81 Frequency Converters and Motors U [V] T [Nm] 800 T U x l f 1 f max. torque decrease with factor 400 Star configuration 10% M N T Load 230 Delta configuration T motor 87 Hz T motor 0 50 87 f [Hz] 0 50 f [Hz] 87 a) 87 Hz U/f characteristic control (ideal) b) T/n characteristic (ideal) Fig. 3.2 87 Hz characteristic This means we have the rated fl ux density (Φ) up to 400 Volt even the motor is confi gured for 230 Volt. With this higher voltage we can increase the maximum frequency with rated fl ux to 87 Hz. The use of this knowledge presupposes the following:
• The selected FC must easily be able to handle the higher delta current (48.70 A) • The motor must be wound such that it can withstand the required operating voltage (typically higher in the star confi guration) supplied by the FC (i.e. with a 690 V supply voltage and a 690 V FC, this application is only possible with a motor wound for 690 V / 400 V Y/Δ) • The torque on the motor shaft remains the same for both confi gurations up to 50 Hz. Hz. Above 50 Hz, a star-connected motor enters the fi eld weakening range. When it is delta-connected, this does not happen until approximately 90 Hz.
If the ±10% tolerance of the FC is used, the fi eld weakening range begins at 55 Hz or 95 Hz respectively. The torque decreases because the motor voltage is not increased The benefi ts of this increased motor capacity utilisation are: • An existing FC can be operated with a greater speed control range. • A lower-power rating motor can be used. Such a motor can have lower moment of inertia which allows higher dynamics. This improves the dynamic characteristics of the system. Please note that operation of a 400V/230V Y/Δ motor in delta at 400 V is only possible on a FC because of the higher feeding frequency of 87 Hz. Operation direct on 400 V/ 50 Hz mains will destroy the motor!