How to find the maximum inrush current of a transformer
Reiner Goetz
The transformer is a 160VA type. 230V AC 50Hz .
B = µ * H
magnetic induction = µ * field strength (sorry! I hope this are the
right word's)
If I switch on the transformer when the input voltage is going trough
zero and there is no flux stored in the transformer, B will follow the
magnisation graph.
I think, the result is the maximum inrush current.
If there is a flux stored in the transformer and I switch it at the zero
point on, it may occur, that the residential flux will add to the "new"
flux. The transformer can lost is inductance and the current will only
limited by the Ohm resistance of the transformer.
Is that right?
Can we say: to switch on the transformer at the zero point of the input
voltage, we produce the maximum input current witch can occur.
kind regards
Reiner
Kevin Aylward
"Reiner Goetz" <reiner...@n.zgs.de> wrote in message news:3C4AE792...@n.zgs.de...
If the transformer is a power supply one driving a diode capacitor arrangement then the above might be mute. There will be a large
current surge to charge the capacitor up, limited only by transformer resistance etc.
Kevin Aylward , Warden of the Kings Ale
ke...@anasoft.co.uk
http://www.anasoft.co.uk - SuperSpice "Seriously...Why pay more?",
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John Popelish
>
> I have a question about the inrush current of a transformer.
> The transformer is a 160VA type. 230V AC 50Hz .
>
> B = µ * H
> magnetic induction = µ * field strength (sorry! I hope this are the
> right word's)
>
> If I switch on the transformer when the input voltage is going trough
> zero and there is no flux stored in the transformer, B will follow the
> magnisation graph.
>
> I think, the result is the maximum inrush current.
>
> If there is a flux stored in the transformer and I switch it at the zero
> point on, it may occur, that the residential flux will add to the "new"
> flux. The transformer can lost is inductance and the current will only
> limited by the Ohm resistance of the transformer.
>
> Is that right?
Almost. The current will be limited only by the resistance and air
core inductance of the primary for the part of the half cycle after
the core saturates.
> Can we say: to switch on the transformer at the zero point of the input
> voltage, we produce the maximum input current witch can occur.
>
> kind regards
> Reiner
I think so.
--
John Popelish
Bill Sloman
I think the second proposition is correct - there can be enough flux
stored in the transformer core for an unfortunately timed switch-on to
drive the core into saturation.
One the core is saturated, the only limitation on the primary current
is the resistance of the primary windings. It is a long time since I
saw the analysis of this condition and its consequences, but IIRR, the
condition would sort itself out over a bout ten cycles if the primary
current didn't heat the core above its Curie point in the process. I
got a lot more interested in "soft-start" and "anti-inrush" circuits
after that.
---
Bill Sloman, Nijmegen
John Woodgate
<ke...@anasoft.co.uk> wrote (in <xwC28.56573$WQ1.8...@news6-win.serve
r.ntlworld.com>) about 'How to find the maximum inrush current of a
transformer', on Sun, 20 Jan 2002:
>If the transformer is a power supply one driving a diode capacitor arrangement
>then the above might be mute.
moot (The OP is not a native English speaker, so might be confused. I
don't 'do' typos unless for a good reason.)
> There will be a large
>current surge to charge the capacitor up, limited only by transformer resistance
>etc.
Indeed, and *that* inrush current is highest if switching occurs at peak
voltage. With a storage scope, and preferably with a switch controlled
by a zero-crossing detector, you can see the combined effect of both
inrush phenomena, with a quarter-cycle timing difference.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
After swimming across the Hellespont, I felt like a Hero.
John Woodgate
wrote (in <3C4AF560...@rica.net>) about 'How to find the maximum
inrush current of a transformer', on Sun, 20 Jan 2002:
>core inductance of the primary for the part of the half cycle after
>the core saturates.
The air-core inductance is usually negligible.
John Woodgate
<reiner...@n.zgs.de> wrote (in <3C4AE792...@n.zgs.de>) about
'How to find the maximum inrush current of a transformer', on Sun, 20
Jan 2002:
>The transformer is a 160VA type. 230V AC 50Hz .
>
>B = µ * H
>magnetic induction = µ * field strength (sorry! I hope this are the
>right word's)
>
>If I switch on the transformer when the input voltage is going trough
>zero and there is no flux stored in the transformer, B will follow the
>magnisation graph.
>
>I think, the result is the maximum inrush current.
Yes, apart from the case you describe next.
>
>If there is a flux stored in the transformer and I switch it at the zero
>point on, it may occur, that the residential flux will add to the "new"
>flux. The transformer can lost is inductance and the current will only
>limited by the Ohm resistance of the transformer.
>
>Is that right?
Yes.
>
>Can we say: to switch on the transformer at the zero point of the input
>voltage, we produce the maximum input current witch can occur.
>
Yes. In the second case, the inrush current is highest, of course.
Phil Allison
"Kevin Aylward" <ke...@anasoft.co.uk> wrote in message
news:xwC28.56573$WQ1.8...@news6-win.server.ntlworld.com...
>
>
>
> "Reiner Goetz" <reiner...@n.zgs.de> wrote in message
news:3C4AE792...@n.zgs.de...
