alternator performing poorly

[Wil]

Hi. I'm trying to troubleshoot a problem with the alternator on my narrowboat

 

When the engine is idling the voltage at the alternator is low, around 12.5 - 12.7. Afaik it should be something like 13.8 - 14.2. Is this right?

 

Does anyone have a list of things to check to start narrowing this down? I'm not quite sure how the to check for a slipping belt but suspect that's worth a look.

 

 

Let me know what info would be useful. I'm not sure what the alternator is but will check.

 

 

Thanks

 

Will

 

 

 

edit - sorry, realised in my sleepy haze that I might not have been that clear about something. To start with, I'm only assuming that the alternator should be charging properly at idle. At higher rpm, the voltage goes up to around 13.9, and the batteries charge. I've just been told that I should not need to do this

Edited August 29, 2013 by Wil

[Arthur Brown]

You say nothing of the type of engine or alternator, at engine tickover (which could be 40 - 800 engine RPM) what speed is the alternator spinning at?

 

What happens when the engine is at a more usual (for you) running speed?

[Ex- Member]

Hi. I'm trying to troubleshoot a problem with the alternator on my narrowboat

 

When the engine is idling the voltage at the alternator is low, around 12.5 - 12.7. Afaik it should be something like 13.8 - 14.2. Is this right?

 

Does anyone have a list of things to check to start narrowing this down? I'm not quite sure how the to check for a slipping belt but suspect that's worth a look.

 

 

Let me know what info would be useful. I'm not sure what the alternator is but will check.

 

 

Thanks

 

Will

 

 

 

edit - sorry, realised in my sleepy haze that I might not have been that clear about something. To start with, I'm only assuming that the alternator should be charging properly at idle. At higher rpm, the voltage goes up to around 13.9, and the batteries charge. I've just been told that I should not need to do this

 

Some alternators only begin charging when they reach so many revvs. Mine does this, If I forget to rev the engine when starting up, it will run without charging, the Red light on the engine control panel stays on. Increase the revs and it kicks in and still charges when back in idle.

 

Most boat engines have ill fitted alternators anyway and don't rev enough to put out any decent amperage. We had ours pullied up to near 5 to 1 ratio so when cruising as say 1000 RPM the alternator is spinning at near 5000 RPM At this speed most alternators are putting out something near their maximum rated amps output.

 

If a belt is slipping usually some kind of noise will be apparent, Black dust amd or the smell of burning rubber LOL

 

Set your engine revs at 1000 RPM and take a reading. If there's a low amps output it's likely your engine pulley is only around twice that of your alternator pulley. or a 2 to 1 ratio.

[by'eck]

Good example of what Julynian is talking about in this video about 1:20 in. Its a chicken and egg situation until the alternator can provide itself with enough field current to induce charge current into its stator, after which it self maintains this state

 

[Wil]

Hi, thanks for the responses.

 

 

I'm aware of the need to rev the engine and do this just after I start the engine. The red light goes out and stays out until the engine stops.

 

I haven't been home since yesterday so still haven't checked the engine/alternator model. I think the engine is made by Perkins.

 

How can I check the rpm of the engine and alternator? There are no dials or gauges for this.

 

The person who was on the boat before me claims the batteries charged fine at idle which is what made me think it may have been a slipping belt.

[nicknorman]

Do you mean there is no rev counter at all? That is unusual. If there were a rev counter, you could measure the diameter of the engine pulley and the alternator pully and thus work out the gear ratio between the two, allowing you to know what the alternator rpm is for a given engine rpm. There are other ways to measure engine rpm but require equipment, such as a stroboscopic rpm measuring device.

 

Can you give us a bit more of a clue when the voltage does start to rise - engine just above idle, or engine screaming etc. You say the alternator light remains out at idle, so I would expect it to be on the cusp of charging at that point and to only require slightly more revs before the voltage rises to a reasonable charging voltage of 13.5+ to 14 point something (depending on the state of charge of the batteries).

 

Check the belt just by seeing how much you can deflect it with your fingers in the middle of the belt run, between the pulleys (engine stopped of course!). Other possibilities are worn brushes on the alternator, or fautly diodes or regulator, or a bad connection in the wiring somewhere, but these are more difficult to identify and fix.

