Solid core iron transformer? No eddy loss for pulsed d.c. <1000Hz?

Thread Starter

Hamlet

Joined Jun 10, 2015
333
Everything is an existential question if you think about it loooong enough, haha! Be glad I don't ask any existentially hypothetical questions...

Ahem: Does a transformer have an eddy loss for a single pulse of d.c. current? Solid core, laminated, ferrite, I don't care (air core?)
 

MisterBill2

Joined Jan 23, 2018
6,727
There is still inductance, since prior to T=0 the current (i) is zero, while after the pulse has started, T>0, the applied voltage is producing some current, and thus creating magnetic flux within the core material.And since there is changing flux there would be "eddy currents", and hence losses. The math might be tedious but the theory is very straightforward.
 

MisterBill2

Joined Jan 23, 2018
6,727
For most applications transformers are used for alternating current with some defined frequency range. So that is what most explanations address. My explanation is applicable for the time between T=0 and the time when the current has stopped changing. That is generally a quite short time period.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
333
There is still inductance, since prior to T=0 the current (i) is zero, while after the pulse has started, T>0, the applied voltage is producing some current, and thus creating magnetic flux within the core material.And since there is changing flux there would be "eddy currents", and hence losses. The math might be tedious but the theory is very straightforward.
Thank you. My math is not great, and most tutorials are either grade-school level or graduate-level.

I am thinking of several projects, both near and far, that require winding transformers, both open, and closed,
for receiving a singe-shot of current from a capacitor bank. If I am going to go to the trouble to wind
a transformer, I wanted to know if it was worthwhile to fabricate my cores from lamination's or not. I've made
several a.c. transformers for past projects, with laminations, and this is not a trivial endeavor. Carving a core
from solid steel or iron would greatly ease construction. I hope this helps impart understanding.

I have more questions related to this. Should I go on? Start a new thread?
 

Thread Starter

Hamlet

Joined Jun 10, 2015
333
For most applications transformers are used for alternating current with some defined frequency range. So that is what most explanations address. My explanation is applicable for the time between T=0 and the time when the current has stopped changing. That is generally a quite short time period.
How short? Is it dependand on size, or current, or is it a fixed constant?
 

BobTPH

Joined Jun 5, 2013
2,499
A square pulse includes very high frequency components and therefore will be distorted by any transformer. Ferrite is likely the better material if you want to minimize this distortion.

The loss / heating due to eddy currents is not an issue for short pulses with a long period between pulses, but repetitive pulses are AC with a DC component and will suffer losses / heating depending on the duty cycle.

Bob
 

MisterBill2

Joined Jan 23, 2018
6,727
How short? Is it dependand on size, or current, or is it a fixed constant?
The process continues until the magnetic material can not be magnetized any more. So usually it is a very few milliseconds. At that point the current will not be changing . The time does depend on how much iron and how many turns of wire and how much resistance in the wire. We derived the equations in my physics class back in 1967. So my recollection of the math is a bit faded.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
333
Thanks for sharing, guys. Looks like steel is better than cast iron.

If one were to build two identical transformers/electromagnets,
one with steel laminations, and one with a solid steel core,
how much loss are we looking at? (I understand Westinghouse
got a patent for laminated transformer design, which made AC
transmission possible, by reducing core losses.)

Obviously, heating would be a problem, over time, but putting
that aside, how much energy is lost to eddy/heat/et al. in a solid
core vs a laminated core? 2%? 12%? 30%?

(I could try refining my question, say, at 50% of max saturation,
or 100% saturation, but I'm not sure that helps, or my understanding
as yet is inadequate for an attempt to narrow the question.)
 

Thread Starter

Hamlet

Joined Jun 10, 2015
333
The losses in a solid steel core are great enough that even the cheapest transformer uses laminations. So they must be quite a bit.
Good point! I am with the understanding that once a transformer reaches saturation, the primary current skyrockets, with zero increase in output current. Yes/No?
 

Papabravo

Joined Feb 24, 2006
14,241
Good point! I am with the understanding that once a transformer reaches saturation, the primary current skyrockets, with zero increase in output current. Yes/No?
The voltage in the secondary is proportional to the field strength (Tesla) created by the primary. when the core saturates the field strength cannot increase further.
 

MisterBill2

Joined Jan 23, 2018
6,727
The voltage in the secondary is proportional to the field strength (Tesla) created by the primary. when the core saturates the field strength cannot increase further.
The secondary voltage is produced by the change in the magnetic field in the core. In addition, once the core saturates the transfer of energy stops, and so the impedance drops and that is why the current rises, because then the current is only limited by the resistance of the wire.
 

Papabravo

Joined Feb 24, 2006
14,241
The secondary voltage is produced by the change in the magnetic field in the core. In addition, once the core saturates the transfer of energy stops, and so the impedance drops and that is why the current rises, because then the current is only limited by the resistance of the wire.
Yes I should have said proportional to the change in field strength. When it stops changing at saturation there is no more there there.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
333
Thank you, one and all! (MrBill2, Papabravo, sparky 1)

As I suspected, there's a lot I don't know about the physics of transformers.
A most fascinating subject to chip-away at. Who knows, maybe this will inspire
me to build an identical transformer set, sans laminated core, and make some
side-by-side tests. The following I found on the educational side of this forum,
I thought it an interesting aspect of transformer behavior to include here:


https://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/practical-considerations-transformers/
Peaking Transformers

Special transformers known as peaking transformers exploit this principle to produce brief voltage pulses near the peaks of the source voltage waveform. The core is designed to saturate quickly and sharply, at voltage levels well below peak.

This results in a severely cropped sine-wave flux waveform, and secondary voltage pulses only when the flux is changing (below saturation levels):voltage-and-flux-waveforms-for-a-peaking-transformer.jpg
 
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