Ferrite Core

Discussion in 'The Projects Forum' started by newbies_hobbyist, Sep 5, 2013.

  1. newbies_hobbyist

    Thread Starter Member

    Jun 4, 2010
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    Experts I'm gonna need some clarification regarding the ferrite core. I have a transformer that both primary (stator) and secondary (rotor) winding are attached in ferrite core and have 1:1 turns ratio. While I was working with it I noticed that primary winding is leaking on its cylinder housing, I used megga-ohmmeter to check the insulation of primary winding to cylinder housing and I got <1Mohm at 500Vdc. While continuing the check I figured out that the ferrite core itself is leaking as well to the primary winding. I tried as well to check the ferrite core itself and I got ~1.5Mohm well I was not expecting that it would give me some resistance as I believe that it has high insulation and it is made of ceramic mixed with either iron, nickel, zinc or manganese but the high insulation turns out to be true in low voltage multimeter only.


    My questions are:

    1. What is the breakdown voltage of ferrite core?
    2. Is there specific insulation value for ferrite core when testing it with megga-ohmmeter? I tested it with 250Vdc and got same value with 500Vdc.
    3. Is it the same material as the low voltage surge arrester?
    4. Is it porous? Because when I wrote a mark on it using temporary marker it sucks the ink.
    5. What would be the effect of bad insulation between winding and ferrite core?
    6. I am thinking of laminating the ferrite core to isolate the winding and eliminate the bad insulation between them, will it be effective?

    For sure I will do some test to see how a bad insulation between winding and ferrite core affect the transformer but before that I need your opinion. I cannot rely on my instincts and my own understanding for now as I really need to understand more deeply regarding this and two or more heads are better than one. I know you all have a big head to help me with my problem.

    Thanks,
    NHB
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
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    The ferrite core should be an insulator (by design to prevent eddy currents flowing in the core). You should always insulate your wires separately of course.

    This is not the same material as used in surge arresters.

    Not normally porous. It's actually a very hard ceramic material.

    Because the ferrite is an insulator bad insulation between the winding and the core will have no effect. But contact between adjacent turns of the winding is very bad.

    It is good practice to insulate you windings from each other and the core. This is in addition to using insulated wire. The wire should be enamel insulated. Put a layer of tape between each winding and the core. Try to wind very carefully so that the turns sit beside each other with no kinks.
     
  3. newbies_hobbyist

    Thread Starter Member

    Jun 4, 2010
    67
    8
    JDT, thanks for the response. Yes as a standard the winding are enamel coated. I agree it is an insulator but why it behaves like the surge arrester that it conducts and gave some resistance when the break down voltage is reached? Yes maybe I can use tape in ferrite core, wrapping a very thin insulation tape would probably help.
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    You are mixing a lot of different terms here that don't normally go together. Stator and rotor are terms used in motors. Primary and secondary are transformer terms. What are you working on, motor or transformer?

    What voltages are you working with? If it is higher than the magnet wire insulation rating it could be sparking between adjoining loops of wire, not to the core. The only thing that the core should have in it is magnetic energy, no electrical energy. Unless the core is also connected to ground, there would be no possibility of sparks. Unless this is high voltage high frequency electricity your talking about.

    Please show schematics or photos of what your doing.
     
  5. newbies_hobbyist

    Thread Starter Member

    Jun 4, 2010
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    Hello shortbus sorry if those terms confused you. Well it is a rotating transformer (rotated by motor) having primary and secondary 1:1 turns ratio, primary winding is attached or glued to the inner hole of rounded ferrite core and arranged inside the stator cylinder chassis or housing and the secondary winding is attached or glued as well to the inner hole of rounded ferrite core and arranged in rotor rod which has ball bearing to the other end (to centralize the rotor inside the stator as it rotates). I don't have schematic but you can take a look at the attached file for the drawing and I hope that explains a bit how the assembly looks like.
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    This is one of the things I love about AAC. Always something new to learn about. Never heard of a rotary transformer .

    If you don't mind my asking, what is this being used for? Are you making it yourself?

    You probably have already seen this; http://en.wikipedia.org/wiki/Rotary_transformer
    The illustration(not photos) shows how the magnetic field has to be made up. The way you are describing with the windings in the center of your core, I don't think will give you the correct magnetic field. This would be a better type of core to use; http://www.dextermag.com/products-page/0f42616ug-linear-ferrite-26x16-pot-core.html

    I'm no expert on this, but unless your using high frequency in the rotary transformer, ferrite isn't a good choice for the core. This is why regular transformers, motors, solenoids or electromagnets aren't made from ferrite. They are "kind of" magnetic without being magnetic, if that makes sense. At low frequencies you need the magnetic properties of the iron core.

