Alternator over voltage

[elessina]

I am concerned that my alternator is charging my batteries at a rather high voltage of 32v (it is a 2 year old, 24v 170amp alternator).

 

This high voltage only occurs when the batteries are fully charged (eg when I leave shore power) together with running the engine at high revs (1,800+). The charge amps are always small, around 5 amps when the voltage is high. The batteries (12 x 2v traction) seem ok, with no sign of water discharge.

 

When the batteries are partly discharged, with alternator running, voltage is ok at around 26 to 28v, and current much greater, around 50a+.

 

All voltage and amp readings taken from my battery monitor system. I do not have an alternator controller.

 

Do I need to be concerned?

 

Ian

DB Elessina

[Sir Nibble]

Morning ellesina,

Do you need to be concerned? well yes, certainly. If your alternator is running unregulated then it will in short order start destroying parts of your electrical system.

All the symptoms you describe lead me to one conclusion, to start with your description of normal running at 28V 50A is perfectly normal, so is a far lower current of about 5A straight after coming off the charger, 32V however is wildly abnormal.

However, the 5A calls the 32V a liar! MOST likely in my opinion is that the 32V is an incorrect reading. I suggest that you beg steal or borrow a decent meter and check the voltage at the battery with that and see how it compares. Don't ignore this.

[elessina]

However, the 5A calls the 32V a liar! MOST likely in my opinion is that the 32V is an incorrect reading. I suggest that you beg steal or borrow a decent meter and check the voltage at the battery with that and see how it compares. Don't ignore this.

Hi Snibble,

I will check battery voltage with my meter over the weekend, but pretty sure battery monitor is correct, in that on 2 instances my Victron combi inverter has cut out (I think it is protected at 32v), when running the engine at high revs and with batteries fully charged. It was this action that prompted me to look for a reason why it cut out, when I realised I had 32v input.

 

I have (I think) an unusual arrangement in that I can turn off the alternator charge. In order to turn the alternator on I have a switch which activates a lamp, to excite the alternator. If I turn the switch off the alternator continues to charge, but when engine revs reduce so not sufficient to produce a charge, it needs exiting again, so switch has to be turned on……. But normally switch is on all the time engine is running (I need the switch to turn off the lamp when engine is not running). Switch and light installed by local marine electrical co. under boat warranty, when I established during installation on house battery bank that the alternator was not working. Never been that sure about it's installation, but that is how it is...... Could this affect / damage the alternator.

 

Ian

[Sir Nibble]

Sounds ok, unusual but not destructive.

Ok, assuming 32V to be correct, measure the voltage from alt output to -ve, and warning light (D+) to -ve. they should be the same or very similar. I would like to know the alternator type, a photo would help.

I think a regulator fault unlikely because the voltage is ok in normal use. A possibility is an insulation fault in the alternator rotor aggravated by centrifugal force.

[frangar]

I had a similar thing recently on a friends boat....when the batteries were low the voltage was within range eg 14.4v (adverc was fitted) however when the batteries reached full charge the voltage went up to 14.9+. I changed the regulator and all was returned to normal, this was easy cos it was on a lucas A127 Frame, depending on Alternator it might be more difficult. I think the reg's suffer on most boat installs due to poor airflow around the unit which are mostly designed to be fitted to a car/truck with a forced airflow from when the vehicle is moving, on a boat they have to rely on the alternator fan. I always tell people to use at least 1200rpm to ensure a airflow. If it was me I would check the reg before anything else. Hope this helps

 

Gareth

[DHutch]

I have (I think) an unusual arrangement in that I can turn off the alternator charge. In order to turn the alternator on I have a switch which activates a lamp, to excite the alternator. If I turn the switch off the alternator continues to charge, but when engine revs reduce so not sufficient to produce a charge, it needs exiting again, so switch has to be turned on……. But normally switch is on all the time engine is running (I need the switch to turn off the lamp when engine is not running). Switch and light installed by local marine electrical co. under boat warranty, when I established during installation on house battery bank that the alternator was not working. Never been that sure about it's installation, but that is how it is...... Could this affect / damage the alternator.

I beleave all alternators have a switch inline with the lamp in the exciter circuit, without it, the alternator would permantly draw current. Usally this switching is done by the ignition switch/barrel.

- As we dont have any ingnition switch, we also have a seperate togle switch to excite the alternator, However, in practice, the alternator acctally retains enought magnatisum to self-excite without ever switching on this switch anyway!

