Electrical energy transmission and transformer

In summary, when transferring electricity from a generator to a factory or any other location, a transformer is used to step up the voltage. This results in a smaller current in the secondary transformer compared to the primary transformer, as determined by the formula Vp/Vs=Np/Ns and VpIp=VsIs. However, if the resistance of the cable is changed, this can affect the values of voltage and current, which are fixed by the coil formula and conservation of energy formula. The power is dependent on the load resistance, and numerical practice can help to understand this concept. Additionally, it is important to specify which current and which circuit is being referred to in equations such as V=RI.
  • #1
sgstudent
739
3
When transferring electricity from a generator to a factory or anything we would use a transformer to step up the voltage right? As P=VI and P is constant in the primary and secondary transformers (ideal case), with an increased V as compared to the primary transformer it will in turn result in a smaller I as compared to the primary transformer.

Then again, the values of V and I are fixed in accordance to the new voltage output by using the formula Vp/Vs=Np/Ns and VpIp=VsIs. in most calculation questions, the R of the cable is given and the values fit nicely to get the total power. However, if i were to change the resistance of the cable what will happen? Since the values of V and I are constant, how will the V=RI values change. This is because the value of the power, voltage and current are fixed to a certain value due to the usage of the coil formula and the conservation of energy formula. So I'm completely clueless about the scenario that i change the values of resistance within the cable.

I'm hoping that you will be able to help me in this. Thanks so much for the help! :)
 
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  • #2
This set-up is a well known for bringing out misconceptions! Here are one point that I think you might be confused about...

Is is not fixed by the transformer equations. Is is given by
Is = Vs/(Rlines + Rload). (For example, if the load resistance is infinite, that is the lines don't have anything connected to their far end, then Is = 0.)
Is then fixes Ip via your second transformer equation.
 
  • #3
Philip Wood said:
This set-up is a well known for bringing out misconceptions! Here are one point that I think you might be confused about...

Is is not fixed by the transformer equations. Is is given by
Is = Vs/(Rlines + Rload). (For example, if the load resistance is infinite, that is the lines don't have anything connected to their far end, then Is = 0.)
Is then fixes Ip via your second transformer equation.

Huh, but then it current is dependant on the resistance, if resistance increases current will decrease? But then again power is fixed so as V is constant and P is constant, how will the values add up again? Thanks for theft help!
 
  • #4
Philip Wood said:
This set-up is a well known for bringing out misconceptions! Here are one point that I think you might be confused about...

Is is not fixed by the transformer equations. Is is given by
Is = Vs/(Rlines + Rload). (For example, if the load resistance is infinite, that is the lines don't have anything connected to their far end, then Is = 0.)
Is then fixes Ip via your second transformer equation.

So does it mean that the power value will change according to the resistance of the whole load?
 
  • #5
Power is not fixed. The only things that are fixed (in the simple transformer treatment you are using) are VP and VS.

The power is indeed dependent on the load resistance.

What will give you confidence is some numerical practice. Try this. A step-up transformer has 2000 turns on its primary, which is connected to an alternating voltage of 1200 V. The secondary has 20000 turns and is connected to a load of 76000 Ω via a pair of long connecting wires, each of resistance 2000 Ω.

Calculate VS, IS, the power input to the transformer, IP, the p.d. across the load, the power used by the load and the power dissipated in the connecting wires.
 
  • #6
Philip Wood said:
Power is not fixed. The only things that are fixed (in the simple transformer treatment you are using) are VP and VS.

The power is indeed dependent on the load resistance.

What will give you confidence is some numerical practice. Try this. A step-up transformer has 2000 turns on its primary, which is connected to an alternating voltage of 1200 V. The secondary has 20000 turns and is connected to a load of 76000 Ω via a pair of long connecting wires, each of resistance 2000 Ω.

Calculate VS, IS, the power input to the transformer, IP, the p.d. across the load, the power used by the load and the power dissipated in the connecting wires.

Oh ok! I somewhat understand. However, if the power in the primary step up transformer is reduced, then the current will have to drop in that circuit too. This is because the P is now a constant in this situation. So by V=RI, if my I decreases, my R will increase? Like at first V=10V, R=5ohm and I=2A. So my P=20W. Then when my power drops to like 10W, and V is still constant then my I will reduce. So now I is reduced to 1A. Then since V=RI then the resistance should increase compared to before?
 
