I want to use a syncronous generator on reduced speed

[mottt]

hi, i know this is not an usual application, but i hope somebody could help me... this is a generator on a PTO output of a propeller gearbox.. onto a vessel, cause of the rise of the petrol, each guy wants to save their money as much as they could.. for this, they think about to reduce the speed of the diesel machine that moves the gearbox and the generator... in the way to reduce the consume....
for this the final speed on the generator (nominal 1500 rpm 50 hz) will be a 20 % less, a frequency of 40 hz, and the voltage will be 250 V ac, and then... if i want the same power, i need to increase the current 20 %.
The vessel got a 150 kw generator and the needed power is 50 kw, how do you see it? , in example, i was thinking in use generator that is 400 V 50 Hz 150 kw cos 0,8, on this way 250 V, 40 Hz 50 kw and use a variable frecuency conversor... to use for the vessel... does somebody knows how much power i have to rate for the electronics transformation? or if i´ll get special problems with it? ... i´m just thinking on this option, maybe i was thinking on stupid things... thanks for your attention

 

[waross]

Try a gear reducer to maintain the same generator speed after the shaft speed is dropped.
If you drop the frequency you may have issues with the Automatic Voltage Regulator. A drop in speed of more than a few percent will put it into Under Frequency Roll Off. UFRO serves two purposes;
1> It sheds some load by dropping the the voltage if the frequency drops.
2> It prevents the voltage regulator from burning up if it is operated at low speeds/frequencies.
If you must run at reduced speed, a custom designed/built AVR may be required.
I have not seen one, but I have heard about hydraulic drives that maintain a constant generator speed under conditions of varying input speed. I hope someone will comment on hydraulic generator drives.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Is there anything else being driven by the diesel engine besides the generator? If all you have on the engine is the generator, and you have a load of 50kW you are consuming the fuel necessary to produce 50kW. Changing the speed won't change that but will cause all sorts of other problems. If you have other things, they are probably also expecting a constant shaft speed. I think you are going to create more problems than you solve.

 

[jraef]

First off though, is slowing the speed of the vessel really going to save fuel? I know there is an optimum speed that all vehicles can run at, but unlike automobiles, that is usually a prime concern in marine vessel design. So it is likely getting the best efficiency it can at the speed it was designed to run at. Altering that may end up ultimately costing more in the long run.