Need info on calculating Alternator RPM

[KenEhresman]

Ok, so I made some measurements... the 50DN on my series 60 seems to be 4" and is serpentine driven from the accessory drive pulley on the back of the Series 60 which is 5".

My main crank pulley with the belts to the fan appears to be 8", but not sure that even has any bearings on this conversation but I wanted to add this information anyway. So I guess my question is, what is the accessory drive ratio to the crank rotation? is it 1:1 or some other value?

[Gil_J]

Ken,

The 50DN's rated RPM is 6,500 rpm. The DD-60s maximum RPM is something less than 2,500 RPM without fuel (downhill). That would mean the maximum drive ratio needs to be 2.6:1 or less. The minimum drive ration should be about 2:1 to get the alternator's output near it's low end of it's peak output at cruising speed of 1,500 RPM or greater. The 4 and 5 inch data you provided is only a drive ratio of 1.25:1.

[KenEhresman]

Yep, I measured gain, 5Inch Accessory Drive Pulley and 4" Alternator Pulley, but according to the series 60 docs I can find, there is a 2.41 ratio on the base accessory drive from the engine RPM off the bull gear in the housing... if you take that 2.41 and then use the 1.25 Overdive, then you get about a 3:1 Ratio.... (3.0125)

if accurate, that would push that 50DN plus to a max RPM of about 7,200 RPM if the engine was governed at 2400... does this seem right?

[Gil_J]

Ken,

Great info on the drive ratio of the accessory drive. If I can find my electronic tach, I'll shoot the pulleys and report their ratios.

[Gil_J]

Ken,

Your data and calculations are correct. I just check RPM with a digital meter and read the following:
Main Pulley: 750 RPM
Accessory Drive Pulley: 1809 RPM (2.412:1 of drive speed)
50DN Pulley: 2256 RPM (3.008:1 of drive speed)

I'm not sure why they chose 3:1 given the alternator's maximum speed rating is 6,500 RPM with 7,000 RPM allowed intermittently). This means the alternator is in an over speed condition when the engine reaches 2160 RPM sustained and 2,325 RPM intermittently. Although it's unlikely your engine will see these speeds unless aggressively using the Jake on a steep downhill run, it seems unnecessary to even risk the alternator over speed condition.

If they would have chose a final alternator gear ratio of 2.7:1 then over speed could not have occurred. So, what impact does the lower gear ratio have on alternator output? At fast idle (1,000 RPM) the output would be approximately 240 amps versus approximately 250 amps that it would produce today with the 3:1 ratio. That difference does not seem to make a compelling argument to risk over speed.

[KenEhresman]

Hey Gil, Thanks for validating.... I was pretty sure that data I found on the interweb was correct, but you never know since there can be variations...

I'm sizing the new alternator pulley, so I think since it is a 450A i will size the pulley up to reduce the power output at idle, high idle, and that way I can reduce the risk of both over revving the alternator as well as higher Amp output.... I will run some calculations on the power curve on the emp unit and see based on a 5" pulley on te accessory drive, what is the best option to get decent idle Amperage as well as 350-400 on higher normal cruise 1900-2000 rpm....

Will post back calculations as I get them. Need to see my choices for pulley sizes....
thanks again!

[Gil_J]

Ken,

I need to find the engine's maximum RPM. I know the maximum HP is achieved at 2,100 RPM. I also see a line on the engine's identiplate that I think limits the no load RPM to 2,225. If these are the maximum load and no load numbers then the factory pulleys pretty much match max engine speed with max alternator speed.