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Unmatched voltages on a transformer's primary coils

  1. Apr 23, 2005 #1
    Ok. say you have a transformer hooked up to two AC generators. They each produce the same amount of voltages. For the primary coils on the transformer (the 2 generators..) one is coiled 100 turns and the other 10 turns...the result?

    Continuation of question....
    now assuming that this means one source will be transformed to have a high voltage and low current. and the other will have a high current and low voltage, this would mean that they would "fight" each other. But what would this "fight" result in? Loss of power in the form of electricity? or mechanical resistance (from teh generator spinning)? I have no idea to be honest...so any help for these questions would be greatly appreciated!
     
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  3. Apr 25, 2005 #2

    Cliff_J

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    infamous, we talked about this in PMs and I'll try to use a mechanical analogy again to make this easier.

    Instead of a transformation of electrical-magnetic-electrical as a transformer would perform, think electrical-airflow-electrical where we have an electic motor fan that is blowing against a wind generator.

    Our input is a certain amount of power that can be measured as a voltage (pressure) and current (amount flowing). When its converted to wind we have the same qualities of a pressure and amount flowing that equate to a certain amount of power. We could measure the pressure and amount and know how it will influence the wind generator.

    Now you are asking about using two different sized fans. If all things are kept equal like wire size and so on either the small fan is going to burn up and/or the large fan will spin very slowly. Each fan should be optimized to make the best use of its available power.

    If we have X volts to work with, then the large fan we apply a large current we'll call Y, giving us an input power of XY. Using X volts on the small fan we'd normally need less power and a smaller current we'll call Z, giving us an input power of XZ. So we know that XY > XZ, and our input power is XY + XZ.

    Now you decide to change the smaller fan to be more powerful by angling of the blades or making it spin faster and now it handles XY power too. Our input power is now 2XY with both fans running. But now the small fan needs to move an aweful lot of air to match the power of the large fan and efficiency likely roars its ugly head. This is likely not as efficient as just using two of the same optimized fans to begin with since it is designed to work with the appropriate level of power and you're not fighting ending up on a poor part of an efficiency curve like trying to spin a small fan super fast.

    Because with a transformer the number of windings and size of wire is very important to ensure the power is delivered properly and not wasted as heat. So its not so much a battle amoungst the two coils but within each coil itself and its ability to transfer the power efficiently. And that is where the choice of wire size and number of windings is figured out and should match the application.

    And even if the phase isn't maintained (fans aren't blowing in the same direction) there is no mechanical fighting per se, its more of a cancellation determined by how much of an angle difference there is in the directions.
     
  4. Apr 25, 2005 #3
    so if using two generators...and one small winding and one large winding. rather than have the one generator try to spin slower per se....the electricity will cancel itself out? so since power in two primary coils add together (50W + 50W will produce 100W in the secondary coil). would they subtract each other? or turn one sort of "negative"?
     
  5. Apr 25, 2005 #4

    Cliff_J

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    Spinning the generator slower creates all sorts of problems. Change only one thing at a time!!

    If both sources are in-phase operating at the same frequency then they will simply add like constructive interference. (see link below)

    However, assuming they are identical generators (and everything else is the same except windings), using different windings will cause massive problems for the lower turn one - the winding needs to have a certain number of turns in order to not work as something close to a dead short. Nothing wants to encounter a short! If the lower number was sufficient and then the larger turn number was well oversized its efficiency would likely be lower - now entering the art of transformer design and some handy tables would be nice to have access to...

    If the two generators were operating at the same frequency but were 180 degrees out-of-phase and powering identical windings then the magnetic fields would cancel each other out since it would be destructive interference.

    http://www.glenbrook.k12.il.us/gbssci/phys/Class/waves/u10l3c.html
     
  6. Apr 26, 2005 #5
    ok...so like in the waves it's constructive interference, just the size of teh waves are different this time around. what happens then? like what problems actually occur?
    oh, also, since this is interference, is it the overall power that interferes or the voltage and current interfering with each other seperately?
     
  7. Apr 26, 2005 #6

    Cliff_J

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    Simple physical analogy to explain this:

    You push a little on a chair next to you. You push but it doesn't move so you perform no work on the chair. This is like having a voltage (pressure or push) but no current flowing, and with zero current you have zero power.

    You push the chair and it moves along at 1 cm/s because the force you apply is equal to the force of friction from pushing the chair. If you push harder you could push a second chair or push the original faster and so on. But the idea is that your pushing force and the amount of friction dictate the speed the chair moves along and thus the rate at which you're doing work - power.

