Your idea that first the transformer starts up, then, once the transformer is all started up and cozy, the capacitors are charged is fundamentally flawed. Sorry dude. Physics don't work that way...
Any current is needed to establish the magnetic field in the core will be added to the current needed to charge the caps. As the magnetic field develops in the core, energy will be transferred to the secondary to charge the caps. You will not see one pulse from the transformer and another from the caps. The two will merge and become indistinguishable from one another. As I mentioned earlier, you can run a sim to convince yourself of this. Or set up an experiment in the lab if you don't believe in math/physics/computer simulation.
Tom
Any current is needed to establish the magnetic field in the core will be added to the current needed to charge the caps. As the magnetic field develops in the core, energy will be transferred to the secondary to charge the caps. You will not see one pulse from the transformer and another from the caps. The two will merge and become indistinguishable from one another. As I mentioned earlier, you can run a sim to convince yourself of this. Or set up an experiment in the lab if you don't believe in math/physics/computer simulation.
Tom
Progress.
You have shown that you knew all along that two separate mechanisms are at play during and shortly after start up.
Yes, the durations of two currents do overlap. They do not occur in separate moments in time. But their timings are different.
When the transformer starts up, it is incapable of giving a secondary current, because the core has saturated.
At this instant in time the primary current is at it's highest. This is the transformer start up current that we use a soft start to limit to a reasonable level such that fuse/MCB does not open.
As the transformer starts to operate the secondary starts to develop an output and charging current begins to flow. This increases rapidly over a few cycles of the mains as the transformer begins to "transform" to the secondary voltage. By now the start up current is down to normal operating current.
This charging current has been transformed from the primary current.
This next seems to be the only sticking point:
I claim:
The primary charging current is less than the primary start up current.
You have shown that you knew all along that two separate mechanisms are at play during and shortly after start up.
Yes, the durations of two currents do overlap. They do not occur in separate moments in time. But their timings are different.
When the transformer starts up, it is incapable of giving a secondary current, because the core has saturated.
At this instant in time the primary current is at it's highest. This is the transformer start up current that we use a soft start to limit to a reasonable level such that fuse/MCB does not open.
As the transformer starts to operate the secondary starts to develop an output and charging current begins to flow. This increases rapidly over a few cycles of the mains as the transformer begins to "transform" to the secondary voltage. By now the start up current is down to normal operating current.
This charging current has been transformed from the primary current.
This next seems to be the only sticking point:
I claim:
The primary charging current is less than the primary start up current.
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AndrewT is right. Plus, you can't readily sim this startup event, because whether or not the core saturates depends on the residual magnetic state of the core and the point on the mains cycle. It's core saturation which causes the (large) inrush current event which blows mains trips, the transformer isn't a transformer in that state.........
I've had toroids and r cores with no secondary load connected generate inrush currents of several hundred amps for a few cycles on 120Vac - more than enough to cause problems with fuse blowing or breaker tripping. Starting at zero crossing seems to be about the worst case scenario for magnetizing current misbehavior.
Ah! Saturation. That makes sense. I wish you'd mentioned that earlier instead of glossing over the details.
Tom
Andrew, you should know me well enough by now to know that I don't follow orders without some grumbling. "Thou shalt follow this procedure" without an explanation why goes nowhere with me. I don't operate by memorization. I operate by learning, understanding, and applying science.
An answer like this would have worked better:
I do understand that some people operate by memorization. I'm not one of those people, though. I cannot push the state of the art that way. I need to understand the underlying principles. I will openly admit that I don't know everything. I will keep learning until the day I die.
You are correct that the bottom line is: Use a soft-start or an oversized fuse with a large transformer.
Tom
An answer like this would have worked better:
This took me about 90 seconds to type, gets the relevant information across efficiently, and would have saved over two pages in this thread.
I do understand that some people operate by memorization. I'm not one of those people, though. I cannot push the state of the art that way. I need to understand the underlying principles. I will openly admit that I don't know everything. I will keep learning until the day I die.
You are correct that the bottom line is: Use a soft-start or an oversized fuse with a large transformer.
Tom
All knowledgable electronics workers already know this. It is repeated in this Forum all too regularly. The Members do not need to go back to first principles every time another queries a statement. The knowledge is already here. It just needs some effort/research to become "enlightened".
Except I don't restrict this to large transformers.
The "oversized fuse" applies to all motors and transformers and similar highly inductive loads.
Fuse = 3times VA / Vac.
It's that 3times factor that gives one the "oversized fuse" instead of the close rated fuse.
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