Making a 60Hz power transformer using existing laminated E core / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Making a 60Hz power transformer using existing laminated E core

1 2 next

Move Thread

LAN_403

nrhoades

Sat May 26 2007, 10:19PM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110

Hey folks. I have obtained a few large laminated E cores on which power inductors were wound. I have taken one apart and I am planning on using it to build a 60Hz 120VAC to 400VAC transformer that can source around 1kW.

Since the only variables I have control over when making it are the number of turns, and the wire diameter, I am trying to figure out the best gauges of wire to buy. (I will assume that the primary and secondary will each occupy 1/2 of the core height because I read that leakage is reduced when this is the case).

I made a quick sheet that shows the relations of different wire gauges compared to the maximum number of turns that can fit in my core... (attached)

My question is:

Though I plan on maximizing the number of turns for each of the two windings, will this "maximum turn" assumption give me the best results that I can physically achieve by making it myself, or do I really need to spend the time to figure out the parameters of the E-core and make optimal calculations. I am just shooting for performance around 400V at 1kW.

Thanks guys!

Steve Ward

Sat May 26 2007, 11:07PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

I would suggest you first calculate how many turns you need. You can assume that your core can support about 1 tesla for a Bmax. You need to balance core loss with copper loss, so you dont want wire thats too skinny, or to fat.

Also, do you know how you will wind the coils? You might not get as tight of fill as you want, so be sure to leave some headroom there too so you dont run out of space.

IamSmooth

Sun May 27 2007, 01:05AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

This link

should help you calculate the turns you need.

Dr. Slack

Sun May 27 2007, 07:38AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Mr. Smooth's link appears to have all the data there, but it looks as though it could be a bit overwhelming to somebody who had to ask in the first place.

Maybe if you start with a few fundamentals, you can work out which bits of that link to use. Your job is a lot simpler than a transformer design shop, who start with a spec and have to work out what matierlas to use. You have the laminations, which shape decides the copper window, and the number of them dtermines the core area.

First determine the turns per volt, this depends on the maximum flux for the core. THERE IS NO SINGLE ANSWER TO THIS. Core losses rise steadily above a threshold flux, and the pearmability drops allowing higher magnetisation current to flow. So, you've got to ask yourself this punk, do you feel lucky, do you want continuouis operation with no cooling, or can you assume fan-blown intermittent, like penny-pinching microwave ovens designers do. Most transformer iron is specified for that range of uses at fluxes around the 1.3T to 1.7T range. So I guess 1.5T max is a good safe compromise. The higher the max core flux, the higher the off-load losses.

Now use the equations to get a turns per RMS (or peak if you are consistent with which is which) volt which gives 1.5T PEAK, at 60Hz. Remember most AC voltages are given as RMS, and your meter is scaled to read that.

Wire thickness. You'd be surprised at how fast the inefficiency builds up. Even a well set up winding machine can't acheive better than 70% of the available area filled with copper, you need to aim for less than 40% if you are hand winding. It's not just the thickness of the PU coating and the hexagonal/circle filling fraction, it's the wires not lying quite flat, terminations, rows not filled, insulation on the core, insulation and sheild between windings.

From your turns per volt and target voltages, you have a number of turns. Take half the total area the total area for each winding, divide by total number of turns, multiply by 0.4 (if you're feeling lucky, 0.3 if not) and that's the copper wire area to use for each winding. The thinner the copper wire, the higher the on-load losses.

Using the core to its practical maximum flux, filling the winding window with the practical maximum area of copper, will give you the highest power you can obtain with your laminations.

Good luck. I've hand wound several transformers in my time, they have always taken more time than budgetted, and look like cr@p when finished, but they all work as designed.

Steve Conner

Sun May 27 2007, 10:27AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The way I like to do this (though I've only ever done it with ferrite, not iron) is to put the core together with, say, a test winding of 10 turns of thick wire on it. You drive this from a variable voltage AC source and use a sense resistor and oscilloscope to look at the current draw.

You can soon get a feel for how many volts/turn the core can take before it starts to grumble. As you go past the saturation point, you hear it buzzing and see large spikes of current on your scope. If you're designing a high quality transformer, you use slightly less volts per turn than this. If you're designing a MOT, you use about half as much again...

You can work out maximum allowable volts/turn at 60Hz as a function of the core area from a standard equation, but this way is more fun

Incidentally, the tradeoff works like this: Less turns of wire allows you to fit thicker wire in, and have less copper losses. But this increases the volts/turn and so increases the iron losses. Your job as transformer designer is to find a compromise between the two.

nrhoades

Mon May 28 2007, 03:59AM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110

Ha! This place is great! Everytime I find time to sit down and think about things, someone posts more helpful information. I've posted what I had quickly done before I read the last two posts (as attached). First calculation is using 1T, and the second is using what I got from saying that the permeability is 7500.

