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AC Components and Circuits
AC RL Circuits
Section 3-3 Parallel RL Circuit Analysis
Currents in Parallel RL Circuits | 
Currents in parallel RL circuits | 
Phasor diagram for parallel RL circuits. | | Equation The total current of a series RL circuit is: Where: - IT = Total current
- IR = Resistor current
- IL = Inductor current
| In most practical situations, however, the resistor and inductor currents are not known. So the application of the equation for total current is preceded by determining IR and IL: If XL is not directly know, you also have to calculate XL = 2pfL. The phase angle for a parallel RL circuit is usually determined from the resistive and reactive currents.
Currents in a parallel RL circuit.
The phasor diagram shows that there is no phase shift for the resistor current (IR) and there is a phase of -90� for the inductor current. The phase shift for the total circuit is thus somewhere between 0 and -90�. | Equation The phase shift for a parallel RL circuit is: Where: -
q = Phase angle* -
IL = Inductor current -
IR = Resistor current | | Procedure | Given: VT, f, R, and L Determine: IT and q |  | 1. Calculate XL XL = 2pfl 2. Calculate IL 3. Calculate IR At this point, you know IL and IR 4. Calculate IT 5. Calculate q Steps 4 and 5 are the values to be determined. | Examples Impedance in Parallel RL Circuits 
| Equation The impedance of a parallel RL circuit is: Where: - Z = Circuit impedance
- VT= Total voltage
- IT = Total current
| Examples Analyzing Parallel RL Circuits A typical analysis of a parallel RL circuit begins with known values for: - Total rms voltage applied to the circuit (VT)
- Applied frequency (f)
- Value of the resistor (R)
- Value of the inductor (L)
The objective, then, is to determine all other relevant circuit values: - Voltage across the resistor (VR)
- Voltage across the inductor (VL)
- Inductive reactance (XL
) Resistor current (IR) Inductor current (IL) Total current (IT) Impedance (Z) Phase angle (q) | | | General Procedure 1. Determine VR and VL from VT 2. Calculate XL from f and L3. Calculate IR by applying Ohm's law to VR and R 4. Calculate IL by applying Ohm's Law to VL and XL 5. Calculate IT from IR and IL 6. Calculate Z by applying Ohm's law to VT and IT 7. Calculate q from IL and IR | Examples Endless Examples & Exercises Work these problems until you are confident you have mastered the procedures. - All angles are expressed in degrees.
- Round answers to the nearest tenth.
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