Mixing dynamos and alternators

[NBMike]

I'm starting to think of running in the electrics for my boat. I have an alternator from a scrapped car.

(the alternator had been in use for only 2 months) My Petter PJ3W at present has a dynamo for charging the starter battery. This is wired positive earth through the engine block. Can I leave this as is for the starter battery and run a seperate system with the alternator for the 'house' batteries or should I scrap all electrics and run a new system with 2 alternators? I can do the mechanical bits to fit alternators but am not sure of the electrical side. I'm gradually printing off Gibbo's site for in depth perusal!

Mike

[Keeping Up]

You can run a positive-earth system on the engine and a negative earth system for the rest of the boat if you wish. Just make sure they don't ever ever come into contact with each other except at the ground potential (ie starter positive and domestic negative). You'd need to either have a separate "ignition" switch for each system, or use a relay to keep them isolated from each other. If you do keep it this way I'd also recommend sticking labels all over the place to remind you and anybvody else who works on the system which bits are which otherwise if you connect the wrong bit to the wrong bit you'll get a big bang.

[dor]

Is it not possible to change the polarity of the dynamo? I seem to remember from long, long ago "flashing" the field coils to change the dynamo polarity.

[bottle]

Is it not possible to change the polarity of the dynamo? I seem to remember from long, long ago "flashing" the field coils to change the dynamo polarity.

 

Yes did this about 40 years or more ago.

 

Do not remember now how to do it.

[Henhouse]

It is possible for you to add an alternator and continue to have your +ve earthed dynamo for the starter circuit. But the potential for disaster will always be there, and really should be avoided.

 

You oould as suggested reverse the polarity of the dynamo, and go all -ve earth.

 

this link will take you to a site with loads of info on dynamo polarity and conversion. A bit obscure, but good advice from some very experienced people.

 

Land Rover Series II Club

 

 

Personally I like simplicity and reliability. So I wouldn't hesitate in starting again from scratch and going for 2 alternators. One for starter circuit, and the other for domestic, and both with the same polarity.

 

Experience with vehicle electrics taught me long ago that the law of Sod dictates that if if is going to pack up it will always do it when it is cold, dark, and raining.

 

The easiest way to avoid this is to do it right and properly from the start.

[paulcatchpole]

Yes did this about 40 years or more ago.

 

Do not remember now how to do it.

 

"The polarity of the dynamo is controlled by the residual magnetism in the field pole pieces and this needs to be reversed by passing current through the field in the new direction. This can be done by holding open the regulator points with a piece of card and connecting a jumper wire from the battery live terminal (now positive) to the F terminal ( the small lower connection) on the dynamo or the regulator, whichever is most accessible.

 

The idea of holding the regulator points open is to prevent excessive current flowing through the regulator and dynamo armature. The jumper wire should give a small spark when it is connected. It is only necessary to let the current flow through the jumper wire for a second or two, after which the polarity should have been reversed. The piece of card may now be removed from between the regulator points. While this operation is being carried out, the points should not be closed by accident; if they are a very heavy current will flow and damage will result."

 

Never done this, mind, so don't blame me!

 

Wouldn't your big problem at the moment be that the new alternator is likely to be negative-earth via it's casing, and hence the engine block, even if it did have an insulated return as well?

 

PC

[Keeping Up]

Wouldn't your big problem at the moment be that the new alternator is likely to be negative-earth via it's casing, and hence the engine block, even if it did have an insulated return as well?

That's not actually a problem. Yuo can have a negatively-earthed alternator with its casing connected to the block and running in a system which is entirely positive of the block, at the same time as having a positively earthed dynamo with its casing connected to the block and running in a system which is entirely negative of the block.

 

Like I say, it's perfectly possible (electrically), but it's risky in practice because you've got 'live' objects, such as the two ignition circuits, which would usually be at the same potential but are actually 24 volts apart and therefre potentially the source of a big bang if you forget yourself for a moment and bridge across them.

 

Edited to add: changing the polarity is a good idea, but if you've got any instrumentation such as a temperature gauge you'll need to change that around too. Now, who's going to be the first person to ask "won't the starer motor run backwards" (hint, it's OK it doesn't"

Edited August 3, 2008 by Keeping Up

[Henhouse]

Now, who's going to be the first person to ask "won't the starer motor run backwards" (hint, it's OK it doesn't"

 

 

Don't even go there !!!!

