UNIVERSITY OF WATERLOO ELECTRICAL & COMPUTER ENGINEERING DEPARTMENT ME269 ELECTROMECHANICAL DEVICES AND POWER PROCESSING. Group # First Name Last Name UserID @uwaterloo.ca Experiment #3: DIRECT CURRENT MACHINES OBJECTIVE: To investigate the speed control of a separately-excited dc motor To study the load characteristics of a separately-excited dc motor SAFETY: 1. Late arrivals will not be permitted to attend the lab session. See the Lab Safety Manual for details. 2. The pre-lab must be completed and submitted on time. PROCDURE: After the presentation, go to the designated lab setup and identify all the components shown in the diagram located in the lab set-up. Also identify the scales and multiplication factors of all instruments, i.e., find what range the meter is set for and what scale to read from properly. Improper reading may result in measurement and calculation errors. Recorded the nameplate data of the DC motor in Table #1. Study and understand the name plate ratings. Table#1: Nameplate details of DC Motors. Parameter Value Unit Rated output Rated Armature Voltage Rated Armature Current Rated speed Rated Field Current Armature Resistance 1
Part A: EXPLAINATION OF THE EXPERIMNTAL SETUP: Figure 1 shows the DC drive control panel. You must familiarize your self with the switches and push buttons and make sure all connections are correctly done before switching any supply to the unit. Figure # 1 DC drive front and side panel view Figure #2 shows a simple wiring diagram of the DC motor connected to the DC drive panel and the loading unit. The drive need to be set to control the speed of the DC motor as instruction in the following sections. The loading unit consist of AC alternator in which the output is controlled as the mechanical power from the DC motor is applied. Figure #2: Wiring Schematic of the Drive/DC Motor Speed Control Setup 2
Make sure that all the controllers and measuring instruments are connected as per the shown schematic. Before conducting the experiment, study and understand the set up and contact the lab instructor or TA to clarify doubts. The DC motor has two windings namely armature and field windings. So the speed can be controlled in two ways. a. Armature Voltage control method b. Field (Flux) control method. Therefore, two separate variable DC power supplies are needed. The two power supplies are incorporated in the DC drive unit which is supplied through a transformer and a circuit breaker. The DC drive unit is a power electronics converter. It converts 3-phase AC supply into DC supply. The DC drive requires 3-phase, 60 Hz, 230V as an input. Since the power panel has 3-phase, 60 Hz, 208V, the transformer is used to have 230V. It is already pre-adjusted to 230V, do not change it. There is a 3- phase breaker (CB) on the power panel to protect the set up from over loading and short circuit conditions. The DC drive controlled output is a variable power dc supply which is connected to armature winding through meters. The variable supply voltage is to be connected to the armature windings of the motor and is adjustable from the DC drive speed dial demand in the front panel. The drive unit also provide fixed power supply to power the field windings. For fine adjustment of the field supply for flux control method of speed control, an external resistance R is connected in series in order to control the field current in the field circuit. The field supply is then connected to the field winding through meters. DC motors have a peculiar property of run away condition in which the motor will try to rotate at dangerously high speed if the field is open while the motor is running. Under such condition, an emergency push button located on the DC drive must be pushed. Alternatively, the 3-phase breaker (CB) on the power panel must be turned-off. The speed control of the DC motor with the DC drive can be done in two ways open-loop speed control and closed-loop control. To select the appropriate control, a selector switch is used to select the mode. Select the switch to AVF position for open-loop speed control and to ENC position for closedloop speed control. To start the motor, simply press the START push button and wait for a while. Do not touch any live terminals which may result electric shock. To vary the speed, vary the speed control pot designated as SPEED Demand. To stop the motor, simply press the STOP push button. Wait for a while to properly shut off the DC drive. The meters in armature circuit measure armature current and armature voltage, similarly the meters in field circuit measure field current and field voltage. Digital tachometer is to measure speed in revolutions per minute (rpm). The torque meter is used to measure torque in Lb-ft. The synchronous generator and associated loading controller are used as a load. Do not worry about how it is working at this point. There is a 1-phase autotransformer to control the load. Make sure it is turned-off and the knob is at minimum position when the load in not being applied. Pay attention to the lab instructor while demonstrating; note important key points and ask questions at the end of the presentation. 3
