Alternator running unconnected.

[JohnSteven]

Hi

I have just got my old BMC engine back after some work was carried out on it and I intend to run it up in my garage prior to it being fitted back in my boat. I do not want to disconnect the fanbelt to the alternator as I want to run the water pump. Will I harm the alternator in any way if I have it turning but not connected?

Regards

John

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[andyball]

No problem there.

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[john_morris_uk]

I disagree. Depending on the make and regulator, alternators can destroy their diodes if run open circuit. At least connect the main b+ terminal to a 12v battery (assuming its a 12v alternator of course!!) Its not hard to wire the battery sense/indicator terminal as well. Better and a lot cheaper than rebuilding the alternator.

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[toad]

It is possible to damage it but unlikely,especially if you keep the revs down.

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[claudio]

Just connect it up. Why take the risk ?

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[muchy_]

Thats what I would do too. Connect it up. I always thought that you would fry the alternator if it were run disconnected.

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[jollyjacktar]

If you have power passing through the field coils, then the alternator will generate electricity with all the problems of open circuits etc. Ergo, no power through the field coils, then no electricity and thus no problems. Un unconnected alternator is just some useless bits of metal thrashing about aimlessly, so no problems. Incidentally, how do you intend to start it, with a battery? If so why not connect everything up as suggested by other threads, and check the whole system, electrics included.

Good luck

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[MainlySteam]

I do not know much about alternators but I tend to agree with Andy purely on the basis of observation.

It would seem to me that if alternators could not be run into an open circuit then there would be a lot of failed alternators around as everytime we had a sick battery which did not accept charge (ie has a very high internal resistance so essentially an open circuit) we would be frying our alternators. That does not seem to be a recognised problem. However, I have seen advice in some documents not to run them open circuit and wonder if that is just to avoid the possibility of someone disconnecting or connecting the load when running rather from any other risk.

I do not know if alternators are still made with delta wound stators, but if so I could imagine circulating currents perhaps being possible in the windings from residual magnetism in the rotor but would find it hard to imagine these could be any more than the currents carried in normal operation.

If anyone has an informed explanation as the reason why alternators can or cannot be run into an open circuit I would be grateful and it would probably also authoratively answer the original quaestion.

{edit - thanks JollyJackTar you have gone part way to answering in a parallel post to mine}

John

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by MainlySteam on 30/12/2003 21:54 (server time).</FONT></P>

 

[serini]

If the alternator starts putting out a charge with no sink to accept it (a battery - nackered or not) then you run the risk of blowing the diodes. If the coils are not energised then there is no magnetism and so no charge but some alternators retain a residual magnetism so that they can self-start. If the alternator has not been run for some time then the risk may be low - but why take the risk?

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[l'escargot]

Boatowners Mechanical and Electrical Manual by Nigel Calder, which is recognised as an authorative book, is quite specific:
"Key aspects of installation are... making sure the alternator is never open-circuited when running." He then goes on to say about blowing diodes if the alternator is disconnected whilst running.
You could argue about disconnecting before start up being different from disconnecting while running but I wouldn't risk it. He does know about alternators. Could be an expensive lesson if you follow the advice of someone who starts their post "I do not know much about alternators" instead!

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[supermalc]

Worst case scenerio. Blown diode, will result in alternator rebuild. Kits are still available which include diode and brushes, and bearings. Relatively inexpensive, and then the full knowledge your alternator will be top notch.

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[MainlySteam]

<<<Could be an expensive lesson if you follow the advice of someone who starts their post "I do not know much about alternators" instead>>>

You are unkindly putting false words in my mouth.

If you read my post properly you will see that I was not advising anyone to run an alternator in any particular way at all. I just stated that I did not know of any reason why not, but had seen documentation (including Calder, by the way) which said otherwise. I then asked if anyone could explain.

At least I qualified my post with the extent of my little knowledge on the subject which is not often the case with others! I look forward to your detailed technical explanation in answer to my question.

John

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[l'escargot]

They were a direct quote as indicated by " " , not "false words", and there was no intended unkindness in my post. I also made no claim of personal expertise and as such am not qualified to give a technical explanation, so I won't; but I did quote from what is considered by many as an authorative source. I am sure the poser of the original question can decide for himself which advice he wishes to follow.
Don't take offence, but do think through the consequences to others of well intentioned but uninformed advice. I still wouldn't risk it.

