EE 448 Fall 2006 Lab Experiment No. 1 Single Phase AC Circuits

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EE 448 Laboratory Experiment 1 Single Phase AC Circuits I. INTRODUCTION OBJECTIVES: • Study the phasor relationship between Voltage and Current in a single phase AC Circuit. • Study the concept of real power (P), reactive power (Q), apparent power(S) and power factor (cosΦ). • Identify a method to improve the line side power factor with the help of a capacitor bank. BACKGROUND SUMMARY: AC circuit elements consist of resistors (R), inductors (L) and capacitors(C) which can be fed from either a 3 phase or 1 phase 60 Hz, 120V source. Resistor and inductor combination connected to a single phase AC source results in a lagging current with respect to voltage. If R & L are connected in series, the phasor sum of the voltages across L and R equals the source voltage. In contrast if they are connected in parallel the phasor sum of the currents drawn by R & L equals the source current. Power factor of any load (source) is defined as the cosine of the angle between the load(source) current and corresponding load(source) voltage. By connecting a capacitor bank in parallel with such a RL circuit can improve the power factor which in turn reduces the current drawn from the source for a given power drawn by the resistor. Power relations in a single phase system Real power =V rms *I rms cosΦ in watts (where Φ is angle between V and I) Reactive power = V rms *I rms * sin(Φ) in VARs Apparent power = V rms *I rms in VA INSTRUMENTS and COMPONENTS: Power Supply Module EMS 8821 AC Voltmeter Module EMS 8426 AC Current Meter ModuleEM.S 8428 Resistance Module EMS 8311 Inductance Module EMS 8321 Capacitance Module EMS 8421 2
EE 448 Laboratory Experiment 1 Single Phase AC Circuits II. Pre-Lab Test Questions and Calculations 1. The machines we will be working with in this lab have these resistances: R 1 = 300Ω, R 2 = 600Ω and R 3 = 1200Ω. Identify the parallel combinations of two of the resistors at a time to get equivalent resistances of 200Ω, 240Ω and 400Ω. This will make using the lab equipment easier. 2. If R 1 = 300Ω is connected in series with an inductive reactance of X 1 = j300Ω, what will be the impedance angle of this series combination 3. In Fig. 2, If R 1 and X 1 are connected in parallel across a single phase source. What capacitance C value should be connected in parallel to get unity p.f. Assume the frequency of supply is 60Hz. 4. Draw the phasor diagrams for the voltages in figure 1. Take voltage across the resistor (V r ) as the reference vector. 120Vrms 60Hz V1 300 1 j300 2 Figure 1 5. For the circuit in Fig.2, draw the phasor diagram for the three currents I s , I r and I 1 and prove that I s = √2(I r ). 120Vrms 60Hz V1 Is Ir 300 1 Il j300 2 Figure 2 3
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EE 448 Fall 2006 Lab Experiment No. 1 Single Phase AC Circuits

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