circuit e Build the top circuit shown opposite e Set the three phase generator frequency to 150Hz and amplitude to 6V Switch it on amy e Use the same Picoscope settings as in the previous worksheet e Obtain and save a trace showing the supply voltage and current through the circuit e Switch off the generator and the current clamp Picoscope channel A Picoscope channel B Picoscope channel B Picoscope _channel A An R L circuit e Build the middle circuit e Use the same settings as before for the three ohase generator and for the Picoscope e Switch on the gener
4. a combined resistance of 0 5kQ four in parallel have a combined resistance of 0 25kQ For your records e Draw the circuit diagram for a three phase half wave rectifier and include capacitor smoothing e Explain why the output of this circuit is considered to be DC e Explain the term ripple voltage and explain why adding a smoothing or reservoir Capacitor reduces the ripple Copyright 2014 Matrix TSL Limited locktronics Page 22 Worksheet 8 Introduction to Full wave rectification Three phase Power A common use of rectification is in motor vehicles These generate three phase electricity using alternators which are smaller and lighter than equivalent DC generators The majority of the vehicle s electrical system requires a DC supply and so built into the alternator is a full wave rectifier The previous worksheet looked at half wave rectification leading to a DC output voltage but with a ripple voltage This is often undesirable as it can cause audible hum in audio equipment and spurious effects in digital systems Full wave rectification reduces this ripple voltage at the cost of additional components Over to you e Build the circuit shown opposite Notice the Picoscope ground clip Make sure that all diodes are connected the right way round Without specialised probes you can not obtain trac es of all phases AND the full wave output simultaneously that would short circuit par
5. amplitude to 6V e Switch iton and observe the lamps They are neither in step nor completely out of step but reach maximum brightness at different times The diagram that follows shows two ways to represent this circuit using different symbols for the three phase generator Pht N E N e Build the second circuit connecting the three phase generator to Picoscope as shown Again notice the ground clip connecting Picoscope ground to terminal N Picoscope settings A AC generator generator winding coil Ph Timebase 5ms div x Channel A B and C Auto F Trigger Auto 6 Threshold OV 8 Pre trigger 0 e Set the generator frequency to 50Hz and amplitude to 6V and switch it on e Obtain a trace showing the three outputs and save it for your records Copyright 2014 Matrix TSL Limited locktronics Page 11 Worksheet 2 Introduction to Three phase AC Three phase Power So what The diagram opposite shows typical output from the Picoscope Each phase shown in different colours has the same time period 20ms They peak at times separated by 20 3 6 3ms Phase angle Imagine a bar rotating around one end Above it a light shines down creating a shadow of the bar As the bar rotates the shadow gets longer then shorter then zero then longer in the other direction and so on ai g e270 ean As the bar continues to rotate the shadow ee sequence repeats One com
6. clamp e generates a voltage that reflects the size and direction of the AC current e is connected to the digital oscilloscope which displays voltage time graphs but cannot display AC current directly Multimeter e measures voltage current resistance and a range of related quantities e cannot follow rapidly changing signals e displays rms root mean square values of AC quantities usually Copyright 2014 Matrix TSL Limited locktronics Page 4 Introduction Introduction to Three phase Power Using the hardware Three phase generator e has controls that allow the user to adjust e signal amplitude to 6V or 9V e signal frequency to 0 1Hz variable 0 1Hz fixed 1 Hz 50Hz and 150Hz e has red green and yellow sockets for the three phases and blue for the neutral connection e can deliver currents up to 0 8A from each phase e has LEDs on each phase that indicate overloads or short circuits Three phase motor e has three sets of coils U V and W that use the three phase signals a any to produce rotating magnetic fields Each creates four magnetic poles two north and two south and so it is known as a four pole motor e has a squirrel cage rotor in which currents are induced by the rotating w2 Tu2 v2 magnetic fields e can operate as a single phase motor see worksheet 4 e has a stroboscopic disc to facilitate measurement of rotational speed Digital oscilloscope e monitors signals usually time
7. investigation students build three systems two using a single phase power supply with a R C and then a R L network Then they connect up the three phase motor in delta configuration to the three phase output from the generator The best way to do this is to connect the three coils of the motor to each other in head to tail fashion U2 to V1 V2 to W1 and W2 to U1 Then the three phases from the generator are connected to the nodes U1 V1 and W1 The instructor may wish to check this wiring before students turn on the power supply In all three cases they obtain traces showing the relationship between applied voltage and current In the case of the motor this is done for one phase only any phase The motor is a balanced load and so the total power delivered is three times the power delivered to one phase The So what section shows how to obtain phase angle from the traces There is likely to be considerable electrical noise on the current trace because of its small amplitude This makes determining the phase lag difficult but we can still get a reasonable estimate The instructor may wish to hand out traces which are free from noise to check that students are following the correct procedure This is e Measure the distance between adjacent peaks on the voltage curve and between one voltage peak and the nearby current peak These distances can then be converted into a phase angle From there the power factor ca
8. of the current There are two types of current clamp Both rely on the magnetic fields that accompany any current flow That magnetic field intensifies as the current increases and reverses when the current reverses In one type the jaws contain a coil of wire A varying magnetic field induces a current in this coil rather like the behaviour of a transformer This type is suitable only for AC measurement The other type has a Hall effect sensor in the jaws The magnetic field bends the current flow in the sensor producing a voltage across the sensor that is a measure of the size and indicates the direction of the current under test It can be used with AC or DC current measurement The students are challenged to carry out the same investigation using a capacitor and then an inductor Interpretation of the results is important as it reveals the phase relationships between voltage and current in these devices Typical results are given in the So what section They can be explained as follows e When an uncharged capacitor i e voltage across it OV is connected to a voltage source there is an initial rush of charge onto the plates i e the current is a maximum As the capacitor charges the voltage across it ris es deterring further charge transfer i e current falls Eventually the ca pacitor is fully charged i e voltage is maximum and then no more charge transfer takes place i e current 0 In other words curre