 

I remember someone once thinking that the indicators on their car would work backwards if they reversed the polarity of the dynamo.

 

ie: if switched to turn right, the left hand indicator would flash, and vice versa.

[paulcatchpole]

That's not actually a problem. Yuo can have a negatively-earthed alternator with its casing connected to the block and running in a system which is entirely positive of the block, at the same time as having a positively earthed dynamo with its casing connected to the block and running in a system which is entirely negative of the block.

 

Like I say, it's perfectly possible (electrically), but it's risky in practice because you've got 'live' objects, such as the two ignition circuits, which would usually be at the same potential but are actually 24 volts apart and therefre potentially the source of a big bang if you forget yourself for a moment and bridge across them.

 

Ta for explaining that! I'd not really gotten my head around the realities of running it like that. Is that only the case if you're running the alternator as part of a separate neg earth system for the domestic, with the engine/dynamo/starter-battery separate and pos-earth?

 

PC

[Gibbo]

I'm starting to think of running in the electrics for my boat. I have an alternator from a scrapped car.

(the alternator had been in use for only 2 months) My Petter PJ3W at present has a dynamo for charging the starter battery. This is wired positive earth through the engine block. Can I leave this as is for the starter battery and run a seperate system with the alternator for the 'house' batteries or should I scrap all electrics and run a new system with 2 alternators? I can do the mechanical bits to fit alternators but am not sure of the electrical side. I'm gradually printing off Gibbo's site for in depth perusal!

Mike

 

Positive earth systems are HIGHLY prone to causing stray current erosion and corroding your hull. They were bad enough on cars that aren't sat in a bath of water which is why they stopped making them that way. On a boat they are a nightmare.

 

It's a trivial job to repolarise a dynamo. Just do it that way and have both systems as negative earth.

 

Gibbo

[Big COL]

.

 

Edited to add: changing the polarity is a good idea, but if you've got any instrumentation such as a temperature gauge you'll need to change that around too. Now, who's going to be the first person to ask "won't the starer motor run backwards" (hint, it's OK it doesn't"

 

Alan

 

How did the early golf carts get to reverse. I was always under the impression that they reversed the polarity which spun the starter in the opposite direction, and being two strokes just fired them up backwards.Which then gave it a reverse, these were belt driven through a sprung loaded variable speed pulley, no gearbox. Today the new ones have a gearbox.

[Gibbo]

Alan

 

How did the early golf carts get to reverse. I was always under the impression that they reversed the polarity which spun the starter in the opposite direction, and being two strokes just fired them up backwards.Which then gave it a reverse, these were belt driven through a sprung loaded variable speed pulley, no gearbox. Today the new ones have a gearbox.

 

Permanent magnet motors do indeed go the other way if you reverse the polarity. However very few starter motors are permanent magnet motors. Therefore most starter motors run in the same direction whatever the polarity. They will run on AC quite happily. With this type the polarity of the electromagnet has to be changed with respect to the polarity of the rotor.

 

It could be that the ones you refer to used a permanent magnet starter motor or that the polarity of the stator was reversed.

 

Gibbo

[Timleech]

Permanent magnet motors do indeed go the other way if you reverse the polarity. However very few starter motors are permanent magnet motors. Therefore most starter motors run in the same direction whatever the polarity. They will run on AC quite happily. With this type the polarity of the electromagnet has to be changed with respect to the polarity of the rotor.

 

It could be that the ones you refer to used a permanent magnet starter motor or that the polarity of the stator was reversed.

 

Gibbo

 

I'll hazard a guess that they might have used some sort of Dynastart arrangement, possibly built in to the flywheel. Reversing ordinary pinion-drive starter motors isn't a simple matter, as they have Bendix drives or some other sort of freewheeling/disengaging device built in.

 

Tim

[Keeping Up]

Maybe they were small Bolinders? If it fired up forwards you injected just before TDC to kick them back and then run backwards? I reckon that would be easier than trying to put that little ball down a tiny hole hundreds of yards away

[NBMike]

Thanks for all those replies. As the dynamo was 'Free' with the engine, I will try to reverse polarity. If I blow it up I'll go for two alternators.

It may be a while before I get round to it though as I still haven't actually got my replacement camshaft out of the replacement engine and into my boat engine yet. Oh Just a thought- I've probably got a spare dynamo on the other engine too

Mike

[ditchcrawler]

Is it not possible to change the polarity of the dynamo? I seem to remember from long, long ago "flashing" the field coils to change the dynamo polarity.

