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Electrical Machine (EET 306/4 - Software Version) Laboratory Module EXPERIMENT 4 TITLE: SEPARATELY- EXCITED, SERIES, SHUNT AND COMPOUND DC MOTORS OBJECTIVES 1) To demonstrate how the field current affects the characteristics of a separately-excited dc motor using the DC Motor/Generator module. 2) To demonstrate the main operating characteristics of series, shunt and compound motors. EQUIPMENTS EMS Workstation Model 8110, DC Motor/Generator Model 8211, Resistive Load Model 8311, Power Supply Model 8821, Prime Mover/Dynamometer Model 8960 and Data Acquisition Interface Model 9062. INTRODUCTION Separately-Excited DC Motor It is possible to change the characteristics of a separately-excited dc motor by changing the strength of the fixed magnetic field produced by the stator electromagnet. This can be carried out by changing the current that flows in the stator electromagnet. This current is usually referred to as the field current (IF) because it is used to produce the fixed magnetic field in the dc motor. A rheostat connected in series with the electromagnet winding can be used to vary the field current. Figure 4.1 illustrates how the speed versus armature voltage of a separatelyexcited dc motor is affected when the field current is decreased below its nominal value. Constant K1 becomes greater. This means that the motor can rotate at higher speeds without exceeding the nominal armature voltage. It is also possible to set the field current of a separately-excited dc motor above its nominal value for short time intervals. The effect on the speed versus armature voltage relationship is reversed, i.e. constant K1 becomes smaller. As a result, the motor can develop a higher torque during these time intervals but the speed at which the motor can rotate, without exceeding the nominal armature voltage is reduced. KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 1 Electrical Machine (EET 306/4 - Software Version) Laboratory Module Figure 4.1 Decreasing Current IF below its Nominal value Affects Constants K1 Series Motor The series motor is a motor in which the field electromagnet is a series winding connected in series with the armature as shown in Figure 4.2. The strength of the field electromagnet varies as the armature current varies. As a result, K 1 vary when the armature current varies. Figure 4.2 shows the speed versus torque characteristics of a series motor when the armature voltage is fixed. This characteristics shows that the speed decreases non linearly as the torque increases, i.e. as the armature current increases. Figure 4.2 Series Motor and its Speed versus Torque characteristics The series motor provides a strong starting torque and a wide range of operating speeds when it is supplied by a fixed-voltage dc source. However, the speed, torque and armature current depend on the mechanical load applied to the motor. The series motor has non-linear operating characteristics as suggested by the speed versus torque relationship in Figure 4.2. As a result, it is difficult to operate a series motor at a constant speed when the mechanical load fluctuates. Finally, a series motor must never run with no mechanical load because the speed increases to a very-high value which can damage the motor (motor runaway). KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 2 Electrical Machine (EET 306/4 - Software Version) Laboratory Module Shunt Motor The shunt motor is a motor in which the field electromagnet is a shunt winding connected in parallel with the armature, both being connected to the same dc voltage source as shown in Figure 4.3. For a fixed armature voltage, constant K1 is fixed, and the speed versus torque characteristics is very similar to that obtained with a separately-excited dc motor powered by a fixed-voltage dc source, as shown in Figure 4.3. As in a separately-excited dc motor, the characteristics K1 of a shunt motor can be changed by varying the field current with a rheostat. It is difficult to change the speed of a shunt motor by changing the armature voltage because this changes the field current and thereby the motor characteristics in a way that opposes speed change. Figure 4.3 Shunt Motor and its Characteristics The main advantage of a shunt motor is the fact that only a single fixed-voltage dc source is required to supply power to both the armature and the shunt winding. Finally when the shunt winding opens accidentally, the field current I F becomes zero, the motor speed increases rapidly and motor runaway occurs as suggested by the speed versus field current characteristics shown in Figure 4.3. Compound Motor It is possible to combine shunt and series winding to obtain a particular speed versus torque characteristics. For example, to obtain the characteristics of decreasing speed when the motor torque increases, a series winding can be connected in series with the armature so that the magnetic flux it produces adds with the magnetic flux produced by shunt winding. As a result, the magnetic flux increases automatically with increasing armature current. This type of dc motor is KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 3 Electrical Machine (EET 306/4 - Software Version) Laboratory Module referred to as a cumulative compound motor because the magnetic fluxes produced by the series and shunt windings add together. Shunt and series winding can also be connected so that the magnetic fluxes subtract