  • #7
sgstudent said:
Oh ok! I somewhat understand. However, if the power in the primary step up transformer is reduced, then the current will have to drop in that circuit too. This is because the P is now a constant in this situation. So by V=RI, if my I decreases, my R will increase? Like at first V=10V, R=5ohm and I=2A. So my P=20W. Then when my power drops to like 10W, and V is still constant then my I will reduce. So now I is reduced to 1A. Then since V=RI then the resistance should increase compared to before?

I'm afraid I find this very confusing...
(1) For 'primary step-up transformer', you mean 'primary coil of the step up transformer'.
(2) There are two currents involved: Ip in the primary circuit and Is in the secondary circuit. You must always say which you're talking about.
(3) It's no use quoting V = IR unless you say what you're applying it to (load or whatever).
(4) "So by V = RI, if my I decreases my R will increase?" The currents don't affect the resistances (assuming the wires don't heat up).
(5) "This is because the P is now a constant in this situation." Power in general isn't a constant for a transformer; it depends on the load resistance. Unless by constant P you mean that the input power VpIp and the output power VsIs are equal.

I strongly suggest that you tackle the problem I gave now. It'll teach you a lot more than this sort of interchange of words. If you need hints, ask.
 
  • #8
Philip Wood said:
I'm afraid I find this very confusing...
(1) For 'primary step-up transformer', you mean 'primary coil of the step up transformer'.
(2) There are two currents involved: Ip in the primary circuit and Is in the secondary circuit. You must always say which you're talking about.
(3) It's no use quoting V = IR unless you say what you're applying it to (load or whatever).
(4) "So by V = RI, if my I decreases my R will increase?" The currents don't affect the resistances (assuming the wires don't heat up).
(5) "This is because the P is now a constant in this situation." Power in general isn't a constant for a transformer; it depends on the load resistance. Unless by constant P you mean that the input power VpIp and the output power VsIs are equal.

I strongly suggest that you tackle the problem I gave now. It'll teach you a lot more than this sort of interchange of words. If you need hints, ask.

oh sorry for being too vague. (1) Before i attach a load, my VpIp=VsIs=20W for example, Vp=10V and Is=2A. (2) So when i increase the resistance of the cables it will result in a lower power for both the primary and secondary coils. So, for example my VpIp=VsIs=10W, as my Vp will still be 10V, so my Ip will drop to 1A right? So before the increase the resistance (1), my Vp=RpIp=5ohmX2A. then when i increase my resistance of the cables/load (2), the voltage in the primary coil still remains the same. however, due to the decreased power, the current has to drop to get the 10W, 10W=10VX1A. But then the primary coil's resistance does not change so when using the formula Vp=RpIp, since V and R is constant, then how can the current change?

(talking about the primary coil now)Because now i have the added resistance in the secondary coil's part. So there is a lower power in both the coils. so by P=VI, since V is constant so I will drop. but since V and R is constant, then how can the current drop unless the resistance drops?

thanks for the help. I'm doing the calculations now. really appreciate the help given here. :)
 
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  • #9
Good! I agree with almost everything in your last posting. The only thing that sounds a bit suspect is

sgstudent said:
Then again when I use V=RI for the primary transformer it would result in an increased resistance too? Since I is reduced.

The transformer equations you're using assume that the resistances of the primary and secondary coils are both zero. So when you apply V = RI to the primary coil, in the form R = V/I, you are not, literally, finding the resistance of the primary coil. What you are finding is a sort of 'effective resistance' or 'transferred resistance'. This is quite a sophisticated idea, and, at this stage, I'd keep off it. Just don't go there, that is don't apply V = RI to the primary coil.
 
  • #10
Philip Wood said:
Good! I agree with almost everything in your last posting. The only thing that sounds a bit suspect is



The transformer equations you're using assume that the resistances of the primary and secondary coils are both zero. So when you apply V = RI to the primary coil, in the form R = V/I, you are not, literally, finding the resistance of the primary coil. What you are finding is a sort of 'effective resistance' or 'transferred resistance'. This is quite a sophisticated idea, and, at this stage, I'd keep off it. Just don't go there, that is don't apply V = RI to the primary coil.

Oh ok,so actually changing the resistance will affect many variables? But I thought that even if I don't increase the resistance, then P=VI, the V is equal to resistance times current? But now the change in resistance is affected by this complicated concept? Anyways at O levels will there be such questions whereby they change the resistance? Thanks for the help!
 