9. so the voltage scale is marked off in 4V steps The trace still has a maximum value of around 8 7V Trigger Auto SO Still shows any changes in the signal as they happen Ch A SO Still looks at the signal on channel A to decide when to start the trace Falling SO now waits for a falling voltage to reach the threshold Threshold 4V SO Starts the trace when the signal on channel A falls to 4V Pre trigger 0 so the display starts with the very first data captured More information about using Picoscope is given in the Picoscope User manual found on the CD ROM that comes with the instrument or on the website www picotech com Copyright 2014 Matrix TSL Limited locktronics Page 7 Introduction Introduction to Delivering power Three phase Power In elementary science and engineering courses electrical power P is calculated using the formula P IxV In DC circuits there is no problem but in AC circuits the current may not be in phase with the voltage In a capacitor and an inductor for example there is a phase angle of 90 between current and voltage These devices dissipate no power on average When charging the capacitor stores energy in the electric field between its plates When it discharges it returns that energy Nett energy transfer nil The inductor works in a similar way but with a magnetic field In resistors on the other hand voltage and current are in phase When a current flows through them th
10. symbol are found in literature The first is useful in showing that three phase can be treated as three separate sources of AC power The second shows that three phase power is often produced by three sets of coils inside a generator Although the traces produced by the students are voltage time graphs of the signals it is often useful to use angle rather than time on the horizontal ax is The So what section attempts to illustrate why this can be done The in structor should ensure that the analogy works for the students The important result is that the three phases of the supply are separated by phase angles of 120 The So what section obtains this result from the time difference between the phases As before the instructor should check that students understand this argument More examples using other frequencies and numbers of phases will help In this worksheet students use the current clamp for the first time Although it measures currents as low as 10mA electrical noise can make the trace indistinct and for that reason it is advisable to use as large a current as is feasible The 100Q 1W resistor allows an appreciable current without over heating though it may get hot to the touch the height of the trace can be increased by coiling the wire from the generator to the resistor and trapping three or four turns of this coil inside the current clamp Not advisable if you are wanting to calculate the amplitude
11. system This worksheet tackles the issue of phasors and how to combine them This topic needs a great deal of additional tuition by the instructor if the students are required to master the techniques The investigation looks at driving the three phase motor initially using all three phases from the supply and then using a single phase with the help of a capacitor to excite a second coil with appropriate phase angle Students are asked to compare its performance under each condition by subjectively sensing the torque produced They also vary the supply frequency At 150Hz the motor cannot keep up with the rotating magnetic field and so stops In the So what section the concept of vectors is introduced along with the parallelogram rule for combining them Here this rule is applied by drawing a scale diagram though mathematical methods can be applied instead This rule allows physicists to manipulate quantities which have both a magnitude size and a direction e g forces and velocities However in the case of AC electricity the size and direction are changing continuously Vector arithmetic is not enough Instead we use phasors Like vector quantities these use the length of an arrow to represent the size of the quantity and the direction of the arrow to indicate direction However the arrow rotates anti clockwise by convention The size and direction of the quantity at any instant is given by projecting the phasor onto t
12. that students have followed the Electricity Matters 1 and Electricity Matters 2 or have equivalent knowledge covering basic electrical concepts current voltage resistance and the construction and testing of circuits using a range of measuring instruments including multimeters and oscilloscopes Learning Objectives On successful completion of this course the student will be able to explain the meaning of the terms amplitude period and frequency in the context of AC electrical signals state and use the relationship between period and frequency recognise and use the units for these quantities measure these quantities given a sinusoidal waveform of a signal distinguish between peak and r m s values for a sinusoidal electrical signal and use the formula that relates them use a digital oscilloscope such as Picoscope to obtain voltage time graphs of AC electrical signals draw a circuit diagram showing a three phase power source describe five advantages of three phase power supplies over single phase supplies explain the term phase angle and justify that the phase angle between the three phases in the supply is 120 draw two sinusoidal signals separated by a given phase angle use a current clamp to obtain a current time graph of AC electrical signals state the phase relationships between current and voltage in resistors inductors and capacitors distinguish between the terms phas
13. the link between them e A pure inductor no resistance is added in series with the resistor and capacitor Draw the circuit diagram and phasor diagram for the new circuit Choose equal sizes for Ve and vL When added together they cancel each other out Copyright 2014 Matrix TSL Limited locktronics Page 16 Worksheet 5 Introduction to Its a star Three phase Power There are two ways to connect up three phase supplies and loads the star or Wye because of its resemblance to the letter Y and the delta configurations The diagram shows a star connected three phase supply driving a star configured load In the star configuration there is a neutral point where all phases or loads are connected together Over to you e Build the circuit shown opposite e On the three phase generator set the frequency to 50Hz and the amplitude to 6V and switch on Voltages in a star configuration Phase voltages e Use a multimeter on the 20VAC range to measure the red phase voltage Vr across the top resistor e Similarly measure yellow phase voltage Vy across the middle resistor and green phase voltage Vp across the bottom one Voltage e Enter all values into the table Red phase voltage Vr l Yellow phase voltage Vy T Line voltages l Green phase voltage Ve e With the same settings measure the line voltage Vry between the red phase and th
14. the signals they encountered The significance of pre trigger may need additional explanation It allows the us er to view a Signal some time after triggering takes place The instructor should encourage students to experiment with the Picoscope or other oscilloscope settings as described in the Challenge section so that they feel more comfortable and confident in using the device The So what section examines the meaning of the terms amplitude period and frequency and gives the formula relating the last two The instructor should dwell on this and give examples of its use This may be the time to practice the use of multipliers especially milli and kilo lt also looks at the question of electrical power delivered by the supply This section may prove difficult for some students The instructor should test whether students have grasped its significance perhaps through informal questioning The relationship between r m s and peak values given and students should work through examples using it It is true only for sinusoidal signals As the text says it is best to think of r m s as the equivalent DC voltage supply which would deliver the same power The name root mean square comes from a mathematical treatment of the signal e Power can be calculated using the formula V7 R e In this negative and positive voltages both contribute equally as any signs disappear when squared This is th
