View Single Post
Old 12-21-2011, 04:46 AM   #26
RickB
Scraping Paint
 
City: Fort Lauderdale
Vessel Model: CHB 48 Zodiac YL 4.2
Join Date: Oct 2007
Posts: 3,804
RE: Engine Room Exhaust Fans

Quote:
sunchaser wrote:
An AC generator has little tolerance for RPM variation
*Those days are long gone. Variable speed generators (VSGs) are becoming more common at the low end of power generation. The same technology that lets yachts to use shore power of any flavor and frequency from daylight to DC also allows a turbocharger or a propeller shaft, or even a microturbine to drive a generator at varying speeds.

Jet engines used to use complex constant speed devices to drive generators but now a small box of electronic bits does a far better job much more efficiently. The same technology that Boeing uses to produce electricity on the 787 is available on small boats.

Marin's note of how a conventional marine generator responds to a heavy load doesn't really apply. The typical marine generator operates at full rated speed and is capable of delivering full rated load as quickly as the governor can deliver fuel. The generator follows the load, it can't anticipate it and that fact creates the effects you see and hear.

A variable speed, electronically regulated, generator has an output relative to the speed it is operating. If it is operating at half speed, it can only produce partial power. It only produces as much power as is available at the input shaft when the load is applied. The generator control does not tell the engine to dump fuel in the cylinder to try and maintain a synchronus speed. If the VSG is driven by the propeller shaft and an additional electrical load is applied, the engine will add fuel to maintan shaft rpm but the electrical output follows the amount of power available, it is not driving it.

What this means in real life is that even if the shaft generator is a 50kW unit, you can't always get 50kW from it. You can only take off as much power as the propeller is not using. When the engine rpm (which determines power available) drops below a certain point, the generator can no longer supply enough power to be useful and will go offline. It will not act like a standard generator where a very heavy load can actually overload the engine and even stall it. The generator control only lets the generator convert the amount of power that is available at any given moment.

*
RickB is offline   Reply With Quote