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Old 24th August 2008, 01:17 AM   #1
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Default Testing Stator Insulation

I have a pair of transformers that came out of a florescent tube unit. I planned on using an old stereo amplifier and hook one channel up to the primary end of the transformer and hook the secondary end up to the coated steel stators and ground the center-tap. Then observe for any potential arcing issues. These transformers will not be the actual ones I will use. Can you suggest anything else I should think about?

Right now, I have two sampled pieces of the steel coated with a spray on called "Plasti-Dip". I contacted the manufacturer of this spray and they informed me that it should have a dielectric strength of 1400 Volts/mil. They also said that a sprayed coat would have a thickness of 2-4mil. I put five coats on both sides of the sampled steel, so I should be ok under 14,000 volts. I'm not so much worried about the coating, but rather the ability to coat any burrs that were missed from the perforation process. With my plates already curved can you describe how the etching process works? Any other methods for getting these burrs out?

Thanks,
Bryan
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Old 25th August 2008, 09:36 AM   #2
Calvin is offline Calvin  Germany
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Hi,

You can test every sheet on its own. Use a HV-supply eg. the HV-cascade. Connect one end of the cascade to the stator.
Connect a wire to a sheet of household alumnium foil
Lay the Aluminium onto the surface of the stator.
Connect the wire to the foil through a resistor of max. 10Megohms between cascade and foil. Start with a lower voltage point of the cascade.
Switch the cascade on (the foil will be ´sucked´ to the stator surface) and listen. As long as there is silence or just some tickling sounds everything is ok. The ´treshold´ is reached when the flashovers are becoming louder and sound like small gunshots (You will see the foil move and eventually small bumps appear in the foil surface when the arc´s energy is high enough. These bumps make it easy to localize the point where the stator insulation failed and where the insulation has to be repaired.) Raising the voltage even more the single arcings become a constant arcing.

The arc-free voltage should be minimum the max. peak signal voltage of the panel....with 1mm d/s ~2kV for example.

The problem with the data from the datasheets of the manufacturers is that it neither takes burrs and sharp edges into account nor higher signal frequencies. Those figures are only correct for plane, massive surfaces and DC-voltages or below frequencies of 400Hz. As You may see in some capacitor manufacturer´s datasheets (WIMA eg) the flashover treshold voltages sink with rising signal frequencies. With insulation laquers its up to You to evaluate the figures for higher freqs. I have yet to come across one laquer-manufacturer who knows and actually has tested these figures!

jauuu
Calvin
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Old 25th August 2008, 07:59 PM   #3
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Calvin,
I can try this. For my HV supply I have the Yamaha P2075 power amplifier, that in bridge mode gives 150 watts of power (i have to recall into what ohms) I planned on using a step-up for testing. What would be an appropriate step -up here. my d/s spacing is about 90-100mil. the acrylic spacers are 80 mil thick. This is curved design, should I just get 60 mil thick acrylic and then realize with insulation coating and glue bonds will make d/s somewhere closer to 80mil? What voltages should i try for?
thanks
bryan
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Old 26th August 2008, 10:42 AM   #4
Calvin is offline Calvin  Germany
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Hi,

the HV-supply is to supply for the ´bias voltage´ or also called ´polarizing voltage´ of the membrane and which is a constant DC-voltage of ~1-5kV value (depending on the panel´s dimension). Since it only provides enough charge to counter for leakage of the charge on the membrane it doesn´t need high power. Less than 1W is normally more than sufficient. Look for ´HV-cascade´, or ´Cockroft-Walton´.
You were talking about the audio signal voltage (AC!) that is stepped up by a tranny and which is connected to both the stators. A completely different matter!

For a curved panel I´d regard a ´real´ 60mil d/s-value as reasonable maximum. The optimum d/s is normally even less at around 40-50mil.
You must realize -unfortunately this is the most common design failure with beginners- that a large d/s is always a bad solution.
It looses on safety , efficiency, dynamics and sound.
With a d/s around 50mil You just need 2-2.5kV of bias. This value defines the maximum peak-voltage of the music signal too. 2.5kVpeak equals app. 1.8kV rms. A tranny with a stepup ratio of 1:60 would have to deliver 30Vrms -->110W@8Ohms. In praxis the panel will limit mechanically earlier (say at 50W). But since the efficiency of the panel is high (because of the small d/s and the large membrane area) You can reach SPLs of >>100dB.

jauu
Calvin
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Old 26th August 2008, 11:47 AM   #5
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very helpful. The thinnest acrylic sheets i can pick up are 60 mil. I was leaning on using tape for the spacers instead. I can get thickness from 20 mil to 60 mil.

From what you said it wont matter if i test with ac voltage or dc voltage. I like the lower potentials with a smaller d/s. The tape I may end up using is 45 mil thick 3M very high bond. Do you recommend the tape. It certainly would be easier than having to cut and shape the acrylic to lay on the curved stators. Have builder's seen good longevity with tape?

I'm sure your already familiar with Sander's book, I was pulling the numbers there. He and Waldron suggested a larger spacing to keep the film stable and to prevent film collapsing into a stator.

bryan
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Old 27th August 2008, 08:01 AM   #6
Calvin is offline Calvin  Germany
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Hi,

tapes made by 3M have been used, are used and probabely will be used. 3M manufacture tapes that fit this application excellently as well as they manufacture some which are not prime choice for this application. So just choosing the tape by thickness won´t guarantee success and long lifetime. The choice depends on the bonding partners, which means the insulative material of the stators or spacers and the diaphragm material.

I´d rather test with DC voltages. The reason is that the flashover phenomen is a bit time-dependant, meaning that arcing may occur first aver several seconds or even minutes. You should test with higher DC than AC voltages since of the reduction of flashover voltage with increasing frequencies. Generating the DC is very easy done by cascading two HV-supplies or adding cascade-sections.

jauu
Calvin
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Old 27th August 2008, 08:01 PM   #7
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Hi Calvin,

My equipment that will be used for the testing is as follows:

1. 2-20 volt DC that will serve as input to voltage multiplier.

2. Regulated voltage multiplier, 50-5,000VDC, 1.5 Watts of power .2 mA.

3. I will connect aluminum foil to one contact the other to my soldered contact on the steel stators.

You saved me, at first I was going to test the stators together, but the odds that the insulation will fail in a region close where the other stator coating fails is small. Your right use aluminum foil to test stators independently.

As far as insulating the stators, I picked up Plasti-Dip, I contacted the manufacturer and they informed me that it has a dielectric strength of about 1400V/mil, a sprayed coating will have a thickness of 1-2mil so I will put five coats on to play it safe.

What coating do you use for the stators? What coating would be good to ensure good "bonding partner" to the 3M foam tape.

I appreciate your advice, this is my first build, and I have to say that this forum has been extremely helpful and intellectual.

Thanks
Bryan
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Old 28th August 2008, 09:05 AM   #8
Calvin is offline Calvin  Germany
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Hi,

the tapes glue surfaces are modified to bond to different bondding partners. There are high surface energy partners to low surface energy partners. 3M´s datasheets will help

The coating of my choice is already disclosed in a thread of Your´s.

jauu
Calvin
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