Stupid question alert! Alternators and reversing polarity.
Discussion
A friend of mine has (provisionally) bought a car, and is very excited about picking it up.
The seller had to put it through an MOT after coming back on holiday, which he has done and it has apparently passed with no advisories.
The seller also mentioned changing the battery, as the car had been sat for a bit and he thought the battery might be knackered.
My friend's just told me that the seller changed the battery, but at the same time he accidentally "touched the alternator with a spanner" (friend's words) and "reversed the polarity" of the alternator. The car supposedly runs fine but he's now going to change the alternator.
Now, I consider myself relatively knowledgeable about car engines and electrical systems, but I'm not 100% sure about this one.
Is that even possible? I know you can reverse polarity by hooking a battery up the wrong way around, but touching it with a spanner? Surely that wouldn't do anything?
I'm sure it'll all be fine if the battery and alternator is changed anyway, but I just want to find out if the seller's telling porkies to mask his own stupidity or something
The seller had to put it through an MOT after coming back on holiday, which he has done and it has apparently passed with no advisories.
The seller also mentioned changing the battery, as the car had been sat for a bit and he thought the battery might be knackered.
My friend's just told me that the seller changed the battery, but at the same time he accidentally "touched the alternator with a spanner" (friend's words) and "reversed the polarity" of the alternator. The car supposedly runs fine but he's now going to change the alternator.
Now, I consider myself relatively knowledgeable about car engines and electrical systems, but I'm not 100% sure about this one.
Is that even possible? I know you can reverse polarity by hooking a battery up the wrong way around, but touching it with a spanner? Surely that wouldn't do anything?
I'm sure it'll all be fine if the battery and alternator is changed anyway, but I just want to find out if the seller's telling porkies to mask his own stupidity or something
Don't know what the guy is up to. AFAIK dynamo's may need it. Alternators don't.
This link refers to polarising dynamo's for positive or negative earth. http://www.dynamoregulatorconversions.com/polarisi...
Or this link explains things quite well, http://www.vw-resource.com/alternator_generator.ht...
Generator Polarization
When a generator has been left unused for a long time (months or years), the metal core looses it's magnetism, and it NEEDS a little residual magnetism to start the charging process. Polarizing just gives it enough "boost" in magnetism to start it charging properly. When it's being used, it keeps its core polarized core so no need to do it again.
Note: Why do generators need to be "polarized"? Automobile generators need some magnetism to get started. This "residual" magnetism remains in the field pole pieces even after the engine has stopped. The next time the generator starts up, the residual magnetism creates a small voltage in the armature windings. Not enough to charge the battery, but enough to allow the field windings to draw current. As the field current increases, the pole pieces create even more magnetism. That makes even more voltage in the armature, and the cycle continues until the generator is capable of producing maximum output.
What happens though to a generator which has been stored a long time or is freshly rebuilt? The residual magnetism may have decreased to the point where it can no longer get the generator started producing voltage. In the case of a new generator or one which has been mis-treated, the residual may even be of the wrong direction (North and South poles reversed). Polarization is a simple process used to restore the field pole residual magnetism and ensure the magnetic direction is correct.
Note: Voltage regulators do not need to be polarized -- they are not polarity sensitive. Even if voltage regulator came with instructions to polarize it -- these instructions actually polarize the generator, not the regulator. The regulator manufacturer simply wants to make sure that your generator will work properly so you don't blame the voltage regulator.
Neither (d)o alternators need to be polarized. Alternators use a trigger current to start the process of charging - electromagnetism, so don't need any internal magnetism to begin with, like a generator does.
That trigger current is supplied via the alternator dash light (it's a tiny current and won't make the light glow when the engine is running) so if that bulb blows - the battery won't get charged and it will go flat. Curious circuitry VW used, but it works just fine. So that means that with an alternator equipped VW, you should always glance at the dash as you turn the key and make sure the Alt light does glow with just the key on, just to make sure that that circuit is intact.
This link refers to polarising dynamo's for positive or negative earth. http://www.dynamoregulatorconversions.com/polarisi...
Or this link explains things quite well, http://www.vw-resource.com/alternator_generator.ht...
Generator Polarization
When a generator has been left unused for a long time (months or years), the metal core looses it's magnetism, and it NEEDS a little residual magnetism to start the charging process. Polarizing just gives it enough "boost" in magnetism to start it charging properly. When it's being used, it keeps its core polarized core so no need to do it again.
Note: Why do generators need to be "polarized"? Automobile generators need some magnetism to get started. This "residual" magnetism remains in the field pole pieces even after the engine has stopped. The next time the generator starts up, the residual magnetism creates a small voltage in the armature windings. Not enough to charge the battery, but enough to allow the field windings to draw current. As the field current increases, the pole pieces create even more magnetism. That makes even more voltage in the armature, and the cycle continues until the generator is capable of producing maximum output.
What happens though to a generator which has been stored a long time or is freshly rebuilt? The residual magnetism may have decreased to the point where it can no longer get the generator started producing voltage. In the case of a new generator or one which has been mis-treated, the residual may even be of the wrong direction (North and South poles reversed). Polarization is a simple process used to restore the field pole residual magnetism and ensure the magnetic direction is correct.
Note: Voltage regulators do not need to be polarized -- they are not polarity sensitive. Even if voltage regulator came with instructions to polarize it -- these instructions actually polarize the generator, not the regulator. The regulator manufacturer simply wants to make sure that your generator will work properly so you don't blame the voltage regulator.
Neither (d)o alternators need to be polarized. Alternators use a trigger current to start the process of charging - electromagnetism, so don't need any internal magnetism to begin with, like a generator does.
That trigger current is supplied via the alternator dash light (it's a tiny current and won't make the light glow when the engine is running) so if that bulb blows - the battery won't get charged and it will go flat. Curious circuitry VW used, but it works just fine. So that means that with an alternator equipped VW, you should always glance at the dash as you turn the key and make sure the Alt light does glow with just the key on, just to make sure that that circuit is intact.
Scuffers said:
XG332 said:
Should be fine as its all negative earth and 12v from a battery is a very weak current.
Simply check the alternators output at idle and if it's At 14.2v ish then it's working fine.
come again? Simply check the alternators output at idle and if it's At 14.2v ish then it's working fine.
Spanner across the battery/alternator certainly isn't a weak current. More like a welding machine!
Gassing Station | General Gassing | Top of Page | What's New | My Stuff