no-load
Not supplying power; open-circuited.
no-load: machine running but delivering zero output power
A no-load condition occurs when a machine or circuit is energized and running, but is not delivering any useful power to a connected device or load. The machine spins or operates at full or rated speed while driving nothing, or while connected to an open circuit. This is common during startup, testing, or when a motor runs without a mechanical load attached.
In electric motors, no-load operation draws current only to overcome the motor's own internal friction and core losses. A typical three-phase induction motor running at no-load might draw 30 to 50 percent of its rated full-load current, depending on design and speed. This residual current heats the windings and iron core even though no external work is being done. Transformers also have measurable no-load current and loss, caused by magnetizing the core and resistive heating in the primary winding.
Why no-load matters in practice
No-load testing is essential for commissioning electrical equipment. Running a motor or generator at no-load lets technicians verify that the machine starts correctly, reaches rated speed, and carries only expected losses before a load is applied. Comparing a motor's no-load current against its nameplate value detects problems such as bearing drag, internal shorts, or mechanical binding that would otherwise remain hidden. Similarly, a transformer's no-load loss figures help engineers predict energy waste in distribution networks.
In variable-frequency drives and soft starters, no-load operation creates particular challenges. A motor with an open connection or a broken coupling will accelerate uncontrolled until it reaches synchronous speed (or near it), drawing excessive current and generating heat. Some industrial drives include no-load detection algorithms that reduce supply voltage or frequency to prevent damage during unloaded runaway.
The term 'no-load' also applies to electrical sources themselves. A power supply or generator under no-load shows its open-circuit voltage, which is always higher than the voltage it produces when supplying actual current to a load. This difference between no-load and full-load voltage is one measure of a source's internal impedance or regulation quality.