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current surge stalling engine

Discussion in 'General Electronics Discussion' started by davit, Jun 7, 2021.

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  1. davit

    davit

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    Aug 16, 2017
    hi carrying on from my previous post below,
    i made a 24v generator using a 4 stroke petrol engine to charge my bank of batterys,
    my problem is even though its 7.5 hp engine the 24v alternator pulls so much power it stalls the engine, to get it to work i cannot energize ( not good i know ) the alternator untill the engine is hot and reving hard ,
    so my queston is is there a type of rheostat switch i can use to hold back / vary the charge until some power has gone into the batterys, so i could then turn up the switch ,
    the alternator can push out 55amps -- 24v
    many thanks
    davit

    https://www.electronicspoint.com/forums/threads/hi-need-help-with-a-dc-charging-issue.295075/
     
  2. crutschow

    crutschow

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    May 7, 2021
    What do you mean by "energize"?

    Does the alternator have a separate voltage regulator or is it built in?
    If you can access the field wire, you can add a resistor in series to reduce the field current which reduces the alternator output voltage/current.
     
    Last edited: Jun 7, 2021
  3. davit

    davit

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    Aug 16, 2017
    hi thanks for the reply
    the is the modern type all built in with reulator, i have a switch and warning light just like in a car, but also the alternator will self exicite with high rev,s, and charge even with the ign / alt warning light turned off , just need to hold back the the current for a few minutes to a lower reading then turn it up full gradualy, ( to help the engine ) this is what i am hoping for just not sure if this is fesable
     
  4. Bluejets

    Bluejets

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    Oct 5, 2014
    Output would be limited by whatever load you have connected.
    I'd say you have something connected up a-about or the alternator is cactus.
     
  5. davit

    davit

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    Aug 16, 2017
    thank you for the reply
    the batteries would be about 50% discharged when i switch on the alternator causing the engine to stall from a moderate to high rev setting
    thanks
     
  6. Bluejets

    Bluejets

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    Oct 5, 2014
    Can be done but to start with it would be much easier with and external regulator.
    You would need an understanding of the operation of the alternator and be able to sort out the regulator wiring.
    Then make up a controller to manually control the rotor input.
     
  7. crutschow

    crutschow

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    May 7, 2021
    Have you tried connecting the load with the engine at idle, and then increasing it's speed?
     
  8. Bluejets

    Bluejets

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    Oct 5, 2014
    This should get you started.

     
  9. davit

    davit

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    Aug 16, 2017
    yes it instantly stalls
     
  10. davit

    davit

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    Aug 16, 2017
    its a new alternator costing £150 so i will have to stick with it, with there being just the one output + lead i was kinda hoping that if i cut that line and connected wirewound rheostat switch in series that would hold back the charge to a trickle till some juice went into the battery, then slowly turn up the switch allowing more through and hopefully by then engine could take the load,
    thanks for the interesting video link, way beyond my knowledge though LOL
    THANKS
     
  11. Alec_t

    Alec_t

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    Jul 7, 2015
    55A @ 24V is less than 2hp. I'm surprised a 7.5hp engine stalls.
    Could you put a bit of reduction gearing (belt and pulleys) between the engine and alternator?
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

    11,439
    2,627
    Nov 17, 2011
    The 7.5 hp from the engine may be available in the optimum operating point only. Much less will be available during idling or at very low or very high speeds. Do you have the hp vs. rpm characteristic of the motor at hand?
    Here's an example from another site:
    [​IMG]
    As you can see max power in this example is available at 5500 rpm whereas during idling (1000 rpm) only a fraction of ~ 20% is available. For your motor 20 % translates to 1.5 hp only which is less than the power requirementof the alternator:
    Output power Pout = 55 V × 24 V = 1.32 kW equiv. 1.77 hp.
    Taking into account losses within the alternator (efficiency around 60 %) the input power to the alternator would have to be at least:
    Pin = 1.77 hp / 60 % = 2.95 hp.
    If your engine delivers only 1.5 hp during idling, it is no wonder it stalls.
     
  13. davit

    davit

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    Aug 16, 2017
    YES i believe that would work , but i have a lack of space to do that, and weight is getting an issue, thanks
     
  14. davit

    davit

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    Aug 16, 2017
    hi harald
    its a chinese built engine ( honda ripp off ) no spec details available except the 7.5 hp sticker , as i have said before if i rev the machine to almost max (guesing 3-4 rpm) with the alternator freewheeling then switch battery feed on the engine cuts down but accepts the load and keeps running, i know this is really bad for the alternator but its the only way i can get charging,
    do you think my rheostat idea would work ?? allways charging with an intiall trickle going through ,,thanks
     
  15. davit

    davit

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    Aug 16, 2017
    Last edited: Jun 9, 2021
  16. Harald Kapp

    Harald Kapp Moderator Moderator

    11,439
    2,627
    Nov 17, 2011
    Sounds exactly like the problem i described in my post.
    Why not simply put a switch between alternator and battery? Turn off the battery when starting the motor. Once the motor runs at speed, turn on the battery circuit to charge. Why should this be bad for the alternator?
    A rheostat suitable for 55 A will be a messy big thing.
     
  17. crutschow

    crutschow

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    May 7, 2021
    Why do you think that?
    The alternator should handle the sudden full-load without a problem.
     
  18. davit

    davit

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    Aug 16, 2017
    yes this is what i do at the moment , i use an isolater switch, i was hoping to stop the big jolt when i switch on which i have to get right ie engine revs and temperture, i have this coming from ebay
    20k Wirewound Rheostat Variable Resistor
    https://www.ebay.co.uk/itm/20k-Wirewound-Rheostat-Variable-Resistor-/363419254523?nma=true&si=iOGlcuKFP1Iyc7BpihIGMxzKpkM%3D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557
     
    Last edited by a moderator: Jun 9, 2021
  19. davit

    davit

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    Aug 16, 2017
    ok thanks i just assumed it might blow a diode,
     
  20. Bluejets

    Bluejets

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    Oct 5, 2014
    Why won't you just give everyone a break and show what you have there and a diagram of what is connected where.
    While you are at it a link to the specs of the alternator.
    Thanks.
     
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