Largest motor size that can be started on DG set

Hello Guest,

Ask the makers of the DEG set you hope to use with the motor/s and load you have.

The variations between makes, models of DG set, and Starters a plus motors are huge.

Kind Regards....

The answer is you need to calculate starting Kva of your motor and rating of Generator,and you should know basic load on generator or first step load before you start your first motor on generator.if you can send your e-mail id i shall try and send you more information your inputs are not sufficient to help you.

crm

In fact for this type of purpose you mmust directly discuss with your geneset manufacturer that will chosse the best couple of engine & generator,based on the following issue:

- engine: will be choose in order to have the necessary mechanical torque to drive the generator.

- generator: will be choose in order to have the lowest voltage droop during the starting phase

- and the couple engine/generator: will give the inertie of the rotating part, as higher is the inertia, as lower will be the speed droop during starting (and so the frequency droop).

For this purpose you will have to give the most information you can to the manufacturer in order to do this type of calculation:

Motor Voltage/ Minimum Voltage starting/ starting time/ type of mechanical load to be driven/ power factor/ starting power factor/ nominal current/ starting current/ initial load profile (active /reactive load).

The genset manufacturer could also propose and special starting methode, which is to connect directly the motor to the generator before starting the engine, start the engine, and slowly excitate the generator to reduce the starting current. But for this method, you will have to start only the motor (not other load that will not accept voltage variation), and this starting method must be suitable for your mechanical load.

Ten years ago, I had to start a 15 kW motor for a compressor. But we only had a supply from 3 x 25 A 400V. Impossible to start the motor. Increasing the supply to 3 x 40 Amp was to expensive ( installation cost + annual fees).

I took out the star / delta starting system and replaced it with a soft start device.

Additonally I installed a fixed capacitor to increase the power factor untill almost one.

The compressor still works properly...

So I would advise to compensate the reactive current and slowy start the motor to minimize the inrush current.

I use a 125 hp motor with a home made step starter ( the motor was designed for step starting 8 wire ) using a 100 kw generator .

There are exact formulas but this may give you some theoretical answers for your application.

This motor had little drag on initial start up (some hydraulic motors ) guessing 10-15%

I have pushed it further by starting other motors while running with no Ill affects for years apparently initial start up is critical .

I have seen ( do not recommend ) using pony motor to aid in start up .

It depends where you are and what you need to accomplish, farmers have been known to do creative things that engineers might frown on.

Contact your local Cat dealer. They can provide a piece of software called Specsizer which can give you insight into what you are dealing with.

Beware spending inordinate amounts of time designing in detail. Always put in the biggest set you can reasonably - and even unreasonably - get away with. There is no finer instrument in the world than a genset for producing huge quantities of profanity, the diversity of which is inversely proportional to the electrical output.

Last year I was designing 2 x 1500kVA sets in a telephone exchange in Sydney. While busy in the bowels of the building, another division of the unmentioned comms company ripped out and replaced all the aircon on the roof. So before we even got the first set commissioned, the load profile we meticulously prepared was scrap paper. All new "high-efficiency" VFDs installed. To see why this was a problem read this paper from Trane:

www.trane.com/commercial/uploads/library/549/admapn019en_0106.pdf

Cummins also do a nice range of technical papers:

http://www.cumminspower.com/na/technical/papers/

Happy learning.

First, you need to determine what is the short circuit output of your generator. This can be calculated from the transient reactance (x") of your generator windings. In the absence of these information, determine the rated speed of your gen set. As a rule of thumb high speed gen sets (1200 rpm- 1800 rpm) has low capability to absorb impact load. on the average it ranges from a low of 10% to a max of 15% if the gen set is zero loaded. If the gen set loaded during the imposition of the impact load, and assuming the existing load are 100% motor loads, it can only add a max of 25% of the existing load demand to the maximum impact load capability. Example: gen set rating = 1000 kW, 1800 rpm Max impact load is 15% x 1000 kW = 150 kW...this is the max impact load you an squeeze out of the gen set If the gen set is loaded 250kW, the max impact load the same gen set can now carry is 25% x 250 kW + 15% x 1000 kW = 212.5 kW Also, determine how old is your gen set -- may had been so old that the bracings of he windings are no longer that good -- which maybe stressed when you do an impact load on the gen set...