Only 11 Amps !?

[Sven]

I just looked through the Perkins manual to see what the specs are on the standard alternator that comes with the engine. I was amazed to see that it is only 11 Amps ! I don't know what the power curve looks like but I'd guess that means closer to 7-8 Amps when cruising along at 2000 engine RPMs. Coverted (roughly) to watts that would be 90 which is what one solar panel at noon on a clear day, pointed at the sun, would easily put out.

For reference, I think Symon said he was getting a steady 7 Amps out of his towed generator when sailing GOODONYA from SF to Australia.

I definitely need to do an energy budget to see what we really have and use.

Interesting numbers.



-Sven

 

[Emerald]

That is a little guy. Think the smallest I've seen was 35 amps. Any markings on the alternator itself? I'd be tempted to try to look just it up on it's own and see what it is rated at for the sanity check.

 

[Sven]

David,

That is a little guy. Think the smallest I've seen was 35 amps. Any markings on the alternator itself? I'd be tempted to try to look just it up on it's own and see what it is rated at for the sanity check.

The manual isn't internally consistent either. Under performance data it states Max output 11 A but in the testing section it states it should output 35 & 44 Amps. Need to find out which alternator is actually mounted and get its specs.



-Sven

 

[HughHarv]

11 amps

Wow, that's not much juice. Did the manual state the number of rpm's the alternator was spinning at to product those 11 amperes?

 

[bayhoss]

Low indeed!

The output seems to be very low. The alternator that came with my Universal M12 (far from large) is 55 amps.

Best Always,
Frank
E28 Valinor

 

[Loren Beach]

Some of the older Yanmar stock alternators were said to be only 35 amps, and that's about as low a number as I remember seeing for an inboard diesel.

LB

 

[Glyn Judson]

Yanmar alternator output.

All, Loren's right, my 1988 Yanmar 3GMF is fitted with a 35 amp alternator. Some time later they introduced the 3GM30F, a slightly bigger bore, wet sleeved (mine isn't) engine that upped the output from my 22.5HP to 27HP. Oh, the final change was to up the alternator to 60 amps. My best guess is that the power increase was designed to accommodate the larger alternator. Sven's alternator output sounds somehow fishy, hey, Honda and Yamaha alternators crank out about 10 amps so logic indicates that his is closer to the outputs similar to the Yanmar series of the period. Glyn Judson, E31 hull #55, Marina del Rey, CA

 

[Guy Stevens]

Amps in versus amps out

That is not an alternator or a generator amp output. It sounds like you may be reading about the max draw of the field on the alternator.

The stock on most all of the Perkins engines is either 35 amps or 55 amps.

What is the engine that you have, I may have a manual for the engine that lists what the stock alternator was.

Guy

 

[Sven]

I've attached the section of the Perkins workshop manual that made me question the low amperage. As I mentioned, the testing section of the stock alternator mentions getting 40-50 amps so maybe the max in the section I was reading was referring to the max you would get as the alternator cut in at half it's rated speed ?

Anyway, 40-50 isn't as far from the 65 to 120 that the larger alternators I've been looking at put out so the gain isn't as huge as it seemed at first.

Thanks for the replies and sharing my disbelief



-Sven

 

[HughHarv]

Output

Model C40A (I'm betting 40 amperes at cruising speed) the note above the specs refers to new brushes and low rpm. You can't get maximum output at low rpm nor with new brushes. Excitation voltage and amperage is usually very low, just enough to create the magnetic field.

I've attached the section of the Perkins workshop manual that made me question the low amperage. As I mentioned, the testing section of the stock alternator mentions getting 40-50 amps so maybe the max in the section I was reading was referring to the max you would get as the alternator cut in at half it's rated speed ?

Anyway, 40-50 isn't as far from the 65 to 120 that the larger alternators I've been looking at put out so the gain isn't as huge as it seemed at first.

Thanks for the replies and sharing my disbelief



-Sven

 

[Emerald]

Hi Sven,

I'm going to make somewhat of a guess here, but reading this text makes me think this is a reference type specification as if you were spot testing units during production. It notes they are "production test figures and refer to cold machines with brushes only partly bedded". If the brushes aren't fully bedded, I wouldn't expect full output, and hence a lower number. Any thing indicating what the pulley ratio is and resulting drive speed on the alternator as installed? It would be interesting to see what your actual typical RPM is compared to the specified RPM for this test.

