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When the asynchronous machine is supplied by the rated voltage and it operates in no load conditions (no mechanical load), it can be approximated that the mechanical speed is the synchronism one, the slip s is zero respectively.

In these conditions, the rotor current I¢₂ can be approximated to be null and, taking into account the equivalent "p" circuit and the Amper-turns equation, the current the motor absorbs, is only the magnetising current

.

By using a tri-phased measuring group TM (Figure 4), are determined:

• the line-to-line voltage U₁,
• the line current I₁,
• the active power P₁₀.

Figure 4

Taking into account the stator connection (Y), the impedance of the magnetising circuit is computed

.

We have to separate the magnetising reactance X_m. The equivalent resistance of the magnetising circuit can be determined by subtracting by the active power consumed by the motor P₁₀, the stator and the mechanical looses, the result being the so called, iron looses

.

It results the equivalent resistance of the magnetising circuit

,

then the magnetising reactance

and finally, the magnetising inductance

.

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