15. Copyright 2014 Matrix TSL Limited locktronics Page 28 Introduction to Instructor Guide Three phase Power Scheme of Work Worksheet Notes for the Instructor Timing Introduction This course uses a number of devices which may be new to the student The aim of the introduction is to present some information about them and how to use them The third and fourth pages of the introduction focus on the Picoscope instrument using version 6 of the software but parts will be relevant to other types of oscilloscope and using other software Depending on their ability and previous experience some demonstrations of using the kit may be appropriate in addition Finally there is a discussion about AC power in its three aspects real power reactive power and true power At some point the instructor will need to rein force these concepts through further teaching and examples The instructor may choose to photocopy this introduction for each student This worksheet acts as revision of work done earlier on single phase AC and introduces students to the three phase generator and Picoscope The latter may require an additional introduction by the instructor depending on the students experience of oscilloscopes and ADC type instruments This could include the significance of the timebase voltage sensitivity and the use of triggering The beauty of the Picoscope type instrument is that it allows students to create and keep files showing
16. ator e Obtain and save a trace showing supply voltage and current through this circuit e Switch off the generator and the current clamp The three phase motor e Build the bottom circuit P Fe e Once more use the same settings for the three phase generator and for the Picoscope e Switch on the generator e Obtain and save a trace showing supply voltage and current through one phase of this circuit e Switch off the generator and the current clamp N iN lt N Copyright 2014 Matrix TSL Limited locktronics Page 25 Worksheet 9 Introduction to Delivering power Three phase Power So what The traces produced in the three circuits can be analysed to extract the real power delivered to the load devices The procedure is outlined below using a section of a current voltage trace obtained for a R L circuit to extract phase angle 9 i Waun The applied voltage is shown by the blue trace j current by the red The amplitude of the current so trace is small and affected by electrical noise Voltage leads current as we expect in a circuit containing inductance e Draw vertical lines through two adjacent voltage peaks and a current peak as shown 6 e Use the time scale to measure times T and L T is the period of the signal L is the time lag experienced by the current Sample results are T 6 7ms L 1 3ms e Tis the time between points with a phase angle of 360 between them Hence 1 3ms represents a p
17. balanced load as the impedance in this case resistance in each phase is identical 1kQ e As can be seen from the circuit phase current and line current are identical e Notice that the currents in the three phases are identical not surprising as the phase voltages are identical and the loads are identical e What may be a surprise is the value of current in the neutral wire virtually zero This is the result of the phase differences between the currents e They are not in phase e They do not all reach maxima together e They do not add together arithmetically A challenge Remove the neutral wire Does it make any difference to your readings Investigate what happens when the load is unbalanced Swap one of the 1kQ resistors for a different value such as 2 2kQ For your records e Draw the circuit diagram for the system you built for this investigation e Summarise the findings given in the previous section e Explain the mystery of the missing current when a balanced load was used Where has it gone e Explain why electricity is usually transmitted using a four wire system rather than a three wire system Copyright 2014 Matrix TSL Limited locktronics Page 18 Worksheet 6 Introduction to On the delta Three phase Power The other way to configure three phase circuits is to use the delta connection so called because of the similarity of the shape to the Greek letter delta A The diagram shows a t
18. e square term in r m s The voltage varies with time in a repeated pattern so the power delivered is averaged over one cycle of the pattern using calculus This is the mean term another word for average To get back to a voltage from the voltage squared term we take the square root of the quantity Hence the root part of r m s In the final section students take measurements from traces of the sinusoidal signal to obtain amplitude period and frequency These can be obtained by using the measurement and rulers section of Picoscope or with direct measurement The instructor could have additional handouts with other trac es on them to allow further practice Copyright 2014 Matrix TSL Limited locktronics Instructor Guide Scheme of Work Page 29 Introduction to Three phase Power Worksheet Notes for the Instructor The introduction to the worksheet sets out some advantages of three phase power distribution over single phase In the For your records section students are asked to expand on this using information gleaned from sources such as the internet The instructor may wish to reinforce this with some formal teaching on the topic The first part of the practical work is very quick but well worth doing as it shows vividly what is happening in the three phase supply This is reinforced by use of the Picoscope looking at higher frequency signals Both versions of circuit
19. e yellow phase e Similarly measure the line voltages Vre and Vye e Enter all values into the table Currents in a star configuration e Remove connecting link A With a multimeter on 20mA AC measure the phase current Ir and enter the result in the table Req phase current Ip e Replace connecting link A Yellow phase current Iy 0 e Inthe same way measure phase currents ly and Ie Green phase current le Enter their values in the table Neutral current e Remove connecting link D Measure the current in the neutral wire In and enter its value in the table Copyright 2014 Matrix TSL Limited locktronics Page 17 Worksheet 5 Introduction to Its a star Three phase Power So what Phase voltages The magnitudes of the three phase voltages are virtually identical This is because of the way the three phase generator is constructed Line voltages Theory predicts the relationship Line voltage V3 x Phase voltage e Complete the table using your measurements from the previous page Voltage V3 x phase voltage Red Ae voltage Vr A e OOOO line voltage Vey linevoltage Wve linevoltage Ven e Do the results support this relationship Current in a star configuration This circuit has the three phase power source connected in a star and the loads connected in a star The neutral points in each star are connected together In this case the load is known a