Part B: Determining the characteristics of a separately-excited dc motor in different speed controls. 2. Armature Control Method 2.1 Procedure 2.1.1 Make sure the following conditions are met The shunt field rheostat R f should be adjusted to its minimum value; The switch S should be in the AVF position. The 1-phase autotransformer knob on the load side should be in minimum position. The voltmeters ranges should be at 240V. The main 3-phase breaker and 1-phase breakers should be closed. NOTE: The speed control pot controls the value of V a. Do not increase V a above its rated value. 2.1.2 Start the motor by pressing the START push button; adjust the speed control potentiometer; SPEED ; until the shaft speed is about 600. 2.1.3 Increase the motor speed, using the control potentiometer, in five or six steps until nameplate speed is reached. Record V a, I a, V f and N for each adjustment. (Use Table#2) Table#2 : Armature Control Va, Ia, Amps Vf, N, 2.2 Report (Attach your plots and answers to this report) 2.2.1 Using data from Table#2, plot a graph between V a and speed.. Use V a as the horizontal coordinate. Include the plot in the report with full annotation. Use Excel or GNU plot. From the plot, explain the relationship between V a and speed. Explain why the speed of the motor increases as V a increases. Support your explanation with relevant equations and reasoning. 2.2.2 From the data in the Table#2, why is I a not changing as proportionately as V a? 2.2.3 What is the speed range can be achieved with armature control method? Why? 2.2.4 What is the advantage of using armature voltage control in the starting over the direct starting method? 2.2.5 Will the power and torque constant in the armature control method? Explain. 4
3. Flux Control Method 3.1 Procedure: NOTE: To limit max speed on this motor to the rated value, the base speed is set to 1000rpm. 3.1.1 Make sure of the following conditions are met The shunt field rheostat R f should be adjusted to its minimum value; The switch S should be in the AVF position. The 1-phase autotransformer knob on the load side should be in minimum position. The voltmeters ranges should be at 240V. 3.1.2 Start the motor by pressing the START push button; adjust the speed control potentiometer; SPEED ; until the shaft speed is about 1000. Record V a, V f, I f and speed in Table#3. 3.1.3 Increase R f in 6 steps until the motor turns at about rated speed and record V a, V f, I f and speed at each step in Table#3. Table#3 : Field Control Method Vf, If, Amps Va, Speed, 3.2 Report: (Attach your plots and answers to this report) 3.2.1. Using data from Table#3, plot a graph between V f and speed.. Use V f as the horizontal coordinate. Include the plot in the report with full annotation. Use Excel or GNU plot. From the plot, explain the relationship between V f and speed. Explain why the speed of the machine increases as the field current decreases. Support your explanation with relevant equations and reasoning. 3.2.2. What is the speed range can be achieved with flux control method? Why? 3.2.3 Will the power and torque constant in the flux control method? Explain. 3.2.4 If the field winding is open while the motor is running, what will be the speed of the motor? What will happen in no action is taken? 3.2.5 What is the typical application of the flux control method? 5
Part C: Testing the Characteristics of the separately-excited dc motor Under Loading 4.1 Procedure: 4.1.1 Make sure the following conditions are met The shunt field rheostat R f should be adjusted to its minimum value; The switch S should be in the AVF position. The 1-phase autotransformer knob on the load side should be in minimum position. The voltmeters ranges should be at 240V. 4.1.2 Start the motor by pressing the START push button; adjust the speed control potentiometer; SPEED ; until the shaft speed is about 1500. Record the no-load values of V a, T m, I a and N in Table#4. NOTE: The speed control potentiometer controls the value of V a. Do not increase V a above its rated value. If a further increase in speed is required, adjust R f to get desired speed. 4.1.3 Load the motor to about 100% of its rated value. Record V a, T m, I a and N in Table#4. Speed Adjustment using field control: 4.1.4 Now, adjust the motor speed using R f until the speed is about the same as the no-load speed (1500 ). Record V a, T m, I a and N in Table#5. Table#4: Loading the motor Condition No load Ia, Amps Torque, Lb-Ft Speed, Va, 25% of FL 50% of FL 75% of FL 100% of FL Table# 5: Speed Adjustment Ia, Amps Torque, Lb-Ft Speed, Va, 6
4.2.5 Report: (Attach your plots and answers to this report) 4.2.5.1 Using data from Table#4, plot a graph between (a) T m and I a (b) T m and N. Use T e as the horizontal coordinate. Plot both in a single graph. Include the plot in the report with full annotation. Use Excel or GNU plot. From the plot, explain the relationship between T m and I a & T m and N. Explain with relevant equations and reasoning. 4.2.5.2 Using data from Table#6, plot a graph between (a) T m and I a (b) T m and N.. Use T e as the horizontal coordinate. Plot both in a single graph. Include the plot in the report with full annotation. Use Excel or GNU plot. From the plot, explain the relationship between T m and I a & T m and N. Explain with relevant equations and reasoning. 4.2.5.3 Would it be advisable to use the field-weakening method of speed control if the motor is operating at full load? Explain your answer. Hint: use Table#5 data. 4.2.5.4 What would be the starting current if 220V is applied to this DC motor at once? 4.2.5.5 What will happen if the field winding is open in the case of DC series motor? 4.2.5.6 What are the advantages and disadvantages of a closed-loop control over an open-loop control? 4.2.5.7 Calculate the speed regulation of the motor. 4.2.5.8 If the shunt field winding current of a separately-excited dc motor is increased twofold while the armature-winding current is maintained constant at rated value, will the electromagnetic torque double in its value? Explain? 4.2.5.9 From the experimental results, calculate the efficiency of the DC motor at 100% load. 7