    I look forward to learning more about the use of a rotary transformer from you.
     
  7. newbies_hobbyist

    Thread Starter Member

    Jun 4, 2010
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    Shortbus the transformer is the part of equipment we have here in our working place and for so many years we have a lot of issues with this and that is the insulation problem between the windings primary or secondary and ferrite stator chassis or rotor rod. A week ago I decided to take a look on it and try to understand why it always leak and what makes it leak and then found out that it is because the winding is leaking through the core where it the core is in contact in stator (for primary) and rotor (for secondary). So I decided to take an action to improve this. Yes it is use in high frequency in the range of 1 to 5Mhz but low amplitude (not > 20V) so I believe that is the reason potted ferrite core is used instead of a normal core. One more thing is it wont exhibit permanent magnetization unlike the normal core in household transformer. Continuing to the use of transformer, the purpose is simply to transfer the power to the converter circuit and measuring circuit without having cross talk to measuring circuit. Not expert on this matter as well and so I am asking for some experts advice here. By the way I did some test in a winding that has bad insulation to ferrite core and compared it to the one that is good and I got the same result. I exposed both of them in demag coil operated in 60Hz 80Vac and both are showing low amplitude and 60Hz freq. and I think the magnetic strength are in the same level (both sticks to screw driver and feel it while removing the screw driver).
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Newbies, this peaked my interest so much, that went Googleing on it. There is some information on rotary transformer design out there. When you say it's potted, what was used for this? I am assuming the transformer is a 'store bought' component, not something made in house. Can you tell if the windings were vacuum impregnated with varnish/lacquer? Like is done with motor windings and regular transformers.

    At under 20V I can't imagine the insulation breaking down due to voltage. Is the environment for this hot? Is it in a high vibration operation? Is the winding done on a plastic former? Since this is new to me (rotary trans) I'm really just giving guesses, and things I've found about other electrical/mechanical things. I love solving puzzles like this.

    Some PDF's on rotary transformer design to look at.
     
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Just thought of something else, is there another large coil of wire or electro-magnetic thing near the rotary trans? Is the housing of the transformer electrically grounded/shielded to the rest of the machine? It could be picking up an inducted voltage from something nearby.
     
  10. DickCappels

    Moderator

    Aug 21, 2008
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    1. What is the breakdown voltage of ferrite core?

    Zero volts for most of them. (Actually, its not specified, but cores are resistive, so its like asking the breakdown voltage of a resistance.

    2. Is there specific insulation value for ferrite core when testing it with megga-ohmmeter? I tested it with 250Vdc and got same value with 500Vdc.

    No, there is no specific value. If you are making equipment that humans can contact, any ground leakage current needs to be limited to a few milliamps (actual values depend on frequency and use). Use the insulation that gives you much less than tolerable leakage current and you will be ok.

    3. Is it the same material as the low voltage surge arrester?
    No. They are completely difference materials with completely different characteristics.

    4. Is it porous? Because when I wrote a mark on it using temporary marker it sucks the ink.

    Generally no. Your core might not be ferrite.

    5. What would be the effect of bad insulation between winding and ferrite core?

    Shock hazard, loss of efficiency, faster degradation of the insulation near the contact point. No insulation is perfect.

    6. I am thinking of laminating the ferrite core to isolate the winding and eliminate the bad insulation between them, will it be effective?

    You might be thinking about something other than laminating. A laminated transformer core is one made of thin metal places that are more or less insulated from one-another with varnish. That is done to reduce eddy current losses and probably would have minimal effect on your leakage.

    Just dipping the winding in varnish before slipping it over the core can improve the leakage. For better leakage control when winding directly on the core, wrap the core with paper (almost any kind will work but if you can get fish paper that would probably be best) or tape to form an insulating cushion before you wind the windings. For inter-winding leakage control, put down a layer of tape between windings.
     
  11. newbies_hobbyist

    Thread Starter Member

    Jun 4, 2010
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    Shortbus thanks for the file that you attached, the one with fr1803.pdf file name I believe that is similar to the one I have and the gap between primary and secondary is probably arround uM to 1mm. It is true that under 20V it is impossible for it to break the insulation however this transformer is submerged in oil and used in high temperature as per the information I got. I believe the transformer is made by a third party company, well the windings are covered with varnish and enclosed in to some kind of a resin (transparent mahogany color). No extra wires or electro magnetic thing near to it and yes it is grounded electrically. Again thanks for showing some interest to this matter and helping me.
     
