Say you have a typical bench AC electric motor rated at 180W which has reached its maximum RPM's. Does the motor always draw 180W when running at full RPM's ? Or does the motor draw less power once its at full RPM's?
Note that no load is applied in this scenario.
If I imagine this in my mind, it makes sense to me that a motor once at full RPM's and under no load, would draw less power as the momentum should be helping it turn, but I'm not sure so I'm checking with the community.
To a first approximation, the motor draws as much electrical power as it is doing mechanical work.
If there's no load, the power consumption will be low.
In practice, there are electrical and mechanical losses involved in keeping a motor energised and spinning. The motor might also have a fan for cooling itself. These will require some power.
Finally, the motor efficiency will be less than 100%, so it will require slightly more electrical power than it produces mechanical work.
The electrical power drawn by a motor is equal to the mechanical power delivered to the load plus losses in the motor. A portion of those losses is not diminished when there is no load.
The losses consist electrical, electromagnetic and mechanical losses. The electrical losses consist of heating losses in the resistance of the motor windings and other internal parts. The electromagnetic losses consist of hysteresis and eddy-current losses in the rotor and stator iron. The mechanical losses consist of friction and aerodynamic drag (windage). If the motor has brushes, contact resistance and arcing contribute to the electrical losses. In a DC motor with brushes, the magnetic field does not vary much, so the electromagnetic losses are little or nothing. If the motor has a fan, the fan loss is considered to be part of the windage. Many motors have protrusions or fins on the rotor that are intended to augment cooling.
Motors that do not have permanent magnets have magnetizing current that, in many motors, is not reduced when the motor is not loaded. Some of the electrical and electromagnetic losses described above are associated with the magnetization.
There is a problem with the description 'typical bench AC motor'. There is no typical bench AC motor.
Were we talking about a DC brushed motor, then it's easy. No load means lower power consumption, even at rated speed.
Many bench AC motors are induction types, and herein lies the problem. There is a type of induction motor that overheats at no load, and actually requires a load to make it work properly.
An induction motor is partly a rotating transformer, which operates at the slip frequency. As the load lessens, the slip frequency falls, and at a low enough slip frequency, when the rotor Volt.Second product is exceeded, the stator-rotor transformer saturates and draws excessive current, which overheats the stator.
This excess current is a magnetising current, so is not measured as power by the standard domestic meter. In one sense therefore, even this motor is not drawing power. But drawing so much current that it overheats is unexpected.
A more conservatively-sized induction motor that's safe to run at no load will indeed draw less power than it would at full load.
without a load, the AC motor will not draw more power than needed to turn the rotor.
however, that amount of power is not negligible. by comparison, a piston engine at idle consumes the equivalent of 5l/100km just to idle. for this reason we dont see phenomenal gas savers under 5l/100km, unless the engine cc reduces drastically. this is the same for electrical motors. the power draw with no load is less than at full load but not negligible.
