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Ferrite Core & Power capability

Ferrite Core & Power capability

Ferrite Core & Power capability

(OP)
Can a ferrite Core deal with a large power such as 20KVA
power transformer ?
If not, what is the maximum power rating can be acheived with using ferrite core?

RE: Ferrite Core & Power capability

I think they top out about 7kW...

RE: Ferrite Core & Power capability

(OP)
itsmoked,Can you please specify some cores which can handle high power as high as 7 KVA

RE: Ferrite Core & Power capability

Sorry, but I can not.  

You need to talk to the suppliers!
Call them and ask.

Epcos
FairRite
Ferroxcube
MMG
Magnetics
radiuspower
TDK

www.adamsmagnetic.com  is a rep for many of these companies.

These companies are all happy to help and are usually very helpful.

RE: Ferrite Core & Power capability

For large custom ferrite cores try
Ceramic Magnetics, Inc.
www.cmi-ferrite.com

RE: Ferrite Core & Power capability

Yes, absolutely a ferrite transformer can handle 20KW. You may have to build up a core using a number of 'U' sections to get adequate core cross-section to handle the required flux. I built a liquid-cooled transformer handling over 40KVA at 1kHz as part of my degree project, but kinda over-estimated the cooling requirements. Based on winding temperature rise it could have taken considerably more - I forget the figures now, but doubling the original design throughput would have been quite feasible. Air cooled it was good for over 10kVA.

The primary winding was square section enamelled wire; the secondary was two parallel 8mm copper microbore central heating pipes jacketed in heatshrink with an oil-air forced-draft radiator rejecting heat to atmosphere. Each 'U' section was approx 1" square section, 3C8 grade ferrite running at 300mT. The ferrites were from Philips Components, who are now Ferroxcube. The core structure was a shell type using eight cores held together in a frame constructed from aluminium angle and brass studding.

The easiest way to push more power is to raise the frequency. 3C8 (or maybe 3C85) wqs the most common power transformer ferrite and is good for 100kHz or so before the core losses become too significant. Ferroxcube list their U cores in pretty big sizes and in a 3C94 grade which is a better (less lossy) core than those available to me. Try their U93/76/30-3C94 which looks to be the modern equivalent of the cores I used.

You have stirred some memories of a project which was damned good fun to do, and memories of a lot of friends too - thanks!

----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

RE: Ferrite Core & Power capability

That's cheating... He said "a" ferrite core.

That's cool you can stack them "parallel them". Never thought of that.

What did you need a 40kVA high freq for?  Sounds like a nice design BTW.

RE: Ferrite Core & Power capability

Ha-ha, it was a 'core' made up of lots of bits!

It was an induction heating demonstrator rig using a CSI with a resonant load - I also made a cute little controller for the inverter to keep the switching frequency just above resonance to maximise power transfer into the load. The resonant frequency changes as the ferrous metal load reached the Curie point where its magnetic properties change. If the frequency gets too close to resonance the commutation fails because there is not enough time for the off-going thyristors to recover their blocking state before the on-going ones switch in.

The load coil had enough stray flux to disturb CRT monitors in the adjacent lab - I was asked to shut it down a few times so that the scheduled class could take place! Looking back it had a few safety issues: molten metal, live parts everywhere, a huge DC link inductor. Of course when you are that age, you believe you are immortal and safety takes a back seat to meeting the deadline for completion!

----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

RE: Ferrite Core & Power capability

(OP)
For 'U' Core , is it better to have both the primary and secondary windings on one limb or to use one limb for the primary and the other limb for the secondary ?

RE: Ferrite Core & Power capability

Both windings on one limb is usually better.
Primary and secondary windings on separate limbs are sometimes used when high leakage reactance is desired. With primary and secondary on different limbs, there will be high leakage flux. (There shouldn't be a magnetic path from the top of the core to the bottom of the core)

RE: Ferrite Core & Power capability

(OP)
Could you please explain why sometimes high leakage reactance is desired ?
As far as I know , leakage inductace causes the primary to take more current to produce the same flux which is normaly not desired.

RE: Ferrite Core & Power capability

ScottyUK; Thanks for the description sounds like loads of fun!   I remember the strange experiment/systems running in school lab basements at several schools sigh..

RE: Ferrite Core & Power capability

Golly this stirs ancient memories for me as well.  I also used the large Philips U cores (identical sizes are also available from TDK).

You can place four U cores together to make a shell type of transformer that looks (in concept) something like a transformer constructed of E and I laminations. Then you can stack these in multiples of four to easily get a core large enough to handle tens of kilowatts. The core part is easy.

The design and layout of the windings is usually far more of a challenge. Skin effect,leakage inductance, and capacitance, are going to be your biggest problem. Using thin foil windings at a hundred milliamps in a miniature transformer at high frequency is one thing. But having a hundred amps or more to cope with in a monster transformer at a similar frequency can be problematic.