from each other. This connection produces a differential-compound motor which is rarely used because the motor becomes unstable when the armature current increases. Figure 4.4 shows a compound motor and its speed versus torque characteristics (cumulative compound). Figure 4.4 Compound Motor and its Speed versus Torque Characteristics Figure 4.5 is a graph that shows the speed versus torque characteristics of the various types of dc motors. As can be seen, the separately-excited dc motor and shunt motor have very similar characteristics. The main feature of these characteristics is that the motor speed varies little and linearly as the torque varies. Finally, the characteristics of a cumulative compound motor are a compromise of the series and shunt motor characteristics. It provides the compound motor with a fairly wide range of operating speed but the speed does not vary linearly as the torque varies. Figure 4.5 Speed versus Torque Characteristics of Various DC Motors KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 4 Electrical Machine (EET 306/4 - Software Version) Laboratory Module PROCEDURE CAUTION High voltages are present in this laboratory exercise! Do not make or modify any banana jack connections with the power on unless otherwise specified! 1. Install the Power Supply, Prime Mover/Dynamometer, DC Motor/Generator and Data Acquisition Interface (DAI) modules in the EMS Workstation. Note: If you are performing the experiment using the EMS system, ensure that the brushes of the DC Motor/Generator are adjusted to the neutral point. To do so, connect an ac power source (terminals 4 and N of the Power Supply) to the armature of the DC Motor/Generator (terminals 1 and 2) through CURRENT INPUT I1 of the Data Acquisition Interface module. Connect the shunt winding of the DC Motor/Generator (terminals 5 and 6) to VOLTAGE INPUT E1 of the Data Acquisition Interface module. Start the Metering application. Turn on the Power Supply and set the voltage control knob so that an ac current (indicated by meter I1) equal to half the nominal value of the armature current flows in the armature of the DC Motor/Generator. Adjust the brushes adjustment lever on the DC Motor/Generator so that the voltage across the shunt winding (indicated by meter E1) is minimum. Turn off the Power Supply, exit the Metering application and disconnect all leads and cables. Mechanically couple the Prime Mover/Dynamometer to the DC Motor/Generator using a timing belt. 2. On the Power Supply, make sure that the main switch of the Power Supply is set to the O (OFF) position, and the voltage control knob is turned fully counter clockwise. 3. Connect the DAI LOW POWER INPUT of the DAI and Prime Mover/Dynamometer modules to the 24V-AC output of the Power Supply. On the Power Supply, set the 24V-AC power switch to the I (on) position. 4. Display the Metering application. 5. Set up the separately-excited dc motor circuit shown in Figure 4.6. Attention! - Note: If you are performing the exercise with a line voltage 240 V, use the resistive load module to connect a 960-Ω resistor in series with the rheostat. KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 5 Electrical Machine (EET 306/4 - Software Version) Laboratory Module Figure 4.6 Separately-Excited DC Motor Coupled to a Dynamometer 6. Set the Prime Mover/Dynamometer controls as follows: MODE switch ………………………………..…………….…. DYN. LOAD CONTROL MODE switch …………………………… MAN. LOAD CONTROL knob ………………..……… MIN. (fully CCW) DISPLAY switch ……………………………………….SPEED (N) Note: If you are performing the experiment using LVSIM-EMS, you can zoom in the Prime Mover/Dynamometer module before setting the controls in order to see additional front panel markings related to these controls. Speed versus Armature Voltage Characteristics of a Separately-Excited DC Motor 7. Turn on the Power Supply. On the DC Motor/Generator, set the FIELD RHEOSTAT so that the field current IF indicated by meter I2 in the Metering window is equal to the value given in the following table: KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 6 Electrical Machine (EET 306/4 - Software Version) Line Voltage V ac 240 Laboratory Module Field Current If mA 140 Table 4.1 Field Current of the Separately-Excited DC Motor 8. In the Metering window, select the torque correction function for meter T. Meter T now indicates the dc motor output torque. Record the dc motor speed n, armature voltage EA, armature current IA, field current IF, and output torque T (indicated by meters N, E1, I1, I2 and T respectively) in the Data Table. On the Power Supply, set the voltage control knob to 10%, 20%, 30% etc. up to 100% in order to increase the armature voltage EA by steps. For each voltage setting, wait until the motor speed stabilizes and then record the data in the Data Table. 9. When all data has been recorded in Table 4.1 (Results Section), turn the voltage control knob fully counterclockwise and turn off the Power Supply. 10. In the Graph window, make the appropriate settings to obtain a graph of the dc motor speed n (obtained from meter N) as a function of the armature voltage EA (obtained from meter E1). Plot the Graph 4.1 (Results Section). 11. Use the two end points to calculate the slope K1 of the relationship obtained in Graph 4.1 (Results Section). The values of these points are indicated in data table (Table 4.1-Results Section). Describe how does decreasing the field current IF affect the speed versus voltage characteristics and constant K1 of a separately-excited dc motor. In the Data table window, clear the recorded data. Speed versus Torque Characteristics of a Series Motor 12. Modify the connections so as to obtain the series motor circuit shown in Figure 4.7. 