>> Because this is driving a purely resistive load, this doesn't help bring
>> the parameters within the transformer's rating, does it?

> No, a purely resistive load is best case.

 But since the transformer is (presumably) designed with a worstcase PF (or
 at least some PF value greater than 1), can't it be rerated (what's the
 opposite of "derated"?) if used with a strictly resistive load?

 In other words, a transformer rated at 10KVA for a load with PF = x, should
 be able to handle a greater KVA if driving a PF = 1 load. Shouldn't it?
What is relevant for a transformer rating is both amps and voltage in an
independent way. There is a maximum amperage each winding can handle.
There is a maximum voltage each winding (and the core) can handle. That
is why they are rated in terms of VA, kVA, MVA, etc. They are not rated
in W, kW, or MW for this reason. If your PF is 1, then you can get the
most power through the transformer ... e.g. you can run 10 kW through a
10 kVA transformer. But if your PF is only 0.5, then the most power you
can run through a 10 kVA transformer is 5 kW. Low PF forces you to derate
the transformer. Unity PF lets you run it at maximum rated power.
 I am sure you are correct, in theory. However, the oven is computer
 controlled and can't control until it gets power. Darn computers!
Batteries! Capacitors!
It definitely would  required lamination crossection depends on
power and frequency. If it is not enough then it would start
overheat and then transformation ratio would drop due to
insufficient magnetic flux.


REPLY

The turnon transient isn't similar to normal AC operation; a half
cycle
of positive potential in normal AC conditions always occurs
right after the core achieves maximum negative flux. If the
core flux starts at zero (or even slightly positive due
to remnant magnetism) and THEN is powered at the beginning of
a halfcycle of positive potential, the end of that halfcycle of
excitation will occur with 150% of the normal flux in
the core.
If there is a substantial load attached, the flux will be less than
150%,
and (of course) if there is a short circuit attached to the output,
there will be zero flux (but despite the nonsaturation of the core,
that won't help the blownfuse situation much).
True, the turn on isn't the same as steady state and I hope that I didn't
imply that. You are looking at the case when the transformer is energised at
the time when voltage passes through 0 which is a worst case situation. In
the case of a linear core transformer, the "steady state" flux will be at a
negative maximum so there is a bridging transient.
Krause and Wasynczulk, "Electromechanical motion devices" deals with a
transformer situation.
Linear model: no loadpeak current max amplitude 2*root(2)
Short circuited: peak current 7.4*root(2)
Loaded somewhere in between.
Including saturation: no load 8*root(2) peak but only on the first half
cycle.
The primary flux linkages are essentially the same for all cases.
A (rough) transient analysis of a 10kVA 1000/200 V transformer based on what
appears to be reasonable data indicates in a worse case case ignoring
saturation and leakage reactance does show that the magnetising component of
current is offset by somethng under 200% from the steady state peak values
for no load. There is With rated load, and the same data, the analysis
shows a larger time constant and a smaller transient bridging current but
the maximum inrush current is still higher than the no load inrush.
The culprit is saturation. I am not going to try to do a numerical analysis
of this as it is messy,but if saturation occurs, the d(phi)/di or inductance
of the core becomes small, Leakage reactance isn't affected as much because
part of the leakage path is outside the core. The reference above shows an
initial sharp reduction in the secondary voltage due to saturation along
with the limiting effect of primary impedance. Initial load current will
then also be limited so that it may not have the effect that you suggest.
It is a bit messier than you have indicated but, if you have contradictory
references, I would like to see them my observations are pretty much off
the cuff and if out to lunch, I would like to know why (and eat crow in that
case).

