Objective
The purpose of this experiment is to study power dissipation in reactive loads, and how power factor affects the total power consumption in AC circuits with inductive or capacitive elements.
Materials Needed
- AC power source (50 Hz or 60 Hz, standard household voltage)
- Reactive load (e.g., inductive coil or capacitor)
- Resistor (for comparison)
- Multimeter (for measuring voltage, current, and power factor)
- Oscilloscope (optional, for waveform analysis)
- Wattmeter (for direct measurement of power dissipation)
- Connection wires and breadboard (or soldered circuit)
Theory
In AC circuits, power dissipation occurs differently depending on whether the load is resistive, capacitive, or inductive. A purely resistive load dissipates power fully as heat, while reactive loads (inductive or capacitive) cause energy to be stored and returned to the power source periodically. This is why power factor plays a critical role in the actual power consumed in a reactive circuit. Power factor is defined as the ratio of real power (P) to apparent power (S), where:
- **Real power (P)**: The actual power dissipated as heat or work in the circuit, measured in watts (W).
- **Reactive power (Q)**: The power stored in the reactive components (inductors or capacitors), measured in volt-amperes reactive (VAR).
- **Apparent power (S)**: The product of the RMS voltage and current in the circuit, measured in volt-amperes (VA).
The power factor (PF) is given by:
PF = P / S = cos(ϕ)
where ϕ is the phase difference between voltage and current. A power factor closer to 1 indicates more efficient power consumption, while a lower power factor indicates significant reactive power and lower efficiency.
Steps
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Set Up the Circuit
Connect the AC power source to the load (either inductive or capacitive). Start with a purely resistive load for comparison. Make sure to wire a wattmeter in series to measure power dissipation directly. If you're using a multimeter, measure the RMS values of voltage and current.
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Measure with Resistive Load
Turn on the power source and measure the voltage, current, and power with the resistive load connected. Since the power factor for a purely resistive load is 1, the power measured will be equal to the product of voltage and current (P = V * I).
Record the values of voltage, current, and power.
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Measure with Reactive Load
Replace the resistive load with an inductive coil (or capacitor) to create a reactive load. Measure the voltage, current, and power again. In this case, the power factor will be less than 1 due to the phase difference between voltage and current.
Record the values of voltage, current, and power, and calculate the power factor using:
PF = P / (V * I) -
Compare Power Dissipation
Compare the power dissipated in both the resistive and reactive loads. You should observe that, for the same voltage and current, the reactive load consumes less real power (as indicated by the lower wattmeter reading) due to the presence of reactive power.
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Optional: Use an Oscilloscope
If you have access to an oscilloscope, you can connect it to observe the phase difference between the voltage and current waveforms. For resistive loads, the two waveforms will be in phase. For reactive loads, the current waveform will lag (in inductive loads) or lead (in capacitive loads) relative to the voltage waveform.
Data and Calculations
Below is an example of data recorded during the experiment:
| Load Type | Voltage (V) | Current (A) | Power (W) | Power Factor (PF) |
|---|---|---|---|---|
| Resistive (R = 50Ω) | 230 | 4.6 | 1058 | 1.0 |
| Inductive (L = 100mH) | 230 | 4.6 | 800 | 0.72 |
| Capacitive (C = 200μF) | 230 | 4.6 | 720 | 0.68 |
In this example, you can see that while the voltage and current remain constant for all load types, the power dissipation is lower for reactive loads due to the reduced power factor.
Conclusion
In this experiment, we demonstrated the differences in power dissipation between resistive and reactive loads in AC circuits. Reactive loads store energy rather than dissipating it entirely as heat, resulting in a lower power factor and reduced real power consumption. Understanding power factor is essential in designing efficient electrical systems, especially in industries dealing with high-power AC loads.