RATTING FOR ALTERNATOR

The size depends on the type of starter you will be using. The minimum capacity of the alternator should be

4 times the motor capacity - for DOL starting

2-3 times the motor capacity - for Soft starter starting

1.4 times the motor capacity - for VFD starting

This sizing is based on the condition that there shall not be any other load connected to the alternator when starting the 90 HP motor. If other additional loads are connected, add the additional loads to the alternator capacity.

See these links for more details:

http://cr4.globalspec.com/thread/45119

http://cr4.globalspec.com/thread/41125

http://cr4.globalspec.com/thread/41210

http://ecmweb.com/training/electrical_basics/electric_sizing_gensets_large/?smte=wr

- MS

20 years back we used to have slip recovery motors whose starting current can be set to 80%.fl without much loss of torque.dont know if they are still in use.

The equivalent KW rating of the motor is 67 KW (app).

The rating of the alternator is to take care of the motor load only as the alternator would be designed to take care of starting current also.

With a cushion the rating may be concluded for 70KW

Hence the rating of the alternator required is 85 KVA app (70/0.85 pf)

The droop characteristics are to be fixed in the governor and AVR perfectly to take care of this starting inrush.

Sivaguru