Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Explain regulators and floating grounds to me, eg a car alternator

  1. May 20, 2014 #1
    I understand the basics but I am a bit lost on what happens in this example.

    In a car we have an alternator that charges the battery, if the battery is charged to stop it overvolting the regulator essentially shorts power to ground via SCRs (correct me if I'm wrong but this is my understanding) to keep the volts at 13.8 v

    Lets say you have zero electrical applicances in your car and it draws no current other than from the small losses of maintaining a battery at 13.8 v.

    As the car's ground is floating and it has a very small capability to hold charge, does this mean that the cars floating ground will increase in voltage (relative to the earths ground) fairly quickly? Eg if I was to drive my car for 1000 years whilst drawing no current, would my cars 0v slowly rise to say 5000v relative to the actual earth, and the positive rail to 5013.8v ?

    Or have I got this all wrong?
     
  2. jcsd
  3. May 20, 2014 #2

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    You dont understand "ground".

    And you have forgotten a very important part of what is voltage.

    Voltage is a potential DIFFERENCE so when you say "voltage" you must specify between where and where else, that is "difference between what two different places?"

    Now, your car's chassis is isolated from earth(ground) by your tires so its 0V rail (chassis) can indeed be at 5kv potential difference(voltage) from earth, but that has nothing to do with the charging system. More likely static electricity . That's one reason for the carbon lampblack in tires, it makes them weakly conductive to bleed off static electricity.

    The car's charging system maintains a potential difference of around 14 volts between car's positive rail , battery + terminal, and car's 0v rail, chassis.
    It does that by adjusting the output voltage of the alternator. It is oblivious to earth and would work equally well on Mars.
     
  4. May 20, 2014 #3
    Yes I do, it is your 0v source, eg earth (the actual ground) or a floating ground, the chassis of a device which may actually be quite different in voltage to other grounds.

    I specified the two different places, the earth, the car, the + rail??

    I now know the answer but my question was where does this excess energy go when the battery is charged, the regulator has to dump the energy somewhere.

    The answer is via the field winding being adjusted by the sense wire, this varies the alternators output voltage.

    Cheers for the non answer.
     
  5. May 20, 2014 #4

    meBigGuy

    User Avatar
    Gold Member

    You have it all wrong. Very Very wrong. Jim tried to explain, but you are very lost and didn't understand.

    The car's ground is at some potential relative to the earth. That could be 0V, 5000V, or -5000 volts (or a million volts). It doesn't matter, and has no effect on the car's charging system or battery. The car is insulated from earth by the tires, but that doesn't matter either.

    What matters is that the car's chassis serves as a voltage reference for the charging system. The alternator produces a charging current for the battery that is regulated, relative to the chassis, by the car's electical system. You can call that the car's ground, if you wish.

    If the battery is charged, the alternator doesn't overcharge it. There is no "shorting" SCR to dump charge. Not sure where you got that from. The charging system can control the alternator output such that it supplies the needed energy to charge the battery and operate the vehicle.

    There is no excess energy that has to go anywhere, and there is no accummulated charge with respect to the rest of the world.
     
  6. May 20, 2014 #5
    I know the potential of the car to the actual planet doesn't matter, it has nothing to do with it, it was just something I was curious about.

    I came here asking a question about why an alternator doesn't over charge a battery, and what might happen if it did.

    I have since looked up how an alternator works and seen that it uses a sense wire to adjust the output voltage such that no current flows when the battery is charged, before I looked this up I was not aware that the output voltage could be adjusted, and assumed it would keep on charging the battery forever (overcharging), a good example of how this could happen is if you pull the sense wire high eg to say 100v, this would mean the alternator charges the battery forever and there is excess charge.

    I did not know that the output voltage of the regulator could be varied like this and thought it essentially provided a minimum current at all times.

    I was under the belief that should the above happen the regulator would have to do something with this excess current to keep the battery at 13.8v, I couldn't see any other option than to send it to ground.

    So what would happen if you tied the sense wire high on an alternator, but the regulator was still trying to keep the voltage at 13.8v?
     
  7. May 20, 2014 #6
    If the sense wire was detecting a high voltage, wrt to the negative terminal of the battery, such as the 100v you mentioned, the regulator would determine that the battery is above the 13.8v and then would not send any current into the battery to charge it.
     
  8. May 20, 2014 #7
    From what I read it is the opposite? Either way, tie it to whatever voltage would cause it to send an over current to the regulator/battery.
     
  9. May 20, 2014 #8

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You should read about the basics of negative feedback (regulation). Volts too low causes the alternator volts to increase and supply more current, volts too high and the alternator output will drop in volts and it will stop supplying current to the battery.
     
  10. May 20, 2014 #9
    Yes that is what the sense wire is for.
     
  11. May 20, 2014 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    So what did you read to make you appear to disagree with meBigGuy's post?
     
  12. May 20, 2014 #11
    The only thing I ever disagreed with was the tone of LOOK AT ME BIGNOTING MYSELF ON A FORUM WHERE SOMEONE IS TRYING TO LEARN

    I have 5 years experience working as a control systems engineer, I'm not a moron, I just have had literally zero experience working with AC systems or generators/alternators, hence why I posted this thread and got literally zero information explaining what I was having trouble understanding, I have since asked other people and read wikipedia and answered the question for myself.
     
  13. May 20, 2014 #12

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    We may have started on the wrong foot.
    Perhaps it was something to do with the wording of your initial question? It was not clear from what you wrote how the two topics - regulation and PD - were related, specifically. It really didn't help that you described a car alternator regulator in a way that appeared wrong, for a start. What shunt regulation (shorting) do you think is involved? The field current is regulated with a series element - a relay contact, in the older systems.

    A general principle when asking for help is to expect the answer to come back in a way that you may not have expected. Could you rephrase your question to make it more clear what you do know and what you want to know? I know there can be a large gap between the fundamental Maths of control systems and the practical realisation. Were you worried about what comprises the actual feedback signal in the auto system?
     
  14. May 20, 2014 #13

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Now there is a well phrased unambiguous question.
    Had you asked that question you'd have got unambiguous answers.
    Instead you proposed something preposterous. That's why you were accused of not understanding.

    I saw that you had that part right but you ascribed it to cars not motorcycles.
    Automotive alternators use the field to control voltage, as you discovered..

    Permanent magnet alternators as in modern motorcycles and some outboard motors indeed short their windings to circuit common(chassis) partway through each cycle to control the voltage . Just as you described.
    Armature reaction prevents excessive current. You might call that 'dumping charge'. It too is closed loop control because the voltage regulator adjusts the SCR firing angle to maintain battery voltage.
    1960's motorcycles simply dumped current into a huge zener diode on a heatsink , that was open loop control.

    Your proposal that the alternator somehow interacts with earth threw me - it appeared you were oblivious to laws of Kirchoff. So i suggested you start back at the basics, which apparently you did. Congratulations.

    So - by modulating the field the automobile system avoids producing excess energy in the first place. So there's none to dump.
    The permanent magnet field & SCR regulator arrangement that shorts the winding partway through each cycle delivers the excess current into a short circuit(SCR ON), so there's essentially no energy dumped there either . Some amps X zero volts is zero watts.

    The 1960's brute force zeners(as on my 1969 BSA Lightning) did dump the excess energy and that's why their heatsink was so big and mounted way out front in the good airflow.

    Well, you finally asked one we could answer. Next time don't get so snippy.

    old jim
     
    Last edited: May 20, 2014
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Explain regulators and floating grounds to me, eg a car alternator
Loading...