20. e voltage and line voltage in a three phase system drive a three phase motor from a single phase source using a capacitor to create a phase shift extract magnitude and direction for a vector quantity from the vector combine together two vector quantities using the parallelogram construction to obtain the resultant distinguish between vector and phasor quantities draw a phasor diagram to show the relationship between currents and voltages in a series LCR circuit connect three resistors in star configuration distinguish between a balanced load and an unbalanced load in a three phase system explain why the neutral return wire is unnecessary in a balanced load three phase system explain why the neutral return wire is normally used in a three phase transmission system connect three resistors in delta configuration state and use the relationship between phase voltage and line voltage in star and delta configurations state and use the relationship between phase current and line current in star and delta configurations compare the advantages and disadvantages of star and delta configurations explain what is meant by rectification and why it is often a necessary part of a three phase system distinguish between half wave and full wave rectification and recognise them from voltage time graphs draw circuit diagrams for half wave and full wave rectification of a three phase power supply describe what is meant by ripple v
21. ey warm up and dissipate energy That energy is lost to the surroundings In AC circuits then we need to take this into account This gives rise to three different measures of electrical power Real power The only elements that actually dissipate power in a circuit are those having resistance R This real or true power P measured in watts W can be calculated using the formulae P IR P V R Reactive power is the imaginary power Q measured in units called volt amp reactive VAR delivered to components with reactance X i e capacitors and inductors It can be calculated using the formulae Q IFX Q V X Although apparently useless as it delivers zero nett power to purely reactive loads it still flows through the circuit and so the cables etc must be capable of handling it Apparent power Apparent power S measured in units called volt amps VA is the total of real and reactive power In other words it is the power that the AC supply seems to be delivering to the load of impedance Z It can be calculated using the formulae S IZ S V Z or S lms X Vims where lms is the rms current delivered by the AC supply at a rms voltage of Vms Power factor When a phasor diagram is produced for a circuit see worksheet 3 the current flowing I may be out of phase with the voltage applied by the source V by a phase angle The term cos is known as the power factor The real power uses the component of the voltage that
22. hase difference of 1 3 6 7 x 360 or 70 approximately In this R L circuit voltage leads current by a phase angle of around 70 This implies a phase factor of 0 34 i e cos 70 e From the traces peak voltage 12 0V giving Vims 8 4V peak current reading 7mV With the current clamp on the 20A i e 1mV 10mA range peak current is 7 x 10 70MA giving lims 49MA Putting all this together Real power delivered to the system P Vms Irms COS 8 4 x 49 x 10 x 0 34 0 14W Apparent power gt Vas line 0 41VA For your records e Inthe same way determine the real and apparent power for the R C and R L circuits you set up in this investigation e The three phase motor is a balanced load The total power delivered 3 x power delivered to one phase Use your results from the third circuit to determine the real power delivered to the motor Copyright 2014 Matrix TSL Limited locktronics Page 26 Introduction to Instructor Guide Three phase Power About this course Introduction The course is essentially a practical one Locktronics equipment makes it simple to construct and test electrical circuits The end result mirrors the circuit diagram thanks to the circuit symbols printed on the component carriers Aim The course introduces students to concepts used in three phase electrical transmission and utilisation These form the basis for a deeper study of this topic Prior Knowledge It is recommended
23. he horizontal axis In the example given a series combination of resistor and capacitor the same current flows through both as they are in series However the voltage across the resistor is in phase with that current whereas the voltage across the capacitor lags behind by a phase angle of 90 Kirchhoff s 2 law requires that the sum in this case vector sum of the voltages across capacitor and resistor equals the supply voltage This is shown in the diagram which uses arbitrary lengths for the phasors It is drawn at the instant when current reach es its maximum value not that this matters It allows us to measure or cal culate the phase angle between current and supply voltage As stated above students must work through a whole series of similar scenarios in order to feel competent in manipulating phasors Copyright 2014 Matrix TSL Limited locktronics Page 31 Introduction to Instructor Guide Three phase Power Scheme of Work Worksheet Notes for the Instructor The next two worksheets show the two configurations used for loads in three phase systems star or Wye and delta The difference between them results in different relationships between line and phase voltages and line and phase currents This investigation looks at these relationships for the star configuration It requires that students be familiar with the use of multimeters to measure AC current and voltage The instructor may wish to introduce some
24. hree phase power source connected in delta configuration to a load also in delta configuration This time there is no neutral point and only three wires link the power source and load Over to you e Build the circuit shown opposite The delta connection may not be immediately obvious Compare it with the circuit diagram that follows e phases are connected together through 1kQ loads e the three load resistors are connected in a loop Phi Ph3 e On the generator set the frequency to 50Hz and the amplitude to 6V and switch on Voltages in a delta configuration As can be seen from the diagram phase voltage and line voltage are now identical e Use a multimeter to measure the line voltages across i the three 1kQ loads e Record them in the table Currents in a star configuration e Remove connecting link A With the multimeter on the 20mA AC range measure the phase current Ir through the 1kQ resistor connected between phases Ph1 and Ph2 e Enter the result in the table si e Replace connecting link A phase current Ip Se e Next measure phase currents ly and le e Enter their values in the table i i prase coren e e Remove the red wire connecting Ph1 to the load linecurrenth e Replace it with the multimeter and read line current la line currente 8 Enter its value in the table line currents OO e Do the same for the other two line currents l2 and l3 Copyright 2014 Mat
