repurposing a 50Hz AC transformer as an output inductor? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: January 07, 2017, 04:28:19 pm
content: I'm looking for a cheap way to make some large output inductors for my 7Kw AC source project.I will have a pulse width modulated square wave output at around 10kHz, needing to be smoothed down to produce a 45 to 65Hz sine wave. I'd like around 2mH of inductance, and be able to carry around 40Arms without excessive saturation of that inductor. So that's about 1.6 joules of energy to store in the inductor (0.5L x A^2).There are no significant size, weight or efficiency restraints, just a cost one! One idea is to take cheaply available 240V 50Hz isolation transformers like this:Typically these are rated at around 3kW continuous at 240Vrms.And remove the existing winding's and re-use the core/ lams - rewinding on a single winding to suit. The practicality of DIY winding probably means several parallel winding's of something like 12gauge to get sufficiently low copper losses. I'd have to measure up the core, but a guess suggests a core size of about 5x5 cm for 2500mm2. If i measure the existing windings for resistance and inductance, and take a good stab at estimating the number of turns, i can estimate the current magnetic loadings, and see if 1.6 joules could be stored in such a sized core?I'd imagine the losses will be fairly high, but that shouldn't be too much of an issue, and i can always fan cool the inductors themselves.Have i missed anything critical, and is this idea feasible?

author: IanB
author_rank: Super Contributor
timestamp: January 07, 2017, 04:39:54 pm
content: I think a transformer with the secondary winding open will act just like an inductor. So you don't even need to rewind it. If you want a low winding resistance, why not repurpose the secondary of a suitable step down transformer as your inductor and leave the original primary open?

author: max_torque
author_rank: Super Contributor
timestamp: January 07, 2017, 05:41:30 pm
content: It'll need to be rewound because i suspect that:1) it'll have to much inductance as std2) i want to lower the copper losses because i need to put 7kW through a 3kW "transformer"Once i measure such a device up for resistance and inductance it'll be clearer if i can unwind the std windings and rewind with the same wire length,but in parallel etc.

author: johansen
author_rank: Super Contributor
timestamp: January 07, 2017, 06:00:03 pm
content: a 3 or 5% load reactor intended for VFDs is exactly what you need.if you take a transformer core (most of them are welded) and cut a gap in the legs with a hacksaw or a bandsaw, (a 1/16th inch thick cut off disk is probably too wide) then that's another option. however, you will have to deal with high eddy current losses because the lamination thickness is too high for pwm, unless you keep the frequency below about 3khz.

author: Arjan Emm
author_rank: Regular Contributor
timestamp: January 07, 2017, 06:45:13 pm
content: You only need to store a fraction of that 7 kW, because the 10kHz. So you can get away with a pretty small core.A normal transformer has a primary inductance close to 1H you only need 2mH.So i would go for a microwave transformer core, grind it open and you are conveniently left with a solid I and E piece.Put as many turns of awg 12 or maybe even 11 or 10 on it and the inductance is probably way too high. adjust the inductance down by adjusting the air gap between the two core sections. The air gap also vastly improves the energy storing capability.And i don't think eddy currents are a problem, the current through the inductor will just be 50-60Hz not 10kHz.

author: Zero999
author_rank: Super Contributor
timestamp: January 07, 2017, 06:45:54 pm
content: Quote from: max_torque on January 07, 2017, 04:28:19 pmThat's a idea, in theory.Unfortunately those transformers are usually potted and any attempt to remove the resin would destroy the case.What about a microwave oven transformer core? You may need more than one to store that much energy though.

author: max_torque
author_rank: Super Contributor
timestamp: January 07, 2017, 07:26:09 pm
content: I rewound one a couple of years back to use as a low voltage source (took off the 110V secondary, and rewound it in parallel for 55V) and that was filled with a semi resin set find sand, which could be broken up and removed.

author: johansen
author_rank: Super Contributor
timestamp: January 12, 2017, 01:14:11 am
content: its not optimal but you can store 1.6 joules per microwave oven core.but you can't do it at 10Khz or the core will get fairly hot.http://www.ebay.com/itm/MTE-RL-03502-AC-Line-Load-Reactor-3-Open-Type-5-Impedance-Transformer-/302102241475this is approximately what OP needs.iirc i bought a 4 amp 9mH load reactor of the same manufacturer as the ebay listing and it did not have any significant power loss when using delta capacitors after the load reactor run off a vfd at 8khz. (230vac 60hz, 10uF delta)btw, these inductors have no significant common mode inductance.

author: oldway
author_rank: Super Contributor
timestamp: January 12, 2017, 09:13:44 am
content: I have a MOT of a 800W microwave oven and I make some measurements of the magnetic core dimensions:Core area: 75 x 28 mm²Windows area: 15 x 45 mm²NB: not sure of the values because it is not easy to measure on a functional MOT transformer.Current density would be around 4A/mm² up to 5.5A/mm².But to try using this 800W MOT, we will choose still a higher current density: magnetic wire of 9AWG or 3mm diameter, 6.63mm² and current density of 6.0033A/mm²In the windows opening, whe can put 3 layers of 14 turns, or 42 turns.Then whe need a gap of 3mm and inductance will be 1.48mH. (Gap of 3mm = 1.5mm distance between E and I of the magnetic core)It seems to be better to use a MOT of a 1000W microwave oven, but I don't have data of such a transformer.

author: max_torque
author_rank: Super Contributor
timestamp: January 13, 2017, 09:48:36 pm
content: Received and stripped my isolation transformers down to their (large) cores:The complete lam stack is around 85mm thick, and weighs a total of 10.7kg!I've done some basic testing with my LCR meter with turns of wire and with the "I" part spaced off the "E" by around 0.5mm it looks like i'll need around 40 turns to get the approx 1.5mH i need. The window in the lams is 2144 mm2 so there's plenty of room for nice big wire.40 turns at 50A is 2000 ampere turns total. What sort of lamination permeability values should i use to estimate the flux density?

author: oldway
author_rank: Super Contributor
timestamp: January 13, 2017, 10:41:18 pm
content: That's to big for what you intend to do.Acore= 45.9 cm² instead of 21 cm² from a 800W MOT.But anyway, no problem !You need a 2mH inductor for 40 A rms (56 A peak)...For security, we will choose 60 A rms (85 A peak) for max induction of 1 T or 10.000 Gauss.(low carbon steel can be used up to 1 T and silicon steel up to 1.4 T)You have plenty of space in the window, than you can use a current density of 4 A/mm². (copper wire)Wire 7 AWG (diam: 3.7mm)Gap: 2 mm between E and I (real gap = 4 mm)N = 39 turns.