Laboratory Manual: Electrical Machines – I Laboratory

Laboratory Manual: Electrical Machines – I LaboratoryProcedure:1.2.3.4.connect the circuit as shown in the diagram.Note down the readings of all wattmeters, ammeters and voltmeters for given load.Repeat the above test for different values of loadTake at least three readingsObservation Table:S.NO.I1 (AMPS)W1(WATTS)I2(AMPS)W2(WATTS)IL I1 I2(AMPS)WL W1 W2(WATTS)1.Result:The two transformers connected in parallel share the load equally.Discussion:The total load current is distributed on two transformers accordingly.I1 I2 I lThe total wattmeter readings are distributed on two wattmeters accordingly.W1 W2 WlPrecautions:1. Transformers should be connected in such a way that they have same polarity.2. All connections should be neat and tight.3. Connecting leads should be perfectly insulated.Quiz:Q.1 What is the minimum no. of transformers needed toconduct this exp.?A1 TwoQ.2 What is the effect of circulating current in the circuit having twotransformers in parallel ?A2 produces additional copper lossesQ.3 when does the circulating current flow in a circuit of two transformersconnected in parallel?A3 If the two transformers have different voltage ratiosQ.4 How much circulating current can be tolerated for parallel operation oftransformers?A4 10% of rated valueQ.5 why the transformer are needed to beoperated in parallel.A5 If the load is more than rated loadQ.6 What will happen if two transformers are connected in parallel withwrong polarity?A6 Dead short circuit on the transformersPrepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 12

Laboratory Manual: Electrical Machines – I LaboratoryExperiment-05Aim: To Study the Various Connections of Three Phase Transformer.Theory:Three Phase Transformer Configuration.A three phase transformer or 3φ transformer can be constructed either by connecting togetherthree single-phase transformers, thereby forming a so-called three phase transformer bank, or byusing one pre-assembled and balanced three phase transformer which consists of three pairs ofsingle phase windings mounted onto one single laminated core.The advantages of building a single three phase transformer is that for the same kVA rating itwill be smaller, cheaper and lighter than three individual single phase transformers connectedtogether because the copper and iron core are used more effectively. The methods of connectingthe primary and secondary windings are the same, whether using just one Three PhaseTransformer or three separate Single Phase Transformers. Consider the circuit below:The primary and secondary windings of a transformer can be connected in differentconfiguration as shown to meet practically any requirement. In the case of three phasePrepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 13

Laboratory Manual: Electrical Machines – I Laboratorytransformer windings, three forms of connection are possible: “star” (wye), “delta” (mesh) and“interconnected-star” (zig-zag)The combinations of the three windings may be with the primary delta-connected and thesecondary star-connected, or star-delta, star-star or delta-delta, depending on the transformersuse. When transformers are used to provide three or more phases they are generally referred to asa Polyphase Transformer.Three Phase Transformer Star and Delta ConfigurationsBut what do we mean by “star” (also known as Wye) and “delta” (also known as Mesh) whendealing with three-phase transformer connections. A three phase transformer has three sets ofprimary and secondary windings. Depending upon how these sets of windings areinterconnected, determines whether the connection is a star or delta configuration. The threeavailable voltages, which themselves are each displaced from the other by 120 electrical degrees,not only decided on the type of the electrical connections used on both the primary andsecondary sides, but determine the flow of the transformers currents. With three single-phasetransformers connected together, the magnetic flux’s in the three transformers differ in phase by120 time-degrees. With a single the three-phase transformer there are three magnetic flux’s in thecore differing in time-phase by 120 degrees.The standard method for marking three phase transformer windings is to label the three primarywindings with capital (upper case) letters A, B and C, used to represent the three individualphases of RED, YELLOW and BLUE. The secondary windings are labeled with small (lowercase) letters a, b and c. Each winding has two ends normally labeled 1 and 2 so that, for example,the second winding of the primary has ends which will be labeled B1 and B2, while the thirdwinding of the secondary will be labeled c1 and c2 as shown.Transformer Star and Delta ConfigurationsSymbols are generally used on a three phase transformer to indicate the type or types ofconnections used with upper case Y for star connected, D for delta connected and Z forinterconnected star primary windings, with lower case y, d and z for their respective secondaries.Prepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 14

