Output 60amp alternator on Volvo MD2040B

[Tradewinds]

Asking for a friend in Med with poor internet access so cannot do a google etc.

He has a Volvo MD2040B with a standard 60amp alternator as originally supplied on his, new to him, 1997 Oceanis 36cc.

Could someone tell me at what RPM full output theoretically occurs? He thinks it may be 2500 but is not sure.

Also, what max RPM are they rated (ie the RPM the alternator must not exceed)?

TIA /forums/images/graemlins/smile.gif

 

[Alex_Blackwood]

RPM will not have any bearing on the current (amps) output of the alternator. Although you will need the minimum revs required to excite the alternator and produce volts.
The current drawn from the alternator depends on the resistance of the circuit connected. If you have a flat battery the resistance will be low and the alternator current output high. As the battery charges the resistance increases and the current flow will fall off.
Similarly if you have a lot of things connected to your battery, low resistance, the battery current will be high and it will discharge quicker than if you only have one low wattage, higher resistance, device connected.
Hope you can follow that!
Having said all that if you have a high current draw on the alternator you will find that the engine revs drop and you have to increase the fuel to maintain the revs.
This is because amps + volts = watts = horsepower. (you probably wouldn't be aware of the rev drop/fuel increase on a boat or car engine. It is certainly noticeable on large power systems when large loads are applied or shed.)
Afraid I cannot answer the second part of your question with any certainty. However if it is the original alternator for that engine it will cope with any normal running RPM produced by the engine.
Hope you can make sense of all that, Not sure I can!

 

[VicS]

I think this is information that you will only get, and then only if you are lucky, from the alternator manufacturers.

FWIW the maximum speed quoted for some of the range of alternators fitted to my (Volvo) car are 13500 and 15000 rpm. 15000rpm being for a SEV Marchal alternator and 13500 being for Bosch alternators

Some test output figures are also given at 3000rpm. They are:

30A for a 35A SEV alternator at 13V
30A for a 35A Bosch alternator at 13V
48A for a 55A Bosch alternator at 13V
48A for a 55A SEV alternator at 14V
62A for a 70A SEV alternator at 14V
58A for a 70A Bosch alternator at 14V

I assume that is 3000 rpm alternator speed not crankshaft speed, but it is an assumption.

Obviously the output is dependent on speed but I think it rises quite quickly to near the maximum as the speed is increased.

Sorry not much help really. Nor is the workshop manual for the MD2040

It would be interesting to know why the information is required.

 

[Tradewinds]

Hi Vic,

Thanks for the reply.

Haven't got much more background really, all I know is he's having trouble getting 14v to the batteries when the engine is running. He knows it could be one of a few things and this is a piece of information he hasn't got. I know he's got the Volvo manual but, as you say, it doesn't give the information he requires.

Thanks to you & Alex for the answers so far.

 

[Sanderling_]

Had exactly the same problem with the alt on my '97 Moody 36, took all day to charge the batteries, and not very well at that. Ended up splashing out on a Sterling Alt-Bat charger and it has transformed our lives!! It was the best £200 I spent last year and very easy to install.

Tony

 

[VicS]

Presumably he has checked all the connections and checked for volts drop between the alternator and the battery on both the positive and negative sides.

I note that the spec for the alternator is 14.2 ±0.15volts. It would be sensible to check the volts directly between the B+ and B- terminals.

Hopefully he is not trying to charge a battery bank so huge that a 60amp alternator is struggling.

It is of course possible that there is something wrong with the alternator. The regulator could be faulty (I assume the brushes are OK. 5mm minimum length according to the w/shop manual). The rectifier could have some open circuit diodes.

One of the fancy regulators may well be the final answer but the possibility of a fault somewhere must be eliminated or that won't help either.

 

[halcyon]

Alternator normally kick in around 1000 rpm, so with pulley ratio normally charge at tickover, at a very low level. At normal engine rpm for the boat, you should be seeing full output, assumming Volvo have there ratios right.

Check voltage on alternator output stud, is it the same as the battery?

If not you have a volt drop problem, could be blocking diodes for split battery charge, could be a bad connection. Follow charge cables, pos and neg till you come to a big change in voltage, you have your problem.

If alternator is regulating at 14.2 / 14.4 volt it should be ok, if power loss is blocking diodes a VSR will correct most of the problem.

If the output voltage is low, 13.6 volt, get it check for faulty reg, if alternator checks out ok, a smart regulator, or a replacement high voltage reg will be needed.

But first job is to find out what your problem is, otherwise it is how long is a ball of string problem, many possible faults and answers.

Brian