What is load acceptance?
Load acceptance is a defined test in ISO 8528-12 and is for diesel generators the maximum load that can be applied before the speed drops by more than 10%. For diesel generators with synchronous generators at 50Hz this is normally the point at which the under speed protection also activates (often for both the engine and alternator) due to the fact the alternator will struggle to produce voltage above this speed drop.
Load Acceptance on a Diesel generator
Typical load acceptance on a diesel generator is in the range of 50-80%. There may be cases where it is higher or lower than this figure, for very small or very large equipment.
It should be noted that engine manufacturers give the load acceptance ratings at a given point when then engine is already warm. A cold engine will perform worse that the given data. The engine should therefore have heaters and/or glow plugs fitted to assist in starting and load acceptance. Running the engine for several minutes to get the oil and block warm prior to accepting the first load step is also a solution.
Why is it important?
Load acceptance is important because the end user may not realise that 100% load acceptance is not realistic in most cases. If for example the user buys a 1000kVa machine, 800kWE, he would only be able to apply say 70% of this 800kWE, which would give him 560kWe in one step, not the full 800kWe.
Can I get a higher load acceptance?
It isn't normally possible to get a better load acceptance from a given engine, because the manufacturer will of already optimised the unit for the best possible acceptance given the other limiting factors within the engine design. Heating the engine with heaters or running it will make sure you get to he rated load acceptance figure for the engine when testing.
What if my customer wants a higher load acceptance? The specification says I need 100%!
The best thing to do is to discuss this with the customer prior to signing the contract. If they insist on a higher load acceptance, then a larger engine would need to be used. For example, customer wishes to buy a 100kVA generator, but they insist on 100% load acceptance. The normal engine we would use to give 80kWe (after alternator efficiency conversion) has a load acceptance of 75%, so this is not going to be able to take 100% of the customers load.
The next size up is an engine that would deliver 112kWe (after alternator efficiency conversion) and has a load acceptance also of 75%. This would therefore deliver 84kWe in one step and would meet the customers requirements.
However, as with most things, there is a trade off. The bigger engine will cost more, it may consume more fuel at its normal running rating and it may be physically larger, increasing the size of the unit and the canopy (if fitted).
We therefore recommend that you discuss these trade-offs with the client to ensure they get the right unit for their needs.