APPLICATION NOTES TRANSFORMERS
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1. I Voltage Ratio 2K Current Ratio 1 2 5 www cdautomation com C D Automation srl Application Note Transformer V1 0 3 1 1 Indicative value for realistic transformer Heat from core Heat from winding Leakage fluxes Efficiency n Noise 1 5 of Input Power 1 5 of Input Power 1 10 of Flux 90 97 Depending on nominal power With big transformers the efficiency is very high 97 20 60dB Depending on magnetostriction of lamination and windings vibration www cdautomation com C D Automation srl Application Note Transformer V1 0 3 2 The transformer and equvalent circuit Equivalent circuit of a loaded transformer In a ideal transformer M If we assume that that transformer has K 1 we have the following equivalent circuit X r X This circuit is used in next pages to have more intuitive equivalent circuit 7 www cdautomation com C D Automation srl Application Note Transformer V1 0 3 3 Unloaded transformer Equivalent circuit of a unloaded transformer M We can simplify the circuit assuming The unloaded 1 transformer seen by thyristor unit is this means to have secondary open and to apply voltage on primary www cdautomation com 8 C D Automation srl Application Note Transformer V1 0 The magnetizing current J and there a pick of current that is norma2. Complete rotation of 3609 that is complete period One period is 20msec at 50Hz Thus if delay of current is 45 the unit must switch ON with a delay Pod 21 25msec 4b X 360 delay www cdautomation com 20 C D Automation srl Application Note Transformer V1 0 Negative magnetising current Residual positive with 0 Residual negative with 0 H Positive magnetising current In magnetic circuit there is a residual magnetism that can be positive or negative depending on when the current was switched OFF If we have residual flux positive and we switch ON when the current is positive we have the magnetic saturation due the sum of residual positive flux positive In this case the current rise and fuses blow If we switch OFF negative the residual flux is negative and switching ON positive the total flux is smaller than normal one Residual flux flux positive lt Normal flux In this mode we have no saturation 21 www cdautomation com C D Automation srl Application Note Transformer V1 0 Delayed angle depend on the feature of the transformers with delay of 2 5 msec we switch ON when current is zero and after which becomes positive Residualnegative Flux positive Flux value below nominal In this case there are no problems for saturation and there is no inrush current The fuses does not blow www cdautomation com 22 C D Automati
3. SARY F1 r2 le Primary windings resistance Secondary windings resistance Primary windings resistance plus Secondary windings resistance seen from the primary Load Resistance Primary leakage inductance Secondary leakage inductance Primary leakage inductance plus Secondary leakage inductance seen from the primary Primary impedance consisting of the vectorial sum of and Primary voltage Secondary voltage Primary current Secondary current Magnetizing current generating the flux Eddy current in laminations core losses Vectorial sum of Im Ip also called magnetizing current since Im gt Ip on power transformer Number of winding on primary Number of winding on secondary Voltage frequency Electromagnetic force on primary Electromagnetic force on secondary www cdautomation com 2 C D Automation srl Application Note Transformer V1 0 2 PRINCIPLES How works a transformer The principle on which works the transformer is based on general low of electromagnetic induction E dt The magnetizing current generate a magnetic flux that is concatenated with the secondary This flux is variable on time and it is sinusoidal on secondary circuit that is metallically insulated from primary is inducted on electromagnetic voltage that give a voltage between the secondary terminals If we apply to primary a D C voltage the transformer does not work Infact the derivative of a constant value is zero so
4. USER S MANUAL V1 Rev 01 2002 APPLICATION NOTES TRANSFORMERS A a a Hr je lr 1 BEEN Y nov Y Y LA J P ri M o VS CD Automation S r l Via F lli Cervi 42 44 m 20020 Cantalupo di Cerro Maggiore MI Italy RR Tel 39 0331 533512 Fax 39 0331 533516 AUTOMATION e mail info cdautomation com lt web www cdautomation com C D Automation srl Application Note Transformer V1 0 PT Index 1 GLOSSARY 2 2 PRINCIPLES 3 3 THE TRASFORMER 4 3 1 REAL TRANSFORMER 5 3 1 1 INDICATIVE VALUE FOR REALISTIC TRANSFORMER 6 3 2 THE TRANSFORMER AND EQUVALENT CIRQUIT 7 3 3 UNLOAD TRANSFORMER 8 3 4 LOADED TRANSFORMER 10 3 5 TRASFORMER WORKING IN SHORT CIRCUIT 11 3 6 TRANSFORMER WORKING IN SATURATION 12 3 7 FORMULAS 13 3 8 THREE PHASE TRANSFORMER 15 4 THYRISTOR STACKS TO DRIVE TRANSFORMER 16 4 1 THYRIASTOR STACK CONNECTED BETWEN SECONDARY AND LOAD 17 4 2 THYRIASTOR STACK CONNECTED BETWEN PRYMARY 18 4 3 THYRIASTOR STACK CONNECTED TO PRYMARY USING BURST FIRING BF 19 4 4 CD AUTOMATION UNIT TO BE USED 23 4 5 APPLICATION WITH THREE PHASE TRANSFORMERS 24 4 6 SCOTT TRANSFORMERS 25 4 7 WHY USE THYRISTOR UNIT IN FOURNACE 27 1 www cdautomation com C D Automation srl Application Note Transformer V1 0 1 GLOS
5. der of input signal on zone B This can be done in digital mode on CD3000A M family and Multidrive www cdautomation com 28
