Selection of Captive Transformer Rating for Direct-On-Line Motor Starting

In chemical industrial facilities, like for example polypropylene and polyethylene plants, high power motors (typical rated power is between 5 MW and 15 MW) are normally used to drive compressor and extruder machines required by this type of industrial process. In order to be started directly-online, the motor is fed by a dedicated transformer usually named captive (or unit) transformer as per industry practice. The captive transformer is quite similar to a normal power transformer: anyway, due attention shall be paid to its sizing and verification, both in terms of short circuit power delivered by the transformer in order to start safely the motor, as well as in terms of thermal overload and hot spot condition of transformer windings during the consecutive starting duty cycle allowed by the design of the supplied motor. A simplified analytical procedure is presented here to define the rated power of the captive transformer; this by-hand simplified procedure is intended as preparatory before the detail design calculations and factory tests are performed by the manufacturer of the transformer to confirm the selected rating.