Exercise 1: Inductors EXERCISE OBJECTIVE When you have completed this exercise, you will be able to describe the effect an inductor has on dc and ac circuits by using measured values. You will verify your results with an oscilloscope and multimeter. DISCUSSION NOTE: This exercise treats DC as a steady state maximum value. It does not consider the initial current When ac is applied, cemf is produced in proportion to the amount of inductance and the frequency of the applied signal. inductor. a. increase. b. remain the same. c. decrease. also affects the phase relationship between the applied voltage and current. As shown, the voltage across the inductor leads the current by 90. PROCEDURE If necessary, clear the AC 1 FUNDAMENTALS circuit board of all two-post connectors and any other connections. In the following steps, you will determine if inductor L3 produces cemf when a dc signal is applied. measure the coil resistance (R L3 ) of inductor L3. R L3 = (Recall Value 1) FACET by Lab-Volt 59
AC 1 Fundamentals Adjust the positive variable supply to 8 Vdc. NOTE: Be sure to short out current-sensing resistor R2 with a two-post connector. Measure the circuit current (I) with the multimeter. I = ma (Recall Value 2) Measure the voltage drop across inductor L3 (V L3 ) with the multimeter. V L3 = Vdc (Recall Value 3) 60 FACET by Lab-Volt
Use your measured values of I L3 ( ma [Step 6, Recall Value 2]) and V L3 ( V [Step 7, Recall Value 3]) to calculate the resistance of L3 (R L3 ). R L3 V = I L3 L3 R L3 = Ω (Recall Value 4) Compare the coil resistance of R L3 ( [Step 3, Recall Value 1]), measured by using the resistance function of the multimeter and the calculated coil resistance of R L3 ( [Step 8, Recall Value 4]). Based on this comparison, is inductor L3 producing cemf with constant dc current applied? a. yes b. no Connect the circuit shown. In the following steps, you will determine if inductor L3 produces cemf when an ac signal is applied. Adjust V GEN for a 10 V pk-pk sine wave at 20 khz. Use the oscilloscope to set the amplitude and the frequency. Determine the circuit current (I). I = V R2 I = ma pk-pk (Recall Value 5) NOTE: Determine the circuit current by dividing the voltage drop across R2 by the resistance of R2 (10 ). FACET by Lab-Volt 61
AC 1 Fundamentals Measure the voltage drop across L3 (V L3 ) with the oscilloscope. V L3 = V pk-pk (Recall Value 6) Use your measured values of I ( ma pk-pk [Step 12, Recall Value 5]) and V L3 ( V pk-pk [Step 13, Recall Value 6]) to calculate the impedance of L3 (Z L3 ). VL3 ZL3 = I Z L3 = (Recall Value 7) Compare the coil resistance of R L3 ( [Step 8, Recall Value 4]) and the calculated coil impedance Z L3 ( [Step 14, Recall Value 7]). Based on this comparison, is L3 producing cemf when ac current is applied? a. yes b. no When ac is applied, cemf is produced by inductor L3 in proportion to the amount of inductance and the frequency of the applied signal. 62 FACET by Lab-Volt
Monitor the circuit current on the oscilloscope by observing the amplitude of the voltage across current-sensing resistor R2. Be sure to remove the two-post connector across R2. Place CM switch 16 in the ON position to increase the inductance of L3 from 4.7 mh to 14.7 mh. While observing the oscilloscope, toggle the CM switch off and on. Does an increase of inductance increase or decrease circuit current? a. increase b. decrease Increase the frequency of the generator from 20 khz to 50 khz while observing the circuit current on the oscilloscope. NOTE: The value of L3 is back to the original value of 4.7 mh. What is the effect of an increase in frequency on circuit current? a. remains the same b. decreases c. increases If necessary, readjust the generator input for a 10 V pk-pk signal at 20 khz. Connect the oscilloscope s channel 1 input to measure V L3, and connect the channel 2 input to measure V R2. FACET by Lab-Volt 63
AC 1 Fundamentals Make sure both scope probes are set for X10. s. Use the position controls and set the CH1 waveform to start on the left side in the middle of the screen. Observe the phase angle between the circuit current (V R2 ) and V L3 (use V L3 as the reference, CH 1). NOTE: Observe the phase angle of the circuit current by looking at the voltage drop across R2 (V R2 ). The circuit current and V R2 have identical phase angles. Does the circuit current lead or lag the inductor voltage? a. lead b. lag Make sure all CMs are cleared (turned off) before proceeding to the next section. CONCLUSION When a constant dc is applied to an inductor, no cemf is produced and the only opposition to current changing impedance of the inductor. CEMF is produced in proportion to the value of inductance and to the frequency of the applied signal. by 90. REVIEW QUESTIONS 1. a. cemf only. b. coil resistance only. c. cemf and coil resistance. d. emf only. 64 FACET by Lab-Volt
2. Connect the circuit shown. Adjust V GEN for a 10 V pk-pk, 20 khz sine wave. Place CM switch 17 in the ON position to alter the inductance value of L3. Monitor the circuit current by using an oscilloscope to measure the voltage across current-sensing resistor R2. While observing the oscilloscope, toggle CM switch 17 off and on. When the CM is toggled off and on, what can you conclude about the inductance of L3 based on the circuit current? a. The inductance of L3 was reduced in value. b. Changing the inductance of L3 had no effect on circuit current. c. The inductance of L3 increased in value. d. Changing the inductance of L3 decreased the circuit current. 3. Decreasing the frequency of the signal applied to an inductor a. b. increases cemf. c. neither increases nor decreases the current. d. decreases impedance. 4. In an ideal inductor, a. voltage leads current by 90. b. current leads voltage by 90. c. voltage lags current by 90. d. voltage and current remain in phase. 5. An increase in cemf produced by an inductor is seen as a. a decrease in voltage drop. b. an increase in circuit current. c. an increase in impedance. d. a decrease in the coil resistance. NOTE: Make sure all CMs are cleared (turned off) before proceeding to the next section. FACET by Lab-Volt 65