6. e became zero 3 www cdautomation com C D Automation srl Application Note Transformer V1 0 3 THE TRANSFORMER Transformers are used to modify parameters of electric power They receive energy with a certain value of voltage and current primary of transformer and they delivery a voltage on secondary that is K time bigger in voltage and K time lower in current or vice versa The apparance of the transformer are below represented d Tt dt Load gt dt V M gt t Jl Vol V M www cdautomation com 4 C D Automation srl Application Note Transformer V1 0 3 1 Real transformer The transformer represented in the precedent paragraph is an ideal transformer In a real transformer there is a power dissipation and flux dissipation Heating from Heating from Heating from winding 1 magnetic circuit winding 2 pT 1 1 m 4 _ gt l i P gt _ gt 1 Y v v Y Qa Vi V JN J X eem Ce Flux dispersion Flux dispersion on primary 5 Coupling flux The balance of power is represented above If we consider the principle of preservation of energy and in first hypothesis we don t consider power los A EET WLEVIL
7. e equal by maintaining the three voltage on primary equal with Phase Angle Firing This means that if the loads on each phase are equal also the currents will be equal and viceversa If loads are unequal and customer want to adjust the voltage to equalise the power in the 3 zones is not easy and can be done just with cautions and for small percentage Avoid to do it is too dangerous This means that a three phase transformer can not be treaded like three one phase transformers because there is an interaction between the phases A extra complication are the harmonics generated by phase angle To maintain the harmonics inside the transformer is used a delta connection on primary The current in the windings consist of a range of harmonics in addition to the fundamental current The effect of the harmonics an a core transformer is an overheating on the transformer core and on metallic steel case because the lost fluxes try to close via any metals in the vicinity With delta connection the flux of the harmonics will add in a ring round the core with a resultant of zero Anyway in the field three applications where the primary is a star connection Despite the guideline these applications are working with satisfaction probably because the transformers are lightly loaded and designed to work with low induction One of the rules used by CD Automation when a transformer is ordered is e Increase the nominal power of the transformer of 20 e Increase t
8. he nominal value of primary and secondary voltage of 10 The transformer ratio remain the same but the transformer works at low value of induction www cdautomation com 24 C D Automation srl Application Note Transformer V1 0 4 6 Scott transformers The Scott wound Transformer is designed to have one three phase primary normally star connected and two one phase secondaries lt m m 2 secondary c a n __ lt 1 secondary T This type of transformer is not very popular but there are application where there are two zones and is necessary to balance the load on the three phases The solution is a Scott Transformer Typical applications are two zone furnaces and salt baths where for supply reasons energy for only two zones is taken from a three phase main voltage supply All the attentions paid for normal three phase transformers must be used also for Scott Winding Transformer Flux patterns are more critical than in normal transformers and so it s necessary to use a longer soft start It s suggested to have a Soft Start Ramp of 2 sec minimum In this application must be used Phase Angle and the harmonic content in load current are reflected on primary and on primary star connected the current on the three limbs can be different 25 www cdautomation com C D Automation srl Application Note Transformer V1 0 Another precaution is
9. lly bigger than nominal current The fuses blow ie ra X4 In this case the thyristor unit see the transformer as a pure inductive load Very close to 90 delay COS close to 0 To avoid to have the pick current Io can be used two technique 1 Phase angle firing with soft start and current limit 2 Burst firing using the Delay Triggering DT technique To avoid magnetic circuit saturation the thyristor unit switch OFF when load voltage is negative and switch ON only when is positive with a setted delay on voltage zero crossing In this way is possible to switch ON when current is zero 9 www cdautomation com C D Automation srl Application Note Transformer V1 0 3 4 Loaded transformer If the secondary of transformer is closed on a resistive load R the equivalent circuit is X M If we assume zy The unloaded transformer seen by thyristor unit is e The load seen by thyristor is mostly resistive e Power factor 0 8 0 9 e In burst firing is required a small phase delay from 15 to 40 If we consider that a complete cycle of voltage waveform is 20 msec and is equal to 360 220 40 2 1 360 When delayed triggering is used the delay that must be setted is 2 1 msec The electric circuit is based on zero crossing of voltage and the signal to fire thyristor is given with the setted delay In our example is 2 1msec and this allow to fire the thyri
