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What is difference between series and parallel

  1. Feb 8, 2010 #1
    I have a unit giving for example 100-150 volts. I want to reach upto 1 kv. A step up transformer will do the work, but my feeling as a noob is that, if i step up voltage, the amps will go down, if i want amps also up, i need to attach another unit to the existing power unit.

    My question as a noob is that whether i should attach another power unit in series or parallel or how ? will it benefit in my scenario target of 1kva ? how to attach in series or parallel etc ?

    Kindly help this noob.
     
  2. jcsd
  3. Feb 8, 2010 #2

    vk6kro

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    For the same power, more voltage means less current.

    That is because
    Power = Voltage * current.
    So to keep the power the same, if the voltage goes up by 10 times then the current only needs to be 1/10th as great.

    If you had a source of 100 volts AC that could give 10 amps, and you drove a 1:10 stepup transformer with it, you could get 1 amp out at 1000 volts if there were no losses, and assuming the stepup transformer could handle this much power.

    If you put another 100 volts AC in series with the first (and in phase with it) the result would be 200 volts. If you ran this into the same transformer, the output would be 2000 volts.

    If you put another 100 volts AC in parallel with the first (and in phase with it), the transformer would still generate 1000 volts and with the same load, you would still take 1 amp. With a different load, you might be able to get 2 amps at 1000 volts if the transformer was capable of it.
     
  4. Feb 8, 2010 #3

    sophiecentaur

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    You can't get anything for nothing.
    It all depends on what you actually want to do.
    A load (heater, motor, TV) will take a certain amount of current for a certain input volts. They are usually designed for a particular operating voltage - they have a certain resistance, which defines the current.

    A 110V, 1kW device, operating, via a transformer, from a 220V supply, will still consume 1kW (ignoring any inefficiency and assuming a perfect transformer). The supply would not be able to tell the difference between that and a regular 220V, 1kW device.

    I'm not sure exactly what you want. It depends what current your 1kV device needs but, as you say, you will need nearly ten times the current supplied from the 110V mains.

    Putting high power supplies in parallel is not to be recommended because their voltages may not be precisely in step and you could be making a smoke making machine.
    If you really need 10kVA then you have to buy something appropriate - unless you really know what you're doing!
    Good luck.
     
  5. Feb 9, 2010 #4
    my question is very simple for you. I have a homemade generator which generates 100 volts and about 8 amps. How many generators I have to make to meet my below power requirement to run a machine which require power at below requirement. Also comment if i need a step up transformer and whether to connect in series or parallel, if you have any schematic, please put it here.

    110 VAC (+5% / -10%), 50-60 Hz, 15A 765W 1HP Power.

    Hope this helps.
     
  6. Feb 9, 2010 #5

    vk6kro

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    If you could synchronise your generators so that the outputs were the same in voltage, frequency and phase, you could put two generators like that in parallel without any transformer. This would give you 16 amps at 120 volts.

    However as Sophi has said, putting generators in parallel is very dangerous unless you know exactly what you are doing.

    The problem is that one generator can feed all its power into the other one and they could destroy each other.

    It would be far better to buy a commercial 20 amp 120 volt generator and use that on its own.
     
  7. Feb 9, 2010 #6
    Hello,

    I am not putting in parallel, I am asking whether it should be put in series or parallel ! I am a noob yeah. and what about watts and hp, will the 2 connected together will give the watts.

    Please do not ask me "unless I know what I am doing" because I think I have already revealed what I am doing. I am asking for help. You have not commented on watts requirement it is over 750 watts and 1hp as specified above. will the 2 connected together will satisfy my watts and hp requirement.
     
  8. Feb 9, 2010 #7

    sophiecentaur

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    The short answer is "no chance". Small generators will not have the possibility of synchronising together reliably. If they are not synchronised they will trip off or blow up.
    I don't think this is negociable. You will just need a bigger generator.
    The only possible way to get round this would be if you can divide your load between the generators.
    Sorry.
     