One or more really large toroids may be worth considering as well at very high power levels. The advantage of having a very long winding length and fewer layers can simplify some things.

Another sneaky way is to stack many smaller toroids and have a long thin cylindrical transformer. That will enable very few turns to be used which also is sometimes an advantage. This works especially well at very high operating frequencies.

So very much depends on operating voltage, current, required turns ratio, and frequency. Power by itself is not really an important parameter. Other problems concerned with the winding design will usually stop you long before reaching core overheating with a really large design.

RE: Ferrite Core & Power capability

Hi Warpspeed,

I looked briefly at toroids but couldn't get enough ferrite cross section in any sort of reasonable shape. I'd toyed with tape-wound amorphous iron too, but couldn't get hold of a big core in small quantity. I wonder if the long tube shape built up from multiple toroids would work well? At the time I reckoned that it added a lot of winding resistance which could be avoided by a shell-type design as you describe.

The tubular secondary winding I used was an intentional way of combining cooling and minimising skin effect problems. Slightly heavier walled tube would have been almost perfect.

If you are able to make a number of closely matched smaller transformers I guess you could parallel them to a low inductance bus. For the bus I'd suggest a laminated structure, or for really large currents an interleaved laminated design +/-/+/-, to keep your leakage reactance down.

Zacky,

Leakage reactance causes poor regulation in the transformer. In the classic transformer model with all parameters referred to the primary side, leakage reactance and winding resistance are shown in series with the primary winding. Magnetising reactance and core loss 'resistance' are shown as being in parallel with the primary winding.

High leakage reactance can have benefits. A couple of examples:

The inverter that the transformer design I built formed part of needed a commutating reactance to be added to the circuit. A little more leakage reactance from the transformer and that additional component could have been avoided, but since this was a one-off design it was easier to build a low-leakage tranformer and add a reactor rather than get too much leakage and have to worry about getting rid of it. With hindsight I could probably have introduced plastic shims into the core structure of the low leakage design to create tiny well-defined air gaps to give the desired additional reactance.

In power transmission and distribution systems, transformers are often designed with a higher leakage reactance than might otherwise be achieved in order to keep prospective fault currents on the secondary side within limits which switchgear can handle. If the transformers were built with the minimum possible reactance then a fault would cause colossal currents to flow, requiring massively oversized switchgear, oversized conductors, additional mechanical bracing of windings, and so on. It is far more economical to allow for slightly poorer regulation and compensate for it than it is to design everything to withstand very high fault levels.

----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

RE: Ferrite Core & Power capability

Scotty,
The multiple toroid "tube" idea is an easy way to get fairly high inductance with very few turns. The magnetic path length can be kept very short giving a high inductance per pass through each toroid. As you say, the individual turn length will be longer, but far fewer total turns are required, and the voltage per turn can be surprisingly high doing it this way.


Using stacked multiple smaller transformers also has several advantages just as you suggest, but balancing the winding currents can be rather interesting for parallel connection. Windings in series can sometimes work particularly well for higher voltages.

I once built a series trigger transformer for the xenon flash tubes used in a high powered laser. It used twelve high permeability ferrite toroids arranged in a tight rectangle with three per side. Each toroid had it's own one turn primary (twelve primaries) each driven from its own capacitor discharge system running off 1Kv. Each secondary turn looped through all twelve toroids developed close to 12 KV per turn, and there were four secondary turns generating something like a 45kV pulse.

The secondary winding was actually welding cable because it also had to carry the 900 Amp main flash tube current from the pulse forming network and dc supply.

Only a small transformer, but the four turn secondary had to both generate a 45kV trigger voltage, and also carry 900 Amps, an interesting transformer design problem.

Lots of different ways to skin a cat. Designing high power high frequency transformers is definitely an art form and a particularly fascinating design subject to get into.

RE: Ferrite Core & Power capability

(OP)
ScottyUK, as you explained about the benefit of the leakage inductance, is that mean for Battery charger transformer ,the high leakage inductance can be used to limit the charging current so it does't exceed the requied limit ?   

RE: Ferrite Core & Power capability

Warpspeed what kind of laser, neodymium:yttrium-aluminum?

RE: Ferrite Core & Power capability

It was an niobium yttrium aluminium garnet Nd:yag (ruby) laser, peak flash tube power was 1MW. I cannot say exactly what it was for, It was part of a government project.

RE: Ferrite Core & Power capability

Thanks for that info.  I walked though Nova at LLNL where they had lots of those and a huge warehouse full of caps to drive them all, (16trillion watts).  Almost insane in principal.

RE: Ferrite Core & Power capability

I have read in the non classified annual reports about the Shiva and Nova projects at Livermore, mind boggling stuff indeed. The raw electrical power to charge the pulse forming networks is almost beyond belief.