 

[Sven]

Model C40A (I'm betting 40 amperes at cruising speed) the note above the specs refers to new brushes and low rpm. You can't get maximum output at low rpm nor with new brushes. Excitation voltage and amperage is usually very low, just enough to create the magnetic field.

I think the RPM (out of necessity) refers to the alternator, not the engine RPM.

Our alternator is geared close to 3:1, maybe 2:1 which would correspond to not too far from the 1650 number at our max engine RPM of 3500.

I still think the testing numbers of 40-50 Amps is the actual output of the alternator under ideal conditions and I'm convinced that it isn't the 11 A that I first surmised and was surprised by.



-Sven

 

[Emerald]

I think the RPM (out of necessity) refers to the alternator, not the engine RPM.

Our alternator is geared close to 3:1, maybe 2:1 which would correspond to not too far from the 1650 number at our max engine RPM of 3500.

I still think the testing numbers of 40-50 Amps is the actual output of the alternator under ideal conditions and I'm convinced that it isn't the 11 A that I first surmised and was surprised by.



-Sven

Yes, exactly - the rated output broken in and functioning properly is probably 40 amps. The RPM given with the 11 AMP figure is the alternator RPM for this testing before break in. Your final RPM will vary with engine pulley setup etc.

 

[Glyn Judson]

Sizing the alternator to the engine.

Sven, What I didn't mention earlier was that I suspect that Yanmar upped the HP to accommodate the larger alternator. I've read here before that there's a way to fit a larger alternator (presumably for fridge and big screen electronics, etc) that had a cut out switch on it for those occasions when maximum power is desired such as big seas and the like. Is something like a part of your upgrade plans? GLyn

 

[Sven]

Hi Glyn,

Sven, What I didn't mention earlier was that I suspect that Yanmar upped the HP to accommodate the larger alternator. I've read here before that there's a way to fit a larger alternator (presumably for fridge and big screen electronics, etc) that had a cut out switch on it for those occasions when maximum power is desired such as big seas and the like. Is something like a part of your upgrade plans? GLyn

The ballpark figure I've gotten from maxpower (?), Balmer, our mechanic, and basic highschool physics is that each 25 Amps is about 1 HP, taking into account inefficiencies. The actual draw from the alternator when the batteries are charged is actually quite low, but not zero.

Even if we had a 100 amp alternator and fully discharged batteries I don't think the resulting 4 hp power loss is outrageous enough to worry too much about. So, the answer is no, I hadn't thought about the cut-out switch possibility, but I will.

One result of all this digging is that I think we will up our cruising RPMs to 2200+ rather than 2000.

Have you and Marilyn been out lately ? We're dock-bound, or at least sail-less until we get our "stick" back in a few weeks. Originally we'd planned to spend the TG week at the Isthmus but when the rigger said "GO" we ditched that plan and will aim for more time out and about at the end of the year.



-Sven

 

[Guy Stevens]

Ah.....

Ok the c40a is a tractor generator. It was used on chandler combines etc. Indeed it did put out 11 amps max when spinning after starting the engine. This is most likely not the unit that you have fitted to your marine engine.

Here is a photo of a C40A generator. My bet is that yours does not look like that.

These were fitted to the original tractor engines, but were generally not fitted to the marine engines. The Marine workshop manual contains all of the sections from the tractor workshop manual with the additions for the marine components.

Most of the Perkins were fitted with either a Motorola 35 or a 50 amp standard alternator.

Guy

 

[Sven]

Ok the c40a is a tractor generator. It was used on chandler combines etc. Indeed it did put out 11 amps max when spinning after starting the engine. This is most likely not the unit that you have fitted to your marine engine.

I think we have a winner :0305_alar :0305_alar

That finally makes sense from all angles !

Thanks one and all for the fascinating puzzle solving.

Thanks to Guy for the Answer ... now, do you have install pictures from the Cape Horn in the E39 ? Yves said you might, but that would be another thread


-Sven

 

[Guy Stevens]

Not yet

The client with the 39 and the cape horn had an accident on the motorcycle, and we have been delayed on installing it.

Soon I think though.

Guy