The Heyland circle is the current locus diagram of the rotor current during the starting of a multiphase motor with short-circuited rotor in relation to the stator circle. When starting the motor, the full locked rotor voltage acts on the rotor winding with the frequency of the incoming voltage. The rotor’s ohmic resistance and its inductivity are constant. The reactance XL of the circle is calculated as 2 π f L = ωL and is largest at the start of the motor starting. During the starting phase, the rotor attempts to follow the stator field, whereby the frequency and thus also ωL is continuously diminishing. In parallel, also the induced frequency on the rotor drops because it is proportional to the frequency. The rotor current is calculated as l. In this context, cos φ rises from initially 0.2….0.4 to around 0.9 at the end of the starting phase. Voltage and frequency drop to a value that corresponds to the slip of the motor. The rotor current is a vector that changes its size and direction during the starting phase and whose peak forms a circle referred to as the Heyland circle. The value of the rotor current is calculated from

and the angle of aus