25. in a process called rectification but relatively expensive to convert DC to AC Broadly speaking there are two forms of rectification half wave and full wave This worksheet looks at half wave rectification the next at full wave The key to both is the one way behaviour of diodes offering very low resistance to current flow in one direction and very high resistance to flow in the other An alternative is to use thyristors also called SCRs silicon controlled rectifiers which also have this property Students capture oscilloscope traces that show that the supply to the load never crosses the zero volts line and so is DC The instructor may wish to emphasise the difference between DC and smooth DC as this is certainly not smooth A measure of smoothness is given by ripple voltage discussed in the So what section This can be reduced by connecting a capacitor in parallel with the load This acts as a reservoir of charge driving current through the load in those periods when the current from the supply falls vehicle s DC electrical system The investigation follows the same path as the previous one first obtaining a full wave rectified output and then looking at the effect of a smoothing capacitor Unlike the previous one it is not possible to view all three phases and the rectified output on the same trace because of the need to reference the three phases against the neutral terminal but reference the output agains
26. is in phase with the current leading to P Vims lrms COS The reactive power involves the component of voltage that is 900 out of phase with the cur rent and so Q Vims lms SIN Power factor can be calculated from the relationship Power factor true power apparent power Copyright 2014 Matrix TSL Limited locktronics Page 8 Worksheet 1 Introduction to Single phase AC Three phase Power Mains electricity is traditionally distributed as AC The reasons for this include e alternators that generate electricity at the power stations are lighter and cheaper than equivalent DC generators e transformers can be used to step the voltage up or down allowing efficient transmission of electrical power at high voltage and low current over large distances Over to you This worksheet looks at aspects of single phase AC using the signal obtained from one output of the three phase generator via sockets Ph1 and N e Build the circuit shown opposite e On the three phase generator e set the frequency to 50HZz e set the amplitude to 6V e switch on e Picoscope settings Timebase oms div Channel A Auto Channels B C and D Off Trigger Auto Threshold OV Pre trigger 0 e Obtain a trace showing the AC output and save it for your records e Switch off the three phase generator e Now change the settings on the three phase generator to e frequency 150Hz e amplitude 9V e Use Picoscope t
27. motor is running disconnect one of the capacitor Ph2 leads It should continue running Ph3 Again rest a finger on the motor flywheel to judge how N smoothly the torque is applied Does the motor need the second coil or will it run using just the supply to one coil Test by disconnecting one of the capacitor leads Investigate the effects of changing the frequency and amplitude of the supply with and without the Capacitor connected Copyright 2014 Matrix TSL Limited locktronics Page 15 Worksheet 3 Introduction to And now the phasor Three phase Power So what To understand AC circuits containing capacitors and inductors you will need to understand phasors The first step is to look at vectors Vectors vector quantities like forces and velocities are described fully only when both size and direction are given Combining them together is not straightforward For example the diagram shows two tugs pulling a ship The force exerted by each tug is shown as an arrow e its length represents the size of the force e its direction is the direction of the force To find the total resultant force complete the parallelogram and construct the diagonal as shown Its length gives the size of the resultant force its direction the direction of the resulting force Phasors Current and voltage have size and direction like vectors but these change with time They can also be represented by arrows but these rota
28. ms voltage are linked by Vims 0 7 X Vpeak For your records e Copy the diagram of the AC signal and explain the meaning of the terms amplitude period and frequency e Copy the table and use the measurements you took in the first part of theinvestigation to complete it Amplitude in V Frequency in Hz e Copy and complete the statement by calculating the rms voltage In the first circuit the rms voltage was Copyright 2014 Matrix TSL Limited locktronics Page 10 Worksheet 2 Introduction to Three phase AC Three phase Power Electrical power is transmitted around the country using a three phase rather than a single phase system The advantages of three phase include e less copper needed for the transmission cables e devices like motors are less affected by vibration power is delivered more smoothly e three phase alternators are smaller and lighter than equivalent single phase versions e both three phase and single phase devices can be powered from a three phase supply but the reverse is not true rv es This worksheet looks at basic properties of three phase power distribution Over to you The two investigations both look at why three phase is so called the first at low frequency using lamps the second at higher frequency with Picoscope e Build the first circuit using three 12V MES bulbs and holders e On the three phase generator set the frequency to 0 1Hz fixed and the
29. n be calculated Measure peak values for the voltage and current trace The current clamp is set to the 20A range which gives an output of 1mV for every 10mA of current see the Current Clamp Operator s Manual and hence the peak value of current is calculated by multiplying the peakvalue of the current trace by 10 e Convert the peak values into rms values e Apply the formulae to give the estimates of real and apparent power The For your records section asks students to follow this procedure with the three traces they obtained in the investigation to obtain real and apparent power for these systems Copyright 2014 Matrix TSL Limited
30. nd reinforced with written examples assignments and calculations The worksheets should be printed photocopied laminated preferably in colour for the students use They should make their own notes and carry out the tasks identified in the For your records sections They are unlikely to need their own permanent copy of the worksheets but the instructor may choose to distribute copies of the Introduction for students records Each worksheet has e an introduction to the topic under investigation e step by step instructions for the investigation that follows e a section headed So What to collate and summarise results offer extension work and encourage development of ideas through collaboration with partners and with the instructor e asection headed For your records to be copied and completed in students exercise books This format encourages self study with students working at a rate that suits their ability The instructor should monitor that students understanding keeps pace with their progress through the worksheets One way to do so Is to sign off each worksheet as a student completes it and in doing so have a brief chat with the student to assess grasp of the ideas involved in the exercises it contains Time lt should take students between 5 and 7 hours to complete the worksheets It is expected that a similar length of time will be needed to support the learning that takes place as a result