Getting back to the topic of this thread. I was using those large ferrite U cores in multiple parallel flyback supplies, each in the multi kilowatt range. These supplies were to charge the flash tube pulse forming network, and each was run slightly out of phase with respect to all the others, to minimise total input ripple current. That was all twenty years ago, but it still stirs some fairly vivid memories.

The whole power unit averaged out at roughly 12Kw, so yes multiple ferrite cores can be used to couple fairly serious power.

RE: Ferrite Core & Power capability

(OP)
That is a great new idea to have U cores multiple in parallel to increase the power availability, but what would be the professional mounting structure needed to achieve that ?

RE: Ferrite Core & Power capability

Possibly the preferred way is to epoxy everything together into one solid lump. Another method might be to place a clamping band right around the outside periphery of both U cores, rather like a giant hose clip. That could also be used to secure some appropriate mounting feet to the core.

Or maybe be a couple of flat plates with corner holes holding the whole thing together in compression with some long screws or all thread. Many possible ways to do it, but nothing really neat unfortunately. I have even used large rubber bands on a prototype.

One difficulty with the large ferrite U and I cores is the lack of commercial bobbins to fit the cores. That may have changed, but finding something onto which the windings can be placed is another difficulty to be solved.

RE: Ferrite Core & Power capability

zacky,

For your battery charger you will almost certainly need a current limiter measuring the DC output current. A hall-effect transducer or a shunt resistor and diff. amp are the conventional ways to do this. For simplicity the hall effect device from, say, LEM would be the easiest to interface to.

For joining large U cores together with minimal air gap, cyanoacrylate adhesive - superglue - has very low viscosity which will not add an appreciable air gap to the core design. Thicker adhesives may do this and cause the performance to deviate from the design expectations. Obviously adhesives do not make for a good solution when prototyping!

Aluminium angle and stainless steel band clamp may be a possibility for holding it all together. Thin rubber sheet fitted between the angle and the ferrite prevents the ferrite suffering compression fractures. I've never seen commercial hardware for these cores. Do not use standard steel hardware.

----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

RE: Ferrite Core & Power capability

(OP)
scottyUK, I understand that the cyanoacrylate adhesive-superglue will be used to join each two pieces of each U core and not to join all the cores together- is that what you mean ?

RE: Ferrite Core & Power capability

Yes, that's what I would do. The mating faces of the ferrite are machined to a very high standard to give a near-zero air gap when assembled. I found the cores were dimensionally not particularly accurate. Tolerances of about +/- 0.5mm on nominal was about typical. Your clamping arrangement needs to accomodate this, hence the suggestion to use rubber sheeting or similar. Tolerances may have improved over the past few years, I simply don't know.

----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

RE: Ferrite Core & Power capability

Agree with all of the above comments, but a very small controlled  deliberate airgap can sometimes actually have advantages even in forward converters. With zero airgap in a push pull forward converter for example, there can be problems with staircase flux saturation where the push and the pull are not identical. Flux doubling at turn on from stored remnant flux may also create a massive uncontrolled inrush current spike unless very fast current limiting is incorporated.

Current mode control will solve a lot of these problems.

While not a complete solution by itself, a very small deliberate airgap will considerably soften the saturation characteristic of the core material, and add just a little bit more safety for very high power operation. It may not always be needed or even a good idea, but it is something to think about.

Epoxy adhesive if heated, and the parts tightly clamped will squeeze out extremely thinly. The cyano "super glues" have been known to give up and let go after a time, and may not be reliable. They are excellent for holding parts together while the epoxy cures.

RE: Ferrite Core & Power capability

"may not always be needed or even a good idea"

pace maker equipped persons need not apply.

Yes, by the way at HP I made up to 16 layer printed circuit boards that were epoxy/pressed with thin insulator sheet that when complete were just standard 0.0625" thickness so I concur you can make epoxy thin!

RE: Ferrite Core & Power capability

hehehe, I well know that conventional transformers are NOT supposed to have an airgap, but in some circumstances it CAN be advantageous.  Like any design work, all the factors need to be considered, and very slightly gaping a transformer definitely falls into the sneaky tricks department.

RE: Ferrite Core & Power capability

Foreshore!  I can totally see how the gap could be a big advantage for anti saturation purposes!  I can also see a tiny gap in unit "A" that works great!  While the properly gap-less unit "B" doesn't work.

RE: Ferrite Core & Power capability

It also reduces remnant flux.  With absolutely no gap, (ferrite toroid?) when you turn off the current, there can remain some residual magnetism in the core. With a high permeability material this remnant flux can be considerable. The BH loop can sometimes look almost rectangular.

