EGRE 101 DC Motor II

Transcription

1 EGRE 101 DC Motor II Preamble In this week s laboratory exercise you will become familiar with: Converting a circuit schematic to a physical circuit implementation Measuring physical quantities relevant to electrical engineering (resistance and voltage) using a multimeter and an oscilloscope Using an oscilloscope and a multimeter to investigate the operation of the DC motor, in particular the phenomenon known as backemf (electromotive force) Calculating physical quantities from experimental information by using Ohm s law, KCL, and KVL Laboratory notebooks At the end of this laboratory exercise you must have all your procedures, measurements, and observations written down in your laboratory notebooks. In order to receive credit you need to get your laboratory notebook signed off by one of the teaching assistants in your lab section. Equipment and materials HewlettPackard 34401A Multimeter HewlettPackard 54600B 100 MHz Oscilloscope HewlettPackard E3630A Power Supply Oscilloscope probes General probes Magnets Coil made from 26 AWG magnetic wire Breadboard Paper clips 10 Ohm series resistor Procedure overview You need to do the following in a sequential order: 1. Measure and record the resistance of the coil and of the series resistor 2. Follow the procedure in the previous lab to get the motor spinning 3. Use your oscilloscope to take a snapshot over time of the current through the coil 4. Measure the current at various times and calculate the corresponding. 1

2 Step 1: Measure the resistances We use multimeters to measure currents, voltages, or resistances. In this lab, we measure the resistance of the coil and of the series resistor. Using hook probes, connect the coil (lying flat on the table) to the "Ohms" terminals of the multimeter, as shown in Figure 1. Turn on the multimeter and set it in Ohmmeter mode by pressing the "Ohm 2W" button. Record the result,, in your notebook. Repeat for the series resistor,. Step 2: Get the motor spinning Figure 1: Digital multimeter Follow the procedure in the previous lab to get the motor spinning. Figure 2: A standard DC power supply 2

3 Step 3: Take a "snapshot" of the current through the coil Introduction: An oscilloscope, depicted in Figure 3, is a device that allows for observations of oscillations either of voltage or current. In contrast with the multimeter that measures an "instantaneous" value of the physical quantity, an oscilloscope measures the quantity as it changes in time. Input 1 Figure 3: A standard analog oscilloscope To measure the current through the coil, we would normally connect the oscilloscope in place of the ammeter, in series with the coil (see the Motors worksheet we did in class). Unfortunately, the scopes you will be using do not allow for current measurements. I Motor Vsource V Rmotor Rmotor 0 V Rseries Vemf Rseries Figure 4: Equivalent circuit of DC motor with series resistor. 3

4 You will measure the current indirectly, as follows: You include a series resistor in the circuit, as shown in Figure 4. Then you measure the voltage using the oscilloscope (described later) and calculate the current using Ohm's law: With the current known, you can compute using a slightly modified version of the formula we derived in class: (You'll have to prove this in your postlab report.) 3.2. Circuit setup: Build your circuit as shown in Figure 4., and connect an oscilloscope probe across the series resistor and to the scope's "Input 1" Scope setup: Turn on the scope and press the "Autoscale" button. This will set the time and amplitude scales so that the waveform is visible on the screen. Turn the "Position" knob in the "VERTICAL" section, until the waveform's bottom is centered vertically. Turn the "Time/div" knob in the "VERTICAL" section, until the waveform fits comfortably on the screen. Turn the "Volts/div" and "Delay" knobs in the "HORIZONTAL" section, until you can clearly see two periods of the waveform. Press the "Stop" button in the "STORAGE" section to capture the waveform on the screen. Note that you may have to press "Run" and "Stop" a few times until you get a somewhat cleaner picture. Step 4: Measuring the waveform 4.1. Measurements: Once you have captured a full period of the waveform, you will measure the voltage at 12 time instants within a period. 8 of the point will be in the "on" zone and 4 will be in the "off" zone, as shown in the table below. Start by making a similar table in your notebook. Then, for each time instant: Measure the time: press "Cursors" in the "Measure" section, then press the button marked "t1" at the bottom of the screen. Using the knob below the "Cursors" button, move the time cursor to reach the point you want to measure on the waveform. Read the time value from the screen and record it in the table in your notebook. Measure the voltage: press "Cursors" in the "Measure" section, then press the button marked "V1" at the bottom of the screen. Using the knob below the "Cursors" button, move the voltage cursor to reach the point you want to measure on the waveform. Read the voltage value from the screen and record it in the table in your notebook. 4

5 Measured quantities Calculated quantities # [s] [V] [V] [A] [V] 1 (start of period's "on" zone) (end of "on" zone) 9 (start of "off" zone) 10 (middle of "off" zone) 11 (just before end of period) 12 (start of next period) 4.1. Calculations: Calculate a couple of and pairs of values, to ensure that you know how to use the equations in section 3.1. You will complete the calculations in your lab report. Step 5: Checkout Ask a TA to check you out by signing your lab notebook. Turn off all instruments and clean up your workstation. Stay tuned for postlab report instructions. 5