Alternator regulator

Discussion in 'General Electronics Chat' started by ronv, Jul 2, 2014.

  1. ronv

    Thread Starter AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    I want to build a regulator for my little portable generator. (battery, small engine, alternator) In looking on the internet it looks like they just turn the field on below 14.6 volts and off above 14.6. Is it really that simple?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    The methods used in alternators such as the automotive type, was originally this method, using a simple electromechanical regulator that just switched the field to either the supply when below the 14.5 and to common or chassis ground when above the 14.5.
    This was replaced by the equivalent in S.S. switching device but it was found to be very noisy and caused car radio interference, so the present method of fixed switching PWM type was devised to avoids this.
    If you are not worried about the interference aspect, it can be done simply by a LM311 comparator with a pot set up across the battery supply and then compared to the 14.5 across a voltage divider/pot to adjust with, I made this set up for a mobile 230v generator field using appropriate zener and reference divider resistors.
    The LM311 switched a Mosfet to control the field.
    Alternatively an autowrecker may be a source of the 14.5 versions.
    Max.
     
    Last edited: Jul 2, 2014
    ronv likes this.
  4. ronv

    Thread Starter AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    Thanks Max. I'm trying to make it adjustable so I can do two 8's or one 12, hence the dyi. I'm thinking of using FETs because the field current is high (250 amp alternator) so I tried to slow them down a bit, (~15 usec turn on time) bur maybe not enough because when I look at the link Burtus posted they were 75 usec.. My other concern was just "zap" a couple hundred amps into the battery, but I guess the voltage would rise quicky to the trip point so it would run as a kind of PWM anyway?
     
  5. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,036
    1,662
    If it was me I would just buy a common one wire type alternator or at minimal a regulator for a one wire alternator.

    The simplest and cheapest one I know of are the regulators for the Delco SI series alternators.

    They have a new low start up speed design that is very easy to work with and given the designs ruggedness they will turn most any type of non Delco alternator into a one wire design if set up as an external three wire (Field, Stator, Ground are the three connections) regulator.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    I used a single power FET for the 240v 200amp Gen, the field was around 20amps.
    Max.
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,988
    745
  8. ronv

    Thread Starter AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    Thanks guys.
    The big question I am asking myself is the bang bang (full on / full off) ok? I'm thinking to much probably.:rolleyes: The real question is lets say I turn it on because the battery voltage is 12 volts. The alternator will dump full current into the battery. Is that ok? I don't think there is any way to prevent it so I'm guessing the voltage rises quickly to the turn off point and it just toggles around that point. I'm guessing the response time of the battery or the ripple will slow things down enough that the regulator won't go analog on me.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    Remember it won't (or shouldn't) go over 14.5v as does a regular automotive regulator or charger, works the same way.
    14.5v is the manufacturers recommended for automotive batteries.
    Max.
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the output from the alternator is controlled by the current going into the "field" terminal. car alternators acutaly use a rotating field and a stationary "armature" unlike the generators of old. the rectifier diodes change the voltage induced in the stationary " armature" to dc to charge the battey. the other connection supplies current to magnetize the "field" if hyou vary the "field " current, the output voltage changes. the "field " current is not very much, 200 amps is not needed, more like less than 5.
     
  11. bwilliams60

    Active Member

    Nov 18, 2012
    722
    88
    Ronv,
    In an automotive alternator, the output is controlled by the amount of demand on the battery by a transistor and a zener diode basically. The output is going to be what the battery requires to recharge. If the battery is dead, the duty cycle of the regulator is high, keeping the alternator output high. As the battery charges, the current level will slowly decrease. This is a big reason for alternator failure. Too much heat kills diodes in rectifier quickly. If you are going to build something, you must take into consideration the condition of the battery and the load dump you will incur on initial startup. You need a circuit to cushion the initial draw.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    If you design for 14.5v max you should be OK, the alternator in an auto not only keeps the battery charged by outputing 14.5 but also provides power to the rest of the system after the initial start.
    Max.
     
