LABORATORY Experiment 3 Properties of Inductor and Applications 1. Objectives To investigate the properties of inductor for different types of magnetic material To calculate the resonant frequency of a LC tuned circuit To investigate the operation principle of a relay 2. Apparatus and Components 1. Power Supply ( x 2) 2. Digital Multimeter (Fluke) ( x 1) 3. LCR Meter (LCR-815B) ( x 1) 4. 40MHz Oscilloscope ( x 1) 5. Audio Signal Generator ( x 1) 6. Soldering Iron 1. Power Transformer ( x 1) 2. Audio Frequency Transformer ( x 1) 3. Antenna Coil with Ferrite Rod ( x 1) 4. Variable Capacitor ( x 1) 5. LED ( x 1) 6. 12V Relay ( x 1) 3. Background 3.1 Inductors A basic structure of ideal inductor, which has no internal resistance, is an air-core coil of wire or a solenoid with different magnetic material. Inductors of different core material will not have the same properties under the same conditions due to their different saturation point. Inductor will exhibit different properties under d.c. or a.c. conditions. Under d.c. condition, it becomes short-circuited. However, under a.c. condition, it will induce back e.m.f. to oppose changes of a.c. current. The unit of inductance (L) is Henry (H). Core material Coil L Symbol of an Inductor Figure 1 - Basic inductor and its symbol Laboratory Experiment 3 1
The value of inductance of coil is given as 2 µ AN L = (H) (Equation 1) l X L = 2 π fl (Ω) (Equation 2) Ferromagnetic material is one of the commonly used magnetic materials with a high permeability µ, which is defined as below: B µ = = µµ r o(a2) (Equation 3) H where µ o = permeability of free space permeability of air µ r = relative permeability of material B = flux density (Tesla) H = magnetic field strength (At/m) 3.2 Parallel Resonance of the LC Tank Circuit A LC tank circuit is shown in Figure 2. This tank circuit is used extensively in communications equipment such as AM, FM, and television transmitters and receivers. At resonant, the capacitive and inductive reactance of the circuits are equal, and the resonant frequency is given as: C L Figure 2 - LC tank circuit 2 Experiment 3 Laboratory
F r = 1 2π LC (Equation 4) where F r = resonant frequency L = inductance (H) C = capacitance (F) In this experiment, you will study how to use a ferromagnetic material to change the resonant frequency by changing the inductive reactance. 3.2 Use of Relays One common application of magnetic effects is relay. Basically, relays are electromagnetically operated, remotely led switches, with one or more sets of contacts. The equivalent circuit of relay is shown in Figure A3. When the current is flowing through the coil, the internal switch change the contact position from NC to NO. ( NC stands for normally closed, NO stands for normally open. ) Common applications of relays include: remote-switching, status indication and circuit-protection. The main advantage of relays is their ability to be operated from a remote location and can be operated with low DC voltage. relay pole NO NC coil NO = normally opened NC = normally closed Figure 3 Connection of relay Laboratory Experiment 3 3
4. Procedure 4.1 LC Tuned Circuit Inductor Movement of iron core LCR Meter Figure 4 - Measurement of Inductance (a) Using the LCR meter, measure the inductance (L) and resistance (R) of the coil without the steel rod inserted. L = R = (b) Repeat step 2.1 with the steel rod fully inserted into the coil. L = R = (c) Calculate the frequencies of the LC tuned circuit if C = 10 µf Fmax. (with low inductance) = Fmin. (with high inductance) = 4.2 Measurement of Voltage Drops across the Inductors V1 Oscilloscope V2 Inductor 1 Inductor 2 L1 N=1000 L2 L=2500 Signal generator 5V, frequent = variable, sine wave Figure 5 - Measurement of Voltage Drops across the Inductors 4 Experiment 3 Laboratory
(a) (b) Connect the circuit as shown above. Measure L 1 and L 2 of the inductors by LCR meter L 1 = and L 2 = (c) Adjust the frequency of the signal generator starting from 5 khz to 50 khz at a step of 5kHz. (d) Keep the output voltage of the signal generator constant at 5 V. (e) Calculate the relative permeability, µ r, of magnetic material of the inductors 1 and 2. µ r of inductor 1 = and µ r of inductor 2 = Frequency (khz) V 1 (V) V 2 (V) L 1 (calculated) L 2 (calculated) 5 10 15 20 25 30 35 40 45 50 4.3 Applications of Relay R LED relay 10V pole NO switch NC 12V coil Figure 6 Laboratory Experiment 3 5
(a) Calculate the value of the resistance in series with the LED. Specification of the LED is as following: V d = 2 V I d = 5 ma R = (let your supervisor check the value before proceeding to step (b)) (b) Connect the circuit as shown in Figure 6 (c) (d) Use one LED as the load in the lighting fixture. Measure the current required to energized the coil when the switch is closed. (e) Based on the result of step (d), calculate the coil resistance. 5. Discussion 5.1 Based on the results of Part 4.1, calculate the relative permeability of the steel rod (µ r ). 5.2 Based on the results of 4.1, calculate the range of frequency covered by the LC tuned circuit. 5.3 Is the steel rod a good magnetic material? and why? 6 Experiment 3 Laboratory
5.4 If the steel rod is partially inserted, what will be the effect on the tuning frequency? Laboratory Experiment 3 7