Check your battery terminals

vanny · Post by **vanny** » 26 Apr 2005, 20:13

okay, time for the children to have there say! And fortunately i did a lab on this the other day!
You average motor consists of armature winding and field.
Adjust the speed of the motor by changing the field strength.
If you then drop the field strength to zero, the torque produced by the motor rapidly reduces and motor speed increases to infinity, burning out the motor as it goes.
In this circumstance the alternator is simply a driven generator, and as it is driven by the engine it can not spin to rediculous speeds and burn it's self out, so removing the battery wouldn't cause the alternator it's self to burn out!
But, as i understand it, the output voltage from an alternator is DC and controlled by the field in the alternator, which presumably comes from the battery. Hence you couldn't bump start a car and run it without some sort of charge, no matter how small as there would be no field and would result in a couple of magnets and bits of wire moving inside a metal box. So you have to at least start with a charge to get the alternator sefl sustaining. Once the alternator is producing an output you can indeed remove the battery as it provides enough power in the armature to power the field, and so long as the engine keeps running then it will continue to provide power.
The problem would then be in two parts, the output increases with engine speed so has to be regulated, and what to do with surplus power! The output of the alternatr is limited to 14v ish, i pressume this is done by controlling the field voltage, and i suspect it is done with some sort of zenner diode. regardless of what speed the engine turns at, if the field is limited then so too is the output voltage. If there where a significant instantaneous surge of power through the field then this could cause the regulator to fail, or as is more likely a surge thru the out put voltage would cause the same.
If the battery is treated as a giant capacitor (rather than an inductance) then once the capacitor is charged, disconnecting the alternator would allow the stored capacitance to provide the difference between the required current and the alternator current. If however we look from the other side the alternator is a giant inductor (huge big coil) connected to two resistances in series (the battery and the car!), then disconnecting the battery woudl lower the resistance and probably result in the regulator and alternator having to work harder, however the field would still be induced and while a high load might be produced it would not result in back EMF as there is still a load and charge in place.
I guess the regulator will simply adjust the voltage in the field based on the voltage from the armature, and hence the voltage across a battery shoudl remain totally constant if the altenator if in perfect condition?
so am i close?

NiSk · Post by **NiSk** » 26 Apr 2005, 20:24

Itdontgo has the most valid point - in fact the battery works as an extreemly large smoothing capacitor- without it being in circuit, the alternator will be feeding all the electrical consumers with fullwave rectified three-phase DC - containing quite a lot of ripple and dangerous harmonics that will definitly not do the delicate electronics any good - so keep your battery connected or fry your electronics!
The oldest vehicle I have had the pleasure to run and maintain was considerably older than myself - a 1923 Gwynne 8 - that actually had a 12V negative earth system (in 1923!). And a brass switch on the dashboard to select charging off / half / full ( but no speedometer!).
//NiSk

Kowalski · Post by **Kowalski** » 26 Apr 2005, 20:24

If you disconnect the battery i.e. remove the load, the alternator's regulator will take a time to compensate, and the inductive effect of the windings in the armature will mean a massive instantaneous over voltage will be produced, this is probably what kills the rectifier diodes.

oilyspanner · Post by **oilyspanner** » 26 Apr 2005, 20:25

Its magic really, all electrical devices are powered by smoke, any short circuit, wrong polarity incident or overload causes the smoke to get out (I have seen this) and the device no longer works, I have attempted to replace the smoke on a number of devices, generally without success[:D]
Running an alternator that is currently charging a battery then disconnecting it will very often cause a minor smoke escape.
On the subject of six volt systems I used to own a six volt beetle[:)]
The lights were appalling, sort of brown coloured light that used to fall onto the road in front of the car, oncoming motorists were never offended if I did not dip [:o)]
Stewart

AndersDK · Post by **AndersDK** » 26 Apr 2005, 21:34

Before the debate drowns in a lot of good and relevant opinions - I need to thank you all - especially Peter - for a most disciplined debate [8D]
I have to repeat (as many of you are too lazy to read through all the postings) - that my CX ran utterrly well - all electrics worked A1 - with no battery. Just a jumpstart - and off we go.
Now to NiSk : dont use your stereo in such an event - it's howling like mad - as did in my CX [8D]

NiSk · Post by **NiSk** » 26 Apr 2005, 21:54

Anders, fortunately your CX probably didnt have as much hi-tech electronics as a modern Citroën - which is absolutely crammed with low grade electronics (worse than the contents of the average TV set!) Which is why it merrily motored away without a battery.
//NiSk

Stempy · Post by **Stempy** » 26 Apr 2005, 23:14

Blimey, did you ever wish you'd never started anything.[:D]
In my case the escape of smoke occurred after the hours of darkness and it was raining so all conceivable electrical devices were in use and hence quite a bit of current flowing. No doubt there would have been a bit of arcing if the battery terminal was jumping loose, so maybe it was the arcing that did the damage, though I still believe that running an alternator off load isn't the best of ideas as they can kick out 90 or so volts unloaded. Funny thing is there was no sign of flickering lights or any such thing and the charge light on the dash didn't come on even at max smoke level.[?]

vanny · Post by **vanny** » 26 Apr 2005, 23:38

Stempy that pretty much sums up how i think mine went the other week. The lights eventually started getting dimmer and dimmer which was the first warning i had, and it want while i was doing 70(ish) on the motor way so i figure it must has packed in pretty much instantly and then fed off the battery

Post by **Peter.N.** » 27 Apr 2005, 04:01

Phew, I didn't know we had so many egg heads either! I promise this will be my last mail on the matter! The point I was trying to make is that the output current is controlled by the rise in voltage, practically zero at 14v, if you disconnect the battery and the load (which I conceed would have to be pretty heavy) pulls the voltage down much below 12v the rated output current of the alternator could be exceeded, unless of course the reduction in exiter current caused the output to drop, possibly it does, My knowledge of regulators comes mainly from TVs. The other point is that in the days of dynamo's, there was sufficient residual magnetism in the armature to start the system up, is this not the case with the alternators rotor?

AndersDK · Post by **AndersDK** » 27 Apr 2005, 04:21

Nope Peter -
An alternator will never start charge unless connected to a battery - either the local - or the jump starting one. After that it is self containing.

Post by **Peter.N.** » 27 Apr 2005, 04:26

Thanks