How do you calculate the max VA a steel transformer core can transfer? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: December 27, 2023, 02:20:38 pm
content: For plain silicon steel mains transformer cores, does calculating the max VA for the core matter or is the copper loss the main limiting factor there?And how do I calculate the max VA that the core can be used for?

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 27, 2023, 03:59:02 pm
content: Core loss for reasonable quality silicon steel is fairly low. On the order of 1 Watt/Kg at 50Hz if properly used and constructed. So yes, it's safe to assume that most of your limitation will come from copper loss.You obviously need to prevent the core from saturation, so the minimum amount of primary turn will be known from your core's cross sectional area and your maximum input voltage (Np = Vp / 4.44fBAor whatever your favorite formula).Now that you know Np (and consequentially Ns from your design requirement). It's a matter of fitting the largest possible primary and secondary wires within the available winding area. This in turn dictates your copper loses.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 27, 2023, 04:13:23 pm
content: The core loss is about constant, not effected by the loading (except for the very low power ones with < 2 VA). Even a poor core material does not make a big difference - it is only that good material can allow higher magnetization and this can allow for fewer turns for the same voltage and thus less resistance. How much magnetization is actually used also effects the no load loss and noise - so it depends on the application what is suitable and not all transformers are made the same.The limiting factor is the copper loss and temperature rise from this. Here it depends on the isolation how much temperature rise is considered acceptable. A higher temperature class allows for more power, even if physically the same transformer. It however comes with faster aging when actually used at nominal power.

author: ramussons
author_rank: Contributor
timestamp: December 30, 2023, 04:05:57 pm
content: A very old Rule of the Thumb. For 50 / 60 Hz power transformers.1 Sq Inch core area ~ 30 VA,the VA increases as the Square of the AreaSo 2 Sq Inch ~120 VA.Winding is ~ 6 turns / Volt.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 30, 2023, 04:52:03 pm
content: The cross section is the aerea the magentic field flows though, e.g. indide the coild or approximately (may be a little larger) the 2 side arms combined. So in the example more the 14 mm * 17 mm.The voltage per turn depends on the area and also the frequency. The larger the area the fewer turn are needed. Also the maximum magnetization makes a difference - here and with the fill factor modern materials can be a bit better than the old text. A oscilloscope is also a rather special case where they want lower magnetization to reduce the leakage.

author: MrAl
author_rank: Super Contributor
timestamp: December 30, 2023, 07:01:01 pm
content: Quote from: ELS122 on December 28, 2023, 05:34:42 amHello there,Where did that paper come from?I ask because it looks like the formula for Bmax is very wrong. It states the area of the core in square inches when it should be in square centimeters. This agrees with data published by Thomas and Skinner and also some web calculators for Bmax.Since 1 square inch = 6.45 square centimeters, designing with the formula in that paper will lead to a core size requirement over 6 times as large, which would be nuts.So the correct formula is:Bmax=E*1e8/(4.44*F*A*N)with E in volts, F in Hertz, A in square centimeters, Bmax in Gausses, and N the number of turns on the primary.Solving for N we get:N=E*1e8/(4.44*F*A*Bmax)again with A in square centimeters.Also, the power handling capability of the transformer depends on other factors like window area as well as the cross-sectional area, and even the entire surface area of the transformer. It also depends on the thermal conductivity of the cross section of the entire transformer construction. This also includes the windings as well as what is between the windings and how the transformer was potted (such as vacuum varnishing).There are formulas but it gets a little complicated. The way some places used to do this was do a rough calculation and then bench test the design by running at full load overnight. In some cases the transformer winding layer insulation would melt, indicating of course that the design was not acceptable yet.Bmax is usually around 20kG for line frequency laminations but can vary considerably for other types of cores. With 20kG max, a typical design target is 15kG.