The no-load voltage is higher than the full-load voltage due to the electrical behavior of the system when load conditions change.
This phenomenon is primarily related to the internal impedance of the power source, such as a transformer, generator, or power supply, and the effects of loading on the voltage.
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Here are the key reasons:
Internal Impedance of the Power Source
Power sources, such as transformers and generators, have an internal impedance (a combination of resistance and reactance). When a load is connected:
- At no load, the current drawn from the source is minimal, resulting in negligible voltage drop across the internal impedance. Therefore, the output voltage remains close to the source’s nominal voltage.
- At full load, the current increases significantly, causing a larger voltage drop across the internal impedance. This voltage drop reduces the output voltage observed at the load terminals.
Voltage Drop in the Transmission Path
The conductors or wires used to transmit power from the source to the load also have resistance and inductance. The voltage drop across these components depends on the load current:
- At no load, the current is very low, so the voltage drop is minimal.
- At full load, the current is higher, leading to greater voltage drop across the transmission path, reducing the voltage at the load end.
Magnetic Effects in Transformers
In transformers:
- The primary and secondary windings have leakage reactance and resistance.
- When the load increases, the leakage reactance causes additional voltage drops due to the inductive nature of the windings, reducing the secondary voltage under full-load conditions.
Voltage Regulation of the Power Source
Voltage regulation reflects the ability of a power source to maintain a stable output voltage under varying load conditions. Poor voltage regulation results in a significant drop in voltage from no-load to full-load due to:
- The inherent design of the source.
- The load’s power factor, with inductive or capacitive loads causing additional voltage drops due to reactive power.
Example
If a transformer is rated for 230V:
- At no load, the output voltage might be around 235V, as there is minimal current and negligible voltage drop.
- At full load, the output voltage could drop to 230V or lower due to the increased current and the resulting voltage drop across the internal impedance.
Summary
The difference between no-load and full-load voltage is primarily due to the internal impedance of the power source and the transmission path, as well as the load’s characteristics. This variation highlights the importance of voltage regulation, ensuring that the voltage drop remains within acceptable limits to provide stable power to connected devices.