Laboratory Manual: Electrical Machines – I LaboratoryThen, Star-Star would be labeled Yy, Delta-Delta would be labeled Dd and interconnected star tointerconnected star would be Zz for the same types of connected transformers.Transformer Winding IdentificationWe now know that there are four different ways in which three single-phase transformers may beconnected together between their primary and secondary three-phase circuits. These fourstandard configurations are given as: Delta-Delta (Dd), Star-Star (Yy), Star-Delta (Yd), andDelta-Star (Dy).Transformers for high voltage operation with the star connections has the advantage of reducingthe voltage on an individual transformer, reducing the number of turns required and an increasein the size of the conductors, making the coil windings easier and cheaper to insulate than deltatransformers.The delta-delta connection nevertheless has one big advantage over the star-delta configuration,in that if one transformer of a group of three should become faulty or disabled, the two remainingones will continue to deliver three-phase power with a capacity equal to approximately twothirds of the original output from the transformer unit.Transformer Delta and Delta ConnectionsIn a delta connected ( Dd ) group of transformers, the line voltage, VL is equal to the supplyvoltage, VL VS. But the current in each phase winding is given as: 1/ 3 IL of the line current,where IL is the line current.One disadvantage of delta connected three phase transformers is that each transformer must bewound for the full-line voltage, (in our example above 100V) and for 57.7 per cent, line current.The greater number of turns in the winding, together with the insulation between turns,necessitate a larger and more expensive coil than the star connection. Another disadvantage withdelta connected three phase transformers is that there is no “neutral” or common connection.Prepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 15

Laboratory Manual: Electrical Machines – I Laboratory.In the star-star arrangement ( Yy ), (wye-wye), each transformer has one terminal connected to acommon junction, or neutral point with the three remaining ends of the primary windingsconnected to the three-phase mains supply. The number of turns in a transformer winding for starconnection is 57.7 per cent, of that required for delta connection.The star connection requires the use of three transformers, and if any one transformer becomesfault or disabled, the whole group might become disabled. Nevertheless, the star connected threephase transformer is especially convenient and economical in electrical power distributingsystems, in that a fourth wire may be connected as a neutral point, ( n ) of the three starconnected secondary’s as shown.Transformer Star and Star ConnectionsThe voltage between any line of the three-phase transformer is called the “line voltage”, VL,while the voltage between any line and the neutral point of a star connected transformer is calledthe “phase voltage”, VP. This phase voltage between the neutral point and any one of the linePrepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 16

Laboratory Manual: Electrical Machines – I Laboratoryconnections is 1/ 3 VL of the line voltage. Then above, the primary side phase voltage, VP isgiven as.𝑉𝒑 𝟏 3 𝑉𝐿 1 3 100 57.7 𝑣𝑜𝑙𝑡𝑠The secondary current in each phase of a star-connected group of transformers is the same as thatfor the line current of the supply, then IL IS.Then the relationship between line and phase voltages and currents in a three-phase system canbe summarized as:Three-phase Voltage and Current:Where again, VL is the line-to-line voltage, and VP is the phase-to-neutral voltage on either theprimary or the secondary side.Other possible connections for three phase transformers are star-delta Yd, where the primarywinding is star-connected and the secondary is delta-connected or delta-star Dy with a deltaconnected primary and a star-connected secondary.Delta-star connected transformers are widely used in low power distribution with the primarywindings providing a three-wire balanced load to the utility company while the secondarywindings provide the required 4th-wire neutral or earth connection.When the primary and secondary have different types of winding connections, star or delta, theoverall turns ratio of the transformer becomes more complicated. If a three-phase transformer isconnected as delta-delta ( Dd ) or star-star ( Yy ) then the transformer could potentially have a1:1 turns ratio. That is the input and output voltages for the windings are the same.However, if the 3-phase transformer is connected in star–delta, ( Yd ) each star-connectedprimary winding will receive the phase voltage, VP of the supply, which is equal to 1/ 3 VL.Prepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 17