Edited August 30, 2013 by nicknorman

[BEngo]

To check the belt tension either put a spanner or allen key as appropriate on the alternator pulley end and try to turn it. ( Engine stop pulled out.) If the alternator turns but the engine doesn't the belt needs to be tightened. If the engine turns until it hits compression then stops the tension is probably OK.

 

 

Look at the belt. If it has a ridge in the part that runs in the groove it's been slipping and is scrap. If you can see 'rope' like bits in the belt its worn out too. The belt should sit about 1-2 mm proud of the groove in the pulleys. If it sits right down in the groove it's worn and needs to be replaced.

 

N

[by'eck]

Modern engines idle around 800 rpm. Most alternators don't start producing close to their maximum current until 3000 rpm at least.

 

This pans out as a requirement to run the engine (when boat not under way) at above idle, say 1200-1500 rpm on a modern one, plus having ideally a 3:1 pulley ratio to increase the alternator revs pro-rata.

 

The higher engine speed when boat is stationary and just driving the alternator, will help load the engine which is beneficial to its longevity.

[nb Innisfree]

Be aware that some larger alts can overload an idling engine at full output, our large frame 24v 100 amp Leece Neville is pulleyed up at a 2:1 ratio, our LPWS4 needs to run at 1200 rpm to comfortably drive alt at full output.

 

Alternator performance graph indicates 10 foot pounds and over 7bhp to produce 100amps @24v.

[smileypete]

As others say, much lower than rated current output at rated RPM into flat batts once the light has gone out, may be damaged rectifiers, a slipping belt, bad brushes, malfunctioning regulator, burnt stator, broken connection somewhere. Most likely to be either of the first two though.

 

cheers, Pete.

~smpt~

[Wil]

Thanks. I should have some time to try these suggestions out over the next few days and I'll report back.

 

I did think I could probably make a nice digital rev counter using an led and ldr + microcontroller

Edited September 2, 2013 by Wil

[nicknorman]

Ldr = slow response time, better with a photodiode /phototransistor. But easier to use the alternator W terminal (if it has one) on which you'll find an AC voltage whose frequency is proportional to the alternator, and hence the engine (after allowing for belt pulley ratios) rpm. That's how most boat rev counters work.

[Wil]

Ldr = slow response time, better with a photodiode /phototransistor. But easier to use the alternator W terminal (if it has one) on which you'll find an AC voltage whose frequency is proportional to the alternator, and hence the engine (after allowing for belt pulley ratios) rpm. That's how most boat rev counters work.

 

Right yes, it did occur to me that I could use two matched leds, one as an emitter, the other as a photodiode. Didn't realise ldrs had slow response time.

 

Interesting about the ac waveform! What is the w terminal for on an alternator?

[Tony Brooks]

First of all its half wave rectified DC.

 

It just produces a series of speed dependant pulses that a rev-counter can count over time and convert to a reading on the dial. It may also be used to drive things like a split charge relay.

[nicknorman]

What is the w terminal for on an alternator?

Primarily for running a rev counter.

[Wil]

Primarily for running a rev counter.

 

[by'eck]

First of all its half wave rectified DC.

 

It just produces a series of speed dependant pulses that a rev-counter can count over time and convert to a reading on the dial. It may also be used to drive things like a split charge relay.

 

Hi Tony - can I ask if all "W" terminals on automotive alternators are fed with half wave rectified AC from one of the stator phases?

 

Have Googled this issue and can't find a definitive answer other than the W terminal is the phase terminal.

 

Many seem to be of the opinion that its unrectified AC which was my understanding, but I am happy to be corrected.

[Tony Brooks]

As far as I am aware they all produce half wave rectified DC. However having said that I have never had a scope on one. I am almost sure I have measured the voltage on a DC scale and got 6v+. To me this would indicate half wave rectification because if it was AC, and ignoring inaccuracies caused by the meter sampling rate, I would have expected close to zero each time.

 

If you tap a phase with a single wire you still have the main negative diode associated with that phase in the "return" circuit so 1 diode = half wave rectified.

[by'eck]

As far as I am aware they all produce half wave rectified DC. However having said that I have never had a scope on one. I am almost sure I have measured the voltage on a DC scale and got 6v+. To me this would indicate half wave rectification because if it was AC, and ignoring inaccuracies caused by the meter sampling rate, I would have expected close to zero each time.

 

If you tap a phase with a single wire you still have the main negative diode associated with that phase in the "return" circuit so 1 diode = half wave rectified.

 

Many thanks Tony. Your last sentence explains it all.