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DC Machines Lab Manual

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   NIE Institute of Technology, Mysore - 18  DC Machines & Synchronous Machines Lab (06EEL67) Department of Electrical and Electronics Engineering 1   NIE Institute of Technology Department of Electrical and Electronics Engineering List of experiments for DC machines and Synchronous machines laboratory   Subject code:06EEL67 IA marks: 25 Exam Hours:03 Exam marks: 50 I cycle Experiments 1 Speed control of a DC motor by armature voltage control and flux control. 2 Load Characteristics of a DC Shunt and Compound Generator 3 Hopkinson's test.. 4 Field test on a DC Series motor.. 5 Swinburne's Test 6 Ward Leonard method of speed Control of a DC motor. II cycle Experiments 7 Load test of a DC motor-determination of speed-torque and HP-efficiency characteristics. 8 Retardation test  –  electrical braking method. 9 Slip test. 10 V' and inverted 'V' curves of a synchronous motor. 11 Voltage regulation of an alternator by EMF and MMF method. 12 Voltage regulation of an alternator by ZPF method. 13 Performance of synchronous generator connected to infinite bus, under constant power and variable excitation & vice  –   versa. Lab Incharge Teaching Staff: 1. Smt. Ushasurendra   2. Sri. B.S. Srikanthan 3. Mr. Sandeep kumar .K.J 4. Mr. Mohan .N Technical Staff: 1. Sri. Arun Kumar .L.S 2. Mr. C. Suresha. 3. Mr. Arun .M HOD E&EE   NIE Institute of Technology, Mysore - 18  DC Machines & Synchronous Machines Lab (06EEL67) Department of Electrical and Electronics Engineering 2   Contents Sl No Name of the experiment Page no 1 Speed control of a DC motor by armature voltage control and flux control. 3 - 6 2 Load Characteristics of a DC Shunt and Compound Generator. 9 - 16 3 Hopkinson's test. 19 - 23 4 Field test on a DC Series motor. 25 - 27 5 Swinburne's Test 29 - 32 6 Ward Leonard method of speed Control of a DC motor. 35 - 36 7 Load test of a DC motor-determination of speed-torque and HP-efficiency characteristics. 39 - 41 8 Retardation test  –  electrical braking method. 43 - 45 9 Slip test. 47 - 50 10 V' and inverted 'V' curves of a synchronous motor. 53 - 55 11 Voltage regulation of an alternator by EMF and MMF method. 57 - 61 12 Voltage regulation of an alternator by ZPF method. 63 - 66 13 Performance of synchronous generator connected to infinite bus, under constant power and variable excitation & vice  –   versa. 67 - 69 Viva questions. 71 - 73   NIE Institute of Technology, Mysore - 18  DC Machines & Synchronous Machines Lab (06EEL67) Department of Electrical and Electronics Engineering 3   Experiment No.01 Speed control of a DC motor by armature voltage control and flux control Aim:  To control the speed of a DC motor by 1) armature voltage control method 2) flux control method CIRCUIT DIAGRAM Name Plate details of the machines: Motor: Apparatus Required: Circuit Ref. Description Rating Quantity A 1  Moving coil ammeter 0-2.5A-5A 2 V 1  Moving coil voltmeter 0-250V 1 R  fm  Rheostat 200Ω,1.7A  2 R  am  Rheostat 50Ω,5A  1   NIE Institute of Technology, Mysore - 18  DC Machines & Synchronous Machines Lab (06EEL67) Department of Electrical and Electronics Engineering 4   THEORY:  The working principal o f a Dc motor can be stated as “ when a current carrying conductor is placed in a magnetic field it experiences a force. In the practical DC motor, the  permanent magnet is obtained by a field winding which produces the required flux is called the main flux and all the armature conductor mounted on the periphery of the armature drum, get subjected to the mechanical force. Due to this overall armature experiences a twisting force called torque and armature of the motor starts rotating. As the armature starts rotating it cuts the main flux and hence an emf gets induced in the conductors the direction of which is against the supply voltage and hence an emf gets induced in the conductors, the direction of which is against the supply voltage and hence it is termed as back emf E b . ( E b =   ) Therefore, the supply voltage has to overcome this back emf to keep the conductor rotating. The speed of the motor automatically adjusts its self to the load so that the electrical power required to drive the current through the armature is equal to the mechanical power required to drive the load. The back emf is always less than the supply voltage V. Therefore E  b  = V - I a R  a  ------------ voltage equation  N ά 1/Ф -------------------- Speed equation (voltage kept constant) From the above equations it can be seen that the speed is inversely proportionally to the flux per  pole. Thus, the speed of the DC motor can be increased or decreased by varying the flux.  Armature control method: (rheostatic control) in this method an adjustable resistance „ R‟ is  placed in series with the armature resistance R  a  making the total resistance in the armature equal to (R + R  a ), then the back emf for any armature current I a  is given by E b = V- (R + R  a ) I a  it can  be seen that maximum drop and the voltage which actually gets impressed is minimum and hence the speed is minimum. Similarly when the resistance is fully cut out, then speed is maximum. The draw backs of this method are 1) for a given value of the resistance the speed is not a constant but a function of the load current. The value of the resistance has to be changed for a rapidly changed load if the speed is to be kept constant. 2) there is considerable wastage of  power and the power wasted is proportional to the reduction of speed (current drawn will be more at lesser speed) 3) only speeds below the rated speed can be obtained. Field control method: (flux control)  it is seen that when the flux is varied speed will also vary. The resistance of variable rheostat R  f   is fully cut out, the speed is minimum (equal to the rated speed ) the field is current maximum and maximum flux is produced. This is accomplished by means of shunt regulator in the case of a DC shunt motor and a diverter in case of a series motor as shown in figures. This method of speed control is both convenient and economical, but obviously it gives speed greater than the normal. By a combination of both rheostatic and field control methods speeds  below or above normal can be obtained.
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