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  1. #41
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    Quote Originally Posted by SND View Post
    PP is working on upgrading to 480V caps. The PT-380 and up units now use them as far as I understand and it sounds like the other units will switch to it at well soon, they're different uF for the 10hp and 20hp models.
    I'll upgrade to 480v's as soon as they're available.
    That's much less extreme a change as to space and cost than going over to oil-filled, but they've BEEN "available" in the general market for Donkey's Years. Try the industrial suppliers to large-project commercial HVAC contractors, among the more competitive of sources.

    As to P-P's own supply-chain? The last time I bothered to check, their incumbent CEO had a background in the furniture bizness, of all things.

    Unit-cost will be more important to them than to any of we Chikn's doing the odd onesies and twosies for our own use.

    Dunno if it would "sell well" but if P-P were to at least offer a "severe duty option" it might at least help them as to reality check on what units without such could expect under Warranty?

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    Quote Originally Posted by SND View Post
    PP is working on upgrading to 480V caps. The PT-380 and up units now use them as far as I understand and it sounds like the other units will switch to it at well soon, they're different uF for the 10hp and 20hp models.
    I'll upgrade to 480v's as soon as they're available.
    If, as I would suppose, they are the same uF value as used before, they will be good for more current. That would appear to be what is needed, as short life could be caused by heat, and the lower voltage parts would not need to be designed for as much current, so are likely higher resistance (ESR) than the 480V, and so will heat up more.

    the higher voltage parts will be larger, and may not fit in the same spaces as the old ones. There might be a new bracket or other adapter needed to install in older units. No clue if they would provide an upgrade kit.

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    Quote Originally Posted by JST View Post
    If, as I would suppose, they are the same uF value as used before, they will be good for more current. That would appear to be what is needed, as short life could be caused by heat, and the lower voltage parts would not need to be designed for as much current, so are likely higher resistance (ESR) than the 480V, and so will heat up more.

    the higher voltage parts will be larger, and may not fit in the same spaces as the old ones. There might be a new bracket or other adapter needed to install in older units. No clue if they would provide an upgrade kit.
    Sorry, but the impedance of a capacitor is strictly a function of capacitance and frequency. Voltage comes into play for the withstand of the dielectric. Heating comes from the internal resistance and the vibrational loss of the dielectric.

    Tom

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    Quote Originally Posted by TDegenhart View Post
    Sorry, but the impedance of a capacitor is strictly a function of capacitance and frequency. Voltage comes into play for the withstand of the dielectric. Heating comes from the internal resistance and the vibrational loss of the dielectric.

    Tom
    yes the higher voltage rated cap should dissipate the same heat as the lower voltage cap when used at the same voltage. but it will have been wound with thicker metalization or thicker metal foil, the thicker dielectric is likely the bulk of the capacitor by weight (self healing plastic caps are something like 99% plastic, the metalization is so thin you can see through it)

    so i can understand his point that the higher voltage capacitor will have lower esr, but its probably a wash for these type of capacitors.

    allegedly the life of a capacitor in a typical single phase heat pump (50-100uf, 370vac typical) is on the order of 6 years nowadays and you can find plenty of ac technicians saying if the old capacitor is good leave it, don't replace it with a new one. most of that data probably comes from the hotter states, where that capacitor lives with vibration and 120F ambient temps. if phase perfect is sourcing their caps from Chinese manufacturers that have appeared on the market with stolen ip in the last 20 years then its no surprise to me that many of them are failing at only 5-10 years service.

    here in Washington state i rarely see failed capacitors, where typical use is at 60-70F.

    seems to me you should still be able to find a plastic film capacitor to last 1 million hours at rated volts, that's 114 years. back when i was too excited about telsa coils i found a guy who had done the research and discovered plastic film caps have a life time that decreases to the 15th power of the applied voltage. so, you can run your 240vac film cap at 480vac for about.. 30 hours (keep in mind thermal melt down limits, so i'm talking about isolating voltage effects from thermal effects). i've run across other research papers that say its a 14th power relationship.

    so regardless why the capacitors are failing, replacing them with either legitimate capacitors rated for the service (240vac plus some amount of ripple current) or simply replacing them with caps rated for double the voltage should fix the problem, to the tune of an order of magnitude of years of service.

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    Quote Originally Posted by JST View Post
    If, as I would suppose, they are the same uF value as used before, they will be good for more current. That would appear to be what is needed, as short life could be caused by heat, and the lower voltage parts would not need to be designed for as much current, so are likely higher resistance (ESR) than the 480V, and so will heat up more.

    the higher voltage parts will be larger, and may not fit in the same spaces as the old ones. There might be a new bracket or other adapter needed to install in older units. No clue if they would provide an upgrade kit.
    They've made new brackets with bigger holes, wheelieking71 already upgrade his PT-3160 to those a few months ago, seems to have made a big difference.
    Fits in the same location, as much of a pain in the "everything" it is to change those damn capacitors in that horrible location...
    Could be even harder to fit them in the 20hp model, we'll see when the time comes.

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    Quote Originally Posted by RustySparks View Post
    My Tektronix 453 which I have had since it was new failed from what I understand is a common problem with these and similar Tek scopes. The HV transformer epoxy insulation becomes conductive over time. Initially you may notice that the trace disappears after being on for an hour or so. Eventually no trace even when cold. One or two guys over on TekScopes (groups.io) have successfully rewound theirs, but otherwise there appears to be no hope. Sad, this was always my favorite scope. If you find 453s on ebay or elsewhere, be aware that most if not all of these will eventually have this problem.
    Fifteen years or so ago, I was able to find a replacement HV transformer for mine. I have no clue where now, or whether they are still available.

    [Edit] These guys might have one, here: Tektronix Electronic Test Equipment Parts - Transformers/Fans/Relays

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    Quote Originally Posted by n2zon View Post
    Fifteen years or so ago, I was able to find a replacement HV transformer for mine. I have no clue where now, or whether they are still available.

    [Edit] These guys might have one, here: Tektronix Electronic Test Equipment Parts - Transformers/Fans/Relays
    They list one removed from a scope they parted out for $40, but it may have the same problem. I will have to look at mine. It is likely impregnated, making it very difficult to count turns. I had a similar situation once where a customer asked me to rewind a transformer for an old digital voltmeter. It belonged to an AT&T installation coupled to a computer. While they were aware that they could get a new one for less, replacing it would require too much reprogramming. The voltmeter was from the transition era before LED and liquid crystal displays came in, so it had low voltage windings for the transistors and high voltage for the nixies. I tried to get winding specs from the manufacturer but they told me to unwind it and count the turns. It was so cheaply made that they didn't even use any impregnant, just sprayed the ends of the assembly with a spray can. I was able to count turns on all 7 windings and made one right with Nomex insulation and vacuum impregnated with baking varnish.

    Bill

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    Quote Originally Posted by n2zon View Post
    Fifteen years or so ago, I was able to find a replacement HV transformer for mine. I have no clue where now, or whether they are still available.
    [Edit] These guys might have one, here: Tektronix Electronic Test Equipment Parts - Transformers/Fans/Relays
    Quote Originally Posted by 9100 View Post
    They list one removed from a scope they parted out for $40, but it may have the same problem.

    Thanks, but I agree used or even new old stock 453 HV transformers seem likely to have the same problem.
    Lots of good information over on the TekScopes group on groups.io. David Wise on that group has posted photos of his rewind here, lots of good info on that site, including turns counts.
    https://groups.io/g/TekScopes/album?id=12876

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    Quote Originally Posted by TDegenhart View Post
    Sorry, but the impedance of a capacitor is strictly a function of capacitance and frequency....
    Tom
    Not technically true. This comment really isn't appropriate for the present topic, but different types of capacitors with
    nominally the same value of capacitence can have different types of construction and different types of dielectrics and will
    have vastly different self-resonant points which is to say they will have vastly different impedance characteristics as
    a function of frequency.

    At some given frequency you can have one capacitor that's an inductor while another type is still a capacitor.

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    Quote Originally Posted by jim rozen View Post
    Not technically true. This comment really isn't appropriate for the present topic, but different types of capacitors with
    nominally the same value of capacitence can have different types of construction and different types of dielectrics and will
    have vastly different self-resonant points which is to say they will have vastly different impedance characteristics as
    a function of frequency.