  13. ronv

    Thread Starter AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    OK, I think I got it. Found a curve for battery voltage under charge that I attached. The voltage does rise up at high charge rates. So I can kind of see it being on for a few seconds then a shorter and shorter time as it charges up. Like you say Max in a car it should always be pretty well charged but also as BW said if it's dead it will suck it up. Guess that's why you don't want a battery to go dead. Hard on the battery and the alternator.
    So anyway I came up with this circuit based on a 1.24 volt reference acting as a comparator for the adjustment powered thru the accessory switch so the little motor can get started easy, then a big PFET since the field is grounded to the case in this alternator.
     
  14. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    The peak (worst case) charge current in automotive charging is limited by lots of factors; wiring resistance, alternator winding resistance, magnetic saturation, bettery internal resistance etc etc. Generally they all add up to quite a significant impedance that limits the worst case current.
     
  15. bwilliams60

    Active Member

    Nov 18, 2012
    722
    88
    Sorry RB, I must disagree. Automotive electrical has been my life and I rebuilt alternators, starters and generators for a living for quite some time. When a battery is in a low state of charge, the alternator will do everything in it's power do recharge the battery straight away and there is no measurable impedance that will stop it. I have had 105 amp alternators charging at 100 amps on startup with nothing on in the vehicle but the engine. The problem was the battery and it is one of the most misdiagnosed problems in auto electrics. Alternators fail, they get replaced and the battery remains only to take out a new alternator. Of course the alternator must have been bad coming off the shelf, right? Wrong. Anyways, wiring resistance in DC wiring on vehicles is minimum if the circuit is clean. We allow up to 0.3VDC drop from alternator to battery. Hope this clears things up a bit.
     
  16. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    I'm agreeing with you. :) A 35A alternator will put approx 35A into a battery worst case. If your 105A alternator was putting 300A into the battery that would be a different matter.
     
  17. WA7PRC

    New Member

    Nov 13, 2010
    4
    0
    For a 12V system, the simplest answer is to use an off-the-shelf automotive regulator. Chrysler's performance parts division has an electronic regulator available to retrofit electromechanical regulators used on alternators produced for pre-1970 model vehicles. Those alternators have one end of the field winding common to ground. Regulation takes place on the high sideof the field winding. You want pn P3690732. Jegs and Summit Racing have them available.

    1970-up alternators had both ends of the field isolated, and the regulator worked the low side of the field to ground. +12V was fed to the high side of the field winding. The same regulator was used no matter the alternator, from 37A to 100A.

    Both regulators use PWM. The conduction angle is such that they can get away with a small pass transistor and very little dissipation. Even if you have a different brand alternator, one of the above regulators should work as long as the field current isn't more than around 5A. IIRC, all my Chrysler 67A and Motorola 100A alternators pull around 3A average field current.

    Bryan
     
  18. bwilliams60

    Active Member

    Nov 18, 2012
    722
    88
    Sorry RB, I must have misread your post. All good :)
    if you are looking for automotive voltage regulators, google a company called Transpo. They are the kings of regulators and rectifiers and then buy that part from a rebuilder.
    Good luck
     
    THE_RB and ronv like this.
  19. ronv

    Thread Starter AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    Thanks 60.

    I think what I learned is that it's a bad idea - in a car at least, to try and charge a dead battery with the alternator. Or if you do leave it at low RPM. That's probably not a problem for me since at 200 amps I would drop probably 14 volts at the alternator stator and the even a dead battery would probably pop up to 12 or 14 volts. I probably can't get 26 volts out of it with my engine.
     
  20. bwilliams60

    Active Member

    Nov 18, 2012
    722
    88
    Ronv,
    Getting 26v out of an alternator is quite simple. Removing voltage regulation will allow the alternator to run up in excess of 300 volts. That is why they make such a good welder. Having said that, there will be large current accompanying that and it spells nothing but disaster for an automotive electrical system and perhaps yourself. Like I said earlier, there are lots of options to achieve your goal, just a question of how elaborate you want to get and how much $$$$ you want to spend.
     
Loading...