  9. Feb 9, 2010 #8
    Hello,

    Thank you for the prompt answer, Well let us assume one small unit (let us not mention this unit as generator for the time being) generates 100V and 8 amps.

    My target was to achieve a particular voltage, amps, hp as per given above by putting the units together as much as possible to achieve the target voltage, amps, roughly.

    Now let us put aside whether the unit is small or big. By your answer, do you mean to say by doing anything like putting the desired units together in series or parallel, it is not possible to achieve this voltage ? WE want to inflate the generators to meet the requirement, you said that you need to divide :( , could not digest it, as I want only one machine to run with above specification, how can i divide the input voltage required by that machine through my generator ? what things are essential to synchronize it. please elaborate. I am going for this because I do not have funds for new generator. So buying one readymade is definetely not even an option for me.

    Please specify.
     
    Last edited: Feb 9, 2010
  10. Feb 9, 2010 #9

    vk6kro

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    The machine you want to power needs 15 amps and your generator can only supply 8 amps, so no transformer is going to get you more current from the one generator, at the same voltage.

    If there were two 120 volt 8 amp generators on the same shaft, they could be put in series to give 240 volts at 8 amps which you could then step down to 120 volts at 16 amps in a step-down transformer.
    Putting them on the same shaft would synchronise their frequency and phase and the voltages would not have to be identical if they were in series.

    It would be far better to buy a commercial 20 amp 120 volt generator and use that on its own.
     
  11. Feb 9, 2010 #10
    Thanks vkpro,

    one more thing if i attach a 3rd unit (on the same shaft) and by using a step down transformer, it will reduce voltage and increase amps, now suppose the amps of generator are exceeding now if you compare it to the machinery input requirement. Will it work ?
     
  12. Feb 9, 2010 #11

    vk6kro

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    As long as you only draw 8 amps from the series generators, each generator will also only supply 8 amps.

    However, don't forget that the engine driving all this (ie making the generators turn) will have to work harder and use more fuel to maintain the same speed of rotation.

    Also the generators would have to be mounted on the shaft so that their outputs were synchronized, ie they produce their maximum output at the same point in the rotation.
     
  13. Feb 9, 2010 #12
    thanks vk6pro once again.

    I know and understand what you want to say. but I will take care that all the units put together will function at fullest possible extent and they really work hard, they will definetely maintain the speed of rotation is my assurance to you. they will be mounted in series as you say and perfectly synchronized, here let us assume that the units of 110v and 8 amps are performing more or less slightly fine, now as you say it should be connected in series is ok.

    My last question is that if the power goes above i.e if the generator gives more amps and more volts like if 16 amp is requirement and we are getting 20 amps and if 110 v is desired and we are getting 240 volts, then whether it will suit the machine which we are going to drive on this power ?
     
  14. Feb 9, 2010 #13

    vk6kro

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    My last question is that if the power goes above i.e if the generator gives more amps and more volts like if 16 amp is requirement and we are getting 20 amps and if 110 v is desired and we are getting 240 volts, then whether it will suit the machine which we are going to drive on this power ?

    You understand that the two generators would give 240 volts at up to 8 amps depending on the load.

    This must then be stepped down to 120 volts again to avoid damage to the 120 volt 15 amp machine, using a suitable 240 volt to 120 volt transformer.
     
  15. Feb 9, 2010 #14
    Thanks Vk6kro,

    for your valuable advise and time extended. :)
     
  16. Feb 9, 2010 #15

    sophiecentaur

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    But you cannot ignore the fact that two AC generators need to be synchronised in order to connect them (series or parallel). If they are not synchronised then one will just drive current into the other.
    The National Grid has very sophisticated control in order to achieve this and your small domestic generators will almost certainly not be able to operate synchronously.
    Do you not realise how relevant this is?
    Or is this just a thought exercise?
     
  17. Feb 9, 2010 #16
    Thanks bro,

    Let me try it, unless i try, i will not realize my mistakes and learn anything new. :) thanks for your co-operation and time extended.
     