 

Co-incently, we also have a 24v system, and i dont think ive ever seen it go over about 28volts. Mine you, ive also never seen it got over about 8 amps eather!

 

Daniel

Edited June 22, 2006 by dhutch

[elessina]

Sounds ok, unusual but not destructive.

Ok, assuming 32V to be correct, measure the voltage from alt output to -ve, and warning light (D+) to -ve. they should be the same or very similar. I would like to know the alternator type, a photo would help.

I think a regulator fault unlikely because the voltage is ok in normal use. A possibility is an insulation fault in the alternator rotor aggravated by centrifugal force.

The alternator is a Leece Neville 175amp 24v, 110-258, 8SC3009ZA, dated 200318. It was supplied with engine (Perkins) 1 year and 49 weeks ago (engine has 2 year warranty, not sure if this covers alternator, I suspect not!!.).

 

Measured voltage of alternator output to –ve = 31.4v

Terminal 'R' (excite lamp?) = 15.8v

 

Voltages taken on mooring after disconnection of shore power / charger (so batteries fully charged). Engine revs around 2000rpm (very close to max).

 

Battery monitor was around 31.4v (could not see both at same time), indicated charge amps in = 40.

 

The high 40amp surprised me, although I suspect they would reduce very quickly as time progressed, possibly with an increase in voltage, as I could not get 32v. I have only seen low amps, high voltage before. I have not run the engine at high revs on mooring before, only whilst cruising and I did not really want to take direct alternator voltage readings whilst cruising in case loosing balance from unseen wash.

 

Any further help / advise appreciated.

Ian

Elessina

[Sir Nibble]

Righto,

If you will forgive me, I presume a total lack of knowledge so that all can understand.

A rotating electro magnet (rotor) induces current in a stationary winding (stator). The stator output is a 3 phase AC, that is, 3 wires output +ve and -ve in sequence. Connected to each of these 3 wires is a pair of diodes, (diodes can be thought of as electrical one-way valves) one connected so as to pass current to battery +ve when the wire is so polarised, and one to take return current from the battery -ve, and so supply a DC output. In addition to these 6 diodes, a further 3 diodes are connected (one to each phase) to supply a second +ve to supply and thereby magnetise the rotor. In a machine sensed alternator it is the voltage here that is sampled by the regulator in order to control output.

Theory:-

Almost certainly one (or more) of these 3 auxiliary diodes has failed, the result is that in attempting to drive this voltage to 28 the regulator is "full fielding" the rotor resulting in an over voltage output. When the batteries are less well charged the rotor magnetism is insufficient to induce enough current to drive the overvoltage against the load of the batteries, hence the symptoms you describe.

 

Take it to bits, the 3 auxiliary diodes are located on the base of the D+ terminal accessed by splitting the cases.

[elessina]

Take it to bits, the 3 auxiliary diodes are located on the base of the D+ terminal accessed by splitting the cases.

Many thanks for that.

 

Sounds like it may be best if I get a local electrician to come and replace the auxiliary diode, as I am not so sure about taking it to bits myself. Are these diodes standard off the shelf bits, or are they specific to each alternator type? Out of interest how would one know which of the 3 diodes has failed.

 

Ian

Elessina

[Sir Nibble]

The terminal and diodes come as an assembly. I have had a hunt around and found one of these units lying mortally wounded in a corner of the workshop, I will cheerfully send you the part robbed from this if you like. Have a go at the alternator, there's nothing to be frightened of, all you need is a 1/4 drive socket set, a soldering iron and common sense and I am quite happy to help.

[elessina]

The terminal and diodes come as an assembly. I have had a hunt around and found one of these units lying mortally wounded in a corner of the workshop, I will cheerfully send you the part robbed from this if you like. Have a go at the alternator, there's nothing to be frightened of, all you need is a 1/4 drive socket set, a soldering iron and common sense and I am quite happy to help.

Many thanks….. but I spoke to my local marine electrical engineer and he said he would not do it and all he would do is take it to a local Auto electrical shop (ex Lucas) for them to do the job…. I can do that!.

 

Shame we are a hundred odd miles away

 

Ian

[Sir Nibble]

Shame we are a hundred odd miles away

You could always consider sending the unit down to me, I bet it would be cheaper than paying for the repair!