Use to do it all the time when transister car radio/tape players became popular. Just flash the fiels of the dynamo and it will pump the volts out with the opposite polarity

[James T Berk]

If you repolarise your dynamo do remember to do it with the dynamo disconnected otherwise you will be also passing current through the regulator contacts to the armature and that's no good at all oh no no but better than leaving the dynamo +ve earth it's something to do with jellies I think or was it broomhandles.

[Big COL]

Permanent magnet motors do indeed go the other way if you reverse the polarity. However very few starter motors are permanent magnet motors. Therefore most starter motors run in the same direction whatever the polarity. They will run on AC quite happily. With this type the polarity of the electromagnet has to be changed with respect to the polarity of the rotor.

 

It could be that the ones you refer to used a permanent magnet starter motor or that the polarity of the stator was reversed.

 

Gibb

 

GibbGibb

 

If the armature had been set up for neutral, rather than giving it a leading field. changing the polarity of the field would reverse the motor, would it not? I appreciate that in this set up you lose some of the motor's power. God its been a lifetime since I had to think of DC machines, and I hated them then, I think I am right with this , if not I'm off for a lay down in a dark room. Nurse why is this jacket got its sleeves sewn up and why am I cuddling myself?

Medication what medication?

[Gibbo]

GibbGibb

 

If the armature had been set up for neutral, rather than giving it a leading field. changing the polarity of the field would reverse the motor, would it not? I appreciate that in this set up you lose some of the motor's power. God its been a lifetime since I had to think of DC machines, and I hated them then, I think I am right with this , if not I'm off for a lay down in a dark room. Nurse why is this jacket got its sleeves sewn up and why am I cuddling myself?

Medication what medication?

 

Yes that's right. You have to change the polarity of ONE of them with respect to the other. If you change them both (by simply reversing the connections to the battery) it will still run the same way.

 

Gibbo

[NBMike]

Aw C'mon now! You're just confusing me with all this talk of armatures, leading fields, jellies and broomhandles.

I will try to change dynamo polarity as per Pauls instructions and add that I will try to incorporate some kind of insulation on the mounting brackets so that the proposed new alternator is electrically isolated from the block/boat.

I'll keep you posted with progress.

Mike

[Keeping Up]

I will try to incorporate some kind of insulation on the mounting brackets so that the proposed new alternator is electrically isolated from the block/boat.

Why ???????

[jake_crew]

I've re-polarised dynamos a number of times on my old cars.

 

Firstly change the battery connections to give you the negative earth you want. As has already been said, ensure that all polarity sensitive devices are either changed over too or disconnected.

 

With luck, when you start the engine, there will be enough remnant magnetism to re-polarise and start generating straight away. If your little red light goes out, JOB DONE.

 

 

If the ign warning light stays on, its very simple, with the engine running fast enough to spin the dynamo quickly, simply flash (connect) the large and small terminals on the dynamo briefly together with a length of electrical wire.

 

Just for a second.

 

As with all electrical stuff, if you're not confident, DON'T DO IT.

[NBMike]

Why ???????

Is it not necessary to do that, then?

 

I've re-polarised dynamos a number of times on my old cars.

 

Firstly change the battery connections to give you the negative earth you want. As has already been said, ensure that all polarity sensitive devices are either changed over too or disconnected.

 

With luck, when you start the engine, there will be enough remnant magnetism to re-polarise and start generating straight away. If your little red light goes out, JOB DONE.

 

If the ign warning light stays on, its very simple, with the engine running fast enough to spin the dynamo quickly, simply flash (connect) the large and small terminals on the dynamo briefly together with a length of electrical wire.

 

Just for a second.

 

As with all electrical stuff, if you're not confident, DON'T DO IT.

I don't have a little red or any other kind of light. There is an ammeter though

[Keeping Up]

Is it not necessary to do that, then?

Whether your starter system is positively or negatively earthed, and given that the domestic system is of course negatively earthed, everybody on this thread has been assuming throughout that the case of the alternator (domestic negative, ground, earth, call it what you will) is connected to the engine block, hull, starter system ground whether it is positive or negative.

[NBMike]

I'm sure I read somewhere that it was advisable not to run any power through the hull. Maybe I mis-remembered it. (Or imagined it!)