13. Turn on the Power Supply and set the voltage control knob so that the armature voltage EA indicated by meter E1 is equal to the any value when the dc motor speed is 1500 r/min. The series motor should start to rotate. KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 7 Electrical Machine (EET 306/4 - Software Version) Laboratory Module Figure 4.7 Series Motor Coupled to a Dynamometer 14. In the Metering window, make sure the torque correction function of meter T is selected. Record the motor speed n, output torque T, armature voltage EA, and armature current IA (indicated by meters n, T, E1 and I1 respectively) in the Data Table. On the Prime Mover/Dynamometer, adjust the LOAD CONTROL knob so that the torque indicated on the module display increases by 0.2 N.m increments up to 2.0 N.m. For each torque setting, readjust the voltage control knob of the Power Supply so that the armature voltage EA remains equal to the value set in the previous step, wait until the motor speed stabilizes and then record the data in the Data Table. Note: It may not be possible to maintain the armature voltage to its original value as the torque is increased. The armature current may exceed the rated value while performing this manipulation. It is therefore suggested to complete the manipulation within a time interval of 5 minutes or less. 15. When all data has been recorded, set the LOAD CONTROL knob on the Prime Mover/Dynamometer to the MIN. position (fully CCW), turn the voltage control knob fully counterclockwise and turn off the Power Supply. In the Data Table window, confirm that the data has been stored. The data must be filled in Table 4.2 (Results Section). 16. In the Graph window, make the appropriate settings to obtain a graph of the series motor speed (obtained from meter N) as a function of the series motor torque (obtained from meter T). Plot the Graph 4.2 (Results Section). Briefly describe how the speed varies as the mechanical load applied to the series motor increases, i.e. as the motor torque increases. Compare the speed versus torque characteristics of the series motor to that of the separately-excited dc motor. KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 8 Electrical Machine (EET 306/4 - Software Version) Laboratory Module 17. Set the 24 V – AC power switch to the O (off) position and remove all leads and cables. Speed versus Torque Characteristics of a Shunt Motor 18. Set up the shunt motor circuit shown in Figure 4.8. Make sure that the LOAD CONTROL knob on the Prime Mover/Dynamometer is set to the MIN. position (fully CCW). Figure 4.8 Shunt Motor Circuit 19. Turn on the Power Supply, set the armature voltage EA to the value recorded in step 13. 20. Set the FIELD RHEOSTAT on the DC Motor/Generator so that the field current I F is equal to the value indicated in Table 4.2. Line Voltage V ac 240 Field Current If mA 210 Table 4.2 DC Motor Field Current 21. In the Metering window, make sure the torque correction function of meter T is selected. Record the motor speed n, output torque T, armature voltage EA, armature current IA and field current IF (indicated by meters n, T, E1, I1 and I2 respectively) in the Data Table. On the Prime Mover/Dynamometer, adjust the LOAD CONTROL knob so that the torque indicated on the module display increases by 0.2 N.m increments up to 2.0 N.m. For each torque setting, readjust the voltage control knob of the Power Supply so that the armature voltage EA remains equal to the value set in the KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 9 Electrical Machine (EET 306/4 - Software Version) Laboratory Module previous step, wait until the motor speed stabilizes and then record the data in the Data Table. Note: It may not be possible to maintain the armature voltage to its original value as the torque is increased. The armature current may exceed the rated value while performing this manipulation. It is therefore suggested to complete the manipulation within a time interval of 5 minutes or less. 22. When all data has been recorded, set the LOAD CONTROL knob on the Prime Mover/Dynamometer to the MIN. position (fully CCW), turn the voltage control knob fully counterclockwise and turn off the Power Supply. In the Data Table window, confirm that the data has been stored. The data must be filled in Table 4.3 (Results Section). 23. In the Graph window, make the appropriate settings to obtain a graph of the series motor speed (obtained from meter N) as a function of the series motor torque (obtained from meter T). Plot the Graph 4.3 (Results Section). 24. Set the 24 V – AC power switch to the O (off) position and remove all leads and cables. Speed versus Torque Characteristics of a Cumulative Compound Motor 25. Set up the cumulative compound motor circuit shown in Figure 4.9. Make sure that the LOAD CONTROL knob on the Prime Mover/Dynamometer is set to the MIN. position (fully CCW). Figure 4.9 Cumulative-Compound Motor Circuit KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 10 Electrical Machine (EET 306/4 - Software Version) Laboratory Module 26. Turn on the Power Supply, set the armature voltage EA to the value recorded in step 13. 