I like Steve Conner's idea because I like playing with things, and I'll pay attention to what NeilThomas said.

From my calculations that I attached, I'm "guess-timating" that I need a primary of atleast 200 turns to start moving away from core losses.

Hows 16AWG for pri/ 20AWG for secondary sound for starters?

Dr. Slack

Wed May 30 2007, 07:41AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I notice you have some inductance calculations. I really wouldn't bother with those. If you do a calculation assuming 0.1mm air-gap, then another with 0.2mm air gap, you'll see that the inductance is dominated by the tightness of fit you can acheive on re-assembling the core, rather than any intrinsic core permeability parameters. In a power transformer, inductance only really affects part of the magnetising current, so is pretty irrelevant.

Incidentally, when I first saw transformers with Es and Is interleaved, I thought it was a much better idea than a block of Es and a block of Is, as it appeared to allow a much larger area of airgap for the flux to get from one bit of iron to the other, to allow me to be more sloppy with the air gap. However, except for flux < 50% saturation, that's wrong. The angle that the flux takes through the lamination tilts, increasing the effective eddy current area and so core loss. In addition, the 50% area part of the iron will saturate, forcing the remaining flux through the air anyway. So you gotta keep the airgap small. Choose interleaving or a block of each for your own personal taste, or available hardware.

Hmm, I've just re-read your first post. You probably mean the right thing, but haven't said what you mean. You don't want to maximise the number of turns. Within the constraint of a maximum permissible flux, which limits the off-load iron loss, you want to minimise the turns per volt and hence number of turns. This minimises the length of copper wire you need. Then maximise the copper fraction in the winding window, which maximises its area. This then gives minimum copper loss, which maximises your output power to on-load loss ratio.

Sulaiman

Wed May 30 2007, 10:19AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have a few observations;

1) Since you have E cores (as opposed to an E I core) you should wind half of the primary on each core to reduce leakage inductance, not primary on one E and secondary on the other E.
This will require reasonable insulation between the primary and secondary on each bobbin.
It will also require the number of turns on each primary to be the same.
2) If you wind each primary for 120 Vac you can wire them in parallel for 120 Vac, or connect them in series for 240 Vac if required in future.
3) You will also now have two secondaries, which can be connected in series or parallel for a choice of output voltage.
4) When used as an inductor the 'flatness' of the E cores where they meet was not important, when used as a transformer you should ensure that the mating surfaces of the two E cores are very flat to minimise the airgap.
A small airgap is required to have low magnetization current - current drawn when there is no load on the secondaries.
5) Your transformer will buzz/hum quite a bit unless you glue the two halves together (once fully tested)
Apply power to the transformer whilst the glue is setting to pull the two halves together tightly.
6) Considering power throughput, you should use the maximum volts/turn (flux density) that your transformer can handle, operating at 1.2T instead of 1.0T gives a 20% increase in power throughput etc. Provided there is not too much saturation.
That's all that comes to mind at the moment.

nrhoades

Wed May 30 2007, 03:10PM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110

I have a laminated E-I core, where the E and I are separate parts and are not interleaved. Sorry for not being more specific.

NeilThomas posted:

This minimises the length of copper wire you need. Then maximise the copper fraction in the winding window, which maximises its area. This then gives minimum copper loss, which maximises your output power to on-load loss ratio.

Since you might have thought that I was talking about two separate E cores, does your advice still hold merit for an E-I core? I.E. since you said that I should maximize the windings related to the winding window, would it be more benificial to wind the secondary on top of the primary? or simply next to each other? It would make sense to me to do the first because of tighter coupling and less leakage?

I have also noticed that there is some sort of thermal paper between the original transformer windings... what does this do?

I'm going to buy 16 AWG wire and start from there.

Dr. Slack

Wed May 30 2007, 03:30PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

There is a small theoretical advantage in coupling efficieny in putting one winding over the other, but given that the permeability of iron is essentially infinite compared to air making all points in the copper window essentially equivalent, the difference is so small that you will want to think about winding convenience, or inter-winding insulation first. Given that the secondary is 1000V, it's probably best to think insulation first. Have you ever seen a MOT construction?

The first time I read Sulaiman's post I misunderstood it and thought he was saying that it is required to have a "small airgap" to have low magnetising current. The ideal of course is no airgap, but there is always an inevitable space, just keep it as small as you can.

Thermal paper? or electrical insulation paper?

1T would be super-conservative, I'm sure any iron that was worth stamping into "Es" would be happy at 1.5T, and if you can measure it in the way that SteveConner suggested, then go for more.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob