Electrical Machines I
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www.jntuworld.comwww.jntuworld.comBRAKE TEST ON DC COMPOUND MOTOR AIM: To conduct the break test on dc compound motor and draw performance curves. APPARATUS: S.NOAPPARATUSRANGETYPEQUANTITY 1VOLTMETER(0-300)VMC1 2AMMETER(0-20)AMC1 3RHEOSTAT570/1.2A_1 4TACHOMETER(0-1000)RPMDIGITAL1 5CONNECTING WIRES __AS PER REQUIRED THEORY: Another method of testing a dc motor is brake test method.This is direct method of testing the motor.In this method the motor is putting on the direct load by means ofa belt and water cooled pulley arrangement..By adjusting the tension ofbelt.The load is adjust the various values of currents.The load isfinally adjusted to get full loadcurrent the power development gets wasted against the friction b/w belt and shaft.due to the www.jntuworld.comwww.jntuworld.combraking action of belt the test is called brake test.In the brake test compound motor we are mainly used in long shunt compound motor .In this type the shunt field winding is connected across the combination of armature and series of field winding.The resistance of field winding in series Rse and shunt field winding Rsh.The total current dfrawn from supply is I L I L=Ise+Ish, Ise=Ia Long shunt compound motor is compared to shunt mot so the torque produce in the motor is Ia. PROCEDURE: 1. Connections are made as per the circuit diagram.. 2. Make sure that noload is applied onthe motors and the motor field rheostat should be in minimum position . 3.Start the motorby using 3-point starter and adjust the speed of the motor in the rated value by using rheostat of motor 4.take the readinrs of ammeter,voltmeter and speed of motor on noload. 5.By gradually applying load and take reading of ammeter, voltmeter,spring balance and speed of motor for each and every load. www.jntuworld.comwww.jntuworld.com 6.Increase the load up to full load that means up to reading of ammeter read as rated value. 7.By slowly varying the load on the motar to zero and stop the motor. 8.Calculate the torque and efficiency using the formulae. 9.Draw the performance charecterstics curves. MODEL CALCULATIONS:
Spring balance reading S1=S2= Torque(T)=9.81(S1-S2)r N-M
Outputpower=2NT/60 Efficiency=output/input100 www.jntuworld.comwww.jntuworld.com TABULAR FORM: S.NO LOAD CURRRENT (IL) VOLTAGE () SPEEDSPRING BALANCE S1S2 KGS TORQUE 9.81(S1-S2)r INPUT (VIL) OUTput 2NT/60 =o/p/i/p100 PRECAUTIONS: 1.Avoid loose connections. 2.Readings are taken without parellox error. 3.check the position of rheostat before startinr the experiment. www.jntuworld.comwww.jntuworld.com APPLICATIONS: 1.Stamping Processes. 2.Rolling mills. 3.Conveyers. 4.Elevators. 5.Punches. 6.Hoists. RESULT: www.jntuworld.comwww.jntuworld.comwww.jntuworld.comwww.jntuworld.comLOAD TEST ON DC SERIES GENERATOR AIM: To conductload test on Dc series generator and to draw the characteristics of series generator. APPARATUS: S.NOAPPARATUSRANGETYPEQUANTITY 1AMMETER0-20AMC2 2VOLTMETRE 0-300VMC1 3RHEOSTAT18/1.2AWIREWOUD2 4TACHOMETER (0-10000)RPM DIGITAL1 THEORY: Incaseofseriesgeneratorarmaturecurrent,seriesfieldcrrent,loadcurrent are equal, Ia=Ise=I L As load current increases,Ise increases.the flux is directly proportional to Ise sofluxisalsoincrease.Theinducedemfisproportionaltofluxhence inducedemfalsoincreases.ThusthecharactrsticsofEagainsti..einternal charactersticsisofincreasingbutitseffectisnegligiblecomparedto increaseinE.Butforhighloadcurrent,saturationoccursandfluxremains constant.In such a case,due to the drop Ia(Ra+Rse) increases. Now as I L=Ia increases thus the drop Ia(Ra+Rse)increases. V t =E-Ia(Ra+Rse) Thus the external characterstics is also of rising nature asE Increasesbut it will be below internal characterstic due to drop Ia(Ra+Rse) www.jntuworld.comwww.jntuworld.comPROCEDURE: 1.connections are made as per circuit diagram. 2.Before starting the experiment maintain rheostat at maximum position. 3.Switch on the power supply,start the motor using 2-point starter without applying load . 4.By varying the rheostats made the speed to rated speed i..e 1500 rpm. 5.Now apply load ,one by one each and every time maintain the speed to rated speed i..e 1500rpm by using rheostat take voltmeter and ammeter readings. 6.Increase the load up to rated current. 7.Before stop the motor the rheostats are comes back to original position. 8.Draw the internal&external characterstics of generator. MODEL CALCULATIONS: Ia= Ra= V= Rse= Drop=Ia(Ra+Rse) E=V+Ia(Ra+Rse) www.jntuworld.comwww.jntuworld.com TABULAR FORM: S.NO CURRENT Ia VOLTAGE (V) Drop Ia(Ra+Rse) Eg=V+Ia(Ra+Rse) PRECAUTIONS: 1.Avoid loose connections. 2.Readings are taken without parllox error. 3.For this first starting at no-load. www.jntuworld.comwww.jntuworld.comAPPLICATIONS: The series generators have raising external charecterstics .Due to this raising charecterterstics it is used as booster to compensate for the resistance voltage drop in the feeders of the d.c distribution system. RESUL www.jntuworld.comwww.jntuworld.comwww.jntuworld.comwww.jntuworld.comBRAKE TEST ON DC SHUNT MOTOR AIM: To conduct Brake test on dc shunt motor and draw the performance characterstics. APPARATUS: S.NO APPARATUS RANGE TYPE QUANTITY 1 2 3 4 5 .
Voltmeter Ammeter Tachometer Rheostat Connecting wires (0-300v) (0-20A) (0-10000) 570/1.2A - MC MC Digital Wire wound - 1 1 1 - - THEORY: In this direct method and consisting of applying a brake to a water cooled pulley mounted on the motor. The brake band is fixed with the help of wooden blocks gripping the pulley. One end of the band is fixed to earth via a spring balance s and the other is connected to a suspended weight W1. The motor is running and the load on the motor is adjusted till it carries its full load current. The simple brake test descrided abovecan be used for small motors only. Becauseinthecaseoflargemotoritisdifferenttodissipatethelarge amount of heat generated at the brake. Another simpledmethod ofmeasuringmotoroutput isbythe use of poney brake one form of the diagram. The tension of rope can be adjusted with the help of swivels obviously. www.jntuworld.comwww.jntuworld.com PROCEDURE: 1.Connectios are made as per circuit diagram. 2,Make sure that no-load is applied to moto and motor field rheostst should be in minimum position. 3.Start the motor by using 3-point starter and adjust the speed of the motor to its rated speed by using field rheostat of motor. 4.Take readings of the ammeter and voltmeter and speed of the motor for each and every load. 5.By gradually increasing the load take readings of ammeter,voltmeter,spring balance measure up toits full load. 6.By slowly varying the load on the motor to zero and switch off the supply . 7.Calculate the torque and efficiency and draw the performance characteristics curve. www.jntuworld.comwww.jntuworld.com MODEL CALCULATIONS: Speed(N)= Spring balances S1= S2=Radius r= Torque T=9.81(S1-S2)r N-M Inputpower=VIwatts Outputpower=2NT/60watts Efficiency() =o/p/i/p100 www.jntuworld.comwww.jntuworld.com PRECAUTIONS: 1.Avoid loose connections. 2.Readings are taken without parallox error. 3.Do not touch wire terminals. APPLICATIONS: 1.For driving constant speed line shafting lathes. 2.Centrifugal pumps. 3.Machine tools. 4.Blowers and fans. 5.Reciprocating pumps. RESULT: www.jntuworld.comwww.jntuworld.com www.jntuworld.comwww.jntuworld.com www.jntuworld.comwww.jntuworld.com LOAD TEST ON DC SHUNT GENERATOR AIM: To conduct load test on dc shunt generator and to draw performance characteristics curves at the given motor. APPARATUS: S.NO APPARATUS RANGE TYPE QUANTITY 1 2 3 4 5 .
Voltmeter Ammeter Tachometer Rheostat Connecting wires (0-300v) (0-20A) (0-2A) (0-10000) 570/1.2A 18/12A - MC MC MC Digital Wire wound Wire wound - 1 1 1 1 2 1 - THEORY: Load test on dc shunt generator consist of two charactersticsthey are 1.Internal characterstics 2.External characterstics Internal characterstics: www.jntuworld.comwww.jntuworld.comIdeally the induced emf is not dependent on the load currents IL (or) armaturecurrentIabutasloadcurrentincreases,armaturefluxIa increases to supply the load demand.As Ia increases,armature flux increases. Due to the armature reaction , main flux pattern gets distorted. Hence lesserfluxgetslinkedwiththearmatureconductors.Thisreducesthe induced emf. External Characteristics: For dc shunt motor E= Vt+IaRa neglecting the other drops. So as load currentincreasesIaincreases.ThusthedropIaRaincreasesandterminal voltages. Vt= E- IaRa decreses. But the value of armature resistance is very small, the drop in terminal voltages as load current changes from no load to fullloadisverysmall.Hencedcshuntgeneratoriscalledconstantvoltage generator . PROCEDURE: 1.Connect the circuit as shown in the circuit. 2.Before starting the experiment the motor field rheostat must be at minimum position and generator field rheostat must be at maximum position. 3.Give the supply and start the motor by using 3-point starter. 4.Adjust motor speed by varying the motor field rheostat upto rated speed. 5.By varying generator field rheostat obtained rated voltage i.e 230v. 6.By gradually applying load one by one, every time by changing the rheostat of motor to its rated speed take the readings of ammeter,voltmeter at each load. 7.Apply the load upto rated current . 8.Before stop the motor ,remove the load and bring back the rheostat to its original position. 9.Stop the power supply. www.jntuworld.comwww.jntuworld.com 10.Draw the internal and external characteristics of shunt generator. MODEL CALCULATIONS: Ra = Voltage (V) = Load current (IL ) = Field