Running engine with alternator not connected electrically

[lw395]

plus 1
they read it off the net and spout off

I cant tell you how many times I was called out in the old days to marine building plant or cars that had a knackered battery which will of course not put the 12 or 24 volts into the alternator to excite the field. Many times with building plant the engines being diesel would be started by jumping and run for the day and not once did I need to change an alternator because of a faulty battery even though they may have been used this way for some times weeks before the site agent called to get it fixed

The faulty battery will still stop the alternator from putting out much over 12V.
Did these alternators generate any volts without the battery exciting them?
If so, it kind of proves that alternators can self start, which we knew already.

Why advise other people to take risks with their kit, even if you've got away with it in the past?

Most of what I know about magnetism, motors and so forth comes from a degree education and a career designing stuff by the way.

 

[rob2]

Never mind the sniping and personal insults, please. It can be a very useful area of knowledge to know how to disable the equipment on your boat (or in your house...). It would certainly be safer to put a battery across the output in case the alternator does self excite, but would any other load be sufficient? Likewise if a load were applied across the excitation feed, would that prevent the alternator fromgenerating based on residual magnetism?

I'd prefer argued suggestions, as would the OP, I assume. The whole purpose of asking such questions is to allow one to proceed with the work at hand without needing to experiment with the ancillaries, reading voltages and currents through a test programme...

Rob.

 

[lw395]

A battery is the best load.
The alternator is capable of a lot of current, so any simple load would need to be high power.
We know the alternator reg is designed to work with a battery.
We don't know how it will react to other loads.

If a battery is not an easy option, then maybe I'd take the regulator out of circuit? Then load the output with some high power resistors which I happen to have?

When I was last bench testing an alternator, I spun it up with a power drill.
Maybe an idea to try that if in doubt, as you can build the revs gradually.
But if it self-starts, the current will kick in suddenly.
I've been through this with BSA dynamos.

 

[ghostlymoron]

Lw39,
" Most of what I know about magnetism, motors and so forth comes from a degree education and a career designing stuff by the way."
I'm impressed.

 

[reeac]

Good plan! Yes, I was wondering about that. It's an 8hp Honda 4 stroke with a pull start and I was wondering how hard it would be to start with the load of the belt, the inertia of the pulleys and any alternator drag.

I used to have a Hayter Condor mower which had a Honda 11 bhp. 4 stroke motor which was very easy to start. I believe that they have an automatic decompressing valve which closes as soon as the engine starts to rotate.

 

[DavidGrieves]

Hi Avocet

What tests are you doing? Could you run engine with the alternator belt removed?

David

 

[Avocet]

No, it would be to see how the belt runs between the engine an alternator, and whether the tensioner is all running true, as well as whether I can pull start it easily enough when it's also turning the belt and alternator. (This is me trying to build a generator, it's not actually a boat engine). I've pretty much made the decision to at least connect the alternator to the battery (perhaps without any field coil excitation) just to try all that though. It won't take long to just run those wires.

 

[William_H]

To demagnetise iron you put it in an alternating magnetic field which is exactly what there is inside a working alternator. I m not saying that I couldnt detect any residual magnetic field at any strength back in my university physics lab, but I am sure that it would be trivial in practical terms

Sorry it is not an alternating magnetic field in the field coil. It is a steady or sometimes pulsed DC and so steady magnetic field which happens to rotate. The alternating magnetic field is in the outer bopdy where the stator or pick up coils are.
Now the conjecture seems to be as to whether this iron centre of the field coils can retain magnetism and so self excite.
Back in my aviation days light aircraft usually have a switch for the laternator. (in the field circuit). We mandated that there must be a sign at the switch saying do not switch the alternator off in flight except in an emergency. The reasoning being that if the alternator was switched off in flight the battery would go flat with normal load and the laternator could not be brought back charging because there would not be enough battery power to re excite the alternator. somene thinks there will not be any residual magnetism in the core of the field coil. Again it is conjecture about whether there will be or not.
Interesting the old generators with the field coil on the outside presumably made of ordinary iron did retain magnetisme enough to generate enough voltage to pull ina cut out relay to connect to the battery. Now presumably modern alternators ahve a field coil core (and shaft) made of similar metal but apparently (possibly) they don't retain enough magnetism.

To the OP is the question is there a regulator connected when he tries to run the engine and alternator with no load. Many vehicle alternators have the regulator built in. In which case the residual field could get the alternator up and charging via the regulator which however should regulate the internaly produced field current to produce 14v output.
However there is risk that the regulator may not be able to regulate and output voltage could run high enough to damage diodes or regulator.
If there is not (external) regulator connected then there is not chance of damage as self generated excitation voltage can go no where so no field current.
So to me it is conjecture as to whether the residual magnetism can get the alternator up and running without the aid of the usual ignition light current and further conjecture as to whether the voltage will run high and damage the diodes etc. My guess is that it is worth the risk so just run it but I may be wrong good luck olewill

 

[nedmin]

Not worth the risk. Disconnect the belt. When a battery is dead flat the alternator will start charging this is because of the slight residual magnetism in the alternator.

 

[VicS]

Not worth the risk. Disconnect the belt. When a battery is dead flat the alternator will start charging this is because of the slight residual magnetism in the alternator.

That would rather defeat the OP's aims. He has already explained the reasons twice!

I was just wondering if I'd be OK to run it to check belt run and tensioner before I add a battery

it would be to see how the belt runs between the engine an alternator, and whether the tensioner is all running true, as well as whether I can pull start it easily enough when it's also turning the belt and alternator.

