24 volt "Ignition" light

[ditchcrawler]

When I first fitted out our boat I used a second hand insulated return 24 volt alternator from a commercial breakers yard, It was a good sized beast physically and would happily self excite at about 600 rpm engine speed, so I just connected it with two wires -ve & +B

 

When it went to meet its maker in the scrap yard I replaced it with a larger electrically but smaller physically speaking one. This would not self excite so I fitted an "Ignition" light to it, A sealed push in thing that I had in the bits box.

Thinking about it it may have been a 12 volt warning lamp, I cant find any markings on it and its popped. So now I need to replace it which set me thinking.

As its a 24 volt alternator would it need to be a higher wattage than a 12 volt one, I am thinking of the current flow in the windings to get excitation. If so what sort of wattage would be needed? If the one I had was 12 volts then I dont know what sort of impedance the field would be so I dont know how much it was over run by and what its equivalent wattage of the bulb would be.

[Sir Nibble]

The field has no impedance, only resistance. A 3w lamp will do fine.

[ditchcrawler]

The field has no impedance, only resistance. A 3w lamp will do fine.

Thanks for that. What sort of resistance are we talking about, the bulb that has blown has probably been in for over a year, but is only on for seconds at start up and when the engine is shut down before the key is switched off. I am still wondering if it was a 12 volt one that lasted that long.?

[sjc]

The field has no impedance, only resistance. A 3w lamp will do fine.

 

Sorry SirN, but it certainly does, it has inductance and resistance, therefore impedance. It may not be relevant in this case, but it is there.

[Tony Brooks]

 

Sorry SirN, but it certainly does, it has inductance and resistance, therefore impedance. It may not be relevant in this case, but it is there.

 

I doubt that until the regulator starts working, which it will not be on starts up.

 

Brian, watch those plastic push in lamps, some have grain of wheat bulbs in that will not energise many alternators.

[nicknorman]

I doubt that until the regulator starts working, which it will not be on starts up.

 

No, it has inductance and hence impedance whether or not there is an AC component of current flowing through it. Unless you are going all "Schroedinger's cat" on us!

[Graham.m]

The field has no impedance, only resistance. A 3w lamp will do fine.

 

impedance - the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.

Edited December 27, 2015 by Graham.m

[Eeyore]

Hurry up, the Christmas science lecture is about to begin.....

Oh no, it's another repeat; is there nothing to watch at this time of year? ?

 

Belated Chistmas tidings, and a happy new year to all.

 

Steve (Eeyore)

[Graham.m]

Hurry up, the Christmas science lecture is about to begin.....

Oh no, it's another repeat; is there nothing to watch at this time of year?

 

Belated Chistmas tidings, and a happy new year to all.

 

Steve (Eeyore)

lol

[Tony Brooks]

No, it has inductance and hence impedance whether or not there is an AC component of current flowing through it. Unless you are going all "Schroedinger's cat" on us!

 

Well it is as good as a back box so anything could be going on inside but:

 

Agreed it has inductance for a few microseconds as the rotor energise but once energised its a straight DC circuit with no AC component or pulse (as far as the rotor itself is concerned) so inductance does not come into it at this stage.

 

Even as the energised rotor starts to spin it is still a straight DC circuit with no pulses or AC component so still no effective inductance (unless there is slip ring or brush problems).

 

Now as the stator starts to generate thing become more complicated because the fields created by the stator must impinge upon the rotor inducing some sort of AC current in the rotor (in addition to the DC energising current) AND the ripple form the stator via the field diodes add more so at that point there must be inductance.

 

However I till broadly agree with Sir N that for ordinary boaters the rotor should be considered as only having resistance during the initial energisation and that is what the bulb is all about on most alternators.

[bizzard]

I've been ohmically impeded by ohmic impedance lots of times.

[ditchcrawler]

 

I doubt that until the regulator starts working, which it will not be on starts up.

 

Brian, watch those plastic push in lamps, some have grain of wheat bulbs in that will not energise many alternators.