10. nsformer because K value depend on the different connection on primary and secondary Y A A A Y Y A Y etc Solution B Pog tP We consider Pioss negligible P P p 17 1 P 300 000 173 V 173 400 15 www cdautomation com C D Automation srl Application Note Transformer V1 0 4 THYRISTOR STACKS TO DRIVE TRANSFORMER We have seen in previously pages that one transformer is used for following purposes e To have galvanic isolation between primary and secondary e To adapt the main supply voltage to the nominal load voltage Ex main supply 400V load supply 60V you need a tranformer 400 60V www cdautomation com 16 C D Automation srl Application Note Transformer V1 0 4 1 Thyristor stack connected betwen secondary and load I 25A Load e Normally this solution is not used because the current of secondary is higher than primary In our example 1 25 I2 200A It s easy to understand that one unit of 200A cost a lot more than one of 25A e Advantage of this connection is that no attention must be paid to the transformer but just to the load features In this case we can use a normal zero crossing firing Ex CD3000S or CD3000A or CD3000M without option e Same consideration for three phase load and is this case the thyristor units will be CD3000A 2PH or CD3000A 3PH If you need communication use M family 17 www cdautomation com C D Automation srl Application Note T
11. on srl Application Note Transformer V1 0 4 4 CD Automation unit to for one phase applications For this application use CD3000A or M configured for delayed triggering For 3 phase application use CD3000E 2PH configured for delay triggering On CD3000E the enhanced type of CD3000 family is possible to set the delay of firing and the number of half cycle on which to apply it In the majority of transformer is enough to set one half cycle When the delay is not known because we don t know the angle between the vectors of voltage and current set 2 5msec 45 delay The delay is adjustable from zero to 90 5msec The inrush current will be minimized and the benefits of Burst Firing will be realised The use of this technique has been experimented with normal resistive loads and silicon carbide elements Don t use this technique with cold resistance like KantalSuper or other resistance that have a very low value when cold In this case for the thyristor unit is like to switch ON a transformer with secondary in short circuit For this application use Phase Angle with Soft Start and current limit The suggest unit for one phase loads are CD2200 or Cd3200 The suggest unit for three phase loads are CD3000E 3PH CD30 or MULTUDRIVE 23 www cdautomation com C D Automation srl Application Note Transformer V1 0 4 5 Application with three phase transformers It s important to keep in mind that the tree limb fluxes must b
12. ransformer V1 0 4 2 Thyristor stack connected on primary 1 25A gt e this solution is used because the current on primary is less than on secondary and thyristor stack cost less e Attention to this connection must be paid because the inrush current when transformer is switched on can be up to 20 times the nominal current This over current causes fuse failure or thyristor failure if fuses are not properly sized i t of fuse higher that i t of thyristor this can happen only when the original fuses has been substituted without to respect the proper i t e How is possible to reduce the inrush current Is necessary to select a thyristor unit with following features Soft Start Current Limit e Use CD3200 or CD2200 for 1 phase transformer e Use Multidrive or CD3000E 3PH for 3 phases transformer Note For small transformer can be enough to use soft start to avoid inrush current There are not rules to establish when this can be done so we suggest to use every time current limit in addition to Phase Angle Soft Start www cdautomation com 18 C D Automation srl Application Note Transformer V1 0 4 3 Thyristor stack connected on primary using Burst Firing BF This firing mode is used to switch ON OFF a transformer on primary With this circuit the power is controlled in non continuous mode The firing technique used is Burst Firing BF If we set Power at 50 4 Cycles ON 4 Cycles OFF
13. sized for i t will blow also the thyristors e A normal transformer must never saturate e To avoid to have saturation positive and negative must be equal Other ways a dc component will appear DC component a b 20 DC component average value www cdautomation com 12 C D Automation srl Application Note Transformer V1 0 3 7 Formulas P oss Data Incognita V1 2400V l V2 100V KVA 100 PL 80KW Solution 1 VL 1 400 7 2100 800 P V 1 994 0004 400 jae 809 _ 9004 V 100 Solution 2 ti K 00 4 V 100 L k 1 I 1 200A 4 100 The maximum current on primary is Po minal KVA po Pose _ 100 000 6 E 400 We suggest to size the thyristor unit for 250A because in future is possible to add other loads 13 www cdautomation com C D Automation srl Application Note Transformer V1 0 Solution 3 This is the easier solution based on balance of power P We consider Pioss negligible P P P Wy _ P _ 80 000 2004 V 400 www cdautomation com 14 C D Automation srl Application Note Transformer V1 0 3 8 Three phase transformer 100KW Data Incognita Vi 400V V2 100V 300KW P 100 100 100 To found total power it s possible to do arithmetical sum boucherout low With 3 phase loads we don t suggest to use solution 1 2 used for 1 phases tra