Laboratory Manual: Electrical Machines – I LaboratoryThen each corresponding secondary winding will then have this same voltage induced in it, andsince these windings are delta-connected, the voltage 1/ 3 VL will become the secondary linevoltage. Then with a 1:1 turns ratio, a star–delta connected transformer will provide a 3:1 stepdown line-voltage ratio.Star-Delta Turns Ratio:Likewise, for a delta–star ( Dy ) connected transformer, with a 1:1 turns ratio, the transformerwill provide a 1: 3 step-up line-voltage ratio.Delta-Star Turns Ratio:Then for the four basic configurations of a three-phase transformer, we can list the transformerssecondary voltages and currents with respect to the primary line voltage, VL and its primary linecurrent IL as shown in the following table.Three-phase Transformer Line Voltage and Current:Prepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 18

Laboratory Manual: Electrical Machines – I LaboratoryExperiment-06Aim: To perform Scott Connections on three phase transformer to convert it intotwo phase supplyApparatus Required:S.NApparatusSpecifications1 Main Transformer Input: 0-200V (50%) 10%, 50HzInput: 0-200V (50%) 10%, 50HzOutput:0-230V 10%, 50Hz2 Teaser0-115.6V (28.9%) 10%, 50HzTransformer346.4V (86.6%) 10%,50Hz400V 10%, 50Hz0-230V 10%, 50Hz3 Wattmeter600V/10A,UPF4 Voltmeters0-600V,0-300V5 Ammeter0-10A(MI)0-20A(MI)6 Load1PH UPFQuantity11112121CIRCUIT DIAGRAM:Circuit diagram for scott connectionPrepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 19

Laboratory Manual: Electrical Machines – I LaboratoryCircuit diagram for finding phase relationTheory:To convert 3- to2- power, Scott connection is one by which 3-phase to 2-phasetransformation is accomplished with the help of two identical 1- transformers having samecurrent rating. One transformer has a center tap on primary side and it is known as Maintransformer. It forms the horizontal member of the connection. Another transformer has0.866 tap on primary side and known as Teaser transformer. The 50% tap point on primaryside of the main transformer is joined to 86.6% tap on primary of the teaser transformer.Obviously full rating of the transformers is not at all used. Refer to the fig. The maintransformer primary winding center tap point D is connected to one end of the primary of theteaser transformer on secondary side, both the main & teaser transformer turns are used (notonly 86.6%). Hence the voltage per turn will be equal for both transformers.Name Plate Details:a) Main Transformer:KVA ratingLV Side VoltageHV Side VoltageFrequencyNumber of Phasesb) Teaser Transformer:KVA ratingLV Side VoltageHV Side VoltageFrequencyNumber of Phases::::::::::Prepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 20

Laboratory Manual: Electrical Machines – I LaboratoryObservations:for Scott connectionS.No.V1(V)I1(A)W1(W)W2(W)V2I2V3 (V)(V)(A)InputOutput(kW)(kW)%ηI3 (A)for phase relationVa (V)Vb (V)Model Calculations:For reading 2:Input Power (W1 W2 )/1000Output Power (V2 I2 V3 I3 )/1000% efficiency Output/Input*100To Calculate Phase Relationship:(Vb)2 (Va)2 (Va)2 2(Va)(Va)cos(Φ)Model Graph:Prepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 21

