Transformers

[hightower]

Probably a really basic question, but something that has been bothering me never-the-less. Say there's a traditional 2-1 transformer (so couple of windings on a core), and the secondary has no load on it (or is open circuit by a switch etc). What happens in the primary - does it keep using power wastefully? Is the current flow the same in the primary whether or not the secondary has a load?

 

[Bobster]

Probably a really basic question, but something that has been bothering me never-the-less. Say there's a traditional 2-1 transformer (so couple of windings on a core), and the secondary has no load on it (or is open circuit by a switch etc). What happens in the primary - does it keep using power wastefully? Is the current flow the same in the primary whether or not the secondary has a load?

Current flow is proportional to the output through a transformer.

If the secondary is open circuit there is still a load on the primary, just out of proportion.

This gets to being quite a heavy subject fairly quickly, not sure what they even teach you about transformers in college now, so I'm not sure how I should gear an answer for you.

 

[hightower]

Thanks Rob. I suppose what I'm asking is how can the primary adjust based on the load of the secondary - in my mind the primary still has the same resistance and the same voltage at source so surely it should still use a load of power regardless of what the secondary is doing.

I know this statement is wrong, but hopefully it shows you my confusion so you can help me understand better.

We learned about transformers at a basic level at level 2. I hope it's something we cover further at level 3

 

[davesparks]

As a basic view the input current of the transformer varies according to the load on the secondary side. The power input will be equal to the power output plus the total losses within the transformer itself (heat, iron losses etc)

As I said, it's a basic overview but a good starting point

 

[hightower]

Thanks dave, but how does the secondary tell the primary what current it is using so the primary can provide the equivalent, is it the magnetic effect that the secondary gives off that sort of says 'I need a bit more power scotty' to the primary.

Sorry, having real trouble trying to explain what it is exactly in struggling with

 

[telectrix]

bedtime reading.:

Theory of Transformer on Load and No Load Operation | Electrical4u

 

[Bobster]

Thanks dave, but how does the secondary tell the primary what current it is using so the primary can provide the equivalent, is it the magnetic effect that the secondary gives off that sort of says 'I need a bit more power scotty' to the primary.

Sorry, having real trouble trying to explain what it is exactly in struggling with

I can understand exactly what you're trying to ask.

I had similar thoughts when I set off on my career. However after spending 25 years designing and maintaining transformers I'm now struggling to think of how to give a simple explanation.

Give me a bit of time an I'll put something together.

 

[hightower]

Is this something way above my pay scale sort of speak? I mean am I getting my knickers in a twist over something I should just accept is so until further on in my studies?

 

[davesparks]

Is this something way above my pay scale sort of speak? I mean am I getting my knickers in a twist over something I should just accept is so until further on in my studies?

you may want to make it a longer term research project/learning goal, but certainly don't give up on it as beyond your ability.

You might just need this knowledge one day, even if it is the day when a trainee asks you the very same question!

 

[Marvo]

It's because of the magnetic flux in the iron core of the transformer. When there's no load on the transformer the magnetic flux induced into the transformer core by the primary impedes the flow of current in the primary. When a load is applied to the secondary current flows in it and the magnetic flux caused by the secondary winding current opposes the flux caused by the primary which allows current to flow.

As Rob points out it's something of a rabbit hole and I'm also not sure how deep into this you need to go. Do you need to be able to make current calcs for different ratio transformers? Do they teach you Lens's Law and how it relates to transformers?

 

[davesparks]

I'd appreciate it if one of my more learned colleagues here would check this over, but I think I've got it simplified.

With no load on the secondary the primary winding acts as one large inductor with a rather large inductive reactance and therefore high impedance. A large current will flow initially as the magnetic field establishes, but then a very small current will flow due to the impedance.
But of course a potential difference is still created between the secondary terminals due to induction.
Once a load is connected across these terminals current will flow through the load and secondary winding as a result of that potential difference.
This flow of current in the secondary establishes its own magnetic field within the transformer.
The interaction of these magnetic fields (I want to say it has a cancelling effect, but my knowledge is a bit shakey and it doesn't sound right) effectively reduces the inductive reactance and allows a greater current to flow in the primary.

 

[hightower]

Thanks, unbelievably this makes a lot of sense. So effectively it is the magnetic affect from the secondary that acts as speed control on the primary side....

I don't really need to go much deeper than this Marvo, at least for now. This is just something that has been bothering me at a fundamental level - these things seem to pop in to my head one day and then eat away at me until I find the answer.

 

[Marvo]

Thanks, unbelievably this makes a lot of sense. So effectively it is the magnetic affect from the secondary that acts as speed control on the primary side....

Yes, the magnetic effect of the primary is like a handbrake when there's no load on the secondary, the magnetic effect when there's current flow in the secondary opposes the magnetic effect of the primary releasing the handbrake.

It's a bit of a mindset thing where you've got to get it out of your head that pure resistance is the only thing that determines current flow, as you get further on in your studies you'll find that impedance which is also expressed in Ohms is actually more important, it's just that often resistance = impedance in many situations but not with transformers and motors where resistance + reactance = impedance.

 

[hightower]

So the handbrake isn't instant, it can take a little time to gradually release it fully which is by the magnetic affect slowly increasing? Obviously when I say slowly I don't mean an hour or 2 but just that it's not a binary effect such as on/off.

Thanks all for your help, it is sinking in. Just with my learning I find that if I don't know the fundamentals then I struggle to reinforce the why, and I find that knowing the why on the fundamental level is key to understanding on the subject as a whole - if that makes sense.

 

[hightower]

So to add, let's take the secondary out the equation and have just a coil on a plate - essentially it is nothing but a big inductor which is going to restrict current flow?

Bingo- the penny has well and truly dropped.