    In electricity its all dictated by ohm's law: Voltage = Current * Resistance

    Voltage = E
    Current = I
    Resistance = R

    E = I * R

    (c is used for the speed of light, v is used for velocity, E&I were used instead)

    Power = Voltage * Current

    And simple algerbra lets you see how you can solve and see that for example power=resistance * current^2 and how that lets you know only the resistance and current and still know the power (you could also find the voltage). But regardless, the voltage, current, and resistance are all tied together.
     
  8. Apr 26, 2005 #7
    so then....since Pt would technically equal (in its most basic form..) P1 + P2. then Pt would also be (V1+V2)*(I1+I2)?
     
  9. Apr 26, 2005 #8

    Cliff_J

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    No, you have a typo.

    P1 = V1 * I1
    P2 = V2 * I2

    Pt = P1 + P2
    Pt = (V1 * I1) + (V2 * I2)
     
  10. Apr 26, 2005 #9
    aaah thanks.

    Edit:

    would the electricity in the wire look like V1+V2 for teh voltage, and I1+I2 for the current?
     
  11. Apr 26, 2005 #10

    Cliff_J

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    Well, now you need to know how things are hooked up. If this were in series with a simple load it could really be that easy. Like if you measured some AA flashlight batteries it would work this way.

    But if you have just one wire and two sources you would have to figure it out using something like kirchoff's laws of voltage and current in a circuit. And that's just linear relationships, something like a light bulb has a resistance that changes as it heats up so as you flow more current this can change a bunch.
     
  12. Apr 27, 2005 #11
    ok...so say that these two coils on teh generator power the single secondary coil, which is hooked up to a parallel circuit...
     
  13. Apr 27, 2005 #12

    Cliff_J

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    Ok, what happens at the secondary of a transformer is heavily influenced by what's happening at the primary but they are isolated and seperate systems. That's why I used an electric fan and wind generator as metaphors to describe how the two are not directly coupled but instead coupled by air movement. Inside a transformer its a changing magnetic field.

    So the math could be simplified to Power in and Power out to find the maximum voltage/current available to your load if you had an ideal transformer (and maybe oversized to allow for regulation and so on).

    Inside a transformer the Primary's voltage is converted to electromotive force (Gilberts or ampere-turn) and the current is converted to flux (Maxwell or Gauss) and the secondary converts those back to voltage and current. Its like a torque converter in an automatic car transmission, it converts the torque and RPM to a higher torque at a lower RPM to make it easy to get the car moving (along with slipping to make for smooth starts).

    Then the secondary could be thought of as a voltage source to the load with a maximum current capacity of the ideal transformer. So you know its voltage and with the resistance (technically impedance) you can find its current.
     
  14. Apr 27, 2005 #13
    Ok..that makes way more sense, because the changing magnetic field in the transformer induces a current in teh other coil. So what makes a highvoltage magnetic flux different from a high current flux?

    and what would happen if (in parallel or series even..) you had an external power source (like a battery or something) where:
    for parallel: the voltage was higher than teh transformers output
    for series: the current was higher than the transformers output
    what would happen in the transformer?
     
  15. Apr 27, 2005 #14

    Cliff_J

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    high voltage = high EMF 'push'
    high current = high flux 'amount'

    (assuming you don't saturate the core, and if they are both high you're simply pushing around a lot of magnetics)

    Ok, so you're going to hook a battery up to the secondary - Lets assume you have a rectifier bridge to convert AC back to DC then.

    In parallel the higher voltage feeds into the other. This would be typical, its how you charge a battery by having just a slightly higher voltage and why the battery will get warm as it sinks that power. Which means the load will likely not see too much higher of a voltage then somewhere between the output and the battery's resting voltage.

    In series its no simpler - the lower current wins since the current is going to be the same throughout the circuit so the bottleneck is the predominant (probably ESR limited). Of course, in series the voltage is higher so with the same load the current is going to try to be higher and the power has gone up by the product (like a square power) and the fuse blows and/or the components let out their magic smoke. Likely 4:55PM on Friday when the boss has stopped by to chat and you're hungry to boot. :smile:
     
  16. Apr 27, 2005 #15
    ok...so if you had one primary coil with a high voltage and one with a high current youd just have a lot of nothing?

    as for the second thing what if you hooked it up to a capacitor? (one with a high voltage and relatively high current)...i dont completely understand capacitors though..and not at once either. Consider 2 generators...in different phases. (ie. since they basically produce power in a save wave form...we'll use 360º as a full revolution), one produces a high voltage from 0-180º and is disconnected until after the full revolution...the other one produces a high current from 180-360º and does nothing before that...would it charge the capacitor with a high current and voltage so taht the load can then take the power from the capacitor?
     
  17. Apr 27, 2005 #16

    Cliff_J

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    Whole lot of theory and not much practical here...but here goes nothing.

    On the first one, it depends. Its all about the power. If you have 200V and 1mA for one and .1mV and 200A for the other you could light up a few LEDs or a tiny auto light bulb with that tiny amount of power.

    Remeber that voltage and current are connected, you need one to have the other. If you have the same load, if you have high voltage you have high current. If you have low voltage you have low current. You don't get one or the other, you always get both. You could change the load to suit the proportions (or a transformer to make them work out better) but there is no free lunch. If you want high power and high current you need lots of power, about the only way around it is to take a smaller power applied over time and store it like in a battery.

    A capacitor only works on DC for storing power that could be applied to a load. And even a capacitor the size of a soda can could only keep a small flashlight bulb going for a minute or less, so its far less space efficient than a battery by maybe 100x or more.
     
  18. Apr 28, 2005 #17
    but its not liek i could build a battery that keeps recharging through a dc input WHILE mainting power to the load (from teh battery)...could I?

    and yes im aware it's lots of theory..which is why i came here to ask for help, since i (obviously) lack much knowledge in these departments.

    Also, would this battery (the one that could be rechargable..if that even works out) be able to hold up the charge? (high voltage low current in, then high current low voltage in, to build up a source with high current and high voltage?)
     
  19. Apr 28, 2005 #18

    Cliff_J

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    No, but a capacitor works in a similar manner and has its own challenges with charging/discharging (namely the amount of current available).

    Ok, new analogy.

    You have a baseball glove on and someone throws you a baseball and as you catch it the kinetic energy is absorbed into your body. Someone could fire a BB gun and deliver the same kinetic energy, someone else could lighlty throw a 8lb steel shotput ball into the glove and again deliver the same kinetic energy. Can your glove handle all three? Or would you rather have a design optimized for each situation (I'll assume you agree).

    I dislike water analogies because they don't obey closed circuits and so on but it'll work well enough here.

    Your capacitor is a very tall water tank like is used for city water distribution. 1st scenario you have a pump that can pressurize a garden hose to a high enough pressure to fill the thing - it can actually fill it but it'll take a while. 2nd scenario you have a swimming pool - it has no pressure but could fill up the first 6ft very quickly if the elevation allowed it. So it does very little good to fill 6ft of a tall tank. And it'll take forever to fill it with a garden hose. Instead you need a pump and engine (power) like a firetruck to fill it.

    And that is how a city water system works - they pump water into the tower all day long 24/7 but they have planned for people to use water in certain amounts at certain times of the day that average out to a value close to the amount they pump in on average. The tower allows for short bursts of higher flow than the pumps may be able to deliver, but this must be followed by a period of low usage to allow for a recharge. When averaged out over a day, its the same amount.

    The big problem is that capacitors store electricity in a very inefficient manner. Its like a fire truck - it may have 1500 gallons of water on board and that may be all the truck can handle for weight, but it can pump all that water out in just over a minute. It takes a lot longer than that to put out a burning house. So they need the fire hydrant nearby to give them a supply of water (or use it in a more efficient manner) because there is no escaping the need for sheer volume.

    And if you need continous large amounts of power you need a source that can provide it. Period.
     
  20. Apr 28, 2005 #19
    and what if the capacitor is constantly being charged and discharged. BUT the discharge is lower than the max the capacitor can hold (most of the time...it varies) and the charge going in is constant?
     
  21. Apr 28, 2005 #20

    Cliff_J

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    It'd be no different than if the capacitor wasn't there except the ripple (amount voltage varies) would be larger without the capacitor. That's why cities use water towers, so you get constant pressure (and so you have water even if the power goes out). To get current out of a capacitor the voltage MUST drop and it drops like a rock - that's why you need a capacitor 100x larger than a battery to light up a small light bulb, it runs out of voltage very quickly as it discharges. A whole bunch of charge in a capacitor doesn't do anyone any good if it doesn't have any voltage to push it out.

    And the capacitor only works on DC like a battery. Electricity generation and transport is almost all AC so its easy to step up with a transformer to avoid heat losses from sending current and then stepped down to use. Its also easy to convert AC to DC and without any complex components. Edison lost the battle for DC a very long time ago and with good reason. Now capacitors are used in power transmission to correct the phase angle (current lags voltage) of the AC power but the capacitor doesn't really store anything.
     
    Last edited: Apr 28, 2005
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