Why the Output Voltage Varies as the Load Increases

When you connect a load to the bus you are completing the circuit with another impedance. So you have a circuit with a voltage source and two impedances in series. When you draw more loads you are lowering the load impedance. This increases the voltage drop across the source impedance.

How does loading affect the output voltage?

As the load decreases, the output voltage will go up. Conversely, increases in load will result in lower output voltages. Typically, the smaller the VA size of the unit, the greater difference there is between no-load and full-load voltage.

Why does voltage increase when current increases?

The difference in electric potential energy (per charge) between two points is what we have given the name voltage. Thus, the voltage directly tells us which way charges want to move – and if they can, then they will speed up in that direction, so the current will increase.