690 Voltage Transformer

If one secondary winding is star connected and the other delta connected, then there is a phase shift of 30 electrical degrees between them.

If you are going to rectifier this AC signal into a DC signal, you have a much smooter DC current. You can use a 12 points rectifier. The following capacitor to store the energie can be much smaller.

Why 720 Volts in stead op 440V ? Probably because in this case you can hold the relation between volts and herts constant over a bigger range.

A motor designed for 440V 60Hz can be operated at 98 Hz, without lossing shaft torque. If the motor is connected in 440V delta and supplied with 720V 98Hz.

So you can take much more mechanical power out of your motor.

"Why 720 Volts in stead op 440V ? Probably because in this case you can hold the relation between volts and Hertz constant over a bigger range."

Well, maybe in the books but in reality, higher voltages are used in the industry to save on the wire size. The higher the KW of the load, the higher the voltage used. This usually works by class in order to standardize the equipments.

Most plant will use 600V in Canada for the 50KW to 1MW. Above this we will use 4160V. Below this we use 120/208V.

In the case of a power plant, they often use 690V simply because they have standardize on this and most equipment used in this industry is made for this. 720V is not very far from 690V and can probably be used on the same feeder simply by adjusting the transformer taps.