> > I have a question about the inrush current of a transformer.
> > The transformer is a 160VA type. 230V AC 50Hz .
> >
> > B = µ * H
> > magnetic induction = µ * field strength (sorry! I hope this are the
> > right word's)
> >
> > If I switch on the transformer when the input voltage is going trough
> > zero and there is no flux stored in the transformer, B will follow the
> > magnisation graph.
> >
> > I think, the result is the maximum inrush current.
> >
> > If there is a flux stored in the transformer and I switch it at the zero
> > point on, it may occur, that the residential flux will add to the "new"
> > flux. The transformer can lost is inductance and the current will only
> > limited by the Ohm resistance of the transformer.
> >
> > Is that right?
> >
> > Can we say: to switch on the transformer at the zero point of the input
> > voltage, we produce the maximum input current witch can occur.
> >
> > kind regards
> > Reiner
> >
>
> If the transformer is a power supply one driving a diode capacitor
arrangement then the above might be mute. There will be a large
> current surge to charge the capacitor up, limited only by transformer
resistance etc.
** The above point is somewhat "mute". The dominant current surge at
switch on is due to core saturation. In a 160 VA tranny this could be up to
about 25 amps peak from a 240 volt supply. This causes a very large primary
voltage drop so the secondary voltage is reduced for that and subsequent
half cycles of the same polarity. (The magnetic current surge pulses are all
the same polarity each time you switch on.)
A rectifier and filter cap will add to the primary surge current and
this is not mains polarity conscious. Since the secondary winding resistance
and any leakage inductance are now in play the surge current pulses are
somewhat supressed compared to the magnetic ones.
Regards, Phil
John Woodgate
wrote (in <7c584d27.02012...@posting.google.com>) about 'How
to find the maximum inrush current of a transformer', on Sun, 20 Jan
2002:
>saw the analysis of this condition and its consequences, but IIRR, the
>condition would sort itself out over a bout ten cycles
Well, if you mean 'get within a small percentage of the steady-state
condition' then about 10 cycles is right. But the BIG inrush lasts only
one half-cycle; the rest of the current peaks are nothing to get
concerned about.
The insulation would melt before the Curie point was reached, if the
core is silicon-iron.
Reiner Goetz
Reiner
John Woodgate schrieb:
Reiner Goetz
I understand this.
When I have a transformer with a rectifier and a cap two effects are added.
There is the core saturation and the current into the cap.
In both cases the current are limited by the winding resistance.
Is there any way to calculate the maximum inrush current (F(t)), and how many
periods are necessary to bring the transformer in a normal condition!
kind regards
Reiner
Phil Allison schrieb:
Trevor Wilson
> Ok!
> I understand this.
> When I have a transformer with a rectifier and a cap two effects are
added.
> There is the core saturation and the current into the cap.
> In both cases the current are limited by the winding resistance.
> Is there any way to calculate the maximum inrush current (F(t)), and how
many
> periods are necessary to bring the transformer in a normal condition!
>
**Just measure your primary resistance. This will provide you with a worst
case condition. Figure on a couple of cycles, to complete capacitor charging
(unless you use a surge limit resistor).
--
Trevor Wilson
www.rageaudio.com.au
Phil Allison
> Ok!
> I understand this.
> When I have a transformer with a rectifier and a cap two effects are
added.
> There is the core saturation and the current into the cap.
> In both cases the current are limited by the winding resistance.
> Is there any way to calculate the maximum inrush current (F(t)), and how
many
> periods are necessary to bring the transformer in a normal condition!
>
> kind regards
> Reiner
>
A simple worst case figure is to divide the mains peak voltage by the
primary resistance. That is how I came up with the 25 amp figure in my
previous post. The peak current can get close but not exceed this figure.
The number of cycles to recover to steady state conditions is variable
depending on the type of transformer, E-core of toroidal etc. It also varies
with the severity of the first half cycle pulse.
I feel it is really easier to do a measurement or a Spice type model may
work if you have all the magnetic data on the core.
Regards, Phil
John Woodgate
'How to find the maximum inrush current of a transformer', on Sun, 20
Jan 2002:
>There is the core saturation and the current into the cap.
>In both cases the current are limited by the winding resistance.
>Is there any way to calculate the maximum inrush current (F(t)),
It can never exceed the peak supply voltage divided by the d.c.
resistance of the primary winding. But to calculate the waveform is at
lest exceedingly difficult. The system is described by non-linear
differential equations
> and how many
>periods are necessary to bring the transformer in a normal condition!
That's also difficult. You can usually use 'Sloman's Rule' - it takes
about 10 periods of the supply to settle down. (;-)
John Woodgate
'How to find the maximum inrush current of a transformer', on Mon, 21
Jan 2002:
>I measured the inrush current and the voltage at the cap.
How did you trigger the oscilloscope? You must trigger ONLY from the
supply voltage, and use 'chopped', not 'alternate', to display the
waveforms.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
After swimming across the Hellespont, I felt like a Hero.
PLEASE do NOT copy news posts to me by E-MAIL!