Depolarized Alternator of VP 2030

William_H said:
cryan said:
On Large alternators polarity can be reversed if current is fed into it in the wrong direction. This should be protected against by the reverse power circuitry but this can fail and I have seen electricians jump start the polarity back to the correct direction with a battery. Incidentally in such occasions the alternator will jump about on its mounts as if it were mechanically unbalanced. I have never seen it happen on small alternators as fitted to yacht or car engines?

What is this all about? Are others too shy to ask? The output polarity of an alternator is dictated by the diodes and their connections. Voltage alternating from +ve to -ve comes out of the actual alternator before the rectifiers. In the case of a true alternator giving AC then polarity is not a question.
If the DC supply to the field coils of any alternator are revered then it will still provide AC with no concern except the Ac will be out of phase. not a concern un;less there are 2 alternators paralleled. So what is Cryan referring too? olewill
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Your right I have got myself confused. Well spotted.
I was referring to issues surrounding a reverse powering of an alternator with a reverse in polarity.
Infact I got so confused that your query forced me to look up exactly what it was I was thinking about and had witnessed a ships electrician do?
This is what I was on about: REVERSE POWER

If, for any reason, one machine is allowed to slow to a point where the other machine is taking all the electrical load, the zero load generator then goes to a negative value or “reverse power.” This generator has now become a motor. This situation is of particular concern where the machine’s protective scheme has not been designed to operate properly in the motoring situation. In such conditions reverse-current relays are usually employed to trip the generator on detection of reverse power flow.

Results of Motorization of a Generator

If a generator loses prime mover power, it acts as a motor with a dc field on the rotor. The dc field will cause the rotor to try and follow the ac field in the same direction as before. If the mechanical drag on the rotor is heavy, it will fall behind and “slip poles,” inducing a large voltage into the rotor; this can cause insulation breakdown of the windings, flashover at the brushes, and violent shaking of the generator mountings.

If a generator loses dc excitation to the rotor it won't generate, but the prime mover power will still turn the rotor. Now the generator acts as a motor running at no load.

The DC excitation can be re-induced with a battery, I think its called flashing?
Apologies.
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Troubadour said:
Certainly on older engines, yes it should. As Vic pointed out they do not have the resistor.
I don't know whether the resistor is there as a backup in case of bulb failure, or whether the bulb does not supply enough current for larger modern alternators.
I suspect it is just a backup.
Was your failed resistor marked as 33 ohms? I'm surprised it is so low. That might make your warning light bulb quite dim (although you might be able to see it in the ambient light here :()
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The charging light of my instrument panel is integrated in what is called "electronic unit" (lights for temperature, oil, and charging)
Thus I think no bulbs there.
We can infere that these vp engines with electronic units should have an excitation resistor.
 
Chanquete said:
The charging light of my instrument panel is integrated in what is called "electronic unit" (lights for temperature, oil, and charging)
Thus I think no bulbs there.
We can infere that these vp engines with electronic units should have an excitation resistor.
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No, they are 2W bulbs that plug into the electronic module. You remove that by two screws at the rear.
Item 11 on http://www.marinepartseurope.com/en/volvo-penta-explodedview-7746410-30-2486.aspx 4 of them plug into item 7.

(Interesting to know the resistor value, my manual does not give it.)
 
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nigelmercier said:
W = I x V
∴ I = W/V

V = I x R
∴ R = V/I

Combine

R = V/I
∴ R = V/(W/V)
R = V²/W

Insert values V=12, W=2

R = 12²/2
R = 144/2 = 72Ω
Nearest E12 value, 68Ω

However, with a bulb you can see it work!
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Yes I do understand Ohm's law thank you; I meant it was interesting to know that the excitation resistor they fit is 33 ohms!!
 
Chanquete said:
I was mistaken. Thank you again trobadour!. My instrument panel is the deluxe version and in its exploded view following your advice I could find the bulbs.
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I have 4 spare bulbs from an old unit, if you can't get one locally I could post you one if yours has failed.
 
Troubadour said:
I have 4 spare bulbs from an old unit, if you can't get one locally I could post you one if yours has failed.
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Thank you Trobadour. I am very grateful for your generous offer.

I tell you; actually the alternator performs impeccably.
I feel so relieved that I accompanied my wife to church.

Yesterday my boat came out of the water on the shipyard of our club.
While the hull is drying under the sun I will sand lightly the bottom putty some irregularities and in 6 weeks repaint with epoxibitumen.
I love this work in the shipyard (varadero) interchanging nautical recipes with colleagues, discussing, teaching and learning.
So we will have our boat ready for january during summer vacations.

Thank you again Trobadour, if you need something from Argentina, including Malbec and beef, I will glad to help!
 
Sadly a bottle of the excellent Malbec is rather harder to post than a tiny bulb.
If you want one just PM me your postal address. Divine intervention is not necessary!
 
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