Laboratory Manual: Electrical Machines – I LaboratoryResult:Procedure:1. Connect the circuit diagram as shown in fig 8.1.2. Close the TPST Switch. Note down the readings of all meters.3. Close the DPST Switches 1&2 and vary both the loads in steps up to ratedload and note down the readings of all meters in each step. Open the TPSTSwitch.4. Connect the circuit diagram as shown in fig 8.2 to verify the phase relationshipof the twophase voltages.5. Close the TPST Switch and note down all meter readings in secondary side.Precautions:1. Connections must be tight and avoid wrong connections.2. Readings are to be taken without any parallax error.3. Initially all the knife switches should be in OPEN position.Questions/Answers:Q.1 What is the effect on the frequency in the transformer?A. No changeQ.2 What is the medium for the energy conversion from the primary to secondary in thetransformer?A. By the flux.Q.3 What is the main reason for the generation of harmonics in the transformer?A. Saturation of the core.Q.4 Why are the ferrite cores used in the high frequency transformer?A. High resistanceQ.5 What type of winding is used in the 3-phase shell type transformer?A. Sandwich typeQ.6 What is increased in step up transformer?A. VoltageQ.7 What is the effect on voltage in step down transformer?A. Voltage is decreasedQ.8 What is the function of bushings in the transformer?A. To make the external connectionsQ 9 What is the principal of transformer?Prepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 22

Laboratory Manual: Electrical Machines – I LaboratoryExperiment-07Aim: To study the constructional details of direct current (DC) machine.Apparatus Required:S.No.1.Apparatus RequiredDC MachineSpecificationsFour pole sectionQuantity1Diagram:Prepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 23

Laboratory Manual: Electrical Machines – I LaboratoryConstructional Detail:YokeAnother name of a yoke is the frame. The main function of the yoke in the machine is to offermechanical support intended for poles and protects the entire machine from moisture, dust, etc.The materials used in the yoke are designed with cast iron, cast steel otherwise rolled steel.Pole and Pole CoreThe pole of the DC machine is an electromagnet and the field winding is winding among pole.Whenever field winding is energized then the pole gives magnetic flux. The materials used forthis are cast steel, cast iron otherwise pole core. It can be built with the annealed steellaminations for reducing the power drop because of the eddy currents.Pol ShoesPole shoe in the DC machine is an extensive part as well as to enlarge the region of the pole.Because of this region, flux can be spread out within the air-gap as well as extra flux can bepassed through the air space toward armature. The materials used to build pole shoe is cast ironotherwise cast steed, and also used annealed steel lamination to reduce the loss of power becauseof eddy currents.Field WindingsIn this, the windings are wounded in the region of pole core & named as field coil. Whenevercurrent is supplied through field winding than it electromagnetics the poles which generaterequired flux. The material used for field windings is copper.Armature CoreArmature core includes a huge number of slots within its edge. The armature conductor islocated in these slots. It provides the low-reluctance path toward the flux generated with fieldwinding. The materials used in this core are permeability low-reluctance materials like ironotherwise cast. The lamination is used to decrease the loss because of the eddy current.Armature WindingThe armature winding can be formed by interconnecting the armature conductor. Whenever anarmature winding is turned with the help of prime mover then the voltage, as well as magneticflux, gets induced within it. This winding is allied to an exterior circuit. The materials used forthis winding are conducting material like copper.Prepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet LongowalPage 24

Laboratory Manual: Electrical Machines – I LaboratoryCommutatorThe main function of the commutator in the DC machine is to collect the current from thearmature conductor as well as supplies the current to the load using brushes. And also providesunidirectional torque for DC-motor. The commutator can be built with a huge number ofsegments in the edge form of hard drawn copper. The Segments in the commutator are protectedfrom the thin mica layer.Parts of DC MachineBrushesBrushes in the DC machine gather the current from the commutator and supply it to the exteriorload. Brushes wear with time to inspect frequently. Graphite or carbon brushes, sometimesmixed with copper dust, are

Laboratory Manual: Electrical Machines – I Laboratory Prepared by Dr. Gurmeet Singh Grewal, Associate Professor(EIE dept.), Sliet Longowal Page 3 List of Experiments of Electrical Machine-1 (PCEE-514) 1. To perform Open circuit and short circuit tests on a single phase