Mains electrical Q: transformer sizing?

|>> 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 worst-case PF (or | at least some PF value greater than 1), can't it be re-rated (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.
Reply to
phil-news-nospam
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| 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!
Reply to
phil-news-nospam
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 to
Zayonc
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-------------------------------------------------------------------------------------------- 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 half-cycle of positive potential, the end of that half-cycle 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 non-saturation of the core, that won't help the blown-fuse 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 load-peak 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).
Reply to
Don Kelly
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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!
Reply to
Don Kelly
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
Reply to
Lew Hodgett
Yep. It was my habit to do power surge testing on newbuilt electronics with a switch and large nearly-unladen 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 real-world environment. HF from the switch arcing, 1-cycle 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 maximum-voltage switching is another treatment (there are solid-state relays for this).
Reply to
whit3rd
Thanks, Phil. With my convoluted terminology, that is what I wanted to know. Your answer was... elegant, to-the-point.
Reply to
James T.
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Yep. It was my habit to do power surge testing on newbuilt electronics with a switch and large nearly-unladen 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 real-world environment. HF from the switch arcing, 1-cycle 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 maximum-voltage switching is another treatment (there are solid-state 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).
Reply to
Don Kelly
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.
Reply to
Lew Hodgett

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