Alternator Output VS Engine Speed

When it comes to electrical systems aboard boats, Arild Jensen knows his stuff. Here's where to ask the simplest or most complex questions about marine electrics.

Alternator Output VS Engine Speed

Postby boatguy1 » Sat Nov 07, 2009 4:35 pm

Arild, Sir can you tell me how to find or what is the Alternator out put curve for a GM 85 Amp Alternator, I have twin Ford 120 Lehmans, with small frame GM 85 Amp alternators,
I can figure the Alternator speed off of the engine RPM, I just need to know their optimum performance curve. What is the best RPM to run the alternators at?
Ken MV Genesis
Posts: 8
Joined: Sun May 24, 2009 9:03 am

Re: Alternator Output VS Engine Speed

Postby elnav » Sun Nov 15, 2009 11:44 pm

Using any typical curve rescale the vertical output graph so 85 amps is at the peakof the curve. Typically max output of any of the small or medium framse sizes occur with a rotor spindle speed of 5000 RPM. Low cut-in remains at around 1000 RPM. My guess is a 2:1 or 2.5:1 pulley ratio is likely used. Most common alternator pulley size is a 2.9" diameter pylley and the samallest practical size is 2.7" anything smaller is too tight and will cause excessive belt heat build up.
Arild Jensen
Custom electrical design for cruising boats
Telephone: 250 998 4474
Posts: 65
Joined: Fri Apr 10, 2009 4:54 pm

Return to Electrical Systems

Who is online

Users browsing this forum: No registered users and 1 guest