  18. Feb 9, 2010 #17

    sophiecentaur

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    If and when one or both of your generators are destroyed or just won't work together then you can invest in one that will be sure to do the job and sell the two small ones - or what remains of them. Just kidding.

    I found this link which says it can be done but only if you stick to the rules and follow the right procedure! (Wear a hard hat!)
    http://yarchive.net/car/rv/generator_synchronization.html
     
  19. Feb 9, 2010 #18

    vk6kro

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    Putting the generators in series should not cause any destruction, but synchronizing will be needed to get maximum voltage out of the combined generators.

    I should warn you NOT to do this yourself. Your question: What is difference between series and parallel should be a warning that you would need help to get this working.

    120 volts or 240 volts, even from a generator, is quite willing and able to kill you

    It would be much better to save up and buy a proper single generator.
    Buying the parts to make a new small generator and then buying a 2000 VA transformer is just wasting money that could go towards buying a good generator.
     
  20. Feb 9, 2010 #19
    Hi Sophie,

    Great Work ! Thanks for your compliments. I think you are having misconceptions with my unit (as I name it to be) my unit is a small coil which generates 100 volts and 8 amps. Now why are you taking it to be a shaft and thinking of those heavy generators linking in parallel. I have previously only made clear that, kindly take it to be a unit or part (a single or combined coil which you can call) which you can combine in multiples maybe in series to adjust to a particular voltage and amps.

    Hope this helps and clear all your doubts now ! :)

    Thanks vk6pro, I will be having a team of people while building a generator from coils and no ! this is not a big readymade generators in parallel, my unit is just a coil yeah with magnets. Vk6pro, If I had money to buy that big generator, I would have definetely liked it to go for one ! :) I am tight at wallet, and I want to get the work done. I hope you all understand now.
     
  21. Feb 9, 2010 #20
    If their shafts are locked together as vk6kro suggested, they will remain synchronized. They must be locked precisely, though, with no slippage, and at the correct angle to each other so their outputs are in phase. The big multi-generator setups use fancier synchronization methods because it's not practical to lock all the generators together on a single shaft...you would need to stop them all to do something like work on a single problem bearing.
     
  22. Feb 10, 2010 #21

    vk6kro

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    my unit is a small coil which generates 100 volts and 8 amps.

    my unit is just a coil yeah with magnets.


    Do you have any pictures of these small coils?

    .
     
  23. Feb 10, 2010 #22

    sophiecentaur

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    This has suddenly become more fascinating. 800VA can't come from a 'very small' coil and the device must be of an 'interesting' design. I made assumptions because you didn't imply that your generator was anything other than a stand-alone conventional design.

    If all the generators are coupled rigidly and mechanically then my misgivings about synchronism are modified - pity you didn't say so before; I have been losing sleep and picturing you being blown up onto your roof!

    Under these conditions, you can add them in series to produce multiples of volts although their relative phases need to be optimised for a maximum resultant output voltage.

    As long as they have the same number of turns and the same strength of magnets, then they can go in parallel. But, in parallel, in addition to the need for correct phasing there is a serious need for equal voltage output , you will risk having circulating currents. Imagine one coil producing 100V and the other coil producing 101V. The 1V difference will cause current to circulate around the coils - not through the load. This will not occur when (conventional) generators are not mechanically coupled because their relative phases will self adjust away from this condition - they take up the appropriate phase and balance out the load automatically In your case, I guess you could adjust the relative angles to get the same effect - i.e. the currents from each coil adding vectorially so there's no circulation.

    But this is a serious bit of generating equipment you seem to be proposing. From some of your remarks you appear to have limited electrical engineering experience. Be very cautious. You may still end up sitting on your roof or being chased around the room by a loose shaft, rotating at 3600rpm. vk6kro's comments are worth taking notice of.
    .
     
  24. Feb 10, 2010 #23
    Aspardeshi,

    Please note that the English expression "to know what you are doing" has a meaning beyond its literal word-for-word sense. It means not only to know what it is that you are doing, but to have the necessary understanding and skills to do it properly (and in this context, safely).
     
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