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[chippie]

I too have wondered about this. in my younger days when I had older vehicles I frequently disconnected batteries from the engine and reconnected the flat one once the engine had started. I never experienced an alternator failure. I always made sure the engine ran at no more than a fast idle while doing it , perhaps that helped.

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[MainlySteam]

<<<but do think through the consequences to others of well intentioned but uninformed advice>>>

Perserveringly! - I did not give any advice on the matter. I asked a question giving some background as to why I asked it. Sorry if I cannot get that message through to you. Perhaps you are French l'escargot, in which case I apologise for not being able to do so in your first language.

In any event, I trust others understand. If anyone out there has a technical explanation, I would value it.

John

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[andyball]

I imagine that the argument against it is the possibility of an internally regulated alternator producing a high enough voltage to damage one of the diodes or the regulator or ?? .....not possible with an external reg since no field circuit would exist. With enough residual magnetism the alternator might start producing power....which the internal regulator that's meant to reduce field current at 14.4V 'ish might continue feeding back into the field, not noticing that the voltage had gone high enough to damage something.

Although in the above there would be a load on the alternator consisting of the field coil....& the internal reg wd likely work ok.



Usual "don't disconnect while it's running" warning is still to protect the diodes/regulator- an alternator winding that is carrying high current when suddenly disconnected may produce very high voltages for an instant as the current drops ,in addition to the above- I certainly wouldn't do that.




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[Alex_Blackwood]

I agree with what Andy is saying. In theory an alternator with a machine sensing regulator should "see" an output even if disconnected and regulate the voltage. A battery sensing regulator would "see" no voltage output and the regulator would increase it's output from the field in an attempt to raise the alternator output voltage causing damage to the alternator. As Andy said the most dangerous situation is the collapse of the field when open circuited whilst running.
As a previous post suggested if you are going to use a battery to start the engine then connect up the alternator and check the electrics at the same time. In any case there is no point in risking damage to the alternator just to test the engine!
Contrary to a previous post a "sick" or "flat" battery will been seen as a LOW RESISTANCE.
The internal resistance of a battery rises as the charge increases. i.e. Low resistance = High Current reducing as charge (and resistance) increases.
The following is a quote from an engine manufacturer:
IF BATTERY ISOLATORS ARE FITTED THEY MUST BE CLOSED WHEN ENGINE IS STARTED AND RUNNING. THEY MUST NOT BE OPERATED WHEN ENGINE IS RUNNING AS THIS WILL DAMAGE THE ALTERNATOR.
Hope the above is of some assistance.
Happy New Year to all.


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[SlowlyButSurely]

Our boat has a BMC 1.5 diesel which is fitted with a Lucas alternator. Can't remember the model number, but it's the same as fitted to the Mini/Metro etc.

We spent three weeks cruising one year with the alternator leads disconnected (the plug had inadvertently been pulled out of the socket on the back). The alternator did not suffer at all. When I queried this with the Lucas dealer I was told that the alternator will be fine as long as it is not connected or disconnected while the engine is running.

Happy New Year.



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[MainlySteam]

<<<"flat" battery will been seen as a LOW RESISTANCE>>>

Thanks Alex, I agree that is the case for a flat battery otherwise in good health but I was thinking of a battery which had only a limited amount of active material remaining at the end of its life. I don't know but I assumed that while its internal resistance will still be lower when "flat" than when it is charged, albeit will only accept a small total charge, its internal resistance would be quite high compared to a healthy battery that was "flat" and would therefore only accept charge at a low rate ie close to being an open circuit. As I say, was pure assumption on my part and I have never looked to see what happens current wise when a battery has been at the end of its life active material wise and is on charge from an alternator.

From what you say with respect to a battery sensing regulator then in the case of the original poster's question that if the regulator was not connected to the alternator so that there was no excitation, then the alternator would run disconnected from the battery quite happily (or not?)?

Regards

John

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[andy01842]

IF a flat battery has a low resistance and IF a fully charged battery had a high resistance how dose a starter motor pull 100+ amps at 12volts to turn over the engine.
12volts into a 1ohm resistance = 12 amps. 1ohm is a low resistance!
If you connect 12volts to the field windings of an alternator you would be lucky to get much more than 20volts out. Most alternator diodes can handle about 50volts plus.
If you wont to know how to blow an alternator diode put your fingers across the two 12volt connections on a ignition coil, it only got 12vols going into the coil! But the back EMF from the collapsing magnetic field is well over 100volts and you would not wont to do it twice. VARY PANFULL DON`T TRY IT.


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