magikal,

In a transformer with 2 solenoids of different turn number mounted on a magnetic core powered by an AC source, why does the secondary EMF decrease as frequency of the power source is increased? What is the theoretical cause of this?

You have not defined the question very well. A solenoid is not used to couple with another solenoid in order to transform a voltage/current. It is used to move a plunger in order do some physical work. A transformer has no moving parts, and forms an inductive storage system that transforms voltage/current.

Next, what are you asking? Are you inquiring about driving the transformer higher than its design frequency? The voltage will then decrease due to parasitic effects as enumerated by the responders above. Otherwise within its design frequency, the voltage will not change much.

Think of it this way. Suppose dI/dt is 10 amps/1 sec for a value of 10 . A doubling of the frequency will decrease the time to 1/2 sec so that dI/dt = 10/(1/2) = 20 . Doubling of the frequency will also double the inductive reactance of the transformer. That in turn will halve the current so that dI/dt = 5/(1/2) = 10 , which is what we had before. So it balances out until the frequency becomes so high that secondary effects take over.

Ratch

 

magikal,



You have not defined the question very well. A solenoid is not used to couple with another solenoid in order to transform a voltage/current. It is used to move a plunger in order do some physical work. A transformer has no moving parts, and forms an inductive storage system that transforms voltage/current.

Next, what are you asking? Are you inquiring about driving the transformer higher than its design frequency? The voltage will then decrease due to parasitic effects as enumerated by the responders above. Otherwise within its design frequency, the voltage will not change much.

Think of it this way. Suppose dI/dt is 10 amps/1 sec for a value of 10 . A doubling of the frequency will decrease the time to 1/2 sec so that dI/dt = 10/(1/2) = 20 . Doubling of the frequency will also double the inductive reactance of the transformer. That in turn will halve the current so that dI/dt = 5/(1/2) = 10 , which is what we had before. So it balances out until the frequency becomes so high that secondary effects take over.

Ratch

At the risk of being even MORE hopelessly pedantic, you're WRONG! A solenoid is, by definition, the coil of wire. In its conventional use, it is indeed used as the mechanical device you describe. But all the basic electronics literature speaks of the inductance of a solenoid as the form itself.
http://www.w8ji.com/cw bandwidth analysis.htm
http://en.wikipedia.org/wiki/Solenoid

See also, the RCA Radiotron Handbook, for a historic perspective.

Now you know!

Eric

 

And knowing is half the battle.. Go Joe!

 

KL7AJ,

At the risk of being even MORE hopelessly pedantic, you're WRONG! A solenoid is, by definition, the coil of wire. In its conventional use, it is indeed used as the mechanical device you describe. But all the basic electronics literature speaks of the inductance of a solenoid as the form itself.

No, I am not wrong. We are both right. You should read the link you presented, especially the second paragraph. You are presenting the definition in terms of physics, as described in the first paragraph. Whereas I was thinking about the second paragraph which defines it from an engineering viewpoint. Whichever way you define it, the important thing is that a transformer is NOT described as two mutually coupled solenoids. http://en.wikipedia.org/wiki/Transformer

Now you know!

But did you?

Here is an interesting story. Several years ago I went to a GM auto dealership to purchase a replacement for my broken electric door lock. The parts man kept insisting that is was a "motor" even though it most certainly was a solenoid. It was listed in his parts catalog as a motor, and the fact that it did not have a commutator could not dissuade him.

triggernum5,

And knowing is half the battle.. Go Joe!

Knowing what, specifically?

Ratch

 

I believe in England, electromagnets are often referred to as electric motors.

So.....how would you describe concentric solenoids with lots of mutual inductance....such as the "Loose Coupler' of crystal radio yore? Or any link-coupled R.F. circuit for that matter? Other than the fact that there's no iron core, they are still transformers. And solenoids.

Of course, there are transformers that are NOT solenoids, as well. Toroidal wound transmission line transformers, for example. Or parallel-line hybrids. Or even your garden variety directional coupler.

And if you REALLY want to be pedantic, a 1:1 transformer can't really be called a transformer, because it neither transforms current NOR voltage. All it does is transform dollar bills from your wallet into copper and iron.

Ahhh....now where were we?

Eric

 

Using the term 'solenoid' for the coils on a transformer is just fine and correct.

 

KL7AJ,

I believe in England, electromagnets are often referred to as electric motors.

Even when it doesn't have a commutator? So the flippers of a pinball machine are "motorized?"

So.....how would you describe concentric solenoids with lots of mutual inductance....such as the "Loose Coupler' of crystal radio yore? Or any link-coupled R.F. circuit for that matter? Other than the fact that there's no iron core, they are still transformers. And solenoids.

So you are going to call every inductive system within a part or appliance a solenoid? See discussion below.

Of course, there are transformers that are NOT solenoids, as well. Toroidal wound transmission line transformers, for example. Or parallel-line hybrids. Or even your garden variety directional coupler.

According to your link, "the term solenoid refers to a loop of wire". So by that reasoning, just about any transformer can be termed a "solenoid".

And if you REALLY want to be pedantic, a 1:1 transformer can't really be called a transformer, because it neither transforms current NOR voltage. All it does is transform dollar bills from your wallet into copper and iron.

Yes, it can. It transforms the input into an isolated output, so is therefore useful.

Discussion: So if every loop of wire is a solenoid, why even have the word? You cannot make any practical inductive component without looping the wire one way or another. It is a matter of semantics, but I would not call a coil of wire that is closely bound to another coil of wire, as in a transformer, a solenoid. My definition of a solenoid would be an inductance that operates in relative isolation from any other inductance. If two inductances work together, then they should be called a transformer, or coupler, or whatever. But not solenoids.

Ahhh....now where were we?

We were explaining to the OP why the output voltage of a transformer becomes less at higher frequencies. I believe I summed it up previously.

BillO

Using the term 'solenoid' for the coils on a transformer is just fine and correct.

Not if you consider a solenoid to be a independent inductance from an engineering viewpoint.

Ratch

 

It is a matter of semantics,

Physician, heal thyself.

 

thingmaker3,

Physician, heal thyself.

I am not a physician and have nothing to heal.

Ratch