If you then switch it back on in the same direction, the core can violently saturate causing severe distress in the switching transistor. A small gap makes the residual remnant flux fall to a much lower level at turn off. This really helps to stabilize the flux operating point around the centre of the BH curve.

Unfortunately it also reduces primary inductance, and increases the magnetising current. But there ain't no free lunch.

Some of these nasty saturation effects can be why a new prototype can seem to work perfectly, then on odd infrequent occasions spontaneously go bang with no apparent obvious cause. I strongly suspect is a contributing factor to why the switchmode guys invariably get premature grey hair.

RE: Ferrite Core & Power capability

(OP)
I wander, how switching mode power supplies are still not reliable at high power level and most cosumer electronics use them and work fine for so many years !

RE: Ferrite Core & Power capability

Switcher supplies are very reliable.  I have only ever seen one fail.  I have seen about 10 linears that have failed.

RE: Ferrite Core & Power capability

They certainly CAN be extremely reliable, provided they are designed properly with the correct insight. The magnetics needs to be designed to suit the topology. High frequency magnetics is an absolutely fascinating field of study, and experience is all.

Some of the very early thirty year old switcher designs even by such venerated and illustrious companies as HP were far from reliable. But these days far more is known.

Very high power designs are not the sort of thing a beginner should cut his teeth on. But switchmode power supplies are these days just about universal, and the reliability and efficiency is excellent.

RE: Ferrite Core & Power capability

Hey Warpspeed,
You piqued my interest talking about series trigger transformers. I'm just dabbling with a little hobby project to try to control a short arc xenon lamp that I have in some rare strobe lights, albeit quite a bit lower power than your novel monster (150W rated lamp, 100A peak flash current).

My idea is to replace the crude capacitor discharge circuit with an IGBT PWM current mode control so that I can control the current (brightness) and repeat rate. I was thinking of using the secondary of the trigger transformer to be the inductance and to control the current through it and therefore the lamp. I've used the original trigger transformer (5mH secondary) and successfully struck the arc and maintained 1A constant current at a 20KHz PWM freq. So 'all' I have to do is get 100 times more current capability!

The current pulses I want may peak 100A, but the average power has to be kept low for the lamp not to overheat. Am I correct in thinking that if I wind my own transformer I can use thinner than 100A rated wire for the secondary as it's not continuous current? But at the same time won't I have to calculate the core size so that it doesn't saturate based on 100A? I need a much lower inductance than original so that I can ramp the current up rapidly, but at the same time I need the secondary turns ratio to work as a trigger transformer. As I'm a relative newbie to all this I seem to be going in circles knowing where to start. I'm not sure if I'm trying to design a transfomer or a DC choke, or if it's even possible!

Oh, and I thought popular ferrite cores were generally below a few hundred watts rating and not that they could be stacked, so that's two things I've learned here today...so thanks.

RE: Ferrite Core & Power capability

Hi Chaos,

Your idea of usng a PWM system to control lamp current directly should certainly work, once the initial lamp discharge arc has been initiated. Running a lamp in "simmer mode" from a current source at low current is a very effective way to maintain a continuous lamp discharge at extremely low power.

This can most easily be done with a voltage source of several Kv dc, and a large high power series wire wound resistor. That could be quite independent of your PWM, and it may be easier to do it that way. The source impedance needs to be high enough to get a stable simmer action, otherwise getting it to start up may be problematic.

The lamp itself will have a very high negative resistance, and you need enough series impedance (positive resistance) to get a stable simmer current.

I doubt if the same inductor could be used for both trigger and pwm averaging, because the inductance/current requirements are quite different. The main PWM inductor will need to be air cored and maintain a reasonably constant inductance value over a very wide operating current range.

An independent series trigger transformer can then be placed between the main PWM inductor and the lamp. This need be only a couple of turns looped through several very high permeability toroids.

The idea here is that the trigger transformer will generate a very high voltage, with extremely fast rise-time, but from perhaps only a few, or tens of millijoules of trigger power. As soon as real lamp current begins to flow through the trigger transformer, the ferrite toroids massively saturate. The very high initial permeability and no gap will guarantee total saturation at perhaps a few hundred milliamps or less, and above that, the trigger transformer will have almost negligible inductance. Once the xenon lamp is conducting, the inductance of the trigger transformer effectively disappears from the circuit.

For your PWM inductor, consider a flat air cored pancake coil of rectangular section copper transformer wire. Several flat coils can be ganged in series for higher inductance.

A xenon lamp simmering with a single very long fine filament of plasma is a fascinating thing to see. The arc will twist and corkscrew from reaction to local magnetic fields. Rather like one of those decorative plasma ball displays.

I think you will find this is sure to be a fascinating and most memorable project.  

RE: Ferrite Core & Power capability

I like that saturating trigger core trick Warpspeed.  Very elegant.

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