warming up an electrical genarator

Hi,

I can't find appropriate category for my this question. I request you to answer.

why is it necessary to warm up an electrical generator for a few seconds before transferring the load on it? what if we directly put load on the generator without warming it up?

A Quick response will be appreciated alot.

thank you

Qaisar

What type of generator are you planning to use?

If you are planning to employ an internal-combustion-engine powered generator, the reason for warm up is to allow lubricating oil to flow to all of the engine's components, specially the head mounted intake/exhaust valves.
Also depending on the ambient temperature, the oil may take some time to reach its working viscosity.

To sum up......you want the engine parts to be properly lubricated before applying the load.

Thank you.
Yes i am talking about generators whose alternators are rotated with the help of Gas or Diesel Engine.

here another question rises that Electrical part has nothing to do with the mechanical lubrication? what if we put the load on it without warming up?

A loaded engine may not be able to start. Same with a loaded motor.

why is it not so in case of cars? we start them easily even if they are cool. they have same engines as we use in generators?

secondly what is the reason behind that a loaded engine or motor can't start?

In a car you normally don't try to start the engine while in gear. You start it in 'neutral' or 'N' before moving off.
The same applies in a generator, if you try to start it under load, the starter motor has to turn the engine AND drive the load at the same time, it takes a lot more energy to do that.

The reason for waiting a moment is to allow the lubrication oil to reach the bearings and it's viscosity to stabilize. Please bear in mind that most car engines are highly refined machines but most generators are designed for lowest cost which means less precision and more need for lubrication to limit wear.

Brian.

Hi,

Besides the mechanical parts, the regulation of frequency and voltage may take some time to stabilise.

Klaus

Example: my Diesel car manual says the oil should be changed once per year. My Diesel generator manual says once every 100 hours!

Brian.

I guess he needs a quick load transfer from mains power to generator without affecting the load operation (interruption).

I'm affraid it won't be possible without some sort of UPS or on-line inverters (running on battery or a VERY big capacitor - depending on load).

If you think about it, from the moment the mains disappears, first you have to allow at least 5 seconds for the engine to crank over (start), then you have to allow another period of time, say 5 seconds to allow the engine speed to stabilise. Then you can apply your electrical load to the generator. We use 250 KW UPSs to carry the equipment load while the generator was starting. They were good for about 10 minutes.
FWIW Its a good idea to have the fuel valve coming from its own battery, else if the main batteries get old, while cranking the engine, the voltage falls too low for the fuel valve. So the engine just cranks and cranks until the batteries are flat.
Frank

Qaisar Azeemi said:

why is it not so in case of cars? we start them easily even if they are cool. they have same engines as we use in generators?

Click to expand...

One of the reasons is the service factor. On a vehicle's engine it is safe to assume that it will be started every day, or every couple of days. In these instances, a thin oil layer still remains attached to all surfaces, providing enough time for the sump oil to reach them.

On a standby engine, which may be stopped for very long periods, every last drop of oil drains down to the sump. Then, during the first critical seconds, the moving parts are running dry. There is a lot of wear during startup, and you don't want to do further harm by applying a load.

For critical standby generators (say, in a Hospital), the engines use many techniques to become ready faster, including sump and radiator warmers, and in larger engines, oil recirculators.
Even so, there will be a couple of seconds of delay. That is why mission-critical equipment, employ a UPS in addition tho the standby generator.

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An interesting fact related the time standby generators take to stabilize:

The Chernobyl nuclear accident, was caused by the engineers preoccupation with maintaining the reactor core pumps running during an outage, while the standby generators stabilized.
They theorized that the remaining kinetic energy in the massive steam turbines could be tapped to create a "power bridge".
Although the theory may have had some merit, the experiment and its execution were flawed, which caused the largest nuclear disaster ever to occur.