No-Load Current
It is the current the motor draws while turning freely without carrying any mechanical load. All of this current covers friction, brush contact, bearing drag and iron losses. In a healthy motor it stays in the band of 10–25 percent of the rated current and is the fastest indicator of mechanical condition.
Measuring it is simple: disconnect the motor from the load, run it at rated voltage and read the current with an ammeter in series with the circuit or with a Hall-type clamp. Record this value during the first commissioning; it becomes your reference for later measurements.
Interpretation:
- If it increases over time: There is bearing wear, dried-out grease, a bent shaft or a dirty commutator.
- If it is far above the catalogue value: There may be binding inside the gearbox or axial loading caused by the mounting.
- If it is lower than expected: The supply voltage is low or the motor is not reaching full speed.
In systems using current-sensing stop, this value directly affects the threshold setting. The threshold is chosen above the real loaded running current, not the no-load current; however, a no-load current that rises over time also pushes the running current up and after a while causes the motion to be interrupted. When there is a complaint of early tripping, measure the no-load current first.
The context in which this term is used: DC Motor Driver Selection Guide