 

[pvb]

No, it would be to see how the belt runs between the engine an alternator, and whether the tensioner is all running true, as well as whether I can pull start it easily enough when it's also turning the belt and alternator. (This is me trying to build a generator, it's not actually a boat engine). I've pretty much made the decision to at least connect the alternator to the battery (perhaps without any field coil excitation) just to try all that though. It won't take long to just run those wires.

Your project sounds intriguing. What sort of belt are you using?

 

[Avocet]

I'm stuck with what the alternator pulley dictates. Being a car alternator (and yes, Will, it's off a Passat and has a built-in regulator - just an ordinary car alternator), it uses the "K" profile Poly V belt, with 6 grooves.

 

[nimbusgb]

Oh for Pete's sake!

An alternator run without any battery connected will be fine. You could run it for a week without harm.

 

[coopec]

I work on the principle that if you don't know what you are doing, don't do it. In a motor car apparently it could be disastrous.



View attachment 47585



http://www.troubleshooters.com/dont_disconnect_battery.htm

 

[duncan99210]

There's a world of difference between disconnecting a load from an alternator producing current, which will cause damage, and spinning a disconnected alternator, which will not produce any electricity and therefore not damage itself.

 

[lw395]

Oh for Pete's sake!

An alternator run without any battery connected will be fine. You could run it for a week without harm.

How many people who assert this have actually ever spun up a car alternator on the bench?
How many have ever looked at the circuit of a regulator?

I've done both, and I'd not guarantee there will be no damage.
Of course not all alternators are the same, and not all regulators are the same, but the time honoured advice not to do it has some basis in fact.

The worst thing about it is that stressing electronics in this kind of way may not bring instant failure, but can shorten its life, so it fails some time later at the most embarrassing time.

Why are people so keen to advise others to take risks with their stuff?
Particularly when some of these people (not nimbusgb particularly) clearly don't have a great grasp of the basics of alternators, let alone the subtleties?

Things like alternators are highly developed to be used in a certain way, then made at minimum cost.
They don't tend to put in components which would make it more reliable when used in a different application circuit.

 

[JumbleDuck]

Oh for Pete's sake!

An alternator run without any battery connected will be fine. You could run it for a week without harm.

I have twice - to my knowledge - run my 1GM10 with the alternator disconnected from the battery. On the first occasion it ran for several hours without any apparent damage to anything. On the second it ran for ten seconds and blew a couple of transistors in my brand new Adverc. The alternator was fine, mind you.

 

[nimbusgb]

How many people who assert this have actually ever spun up a car alternator on the bench?
How many have ever looked at the circuit of a regulator?

I've done both, and I'd not guarantee there will be no damage.
Of course not all alternators are the same, and not all regulators are the same, but the time honoured advice not to do it has some basis in fact.

The worst thing about it is that stressing electronics in this kind of way may not bring instant failure, but can shorten its life, so it fails some time later at the most embarrassing time.

Why are people so keen to advise others to take risks with their stuff?
Particularly when some of these people (not nimbusgb particularly) clearly don't have a great grasp of the basics of alternators, let alone the subtleties?

Things like alternators are highly developed to be used in a certain way, then made at minimum cost.
They don't tend to put in components which would make it more reliable when used in a different application circuit.

I have. Plenty of times.

As the poster above you said. There is a world of difference between running up an alternator with the battery connected and then disconnecting it and doing what the OP wants to do. Turning an alternator without ever connecting a battery to it.

Even if there was a massive amount of residual magnetism in the stator there is no circuit for the voltage to go round, typical alternator diodes are rated at over 100 Volts and any induced potential would be lost.

The 4-108 diesel engine with 120 Amp alternator that used to be on a stand in my garage was run repeatedly without connecting the alternator. No Problem

The engine in my boat was run for a week on and off without the alternator connected ( numpty forgot to connect the wires ) the solar panels and VSR kept the engine start battery well charged. No problem, no damage.

Even a self exciting alternator can be run this way, if there's no load then the system backs itself off.

But never, ever disconnect a lead, positive or negative from the engine of an alternator equipped engine with the engine already running. You stand a good chance of popping alternator diodes AND external equipment due to the very rapid breakdown of magnetic fields.

 

[lw395]

...
Even if there was a massive amount of residual magnetism in the stator there is no circuit for the voltage to go round, typical alternator diodes are rated at over 100 Volts and any induced potential would be lost.

......

That's not actually true is it?
There is a circuit:
stator >aux diodes > regulator > rotor > chassis > chassis side diodes > stator

So the alternator can start, and the regulator may not work quickly enough without the damping/storage effect of the battery.
It may also not regulate down to zero current, it's not a requirement in normal operation.
It will have been designed to work with a battery.
Just because one particular regulator is OK without a battery, proves nothing about any other make of regulator.

 

[Avocet]

Right chaps,

Just an update on this. It all seems to work now (at least under no load). I'm using the boat battery as it's winter and I have it lying around. Before I start the engine, the battery voltage is about 12.7. I switch on the little illuminated switch that puts a positive feed to the alternator via it's lamp, and pull the string. The engine fires up, the lamp goes out and the voltage goes up to about 14.6. That's as much as I can do until I finish paying for Christmas and can afford the inverter! The only odd thing is that if I switch off the feed to the alternator, the voltage stays at 14.6. Does this suggest a degree of self-excitation after all? (Again, this is only for short periods of 10 seconds or so and under no load).