Running engine with alternator not connected electrically

[Avocet]

Will I do an alternator any damage by running an engine with the alternator not connected to a battery? (i.e. no heavy cables to the alternator and no power to the field windings). Don't want to run it for very long, just a few tests.

 

[earlybird]

You'd probably get away with it, but for myself, I don't think that I'd do so, bearing in mind the cost of a possible repair.
Although there is no field excitation, I'd worry that slight residual magnetism might excite the unit sufficiently to damage the diodes.
Can't you just remove the drive belt, or at least run a temporary wire between the battery +ve and the output, (not forgetting an earth.)?

 

[mtb]

it will be fine, no power in no power out

 

[Sandyman]

it will be fine, no power in no power out

Agreed. Done it many times over the years & never a problem.

 

[Beyondhelp]

Its worth a little caution. If the field windings are disconnected then totally fine, but alternator outputs are surprisingly unstable, without any output smoothing enough so that they could in theory blow a regulator.

 

[lw395]

it will be fine, no power in no power out

Here endeth the warranty!

I disagree on both points.
There is mechanical power in.
And no power out, or more to the point no current.
So any electrical power that's trying to get out, zero current means unlimited volts.

So damage is possible.
It might be unlikely, but when the alternator fails in the middle of next season, who are you going to blame?

I think modern regulators are probably more robust than older ones, but running with no load is just not in the 'wish list' for vehicle alternators, let alone a design 'requirement'.

It might be fair to say it's probably OK, but is 'probably' good enough?

For the sake of running two wires.

 

[Birdseye]

Will I do an alternator any damage by running an engine with the alternator not connected to a battery? (i.e. no heavy cables to the alternator and no power to the field windings). Don't want to run it for very long, just a few tests.

no problem. If there is no connection to the field windings there will be no leccy produced and therefore no problem. I had to do this after my regulator failed open ( ie battery volts went up to 18!) on passage from plymouth to falmout in windless conditions and there was no damage done. New regulator and everything back to normal.

without current flowing through the field windings, there is no magnetic field in the alternator and therefore no electricity can be produced by the stator. No leccy produced and there is nothing to fry the diodes. Just make sure that all loose wires are well taped up.

 

[lw395]

no problem. If there is no connection to the field windings there will be no leccy produced....l.

Unless there is some residual magnetism in the rotor.
Which is not unlikely, as it's made of iron and has had a magnetising coil around it.

 

[Birdseye]

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

 

[VicS]

Will I do an alternator any damage by running an engine with the alternator not connected to a battery? (i.e. no heavy cables to the alternator and no power to the field windings). Don't want to run it for very long, just a few tests.

No problem.

Its disconnecting the alternator output from the battery while the engine is running and the alternator generating that is likely to cause a voltage surge and blow the rectifier and/ or the regulator.

 

[lw395]

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

The 'alternating' field follows the rotor around?
Old style British motorbike dynamos relied on residual field (remanance?) to start, having no 'ignition light'.
It probably is a small effect, but I seem to have heard of far more alternator regs failing on boats than on cars.

I can't see it's a good idea to go around encouraging others to chance, even if we've got away with it ourselves.....

 

[richardabeattie]

If the drive belt is off the alternator won't turn. So I assume it can't be damaged?

 

[lw395]

If the drive belt is off the alternator won't turn. So I assume it can't be damaged?

But the water pump won't work either of course, assuming it's on the same belt.
OK for a minute or two?

 

[PCUK]

It will be fine. Note that no-one has come up with an actual incident to report, just ill advised conjecture.

 

[Avocet]

Thanks all. I should have said, it's not a boat engine. I'm coupling a big (180A) car alternator to a Honda engine to make a generator I was just wondering if I'd be OK to run it to check belt run and tensioner before I add a battery and excitation wire. As has been said though, they're no big deal to add, just wanted to get the mechanical side of it running before I did the wiring, but it is only a couple of wires I guess. What sort of current would run down the excitation wire? I guess I'd need a switch in it or the battery will go flat when it's not used? Should I also put a fuse in it, and if so, what rating for an alternator that size?

 

[VicS]

Thanks all. I should have said, it's not a boat engine. I'm coupling a big (180A) car alternator to a Honda engine to make a generator I was just wondering if I'd be OK to run it to check belt run and tensioner before I add a battery and excitation wire. As has been said though, they're no big deal to add, just wanted to get the mechanical side of it running before I did the wiring, but it is only a couple of wires I guess. What sort of current would run down the excitation wire? I guess I'd need a switch in it or the battery will go flat when it's not used? Should I also put a fuse in it, and if so, what rating for an alternator that size?

When running an alternator is usually self exciting via the field diodes and the regulator.

The excitation wire via the ignition switch and the warning light usually provides a small current ( limited by the warning light) to "get it going".

Id therefore fit a switch and a bulb. Then the fuse, if you bother with, it only has to pass the bulb current. If you dont fit a bulb then I think you will need something like a 50 ohm resistance in the circuit.

 

[lw395]

It will be fine. Note that no-one has come up with an actual incident to report, just ill advised conjecture.

And your knowledge of electronics is......?

 

[lw395]

Thanks all. I should have said, it's not a boat engine. I'm coupling a big (180A) car alternator to a Honda engine to make a generator I was just wondering if I'd be OK to run it to check belt run and tensioner before I add a battery and excitation wire. As has been said though, they're no big deal to add, just wanted to get the mechanical side of it running before I did the wiring, but it is only a couple of wires I guess. What sort of current would run down the excitation wire? I guess I'd need a switch in it or the battery will go flat when it's not used? Should I also put a fuse in it, and if so, what rating for an alternator that size?

Although not much current runs down the wire, it ideally wants to work with a low voltage. A bulb is ideal because it's non-linear.
Normally 1 or 2W bulbs are used, so about 100mA roughly. A 1A fuse might be an idea.
You might actually find soft-starting the alternator helps in your application, to let the engine start, before loading it with the alternator?
So leaving the bulb out, or having a bigger resistor instead might work?
Or as you say, have a switch and not energise the alt until the engine is at working rpm.

 

[Avocet]

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.

 

[mtb]

It will be fine. Note that no-one has come up with an actual incident to report, just ill advised conjecture.

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