Thanks. One question. What is a 'crash transfer' ?

'...most gensets, unless specially designed with larger flywheels (which slows start up time) will stall if there is a load step of more than perhaps 1/3 to 1/2 of the full load...'

Would you mind explaining this in simpler words please? 

What does stall means? Stop?

Also what is a load step?

Sorry for not being able to follow. 

What does stall means? Stop?

Yup. Just like a car (non-electric) if you let the clutch out too quickly and/or with too low revs.

   - Andy.

Hi  there, of course, no probs.

Basically a genset is a lorry engine and a big 3 phase alternator in a box. When you add load to the alternator, the force required to turn it rises - if you do this too fast, like switching on a large load all at once, rather than ramping up and giving time for the speed regulator to respond, then  the motor control part does not have time to increase the fuel rate, and the engine splutters and the volts and frequency fall away and the machine may well stop rotating all together - at which point it all stops dead, and the engine needs re-starting. 

Much like a learner driver, bringing the clutch up too fast without adding enough accelerator to compensate....

In general the worst case is a machine idling almost unloaded - then adding about 30% of the max load all in one switch-on event - the "maximum load step", is a typical  upper limit on a smaller machine, but chat to the genset maker for the exact performance of any given arrangement.

A machine with a larger flywheel has the inertia to be able to ride through more violent load changes without losing speed, but it is also harder, or at least slower. to get up to speed in the first place so when the genset has to cut-in there a longer blackout,

Mind you there are some odd fault cases - a dead short accross a genset in the high power state has a much higher fault current, than the same short when the genset is lightly loaded.

Also, though not in a modern set, I have seen a wiring error that managed to shear a safety pin in the shaft between motor and alternator, leading to crawling into  the box to hammer in a new brass pin. The pin being designed to fail first if a dead short in effect locks the rotor, and the engine wants to keep going - much like the shear pin between a boat engine and the prop. designed to save the engine if the prop runs aground or something

Mike.

Nicely put, except that I am told that I must not dilly-dally when letting out the clutch of my R-R 'cos it will not stall, so perhaps the analogy does not always hold true.

I don't think that gensets have a clutch like an automobile.

no clutch in a genset of course,   but sudden steps in load cause a sudden change in the shaft torque required, needing a change to the fuel supply. to maintain frequency, and  the genset speed controller has a similar problem to that of the learner driver kangaroo-ing down the road of not predicting that correctly.

Mike

I've never driven a roller, mind you stalling a tractor is pretty hard too, unless its towing, but if you bring the clutch up sharply, the wheels rotate all right, but on soft ground  the effect can sometimes be more like that of a paddle steamer in throwing out behind you great clods of earth that were previously underneath you...

Mike