Not so.
Core losses which directly heat the core, are nearly independent of load
current. They are dependent on the effective applied voltage which will
decrease somewhat due to primary impedance voltage drops under load.
The core cross section depends on voltage, frequency, number of turns and
the maximum flux allowable. It doesn't depend on power except that larger
conductors will require more space and a longer core. This is a secondary
effect.
Hence, the core losses will actually be slightly higher at no load than
under full load not enough to normally be significant.
Laminations simply reduce the eddy current component of the core losses.
In the case of a fixed applied AC voltage, the flux is fixed independent of
the core material (Faraday or Maxwell take your pick). Magnetising current
follows as needed.
Yes I am being bitchy!
RE: Subject
This discussion has gotten to the point that maybe it is about time to start
talkingh about flux sucking shunts or even "The Sex life of An Electron",
starring Milly Henry.
Lew
Yep. It was my habit to do power surge testing on newbuilt
electronics with a switch and large nearlyunladen transformer,
because every tenth onswitch of the transformer generated
hum/arcing on the switch and significant powerline transients.
If my box survived that, I figured it'd work fine in the realworld
environment. HF from the switch arcing, 1cycle dropouts
from the transformer going into saturation, maybe a
bit of overvoltage (until the arc formed)... that power
outlet was guaranteed hostile enough for a good test.
The fuse went open with no load, so I assumed the
problem to be saturation of the core, and just looked at the
scenario for that one effect, and found a likely treatment.
I understand that maximumvoltage switching is another
treatment (there are solidstate relays for this).

Yep. It was my habit to do power surge testing on newbuilt
electronics with a switch and large nearlyunladen transformer,
because every tenth onswitch of the transformer generated
hum/arcing on the switch and significant powerline transients.
If my box survived that, I figured it'd work fine in the realworld
environment. HF from the switch arcing, 1cycle dropouts
from the transformer going into saturation, maybe a
bit of overvoltage (until the arc formed)... that power
outlet was guaranteed hostile enough for a good test.
The fuse went open with no load, so I assumed the
problem to be saturation of the core, and just looked at the
scenario for that one effect, and found a likely treatment.
I understand that maximumvoltage switching is another
treatment (there are solidstate relays for this).

While it appears to me that the load will do little to reduce saturation
effects, the concept of maximum voltage switching could be very effective
as such switching will be at flux zeros corresponding to the initial
conditions in the core eliminating the transient (assuming no residual
flux).
Enjoy
+++++++++++++++++++++++++++
SEX LIFE OF THE ELECTRON
(STORY OF MILLIE HENRY AND MICRO FARAD)
ONE NIGHT WHEN HIS CHARGE WAS PRETTY HIGH, MICRO FARAD DECIDED
TO
GET A CUTE LITTLE COIL TO LET HIM DISCHARGE.
HE PICKED UP MILLIE AND TOOK HER FOR A RIDE ON HIS POWER
AMPLIFIED MEGACYCLE. THEY RODE ACROSS THE WHEATSTONE BRIDGE,
AROUND THE SINE WAVE AND STOPPED IN A MAGNETIC FIELD BY A SMALL
FLOWING CURRENT.
MICRO FARAD, ATTRACTED BY MILLIE'S WAVES, SOON HAD HER AT
MINIMUM
RESISTANCE AND HER FIELD FULLY CHARGED. HE ALSO HAD HER
FREQUENCY LOWERED AND PULLED OUT HIS HIGH VOLTAGE PROBE. HE
INSERTED IT IN PARALLEL AND BEGAN TO SHORT CIRCUIT HER SHUNT.
FULLY CHARGED MILLIE SAID MHO, MHO, GIVE ME MHO. WITH TUBE AT
MAXIMUM CONDUCTION AND HER COIL VIBRATING FROM EXCESSIVE
CURRENT,
SHE SOON REACHED PEAK. THE EXCESSIVE CURRENT HAD HER SHUNT
PRETTY HOT AND MICRO FARAD'S CAPACITOR WAS RAPIDLY DISCHARGING
EVERY ELECTRON.
THEY FLUXED ALL NIGHT LONG, TRYING VARIOUS CONNECTIONS AND
CIRCUITS UNTIL HER MAGNETIC FIELD HAD LOST ALL OF ITS FIELD
STRENGTH.
AFTERWARDS MILLIE TRIED SELF INDUCTION AND CHARGED HER FIELD;
HOWEVER, MILLIE REVERSED HER POLARITY AND WHEN MICRO FARAD
STARTED FLUXING AGAIN, THEY BLEW EACH OTHERS FUSES.
POWER HAS BEEN CONSUMED.
WORK HAS BEEN DONE.
SO WATT.
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