31. nt leads volt age Copyright 2014 Matrix TSL Limited locktronics Page 30 Introduction to Instructor Guide Three phase Power Scheme of Work Worksheet Notes for the Instructor 3 e You can t have a current without a magnetic field nor a magnetic field with continued out a current so rather like the chicken and the egg which comes first When an inductor is connected to a voltage source and so voltage is at its maximum even though there is very little ohmic resistance no current flows as there is no magnetic field Slowly the current and magnetic field build up As they do so an effect described by Lens law means that an opposing voltage builds up in the inductor coil Current is increasing while voltage is decreasing Eventually current reaches a maximum and at that point nett voltage is zero Hence the phase angle Students could remember the word CIVIL as a prompt for the phase rules In a Capacitor current I leads voltage V but current I lags behind in an inductor L The So what section then distinguishes between phase and line voltages Students could be given the following definitions Line voltage measured between any two lines in a three phase system Phase voltage measured across a single component in a three phase system Line current the current in any line between a source and load in a three phase system Phase current the current in any component in a three phase
32. o obtain a new trace e Save it for your records with details of the new settings A challenge Investigate the effect of changing the Picoscope settings Copyright 2014 Matrix TSL Limited locktronics Page 9 Worksheet 1 Introduction to Single phase AC Three phase Power So what Amplitude the maximum voltage in the signal Period the time taken to produce one cycle of the wave i e 1 peak plus 1 trough measured in seconds Frequency the number of cycles of the wave produced per second measured in hertz 1 Hz means one cycle produced every second The relationship between them is Frequency 1 period Delivering power At times an AC supply delivers zero volts At other times it delivers higher or lower values A useless fact the average voltage for an AC supply any AC supply is zero This certainly does NOT mean that AC supplies do nothing far from it the average power delivered is NOT zero For a resistor R power delivered V R i e depends on voltage squared Hence the voltage may be negative but the power delivered is still positive A negative voltage simply means that the current flows the other way as you saw in the third circuit For an AC supply a measure more significant than average voltage is rms voltage Although it stands for root mean square voltage it is better to think of it as the DC voltage which would deliver the same power to a load For a sine wave signal peak and r
33. ols on the Picoscope 6 screen ble Edit Views Measurements Jools Help gt aa Tm ales lie 7 at ls msi J ix Ls Ef 91CO Agato Eoc E Bg o a Bq of v 7 B e amp Fo Copyright 2014 Matrix TSL Limited locktronics Page 6 Introduction to Introduction Three phase Power Picoscope continued In this trace Timebase 5 ms div SO the time scale horizontal axis is marked off in 5 ms steps Voltage sensitivity auto so the software adjusts the voltage scale vertical axis to suit the signal The scale on the left hand edge of the image increases in 2V steps The trace shown has a maximum value of around 8 7V Trigger Auto SO will show any changes in the signal as they happen Ch A SO looks at the signal on channel A to decide when to start the trace Rising SO Waits for a rising voltage to reach the threshold Threshold 0 mV so starts the trace when the signal on channel A rises through OV Pre trigger 0 so the display starts with the very first data captured For the next trace the signal is the same but some of the settings have been changed Eile Edit Views Measur ajma ilu d F f 20 msaiv x x1 Ag 20v_ oc 2 Bey orf x Ca lotr New settings Timebase 20 ms div SO the time scale is marked off in 20 ms steps Voltage sensitivity 4V div
34. oltage and explain the role of the smoothing capacitor in reducing it distinguish between real reactive and apparent power and state the units in which each is measured use the formulae given to calculate real reactive and apparent power use traces of applied voltage and current to calculate phase angle and real reactive and apparent power Copyright 2014 Matrix TSL Limited locktronics Page 27 Introduction to Instructor Guide Three phase Power What the student will need To complete the course the student will need the following equipment LK5601 Lead green 500mm 4mm to 4mm stackable 2 HP3920 Three phase motor 1 HP4039 Tray Lid 1 HP5540 Deep tray 1 HP7750 Daughter tray foam cutout 1 HP8525 Crashfoam 1 HP9390 Three phase power supply 1 HP9564 62mm daughter tray 1 LK2346 MES bulb 2V 0 1A 3 LK4025 Resistor 10 ohm 1W 5 DIN 2 LK5202 Resistor K V2W 5 DIN 3 LK5243 Diode power 1A 50V 6 LK5250 Connecting Link 10 LK5291 Lampholder MES LK5603 Lead red 500mm 4mm to 4mm stackable LK5604 Lead black 500mm 4mm to 4mm stackable LK5607 Lead yellow 500mm 4mm to 4mm stackable LK5609 Lead blue 500mm 4mm to 4mm stackable LK5987 33uF non electrolytic capacitor LK6218 Resistor 2 2k 4W 5 DIN LK8900 7x5metric baseboard with 4mm pillars LK6529 Binding post Ra a os HBS NYO FP YD OW Using this course The experiments in this course should be integrated with teaching to introduce the theory behind it a
35. ph The basic controls on oscilloscopes are gt fvottage ny e Voltage sensitivity sets the scale on the voltage vertical axis I5 Spreads the trace vertically if a lower number iS 40 used The diagram shows a setting of 0 5 V div 05 nme imi e Timebase sets the scale on the time horizontal axis 0 gt 10 20 30 40 50 spreads the trace horizontally if a lower number is used The diagram shows a setting of 10 ms div e Trigger sets the threshold signal voltage that starts data gathering can be set for either a rising or a falling signal at that voltage level A computer based oscilloscope like Picoscope is a data logger that monitors voltages at reg ular intervals and passes the results to software in the computer There the data is processed to produce voltage time graphs frequency information etc to be displayed on the monitor stored as a file or printed like other information on the computer The Picoscope uses the oscilloscope controls described above plus e AC or DC shows only varying voltages for AC so centres the trace on OV shows the true voltage levels if DC is chosen e Stop Go Stop the trace is frozen i e showing a stored event Go the trace is showing events in real time Click on the appropriate box to change from one to the other The settings are selected on screen using the drop down boxes provided The following diagram shows some of the main contr
36. plete rotation of length the bar creates one cycle of the sequence op 90 180 270 We can use the same idea to describe phase in alternating voltage signals Here the starting point and end point of one cycle of the signal are separated by a phase difference of 360 Voltage Using the information given above one cycle of each phase lasts 20ms so points which are 20ms apart have a phase angle of 360 between them One cycle Time The peaks of the three phases are separated by 6 66ms In other words the phases are separated by phase angles of ae 270 360 of 360 20 x 6 3 120 For your records e Describe five advantages of three phase over single phase power transmission You may need to carry out a little research on this using the internet for example e Draw two cycles of a sine wave signal Add a second signal where the phase angle between them is 180 e Do the same again with a phase angle of 270 between the two signals Copyright 2014 Matrix TSL Limited locktronics Page 12 Worksheet 3 Introduction to More phase basics Three phase Power Voltage Its complicated The voltages and currents change in size and direction over time There appear to be three separate sources all peaking at different times We need a word to describe all of this phase Time Over to you These investigations focus on the relationship between current and voltage in three diffe
