Month: March 2017

Free Rotor

Friction and ventilation losses in the core are achieved by the free rotor test. We must not make mechanical couplings on the electric motor shaft to feed it at rated voltage and frequency.

To ensure the correct value of friction and ventilation losses, the motor will be in free rotation until the input power stabilizes. Stabilization will be achieved when two measurements of the input power do not vary by more than 3% in a time of 30 minutes. The current is measured at each phase and the mean value is the no-load current, I0.


The free motor losses is the difference between the input power, PEL and the Joule losses in the stator, PJE (RI2, at the test temperature). It will equal the sum of the friction and ventilation losses and the core.

We measure the voltage, current and input power at the rated frequency and reduce the supply voltage from 125% of the nominal value to the point of current increase to define the separation between the losses in the core by friction and ventilation.


The simplest of all types of electric motors is the squirrel-box induction motor that is used with three-phase power. The armature of this type of motor consists of three fixed coils and is similar to the one of the synchronous motor. The rotating element consists of a core, which includes a series of large capacity drivers placed in a circle around the tree and parallel to it. When they do not have a core, rotor drivers look like cylindrical cages used to hunt squirrels.

The flow of the three-phase current inside the coils of the fixed armature generates a rotational magnetic field VEM31115, and this induces a current in the drivers of the cage. The magnetic reaction between the rotating field and the rotor drivers carrying the chain causes it to rotate. If the rotor rotates at exactly the same speed as the magnetic field, it will not have induced currents in it, and therefore, the rotor should not rotate at a synchronous speed. In operation, the rotational speed of the rotor and that of the field differ from one another by 2 to 5%. This difference in speed is called a fall.