[elessina]

The terminal and diodes come as an assembly. I have had a hunt around and found one of these units lying mortally wounded in a corner of the workshop, I will cheerfully send you the part robbed from this if you like. Have a go at the alternator, there's nothing to be frightened of, all you need is a 1/4 drive socket set, a soldering iron and common sense and I am quite happy to help.

Snibble,

 

I managed to 'work around' the high voltage output on my alternator last year. But now (we are not cruising) is the time to fix it.

Since this post there have been others on getting good amps at relatively low revs, which I have followed with interest.

 

I wonder if I am getting the correct output from my alternator. I have an 8" dia pulley on the engine, with 3" dia pulley on alternator. The 8" dia pulley has 6 slots (2 for this alternator belt, 2 for the travelpower generator and 1 for the engine alternator - 1 spare), so I guess the size is a compromise for all 3.

 

The alternator is a Leece Neville 175amp 24v, 110-258, 8SC3009ZA, dated 200318. From memory now, the most output I have seen is around 100amps at 2,100rpm (my max engine revs), but my concern is that I only get around 20 amps at 1,200 rpm (5mph cruising speed). At 1,000rpm I get maybe 5 amps, below that zero. I am fortunate in that I can supplement this output with my TravelPower generator (through the 240v battery charger), and as I have a trolling valve I can do up to 1,200rpm (to get max amps output) and cruise at less than 1 mph, but this is not very efficient!!!.

 

Am I getting the right output, given the pulley diameter ratio?

Is there anything that can be done to the alternator to improve output at lower revs?

 

With it not being in use, I wonder if your offer to repair is still open?

 

Ian

Elessina

[Sir Nibble]

Righto! I've taken the time to bring myself up to speed on this thread again, and to look at the parts I have to confirm that I can modify your alternator. I can.

I shall get back to you asap, possibly by pm.

[Sir Nibble]

Little point in paying too much attention to your output at various speeds if, as I suspect your excitation is crippled by a diode failure. Let's get the beast healthy again before deciding if it is or isn't worth mucking about to get more urge at lower speeds. I have looked out a healthy 2nd hand diode / terminal assembly, should I send it to you? or will you send me the whole shooting match?

[elessina]

Little point in paying too much attention to your output at various speeds if, as I suspect your excitation is crippled by a diode failure. Let's get the beast healthy again before deciding if it is or isn't worth mucking about to get more urge at lower speeds. I have looked out a healthy 2nd hand diode / terminal assembly, should I send it to you? or will you send me the whole shooting match?

Snibble,

 

Many thanks. I am tempted to have a go myself, but not done much soldering. What kind of soldering iron would I need? Any type or any specific?.... I guess a good quality crimp would not suffice?.

 

Will PM you for further info.

 

Ian

[elessina]

Snibble,

 

Your diagnostic skills are spot on.

 

The diodes were indeed broken and with new diodes fitted I have no over voltage and my provisional finding are that more amps are going into the batteries, starting at lower revs (little more than tick-over now gives a charge). I have not done any cruising yet with new diodes, so not 100% sure of the extra power, but it looks good.

 

Broken diode photo here diodes

 

With many thanks

Ian

Elessina

[DHutch]

Yeah, there broken....

- Glad you got it fixed Ian.

 

 

Daniel

[davem399]

Snibble,

 

Your diagnostic skills are spot on.

 

The diodes were indeed broken and with new diodes fitted I have no over voltage and my provisional finding are that more amps are going into the batteries, starting at lower revs (little more than tick-over now gives a charge). I have not done any cruising yet with new diodes, so not 100% sure of the extra power, but it looks good.

 

Broken diode photo here diodes

 

With many thanks

Ian

Elessina

 

Certainly look like dead-odes!

[elessina]

Yes, broken indeed.

 

And yet, had we been limited to canal cruising (at 4mph) I do not think I would ever have known that something was wrong with the alternator. We only had high voltage at highish revs (when river cruising at 6 mph against tide) and although always felt amps were a little on the low side at low revs thought this was more down to low alternator revs through under sized belt / pulley ratio design.

 

Ian

[Gibbo]

Yes, broken indeed.

 

And yet, had we been limited to canal cruising (at 4mph) I do not think I would ever have known that something was wrong with the alternator. We only had high voltage at highish revs (when river cruising at 6 mph against tide) and although always felt amps were a little on the low side at low revs thought this was more down to low alternator revs through under sized belt / pulley ratio design.

 

Ian

 

I must say I'm impressed with Snibble for his "diagnosis via font" on this one.

 

Buy that man a beer.

 

Gibbo