14. stor when current is zero A V d _ gt I www cdautomation com 10 C D Automation srl Application Note Transformer V1 0 3 5 Trasformer working in short circuit Short circuit voltage Vs is the value of voltage on primary able to give a circulating current equal to nominal with the secondary terminals in short circuit In normal transformer is 5 of This means that if is done a short circuit on secondary with nominal voltage on primary the circulating current becomes 20 times the nominal one rl Xl If for example Vs is 5 this means that also E is 5 and the flux is small The power loss are small and negligible because are proportional to 0 05 0 025 Of nominal value when the transformer is loaded M The equivalent circuit assuming is 1 X The current is limited only by impedance of primary an secondary winding e Vs 5 of Vi e Is is 20 time the nominal current with full voltage e Power factor 0 7 0 8 e Current limited by winding impedance only e Leakage inductance is equivalent to an air coil it does not saturate 11 www cdautomation com C D Automation srl Application Note Transformer V1 0 3 6 Transformer working in saturation I r X X This circuit is simplified considering K 22 1 1 e The current is limited by r only e The fuse of thyristor unit will blow and if is not properly
15. the contacts done with precious metals have to be substituted The cost of a medium size contactor 200A is 150 Euro Every year the contactor has to be substituted because the mechanical life is finished The 200A contactor cost is 500 Euro in addition when service or maintenance are done there is a stop of the production with relative costs 27 www cdautomation com C D Automation srl Application Note Transformer V1 0 THYRISTOR UNITS e The thyristor units are more expensive than contactor e The payback is very short because there are no maintenance or service costs and the life of one thyristor unit can be 10 years e In addition is also possible to use a cycle time very short up to 20msec ON and 20 msec OFF this firing is known as Single Cycle very useful with low interthia systems For Burst Firing see our application note on Firing modes available on CD Automation thyristor units e With Thriystor units is possible to adjust the power on different zones when are controlled by one temperature control only je AT LIIS al e A B C The output of the controller is a power demand from the process If above process is a furnace and heating elements A B are equal but the is central and has a lower heating losses we can decide to send the same signal to A and C loads and to reduce the power with a devi
16. to use thyristor rated 1600V reverse pick voltage because can happen to have parasite voltage that damage Thyristers when are in off conditions This happens when the secondary are not galvanically isolated If supply voltage is 400V use thyristor unit for 500V that as standard uses 1600V RPV The Scott Transformers must work at low induction value 1 Wb m A good practice is also to use on primary a filter rated for 50 Hz The CD Automation Thyristor units to drive Scott Transformer are e CD30 e MULTIDRIVE www cdautomation com 26 C D Automation srl Application Note Transformer V1 0 4 7 Why use thyristor unit in fournace Electrical powered fournaces are becoming more important mainly for these reasons a Clean ambient inside furnace b Very high efficiency c High capacity to maintain a stable temperature over a wide range of set points d Capacity to optimising temperature uniformity Anyway to maintain points c and d using contactors is necessary to use a temperature cycle time of 20 sec This means that at 50 power demand that is a normal working condition after the Start up of the furnace the contactors switch ON every 20 sec and in one day 4 320 times considering 24 hours normally furnaces don t stop In one year there are 320 working days total switches 24320 x 320 1 382 400 After 0 5 million of operation the contacts of contactor must be substituted This is a big cost because Every month
17. we will have at 75 12 Cycles ON 4 Cycles OFF see Duty Cycle Tecnique in application note This technique allows to use zero crossing avoiding to have electrical noise and interference typical of Phase Angle Firing Unfortunately Burst Firing used on primary transformer generate an inrush current in the transformer able to blow fuses To avoid this problem CD Automation uses a technique named Delay Triggering DT The most important features of this firing are Switch OFF when current is negative and goes to 0 Switch ON when current is zero and immediately after became positive CD Automation thyristor units with Delayed Triggering switch OFF when voltage is negative and switch ON only when voltage is positive this 19 www cdautomation com C D Automation srl Application Note Transformer V1 0 transformers To understand we have refer to the curve of Flux Intensity B related to Ampere Turns mixl H It s important to consider that the current is not in phase with voltage but has a delay The electronic circuit detect the zero voltage but when the voltage is zero the current has a negative value e The instantaneous value is the projection on y axis I that is a negative value that is in same direction of residual flux and allow excess current to flow with consequent fuse failure The technique used is to switch ON after a delay time from the V zero crossing e The vectors are rotating anticlockwise
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