  • #11
At O level, the type of question you're likely to meet is similar to the one I gave you earlier!
 
  • #12
Philip Wood said:
At O level, the type of question you're likely to meet is similar to the one I gave you earlier!

Oh, then it won't be so complicated as in this case. BTW is the name of the transferred resistance called impadence?
 
  • #13
Well, it is a sort of impedance, but it's mainly resistive, if the transformer has a resistive load connected to its secondary! [A fuller discussion of these matters would, I think, be of no help to you at the moment!]
 
  • #14
Philip Wood said:
Well, it is a sort of impedance, but it's mainly resistive, if the transformer has a resistive load connected to its secondary! [A fuller discussion of these matters would, I think, be of no help to you at the moment!]

hi Philip Wood, I was also thinking if I have a step up transformer which steps up the voltage to 1000V and it has a total effective resistance of 10ohms (including wires and all at the secondary transformer circuit), then the power developed will be P=VI so its 100000W right? But now if I use the same circuit just that I step it down to 250V, then won't my power be lower now (6250W)? So when doing questions how can they assume the total power developed when using the different values of voltage in the secondary circuit (includes power losses in wires)? Won't the power be different in other step up/down voltages?
 
  • #15
Hallo sgs. I'm afraid I find the details of your post quite hard to follow. I can't help but feel that you need to tackle some specific calculations on transformers. They will give a better basis for discussion on this forum, though you may well find that just doing them makes things come clear. You could start with the exercise I recommended for you to do in post 5 on this thread.

I'd keep off 'effective resistance'. It's quite an advanced concept which is likely to confuse you at this stage. Sorry if this seems patronising, but I really think you should concentrate on using the basic equations you mentioned in the second paragraph of your first post.
 
  • #16
Hi sorry for being vague Mr Wood, I meant that if I have the same resistance in my secondary circuit, and when I change the number of turns of the coil (hence changing the voltage output), then will my power also change? I'm able to do the basic stuff just that I'm not sure about these stuff... thanks!
 
  • #17
sgstudent said:
I meant that if I have the same resistance in my secondary circuit, and when I change the number of turns of the coil (hence changing the voltage output), then will my power also change?
Power will increase if you apply a higher voltage to a fixed resistance, that's a certainty.

Power = V² ÷ R

Whatever power is drawn by the load on the secondary side, is the power that is delivered to the transformer on its primary side.
 
  • #18
Ah, this is now clear. If you increase the secondary voltage output, then the power taken by the secondary load will increase, since [itex]P_{load} = \frac{V_{sec} ^2}{R_{load}}.[/itex]

Assuming 100% transformer efficiency, [itex]V_pI_p=V_sI_s[/itex]. So the power input to the primary is also given (approximately) by [itex]P = \frac{V_{sec} ^2}{R_{load}}[/itex].

This is basic stuff!

Challenge you to give me the answers to the exercise I recommended in post 5.
 
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  • #19
Hi Mr Wood, I was thinking about this again. So does it mean I always take the power of each circuit by choosing the secondary one? Since in the primary one there is some unknown additional resistance? Or will the question phrase it such that I can only choose one of the transformers if there are more than 1 in the whole circuit.

For example if they give us two of the individual transformer circuit's voltage and resistance eg A has 100V and 20ohm while B has 200V and 5ohm. So the power assuming no power loss will be A's as it has the lowest power which means in B, there is some of that transferred resistance that you mentioned earlier?
 
  • #20
I'm not sure I was much help last time we discussed these transformer issues. I was never quite sure I'd understood your viewpoint. I'm therefore going to refrain from trying to answer general questions of the sort you pose. What I'd be happy to do is to consider specific problems, perhaps taken from a textbook, with numerical data. If we agree on the answers to these, then maybe we could proceed to more abstract discussion.
 
  • #21
Okay, for example there is primary transformer which is connected to a secondary transformer which then is connected to another transformer. in the first secondary transformer, the emf across it is 200V while the resistance is 10Ω. So the P=200X20=4000W. In the other transformer, the emf across it is 100V and the resistance is 10Ω. So the P=100X10=1000W. So will the actual power across all the transformers be 1000W because there might be some transferral resistance in the first transformer?
 
