80 Frequency Converters and Motors 3.1.2 87 Hz Characteristics Typically asynchronous motors operated with FC’s are confi gured to the nominal voltage of the mains. This means that 400 V/230 V motor will be confi gured in star when operated by a 400 V FC. As described in the previous section a 50 Hz motor will enter fi eld weakening when the voltage can’t be increased any more. For extending the speed range the motor can be confi gured in delta. Example Motor: 15 kW, 400 V/230 V Y/Δ, 27.5A/48, 7A, 50 Hz At 50 Hz the power in star and delta confi guration is 15 kW because of the diff erent mains voltage which result in diff erent motor currents.
PY (50 Hz) = 3 400 V 27.50 A cos φ η = 14.92 [kW] P Δ (50 Hz) = 3 230 V 48.70 A cos φ x η = 15.19 [kW] With delta connection it can be seen in Fig. 3.2 87 Hz characteristic that in contradiction to the start confi guration the motor runs with constant U/f ratio up to 230 Volt, but if the FC is powered from a 400 Volt supply, we are actually able to keep the constant U/f ratio up to 400 Volt and the high current, P Δ (87 Hz) = 3 400 V 48.70 A cos φ η = 26.42 [kW]