27. Set the FIELD RHEOSTAT on the DC Motor/Generator so that the current in the shunt winding is equal to the value indicated in Table 4.2. 28. In the Metering window, make sure the torque correction function of meter T is selected. Record the motor speed n, output torque T, armature voltage EA, armature current IA and field current IF (indicated by meters n, T, E1, I1 and I2 respectively) in the Data Table. On the Prime Mover/Dynamometer, adjust the LOAD CONTROL knob so that the torque indicated on the module display increases by 0.2 N.m increments up to 2.0 N.m. For each torque setting, readjust the voltage control knob of the Power Supply so that the armature voltage EA remains equal to the value set in the previous step, wait until the motor speed stabilizes and then record the data in the Data Table. Note: It may not be possible to maintain the armature voltage to its original value as the torque is increased. The armature current may exceed the rated value while performing this manipulation. It is therefore suggested to complete the manipulation within a time interval of 5 minutes or less. 29. When all data has been recorded, set the LOAD CONTROL knob on the Prime Mover/Dynamometer to the MIN. position (fully CCW), turn the voltage control knob fully counterclockwise and turn off the Power Supply. In the Data Table window, confirm that the data has been stored. The data must be filled in Table 4.4 (Results Section). 30. In the Graph window, make the appropriate settings to obtain a graph of the series motor speed (obtained from meter N) as a function of the series motor torque (obtained from meter T). Plot the Graph 4.4 (Results Section). 31. Set the 24 V – AC power switch to the O (off) position and remove all leads and cables. KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 11 Electrical Machine (EET 306/4 - Software Version) Laboratory Module Name: _________________________ Matrix No.: _____________ Date: __________ RESULTS & CALCULATIONS: 9. Armature Voltage (E1) V Armature Current (I1) A Field Current (I2) A Torque (T) Speed (N) N.m r/min Table 4.1 DC Motor speed, armature voltage, armature current, field current and torque 10. DC Motor Speed versus Armature Voltage DC Motor Speed (r/min) 2000 1500 1000 500 0 50 100 150 200 250 300 Armature Voltage (V) Graph 4.1 DC motor speed versus armature voltage Instructor Approval: _____________________________________ Date: __________ KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 12 Electrical Machine (EET 306/4 - Software Version) Laboratory Module Name: _________________________ Matrix No.: _____________ Date: __________ 11. SLOPE = n2 – n1 = E2 – E1 11. ________________________________________________________________ = r/min_ V ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ 13. EA = _____________ V (n = 1500 r/min) 15. Armature Voltage (E1) V Armature Current (I1) A Torque (T) Speed (N) N.m r/min Table 4.2 Series motor speed, torque, armature voltage and armature current Instructor Approval: _____________________________________ Date: __________ KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 13 Electrical Machine (EET 306/4 - Software Version) Laboratory Module Name: _________________________ Matrix No.: _____________ Date: __________ 16. Series Motor Speed versus Torque Series Motor Speed (r/min) 3000 2500 2000 1500 1000 500 0 0.5 1.0 1.5 2.0 2.5 Series Motor Torque (N.m) Graph 4.2 Series motor speed versus torque ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ Instructor Approval: _____________________________________ Date: __________ KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 14 Electrical Machine (EET 306/4 - Software Version) Laboratory Module Name: _________________________ Matrix No.: _____________ Date: __________ 22. Speed versus Torque Characteristic of a Shunt Motor Armature Voltage (E1) V Armature Current (I1) A Field Current (I2) A Torque (T) Speed (N) N.m r/min Table 4.3 Shunt motor speed, torque, armature voltage, armature current and field current 23. Shunt Motor Speed versus Torque Shunt Motor Speed (r/min) 1500 1000 500 0 0.5 1.0 1.5 2.0 2.5 Shunt Motor Torque (N.m) Graph 4.3 Shunt motor speed versus torque Instructor Approval: _____________________________________ Date: __________ KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 15 Electrical Machine (EET 306/4 - Software Version) Laboratory Module Name: _________________________ Matrix No.: _____________ Date: __________ 26. Speed versus Torque Characteristic of a Cumulative Compound Motor Armature Voltage (E1) V Armature Current (I1) A Field Current (I2) A Torque (T) Speed (N) N.m r/min Table 4.4 Cumulative-compound motor speed, torque, armature voltage, armature current and field current 26. Cumulative-Compound Motor Speed versus Torque Cumulative-Compound Motor Speed (r/min) 1500 1000 500 0 0.5 1.0 1.5 2.0 2.5 Cumulative-Compound Motor Torque (N.m) Graph 4.4 Cumulative-compound motor speed versus torque Instructor Approval: _____________________________________ Date: __________ KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 16 Electrical Machine (EET 306/4 - Software Version) Laboratory Module Name: _________________________ Matrix No.: _____________ Date: __________ DISCUSSION ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ CONCLUSION ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ Instructor Approval: _____________________________________ Date: __________ KOLEJ UNIVERSITI KEJURUTERAAN UTARA MALAYSIA – Exp 4 17