  • #22
sgstudent said:
Okay, for example there is primary transformer which is connected to a secondary transformer which then is connected to another transformer. in the first secondary transformer, the emf across it is 200V while the resistance is 10Ω. So the P=200X20=4000W. In the other transformer, the emf across it is 100V and the resistance is 10Ω. So the P=100X10=1000W. So will the actual power across all the transformers be 1000W because there might be some transferral resistance in the first transformer?
If you have resistors getting hot and dissipating energy, regardless of how they are connected or powered, the powers add. So from the power company you are drawing 5000W.

If I have misunderstood your description, then you need to submit a circuit diagram of what you are suggesting.
 
  • #23
hi i think you got my question wrong. Here is a picture to better put forth my idea
http://postimage.org/image/5bbyqownz/full/

So will the actual power be 1000W for each transformer?
 
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  • #24
There is not much point in drawing a schematic and including a text label RT without showing the resistor itself.

Perhaps you mean that the first transformer is outputting 20A at 200V. In that case, it is drawing 1000W from the supply. That power has to be going somewhere, but nowhere in your schematic do you show a resistance dissipating the power. Power doesn't just disappear for no reason; where is the load?
 
  • #25
oh sorry i meant that the circuit would just dissipiate it. But also, the 2nd secondary has a higher power. So will that transferral resistance come into play here?
 
  • #26
I've fallen at the first hurdle. You say: "Okay, for example there is primary transformer which is connected to a secondary transformer..."

A conventional transformer has (at least) two coils, usually called 'primary' and 'secondary'.
If you talk about primary and secondary transformers, it sounds as if you have two transformers (each with two coils). Perhaps that's what you mean.

Or, when you say "primary transformer", do you actually mean "primary coil of transformer", an issue I thought we'd nailed in post 7 above? If that's what you do mean, then why would you connect the primary (coil) to the secondary (coil)?

I'm confused. As NascentOxygen says, a circuit diagram would be useful. For posting on this forum I hand-draw, scan and attach as a thumbnail. Quick and easy to do. It must be, or I wouldn't be able to do it!
 
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  • #27
sgstudent said:
oh sorry i meant that the circuit would just dissipiate it.
What circuit? Where is the resistance dissipating the power? Why can't you draw it?
 
  • #28
Perhaps you don't understand transformers. The symbol for an ideal transformer is of two coupled windings having a turns ratio of n. All a transformer does is transform voltages (by a factor n) and currents (by a factor 1/n). End of story.

If there is no resistor present as a load, there is no current. No current means no power. No power means ZERO WATTS.

You wrote a resistance value near the transformer, but showed no resistor symbol anywhere. But there are no invisible resistors associated with ideal transformers, no magic power sinks, nothing that unaccountably gobbles power without having a symbol. There are just two coupled windings. If there is zero current in one of the windings, there will be zero current in the other. Zero current equates to zero power.

The diagram you provided shows no resistors, so this means the resistance is infinite. Writing a resistance value in Ohms means nothing when there is no resistance shown to which it can apply. If there is no load on a transformer, we have to conclude the resistance must be infinite. A load can be a resistor (e.g., a light globe), or a motor, or a battery, or any device that draws current and uses power. Your circuit shows none of these.
 

Related to Electrical energy transmission and transformer

1. What is electrical energy transmission?

Electrical energy transmission is the process of transferring electricity from one location to another. This is typically done through a power grid which consists of power lines, transformers, and other equipment.

2. How is electrical energy transmitted?

Electrical energy is transmitted through power lines using alternating current (AC). The power lines carry the electricity from power plants to substations, where it is then stepped down and distributed to homes and businesses through smaller power lines.

3. What is a transformer and how does it work?

A transformer is an electrical device that is used to change the voltage of electricity. It consists of two coils of wire, a primary coil and a secondary coil, which are wrapped around a core. When an alternating current is passed through the primary coil, it creates a magnetic field that induces a current in the secondary coil, thus changing the voltage.

4. What is the purpose of a transformer in electrical energy transmission?

The main purpose of a transformer in electrical energy transmission is to step up or step down the voltage of electricity. This is necessary for efficient transmission and distribution of electricity over long distances. High voltage is used for long-distance transmission, while lower voltage is used for distribution to homes and businesses.

5. What are the different types of transformers used in electrical energy transmission?

The two main types of transformers used in electrical energy transmission are step-up transformers and step-down transformers. Step-up transformers increase the voltage for long-distance transmission, while step-down transformers decrease the voltage for distribution. Other types of transformers used include autotransformers, instrument transformers, and isolation transformers.

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