12 volt

tinman7

tinman7

Member
I just bought a 1930 Town car. The car has been converted to 12 volt. How do I know if it is negative or positive ground? Will it work either way or blow the fuse? The battery was not in the car.
 
Battery cable post clamp size is the best clue. Coil polarity is another. Ammeter wire color: if the wires on each firewall post match, it's positive ground; if reversed, negative.

Everything will work either polarity. As has been said, probably negative ground.

Best to set it up to the polarity you want, then polarize the generator to the battery before starting.
 
Badpuppy said:
Battery cable post clamp size is the best clue. Coil polarity is another. Ammeter wire color: if the wires on each firewall post match, it's positive ground; if reversed, negative.

Everything will work either polarity. As has been said, probably negative ground.
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I've seen this expressed before, and assume it is true but do not understand why the starter does not run backwards and be a problem, since it is a DC motor.



Can you/someone please explain?



Thanks
 
Loosen the belt and bypass the cutout, the gen will motor. If it's a diode cutout without bypass it will motor only with the wrong polarity battery. Best way to determine polarity, but requires a diode.
 
tinman7 said:
I just bought a 1930 Town car. The car has been converted to 12 volt. How do I know if it is negative or positive ground? Will it work either way or blow the fuse? The battery was not in the car.
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Battery was not in the car! 50 - 50 chance I would try neg ground it will not blow the fuse one way or the other, only if you have a short
 
tinman7 said:
I just bought a 1930 Town car. The car has been converted to 12 volt. How do I know if it is negative or positive ground? Will it work either way or blow the fuse? The battery was not in the car.
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Being at 12 volt it's more than likely negative ground, especially if it has an alternator.
 
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Does car have alternator or generator ?

If alternator then you might find the answer on it somewhere. Usually 12v alternators are neg grd as said.

If generator then its a crap shoot but it'll work either way.

The battery cable clamp ends is a good indicator as said.
 
Big hammer said:
Battery was not in the car! 50 - 50 chance I would try neg ground it will not blow the fuse one way or the other, only if you have a short
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My guess is that it is a 90% chance that it is negative ground. All modern electronics expects 12 volt negative ground so people usually make it that way.

Charlie Stephens
 
A person can install a twelve volt battery and a Three OHM coil and they will be running twelve volt positive ground . I run my 31 roadster 12 volt positive ground . I use the Fun Projects 12 volt voltage regulator that takes the place of the cut out . None of the wire connections have to be changed . I adjust the generator for a ten amp maximum charge . A one wire 12 volt alternator could be used in place of the generator but it would probably require changing to negative ground . Changing to negative ground would require reversing battery cable connections , coil connections and the ammeter wires will need to be reversed or it will show discharge when it would actually be charging .
 
30 Closed Cab PU said:
I've seen this expressed before, and assume it is true but do not understand why the starter does not run backwards and be a problem, since it is a DC motor.



Can you/someone please explain?



Thanks
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What drives any electric motor is magnetic attraction and repulsion. Like poles repel and opposites attract. There are two major parts to a starter motor - the armature and the field windings. If you reverse the polarity of both, it runs in the same direction. Reverse only one of them, it will run in the reverse direction.
The field windings set up a magnetic field and the armature sits in the middle of it. Another magnetic field is created by the current flowing through the armature. That causes one side ot eh armature to be attracted to the field, the other is repelled by it. The direction of this field changes as the current reverses direction. That is achieved by the way the commutator is connected to the armature windings. That is, as the armature turns, the brushes are connected to different windings by the commutator. When the timing of the changes is right, the armature spins.
This clip probably explains it better than I did.
https://www.youtube.com/watch?v=CWulQ1ZSE3c
When I was in secondary school, each year we had to make something for display to parents on open night which demonstrated something we had learned that year in our science class. When was 14, I thought I was clever making an electric motor, only to be well and truly trumped by a couple of friends who teamed up to make a device which carried their voices on a beam of light. That might not sound overly impressive now, but in 1962, it was more than most parents could comprehend.
 
The key reason for the starter to always turn in the same direction is that the field winding is in series with the stator. When the direction of current changes both the field and the stator changes so the direction is the same.

If the field winding was in parallel with the stator, then reversal would occur with a change in the polarity.

Marc
 
marc hildebrant said:
The key reason for the starter to always turn in the same direction is that the field winding is in series with the stator. When the direction of current changes both the field and the stator changes so the direction is the same.

If the field winding was in parallel with the stator, then reversal would occur with a change in the polarity.

Marc
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Yup.

Permanent magnet motors also reverse with current reversal.
 
Thanks guys. Badpuppy pointed out why my confusion. I was trying to remember schooling never used from 45 years ago, and I was stuck on permanent magnet motor info. The explanations above refreshed my memory. Thanks again.
 
If has a 12V battery installed, most likely changed to negative ground as others have stated if you have an alternator. The generator will work either way if polarized correctly.
You can make the change to negative ground, and by swapping the cable connections on the batter, changing the coil connections, and amp meter. Starter, horn and lights do not care which way the current flows. BTW when taking the laser optics course for degree in EE in 1970 demonstrated how a laser radar would work.
 
Can you even get a positive grounded alternator? I believe all the alternators I've ever worked on had the ground to the case (none floated), and who would buy special diodes to reverse polarity? I'd bet a 4 pack of Heady Topper that if it's an alternator then it's negative ground.
 
Mister Moose said:
Can you even get a positive grounded alternator? I believe all the alternators I've ever worked on had the ground to the case (none floated), and who would buy special diodes to reverse polarity? I'd bet a 4 pack of Heady Topper that if it's an alternator then it's negative ground.
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https://brillman.com/product/6-volt...MI9oO_mKXn7AIVDYnICh1PuQ3PEAQYASABEgKPevD_BwE

This is probably not for a Model A, but it came up on a quick search. My Model A has a 6 volt positive ground alternator on it.


Sent from my iPhone using Tapatalk
 
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