37. preliminary exercises to ensure that students can do this The So what section analyses results and compares them to the theoretical relationship This can be proved relatively easily using phasor diagrams and trigonometry but here we just use the result of all that It includes a discussion of the term balanced load Essentially there is no need for the return neutral wire when the load is exactly balanced The reason for this is that the three phases are not in phase When one phase is positive one of the others will be negative and will carry the current back to the three phase source This may need some emphasis by the instructor The traces of the three phase supply obtained in worksheet 2 may help with this explanation The For your records section includes a question as to why three phase transmission systems usually use four wires bearing in mind this last result The answer lies in the reality that loads are not always balanced in which case excess current is carried in the fourth neutral wire This worksheet is a mirror image of the last but looking at the delta configuration As previously no attempt is made to derive the relationship between line and phase quantities Students are simply asked whether their results support it Previously students connected the three phase motor in star configuration Now the challenge is to do so using a delta connection and then to compare its performance They sho
38. rent types of load resistive capacitive and inductive The inductive load is created by joining the coils of the three phase motor in series Picoscope channel A This time we are monitoring the AC voltage across the 10Q resistor and the current flowing through it e Clamp the jaws of the current clamp around one of the wires from the three phase generator e Use the same settings for the timebase for channel A and for the trigger Switch channel B to Auto e Switch on the current clamp on the 20A range e Check that the three phase generator is set to the 6V output or the 102 resistor may overheat Switch on the generator e Obtain a trace comparing the AC voltage across the resistor with the current flowing through it and save it for your records e Notice that the two AC signals the voltage across the resistor and the current through it are in step in phase with each other e Switch off the three phase generator and the current clamp A challenge Using the same approach modify the circuit to obtain voltage and current traces for e a33uF capacitor e an inductor made by using one of the motor coils Copyright 2014 Matrix TSL Limited locktronics Page 13 Worksheet 3 Introduction to More phase basics Three phase Power So what l A typical Picoscope trace for the resistor is shown l I A N A A A A i opposite The red trace indicates the current flowing through The resistor and
39. rix TSL Limited locktronics Page 19 Worksheet 6 Introduction to On the delta Three phase Power So what Theory predicts the relationship Line current V3 x Phase current e Complete the table using your measurements from the previous page phase current Ip GA RRR phasecurrentle line currents line currents line currents e Does it support this relationship Summary In a delta configuration Line voltage Phase voltage Line current V3 x Phase current A challenge e Connect the three phase motor using a delta configuration e connect the three coils in delta formation U2 to V1 V2 to W1 and W2 to U1 e connect the three phases to U1 V1 and W1 e What happens when you reverse two of the phases e Compare the performance of the motor in delta configuration with that seen earlier using the star configuration For your records e Copy the circuit diagram showing a delta configured three phase supply driving a delta configured three phase load e Summarise the findings of this investigation e Using an internet search engine or text books draw up a table of relative advantages and disadvantages for star and delta configurations e Draw the circuit diagram for the delta configured motor on the three phase supply e Compare the performance of the motor when in star and in delta configuration Copyright 2014 Matrix TSL Limited locktronics Page 20 Worksheet 7 Introduction
40. t is even more complicated than we made out We saw that the current flowing through the component may not be in phase with the voltage across it What s worse the size and direction of both change over time The language of AC is now extended bringing in the phas Or The worksheet begins by using the three phase motor on a three phase supply and then on a single phase and then turns to phasors and their interpretation not the phaser that belongs in Star Trek Over to you Three phase supply Ph1 U1 Build the first circuit shown opposite Ph2 Challenges Set the frequency to 0 1Hz and the amplitude to 6V Ph3 Switch on the three phase supply Carefully rest one finger on the motor flywheel to judge how smoothly the torque is applied What is the effect of turning the amplitude to 9V What is the effect of increasing the frequency to 50Hz What happens when you increase the frequency to 150Hz C wm HAIN Investigate what happens when you change to phase connections to the motor or leave one disconnected Can you reverse the direction of rotation Single phase supply Build the second circuit shown opposite It uses only one phase of the power supply to drive two sets of coils the U and V coils if set up as in the diagram The capacitor adds a phase shift between the supplies to the two coils Using the same frequency and amplitude of 6V switch on aio When the
41. t of the circuit Instead use only one probe to monitor the output across the load as shown Picoscope Picoscope Picoscope settings Sa A Timebase 5ms div Pht Channel A Auto Channels B C andD Off Trigger Auto Threshold OV Pre trigger 0 e Set the three phase generator frequency to 50Hz and amplitude to 9V e Switch it on e Obtain a trace showing the output across the 1kQ load e Save it for your records with an explanation of what it shows e Connect a 33uF capacitor in parallel with the 1kQ load and repeat the process To see the relationship between the output and phases move the Picoscope channel A probe to examine each phase in turn Copyright 2014 Matrix TSL Limited locktronics Page 23 Worksheet 8 Introduction to Full wave rectification Three phase Power So what The two traces opposite show typical Picoscope results without top trace mofo foo force foc oo foe foc fo MNONO fi MOMEN and with smoothing bottom trace Compare them with those obtained for half wave rectification The amplitude of the ripple voltage is much smaller and the frequency is higher than with half wave rectification A challenge As in the previous worksheet investigate the effect of load resistor size on ripple voltage For your records e Draw the circuit diagram for a three phase full wave rectifier including capacitor smoothing e In investigating this circuit e what indicated that the output was DC
42. t the negative side of the load Trying to do both at the same time results in a short circuit of part of the system The only way to do this is to use expensive differential oscilloscope probes that are not automatically referenced to ground The student is asked to investigate the effect of capacitor value and of load resistance on the size of the ripple voltage As before the instructor should emphasise the importance of connecting electrolytic capacitors the right way round The results should show that increasing the capacitor value reduces ripple as it stores more charge and so can sustain the current for longer Increasing the value of load resistance reduces the ripple because it draws a smaller current from the reservoir capacitor allowing it to maintain the voltage longer This worksheet addresses full wave rectification and begins with a reminder that vehicle alternators generate AC but then internally rectify it for use in the Copyright 2014 Matrix TSL Limited locktronics Page 33 Introduction to Instructor Guide Three phase Power Scheme of Work Worksheet Notes for the Instructor The instructor may wish to direct the students back to the Introduction and its treatment of AC power This ties in with earlier work on phasor diagrams and the meaning of rms quantities All of this may prove difficult for some students and the instructor may need to offer substantial support to ease them through this In the
