How to calculate secondary volts/turn for a saturated mains transformer? - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: January 01, 2024, 06:58:52 pm
content: I'm planning to run an M470-50A UT13x17 54mm length core at 1.6Tesla, at which point it's well saturated. (stacking factor of around 4.1)But at what tesla value can I expect the core to still excite the secondaries?I have around 19.8VA draw from the secondaries. 1.7VA loss @ 1.6Tesla for the core. (0.385kg)I'm expecting 0.22V/turn for the secondary voltages to be right. and my primary is 1000 turns, 80 ohms and powered with 237V 50Hz.@ ~22VA the primary voltage would then drop to around 229V.But how many turns/volt will I still have because of how low the permiability is at 1.6Tesla?Here's the values for the core steel:@ 50 HzJ H[A/m] Ps[W/kg] μr[-]1100 135 2.04 65021150 142 2.21 64231200 152 2.40 62851250 164 2.60 60591300 184 2.81 56251350 217 3.04 49411400 273 3.31 40801450 378 3.61 30491500 634 3.93 18821550 1155 4.21 10681600 2045 4.47 622

author: CaptDon
author_rank: Super Contributor
timestamp: January 01, 2024, 07:38:45 pm
content: Why on earth would you want to push a mains powered transformer that far into saturation? No head room and insane amounts of magnetizing current and I2R losses with no benefits what so ever?

author: johansen
author_rank: Super Contributor
timestamp: January 01, 2024, 08:15:55 pm
content: Quote from: CaptDon on January 01, 2024, 07:38:45 pmthe benefit is the power density of a transformer follows the flux density squared. -its just a trade off between no load losses and full load losses.Quote from: ELS122 on January 01, 2024, 06:58:52 pmfind a 24volt 40va control transformer from the nearest HVAC' companies dumpster, measure it. its pretty similar to what you're designing.

author: Benta
author_rank: Super Contributor
timestamp: January 01, 2024, 08:28:37 pm
content: Secondary voltage will be the same, but you'll literally be running an air cored transformer. Immense magnetising current and not very efficient.

author: Benta
author_rank: Super Contributor
timestamp: January 01, 2024, 09:42:19 pm
content: That's obvious. Primary-secondary coupling will also suffer. But the idea is so outlandish that I have no interest in providing detailed replies.

author: johansen
author_rank: Super Contributor
timestamp: January 01, 2024, 09:56:27 pm
content: I have a small transformer running at 1.4T, its an off the shelf 35VA 24vac transformer.somehow they fit 650 turns of 28 gauge wire for the 120v primary at 19 ohms, and 150 turns of 22 gauge wire for the secondary at 1 ohm.it consumes 3.8 watts at .110 amps at 110vac and a 3 turn secondary makes .5110 voltsit consumes 5.0 watts at .162 amps at 122.5 vac and 3 turn secondary makes .5653 volts.the additional .050 amps times 19 ohms makes for 1 volt disappearing and this is appears to show up in a slight reduction of what the 3 turn secondary should produce.my spreadsheet says my transformer weighs .44 kg of iron. i have to crank the volt amps per kilogram up to 45, to get a magnetizing current of .162amps.the no load copper loss is supposed to be only 0.5 watts.the core is welded with all the E's on one side, and it has generous holes drilled in the 4 corners, so its probably saturating in the corners. but the bulk of the core is at 1.4T flux density for the 19mm by 26mm core cross section.hope this helps.with a 22.5 ohm load the volts drop from 28.7 to 25.6 and my kill a watt meter says its pulling 40 watts out of the outlet at 0.90% power factor.the problem being i only have 30 watts dumped in the load. -this is about 4 watts lost in the core and 6 watts of copper losses, for 75 to 80% efficiency.

author: duak
author_rank: Super Contributor
timestamp: January 01, 2024, 10:33:07 pm
content: Slipping a few turns of fine gauge wire tightly around the windings will allow you to measure the volts/turn with an oscilloscope. Please note that the secondary is still "excited" by the flux set up by the primary winding when the core is saturated. However, in saturation the core cannot support much more flux and since the instantaneous voltage on the secondary is a function of the change in flux, the peak voltage will be limited. You should see this with a scope as a flat-topped, distorted sine wave.Are you trying to make a constant voltage (ie. voltage regulating) transformer?Cheers,

author: johansen
author_rank: Super Contributor
timestamp: January 01, 2024, 11:31:40 pm
content: another crude test i did on a smaller 25va transformer of turns ratio 4.25:1 delivering 28.6 volts open circuit at 122v primary.shorted the secondary and provided it 27.7 volts from another transformer. 1.31 amps flowing in the secondary.according to the resistance of the primary, 44 ohms, and 4.25^2 *1.73 ohms for the secondary, which equals 31.5 ohms, i should have had 0.36 amps flowing in the primary and 1.56 amps in the secondary, instead i only had 1.3