  12. newbies_hobbyist

    Thread Starter Member

    Jun 4, 2010
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    DickCappels since I studied electronics I never thought that ferrite could have some resistance and has infinite resistance only well this is true if you are measuring the ferrite core itself with a low voltage multimeter (both probes in ferrite core) but I just figured it out lately when I decided to use megga-ohmeter and set it to 250V and 500V where both exhibits the same value of resistance insulation and take note I tested it in non conductive table and no human contact so leakage current are totally out of this. To satisfy myself I took a small stepped down transformer with multiple output where it is encapsulated in ferrite core, did the same thing with that ferrite core and I got resistance insulation value as well so from this point I am totally satisfied that ferrite core has breakdown voltage that when reached it would show some resistance. Surge arresters and ferrite core both could have fe, mn, zn and that's what I found while researching the materials used for ferrite and surge arrester. My core is completely ferrite. True there is no perfect insulation but you have to set a reference to know at which insulation level will your system be affected specially in high frequency low amplitude signal. I dont see shock hazard because the stator chassis and rotor rod are both tied to ground. Probably a tape as you suggest would work. One more thing, I did as well a simple experiment using a 24AWG stranded wire with high temp high voltage insulator(in range of 600V) and looped it around the ferrite core (20 turns) then checked the insulation of ferrite core and stranded wire using megga-ohmmeter set to 500V and the insulation is infinite, then after this I cut the 24AWG stranded wire insulator and exposed a small part of the wire and intentionally touch it to the ferrite core then test it again in 500V (wire to ferrite core) and voila insulation is leaking. Anyway thanks for answering my questions, I really love AAC a lot of knowledgeable guys are so willing to help you. Long live for the group.....best groups ever
     
  13. newbies_hobbyist

    Thread Starter Member

    Jun 4, 2010
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    By the way if you have a ferrite core also and megga-ohmeter please try it yourself and see at which DCVoltage it would start to show resistance.
     
  14. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    If I was going to insulate between core and winding, I'd use something better than fishpaper. Kapton has pretty much made the old stuff obsolete.

    Newbies, I'm pretty much tapped out on ideas now. If you find out what the problem/fix is, please post back with what you did, we like to get results and it may help someone else in the future. Sorry I couldn't be more help.
     
  15. newbies_hobbyist

    Thread Starter Member

    Jun 4, 2010
    67
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    Shortbus thats fine, so glad you shared some ideas. Have to be very careful on the thickness of tape as the gap are so small. Sure thing once i finish my experiment i will surely post the result here
     
  16. Metalmann

    Active Member

    Dec 8, 2012
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    I just measured the thickness of a piece of Kapton tape, and it read .0015.
     
    shortbus likes this.
  17. DickCappels

    Moderator

    Aug 21, 2008
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    I like your experiment with the high voltage, high temperature wire, seems to show that some insulation would be of benefit.

    Just for info: I used my old Fluke 73 DVM probes with the DVM on the Ohms setting to measure the resistance across a Ferroxcube 500T400 3E2Q toroid core to be about 13 k Ohms. There are many kinds of ferrite and their conductivity varies greatly, and changes (usually increase) with increasing temperature, so you should always be careful to insulate.

    Surge arresters (Varistors) have a completely different internal structure. Ferrites tend to be homogenius while Varistors are made of compressed oxide-coated grains that conduct when the oxide coating breaks down. But that is not important to what you are doing, but might help you have a clearer picture of what's going on with your core.
     
  18. newbies_hobbyist

    Thread Starter Member

    Jun 4, 2010
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    I did that experiment because I am running out of idea about that leak insulation, I just think that the insulator of wire is the varnish of the transformer winding. Wow I don't know about the toroid core could have resistance in low voltage DVM, in that case your coil winding should have a good coating. Yes in varistor (MOV) that's exactly is the structure but for some other arresters the material is different as well. Soon as I go back to work will continue to do the test and experiment to see if the core to winding bad insulation have great effect. Thanks
     
  19. newbies_hobbyist

    Thread Starter Member

    Jun 4, 2010
    67
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    But that number will double as I said in my earlier post that the bad insulation happens either in rotor or stator and so I have to cover both of them and so the thickness will be double. Anyway I try to find what is the best option since I already get some ideas from all of the guys who responded here.
     
  20. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The Kapton should go between the windings and the core, not between the two cores. The Kapton is a 'buffer' to stop the wire insulation from rubbing on the core due to vibration. Extra insulation between wire and core is a secondary benefit. Electric motors use it between the coils and the stator/core teeth.

    In one of your replies, I think you said this was running in hot oil. Is this so? If the oil is directly on the windings, maybe the enamel insulation is breaking down. Some types of oil can dissolve enamel over time, and heat would speed up the process.
     
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