Minimum Generator Rating to Sustain a Motor Soft Starter

Motor soft starter - Wikipedia, the free encyclopedia

ABB soft starts‎

It totally depends on the nature of the load, because the inertia in the load dictates the required current level and/or the acceleration time. That then becomes part of the calculations with the generator supplier in the generator's transient overload capability. Also, the type of excitation in the generator can have an effect, Permanent Magnet Generator (PMG) excitation is much more stable in terms of voltage drop, but is more expensive so it is not always used in "low-bidder" standby generators if not expressly specified.

All that said, if it is a SINGLE centrifugal pump you can usually get away with a quick "rule of thumb" by sizing the generator at 1.5 x the motor kW as generator kVA. So if you have a 100kW motor you will need a 150kVA generator. Once you have more loads or a high inertia start, it gets more complicated.

This is an excellent article on sizing generators, especially on how to size one for multiple motors and combination loads.

http://ecmweb.com/basics/sizing-gen-sets-large-motor-starting

In fact I have a single motor attached to each generator so I might as well use the rule of thumb you mentioned. However, is this rule dependent on the excitation method? Meaning, if the generator is self excited, do I need to increase the generator rating?

The excitation method affects the voltage droop and recovery rate. If you don't have PMG excitation, you run a greater risk that your output voltage under the starting load will droop below the control circuit threshold of the starter, in which case it shuts down before it finishes. You can also have an issue wherein the generator slows down enough to let the frequency drop, then some soft starters, especially those using phase lock loop zero cross sensing, will misfire, which then adds load to the generator because of the resulting high DC component in the motor windings, and it all cascades down to a failure. Over time many soft starter mfrs have addressed this, but not all. If you already purchased it and tell me what it is, I might be able to give you a heads up on that.

You should probably consult the generator manufacturer, providing a full description of your loads and possible sequences of starting. There are rules of thumb for this, but the regulator type does matter, along with other variables.