43. te A The length of the horizontal component gives the size of the voltage or current at that instant As the phasor rotates this increases to a maximum decreases to zero increases and so on like the rotating bar in worksheet 2 Direction when the horizontal component is on the right hand side of the sre ee se diagram current flows to the right say and when the component is on the rotation component left hand side it flows to the left phasor vertical component Combining phasors also uses the parallelogram rule but the current is being pulled by a number of voltages For example in the circuit described by the diagram opposite T s spi wo Ay eh e the supply voltage Vs is split between the resistor and capacitor J J S C e the current through both is the same as they are in series e current is in phase with resistor voltage Vr but leads capacitor voltage bl Py 90 This leads to the phasor diagram opposite The lengths for Vr and Vc are arbitrary Although all phasors are rotating at the supply frequency the diagram shows the situation when current is maximum e Check the diagram to make sure that you can see the information about Vp Vc and current The vector parallelogram rule gives Vs as the resultant of vr and Vc For your records e Describe how a current phasor contains information about its size and direction e Copy the circuit diagram and phasor diagram given above Explain
44. the blue trace the voltage across it Y V y y V E They rise and fall in step They are in phase _ Resistor When you modified the circuit to investigate the other components you should have obtained traces like the ones shown opposite i NY e Inthe capacitor current reaches peak value when the l _ Capacitor voltage is zero e Inthe inductor the voltage peaks when the current is Zero Phase angle g Inductor Using phase angles we can refine our earlier statements e In a capacitor the current signal leads the voltage signal by a phase angle of 90 e Inthe inductor voltage leads current by a phase angle of 90 Phase voltage vs line voltage In discussing three phase systems two different measures of voltage are used phase voltage and line voltage e Phase voltage measured between a phase terminal and the common N terminal e Line voltage measured between one phase and another The circuit you set up earlier measured phase voltages For your records e Copy the diagram showing the difference between phase and line voltage e Copy and complete the statement For a resistive load AC voltage and current are e What is the phase angle between current and voltage in a resistor e Copy the two bullet points describing phase angles in capacitors and inductors Copyright 2014 Matrix TSL Limited locktronics Page 14 Worksheet 4 Introduction to And now the phasor Three phase Power l
45. to Half wave rectification Three phase Power Some electrical devices require a DC supply and will Voltage not run on AC Rectification is the process of turning an AC supply into DC It relies on the fact that diodes allow nme appreciable current to flow in one direction only oe R ion Comparing rectified three phase to single phase Voltage A 4 The advantage e the result is much smoother i Time The disadvantage e it requires more diodes to accomplish it This worksheet introduces the process by looking at half wave rectification Over to you e Build the circuit shown opposite The circuit diagram is given underneath it It uses the three phase generator in star configuration to drive current through a 1kQ load Each phase has a diode in series to ensure that current flows only one way through the load Picoscope Notice the Picoscope ground clip channel A e On the three phase generator set piesa e the frequency to 50Hz channel B e the amplitude to 9V e Use the following Picoscope settings Timebase oms div Channel A B C and D Auto Bicescops Trigger Auto channel C Threshold OV channa D Earth Pre trigger 0 e Switch on the three phase generator Pie e Obtain a trace showing the three phases delivering power to Pho the load and the half wave rectified output applied to it e Save it for your records Ph3 1kQ e To make it clearer what is happening turn off channels B and r
46. uld find that reversing two phases causes the motor to reverse rotation Part of the For your records section includes a comparison of the relative advantages of star and delta configurations Possible topics include the benefit of lack of a neutral wire in the delta system the ease of extracting a single phase output from the star configuration the extra torque and self starting ability for a three phase motor in delta configuration the availability of two voltage values line and phase in star systems the lack of an earthing point in delta systems The instructor may choose to deliver this comparison or may set it as a research task for the students Three phase sources are usually connected in star configuration If in delta an unbalanced load would cause excess current to flow around the windings of the source creating waste heat and possibly damaging the generator It is quite common to find mixed systems where the source is connected as a star and the load as a delta configuration Copyright 2014 Matrix TSL Limited locktronics Page 32 Introduction to Instructor Guide Three phase Power Scheme of Work Worksheet Notes for the Instructor Timing There are good reasons for generating electrical power as AC alternators are more efficient than equivalent DC generators transformers can use AC but not DC However most electronic devices require DC power Fortunately it is easy and cheap to convert AC to DC
47. varying and presents its results as a voltage time graph e digital oscilloscopes use digital processing techniques to obtain the results Any oscilloscope can be used for this course though at times it needs to monitor four signals simultaneously Further instructions on using the Picoscope are given later Current clamp e has a handle to open the jaws so they can be clamped around the wire carrying the current e can be used on AC and DC currents e produces an output voltage proportional to the current e has two ranges 1mV 10mA and 1mV 100mA allowing measurement up to 20A and 60A e has a switch to select the ranges and on oft Multimeter e For voltage measurement e plug probes into V Q Hz or equivalent and COM e select the range either AC or DC e connect the probes in parallel with the device under test e For current measurement e plug probes into mA or 10A or equivalent and COM e select the range either AC or DC e break the circuit just before or just after the device under test and connect the probes to each end of the break e No reading may mean that the internal fuse has blown Copyright 2014 Matrix TSL Limited locktronics Page 5 Introduction Introduction to Three phase Power Using the hardware continued Digital oscilloscope An oscilloscope CRO monitors signals usually those that vary over time It presents the results as a voltage time gra
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