Power company transformer

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Re: Power company transformer

Sparks, I think you've got it. Imagine an unbalanced resistive load. Currents flow in both halves of the secondary, but the neutral current flows only in the half with the heavier load. The primary does not know the load is unbalanced. It merely provides the reflected load current such that the back emf is maintained.
 
Re: Power company transformer

you can model a transformer as an AC source in certain problems, but its operation is quite different from that of an alternator.
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True.

Although the primary and secondary currents are separate and never "mix", the primary current is actually regulated by the secondary load.

a. With the primary energized, and no load (or fault) on the secondary, the secondary current is zero, and the only current that flows in the primary winding is an excitation current. This is quite small, generally only a few percent of rated current, because the core flux, and thus the primary impedance, is high and is mainly inductive reactance.

b. At constant frequency, the only variable in the impedance of either winding is the inductance which is proportional to core flux.

c. When a load is connected, and current flows in the secondary, it creates a flux that is in opposition to, and therefore reduces, the core flux created by the exciting current. This reduces the primary impedance, allowing the increase in primary current required to supply the load.

d. Any increase in load current in the secondary, creates an increase in the opposing flux, further reducing the core flux, and thus the primary impedance, allowing the increase in primary current required to supply the additional load.

e. Under no fault conditions, for any increase in primary current to occur (in addition to the exciting current), there must be an increase in the load current flow in the secondary. In other words, primary current is proportional to, and is controlled by, the secondary current.

Ed
 
Re: Power company transformer

Wouldn't these same principles apply to a generator, of course we are talking about the usage of fuel to turn the generator instead of primary current?

With a xfmr, the primary flux field is constantly changing, so there is no need for relative motion between the primary coil and the secondary coil.

With a generator, the primary flux field is constant, therefore we have to move the secondary coil to provide the relative motion.

??
 
Re: Power company transformer

Ed,

In your comments on transformers, you should also mention the equivalent resistor in parallel with the primary inductance which accounts for core loss.

Inductance is not a variable! A variable inductance would be useless for computation.

I would say that the primary and secondary load currents are proportional. You have lumped the no load currents in with the load current.

I would also mention the leakage inductances and series resistances which reduce the secondary voltage under load.
 
Re: Power company transformer

Crossman,

I think that the xfrmr/alternator analogy is a poor one. I prefer to tell it like it is and just say that the xfrmr transforms voltages and currents according to the turns ratio.
 
Re: Power company transformer

If we hold the secondary coil stationary and if we put DC on the primary coil and spin it around the secondary coil, the physics is pretty much the same i think
 
Re: Power company transformer

Crossman,

It is all based on the changing magnetic field, but we should not throw two machines at the bewildered student at one time. I have been there, I have been confused, and I still have to go off in a corner sometimes to dope things out.

I spend a lot of time in the corner since I joined this website!
 
Re: Power company transformer

Inductance is not a variable! A variable inductance would be useless for computation.
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If the inductance doesn't vary, what does?

The bottom line is - the impedance of the primary winding must decrease in order for the primary current to increase as load is applied.

The resistance can't change, so it has to be the inductive reactance (XL) component of the impedance that is varying.
I realize that it is mainly the CEMF that is controlling the primary current, and that it is dependant on flux density.

XL = 2PiFL

At constant frequency, what else is there?

Ed
 
Re: Power company transformer

Ed,

Refer to the xfrmr equivalent circuit I posted (with your help) some time ago.

The reflected load impedance changes as you increase the load on the secondary, but none of the transformer parameters change. Of course, in the real world, they do change somewhat, but they are not functions of time. They are not variables. We choose values which we think best suit the problem at hand and understand the probability of error.

Simply put, impedances and resistances must be considered to be constants to be of any value in computation.

Inductance is determined by the number of turns, the magnetic properties of the core, and the geometry of the core. The ideal inductor does not change its value under any conditions. Real inductors do change somewhat, but we still consider the value constant for any given problem.
 
Re: Power company transformer

I think we all have a decent grasp of what is acutally happening, it is just that we have phylosophical differences on how to explain it.

For a single coil inductor, inductive reactance = 2PiFL and this is from "self inductance"

Perhaps for a transformer, because of the interaction of the 2 magnetic fields from the two seperate currents, the 2Pi is what needs to be different?

BTW, this post is a target! ;)
 
Re: Power company transformer

Take a look at Sparks1's last question. . .I think this gets back to the point.
posted February 20, 2005 09:00 PM
Take for example, a transformer with a 120/240 volts delta connected secondary with the center tapped neutral. This carries the unbalanced load to the center tap point and then through the winding of the closed loop as it continues to flow around the closed loop the current is magnetically induced back into the primary and back to the poco in either direction. Is this the correct path!
Thanks.
Phil
Click to expand...
Phil, I think this is a great spot to return to. . .what is the primary hookup of the transformer bank? The simplest, to start with will be a delta primary / delta secondary. Then, the question of what happens to the neutral current, the unbalance on the 120/240 center tap between A & B phases, is clearer.

There is no H2 connection to a primary transmission side neutral.

Yet there is neutral current on the secondary side.

The energy in the primary circuit changes from electrical to magnetic and is in the magnetic circuit of the core. And then the energy in the magnetic circuit is converted back to electrical energy in the secondary.

The construction of the secondary winding(s) determine the nature of the electrical properties on the secondary side.
 
Re: Power company transformer

Crossman,

There you go again, stirring things up!

In a transformer, there is the primary inductance, Lp, which determines the magnetizing current. Then there is a coupling coefficient which tells us how much of the flux is coupled to the secondary or vice versa. In power xfrmrs this coefficient approaches unity.
 
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