phase supply voltage l 1 l 2 l 3 into a constant dc link voltage u dc which is smoothed by a capacitor c dc a three phase inverter bridge inu which consists of igbt switches v 1 v 6 and free wheeling diodes d 1 d 6 and creates a three phase output voltage u v w for the motor m and a control unit cu fig4 presents a known characteristic curve of the relation between the shaft torque t and stator current i of an electric motor at a constant speed at the normal operation area between the nominal torque t n and maximum torque t max the relation is quite linear i n i max respectively fig5 presents a known characteristic curve of the loadability of a frequency converter in relation to time t the loadability is usually expressed as the value of the output current and normally the converter can supply an over load current i ol e g twice the continuous nominal load current i n for a short period t ol e g 2 seconds during a certain load cycle t 0 t 1 e g 30 seconds it is also possible to split the allowed overload time t ol to several shorter periods like t ol1 t ol2 and t ol3 in fig4 a known feature of a permanent magnet electric motor is that its rotor position follows the electric field created by the stator current this makes it possible to adjust the.