That is what the one that failed had, but I suspect it was only 12 volt so a bit higher on the current than specified. I called in a Maplins today and bought a 24 volt bulb. they only had one and no holder so its soldering iron out again

I would just like to cobble something up so I dont have to crawl over the engine every morning. What would be the maximum current to apply to the alternator please.

[nicknorman]

Well it is as good as a back box so anything could be going on inside but:

 

Agreed it has inductance for a few microseconds as the rotor energise but once energised its a straight DC circuit with no AC component or pulse (as far as the rotor itself is concerned) so inductance does not come into it at this stage.

 

Even as the energised rotor starts to spin it is still a straight DC circuit with no pulses or AC component so still no effective inductance (unless there is slip ring or brush problems).

 

Now as the stator starts to generate thing become more complicated because the fields created by the stator must impinge upon the rotor inducing some sort of AC current in the rotor (in addition to the DC energising current) AND the ripple form the stator via the field diodes add more so at that point there must be inductance.

 

However I till broadly agree with Sir N that for ordinary boaters the rotor should be considered as only having resistance during the initial energisation and that is what the bulb is all about on most alternators.

So you consider that a coil only has inductance once some alternating or at least varying current starts flowing in it?

 

So all those coils you can buy with labels saying X Henrys are wrong, since in the shop there is nothing connected to them! Hmmmmmmmmmm,

Edited December 27, 2015 by nicknorman

[WotEver]

I think that what Tony & Snib were saying was that in the context of the exciter current through a warning lamp you can ignore impedance/inductance because it is a purely DC circuit.

[ditchcrawler]

I know what they are talking about which is why I said impedance, if I had said resistance someone would have come up with coils, magnets etc.

 

I have fitted the Maplin 3 Watt lamp (bulbs you set in the garden) and its too good so I may try a 20 Watt in series because I have some. Now as soon as the engine fires its charging. I like to start on tick over, couple of minutes circulating the oil and then put on the load. The one that fused was as Tony suggested and just a grain of wheat type, but did the job I wanted perfectly, I just don't know if it was 12 or 24 volt as it had no markings on it.

[DHutch]

I can't tell you what the bulb is ours is, but it sounds like an of the shelf bulb will do.

 

If in doubt and you want to double check, ring David Edge in Eaton Cheshire in the morning!

 

 

Daniel

[Iain_S]

(snip)

I have fitted the Maplin 3 Watt lamp (bulbs you set in the garden) and its too good so I may try a 20 Watt in series because I have some. Now as soon as the engine fires its charging. I like to start on tick over, couple of minutes circulating the oil and then put on the load. (snip)

 

If what you are trying to achieve is engine start, run for a couple of minutes, then start alternator; how about a switch in series with the bulb?

[Tony Brooks]

So you consider that a coil only has inductance once some alternating or at least varying current starts flowing in it?

 

So all those coils you can buy with labels saying X Henrys are wrong, since in the shop there is nothing connected to them! Hmmmmmmmmmm,

 

 

No - did you not read this part? Agreed it has inductance for a few microseconds as the rotor energise but once energised its a straight DC circuit with no AC component or pulse (as far as the rotor itself is concerned) so inductance does not come into it at this stage.

[ditchcrawler]

 

If what you are trying to achieve is engine start, run for a couple of minutes, then start alternator; how about a switch in series with the bulb?

I might just as well do away with the relay and have a push button as to do that. The 3 Watt is bodged in. I have a 3 watt panel lamp on order and I will try 2 in series. The grain of Wheat one did exactly what I wanted until it popped

[bizzard]

I'I'm not certain but grain of wheat bulbs were only available in 12v. They were I think in use for model railways and the likeke, for illuminating

Model railways illuminating lineside buildings, signals ect, long before they were used for vehicle dashboard illuminations.

Edited December 28, 2015 by bizzard

[ditchcrawler]

I'I'm not certain but grain of wheat bulbs were only available in 12v. They were I think in use for model railways and the likeke, for illuminating

Model railways illuminating lineside buildings, signals ect, long before they were used for vehicle dashboard illuminations.

Wouldn't surprise me at all, as I said at the start I didnt know what voltage it was. It was in the bits box and got used.