    At some given frequency you can have one capacitor that's an inductor while another type is still a capacitor.
    Just so, and a couple of inches of wire can make a huge difference. Ideally, power supply filter capacitors should have four terminals, two for the rectified AC coming and two for the DC leaving, each separately connected to the foil. The same philosophy as Kelvin contacts. As we all know, in a typical power supply, the capacitors are only charged during the peak of the AC.and since they need to get the full current during that brief period the current may easily be 10 or 20 times the average and you can have a surprising amount of voltage built up on a short piece of wire. One time a technician had a power supply with a ripple he couldn't get out. It turned out that instead of connecting the rectified AC to the capacitor's terminals as instructed, he ran it to some other terminals and a couple of inches of wire from there to the capacitor. That little piece of wire inserted in the ground loop caused the regulator to regulate to the ripple so it was doing its job steadfastly following the noise.

    Remember the flap about someone selling Saddam Hussein nuclear bomb triggers? They were simply very low inductance capacitors that would give a fast rise spike.

    back in the tube radio days you could buy bypass capacitors that had built in inductance making them series resonant at 455 Kcs.

    Bill

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    I'm a real EE. :-)

    I have to say that using an oscilloscope, looking at time domain signals, and claiming to be able to give accurate THD figures is just not correct. To get accurate numbers, you need a spectrum analyzer or a dedicated instrument like the Fluke that has just the minimum stuff in it to make the measurement.

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    Quote Originally Posted by TDegenhart View Post
    Sorry, but the impedance of a capacitor is strictly a function of capacitance and frequency. Voltage comes into play for the withstand of the dielectric. Heating comes from the internal resistance and the vibrational loss of the dielectric.

    Tom
    Sorry back atcha... You need to reconsider what you are looking for in terms of specs. The higher voltage part, WHEN USED AT THAT VOLTAGE, will draw more current when connected across the line, as it would be as a PF or filter part.. THEREFORE IT IS CAPABLE OF HANDLING MORE CURRENT. The lower voltage part OF THE SAME VALUE does not need to handle as much current, because at the lower voltage, it draws less.

    Therefore, using the higher voltage part at the lower voltage would be expected to allow a larger current in terms of the harmonics etc, and so the part should last longer than the lower voltage part.

    The largest heating comes from current. And, the higher voltage part will have lower ESR, because it normally draws about double the 60 Hz current when used at its rated voltage, That means 4x the heating. The resistance has to be less to keep down the heating, since it is likely the surface area is not that much more.

    Lower ESR means the part will heat mich less at the lower voltage, and so has more reserve capacity to handle harmonic currents. It is a sensible approach. It would appear that someone tried to "get away with" the lower rated part, and suggest regular replacement as the way to deal with having designed-in the overcurrent and resulting heating that shortens lifetime.

    BTW, this is only true in this way for AC rated capacitors. While DC capacitors may have a lower esr and higher ripple current rating at higher voltage ratings, it is not for the same reason. DC bus capacitors normally have ripple current ratings, but motor run types are less commonly characterized that way, so it is harder to spec a part. You may have to make tests, but then you cannot guarantee that the parts you buy later are the same design and heat to the same degree. Nothing that is not on the spec sheet as a guaranteed spec, can be relied on.

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    Quote Originally Posted by Curt Brown View Post
    I'm a real EE. :-)

    I have to say that using an oscilloscope, looking at time domain signals, and claiming to be able to give accurate THD figures is just not correct. To get accurate numbers, you need a spectrum analyzer or a dedicated instrument like the Fluke that has just the minimum stuff in it to make the measurement.
    In some ways trying to use an oscope to measure the harmonics consumed or generated, is like trying to find out which gear is bad in your transmission... by listening to it with a 16 bit rotary encoder on the crankshaft, rather than a strain gauge on the driveline!

    A current probe with a bandwidth 10x higher than the pwm frequecy is your friend

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    Quote Originally Posted by 9100 View Post
    They list one removed from a scope they parted out for $40, but it may have the same problem. I will have to look at mine. It is likely impregnated, making it very difficult to count turns. I had a similar situation once where a customer asked me to rewind a transformer for an old digital voltmeter. It belonged to an AT&T installation coupled to a computer. While they were aware that they could get a new one for less, replacing it would require too much reprogramming. The voltmeter was from the transition era before LED and liquid crystal displays came in, so it had low voltage windings for the transistors and high voltage for the nixies. I tried to get winding specs from the manufacturer but they told me to unwind it and count the turns. It was so cheaply made that they didn't even use any impregnant, just sprayed the ends of the assembly with a spray can. I was able to count turns on all 7 windings and made one right with Nomex insulation and vacuum impregnated with baking varnish.

    Bill
    I found a new aftermarket one, not a pull. Sorry, I didn't research what was on that site. It just looked promising at first glance.

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    Quote Originally Posted by JST View Post
    Sorry back atcha... You need to reconsider what you are looking for in terms of specs. The higher voltage part, WHEN USED AT THAT VOLTAGE, will draw more current when connected across the line, as it would be as a PF or filter part.. THEREFORE IT IS CAPABLE OF HANDLING MORE CURRENT. The lower voltage part OF THE SAME VALUE does not need to handle as much current, because at the lower voltage, it draws less.

    Therefore, using the higher voltage part at the lower voltage would be expected to allow a larger current in terms of the harmonics etc, and so the part should last longer than the lower voltage part.

    The largest heating comes from current. And, the higher voltage part will have lower ESR, because it normally draws about double the 60 Hz current when used at its rated voltage, That means 4x the heating. The resistance has to be less to keep down the heating, since it is likely the surface area is not that much more.

    Lower ESR means the part will heat mich less at the lower voltage, and so has more reserve capacity to handle harmonic currents. It is a sensible approach. It would appear that someone tried to "get away with" the lower rated part, and suggest regular replacement as the way to deal with having designed-in the overcurrent and resulting heating that shortens lifetime.

    BTW, this is only true in this way for AC rated capacitors. While DC capacitors may have a lower esr and higher ripple current rating at higher voltage ratings, it is not for the same reason. DC bus capacitors normally have ripple current ratings, but motor run types are less commonly characterized that way, so it is harder to spec a part. You may have to make tests, but then you cannot guarantee that the parts you buy later are the same design and heat to the same degree. Nothing that is not on the spec sheet as a guaranteed spec, can be relied on.
    You offer some interesting thoughts that I will compose my reply offline as it will take awhile. In the mean time, what is ESR?

    Tom

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    Quote Originally Posted by TDegenhart View Post
    what is ESR?
    Tom
    Equivalent series resistance. It's a catch-all for capacitor losses.

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    Further to the original comments about higher voltage parts. Yes, the impedance is set by the capacitance and frequency considered, at least through the range in which the capacitor is a capacitor. In fact that is key to the idea I am presenting.

    At the higher voltage, the same impedance will draw higher fundamental (50 or 60 Hz) current. So at 480V vs 240V, a given capacitance will draw twice as much normal, fundamental current. So, the 480V part would start at double current capacity on that basis. Since some percentage over the basic current is allowed in the part design for harmonics, the design current for the higher voltage part should be double the basic and allowance for the 240V part.

    So, if they use a 480V part, they can reasonably expect that the part will be rated for double the current. Since they are somewhat "pushing" the specs of the 240V part, but not so much as to induce VERY quick failure, the 240V total current would be expected to be well within the long-life capability of the higher voltage parts.



    Quote Originally Posted by jim rozen View Post
    Equivalent series resistance. It's a catch-all for capacitor losses.
    While it IS a "catch-all", as a practical matter that does not matter, ANY "losses" develop heat.

    The dielectric losses depend on frequency. Every time the polarity reverses, a certain amount of energy is needed to do that, to flip the molecules in the dielectric. The "power" dissipated will depend on how often the reversals occur. Therefore the power losses are higher at higher frequencies, and harmonics dissipate more than fundamental currents.

    Harmonics also represent resistance losses, since the current flow needed to reverse polarity on a capacitor dissipates energy in the resistance, and doing that more often dissipates more in proportion.

    Also, harmonic currents are added to fundamental currents (not necessarily a straight addition, though). Power lot is proportional to current squared, so if total rms current is increased to 1.5x what it was, then power dissipation and heating is increased by 2.25x.

    Since the higher voltage part was rated for 2x current, the power dissipation allowed will be closer to 4x. The higher voltage part is better able to handle the higher dissipation due to harmonics when used at the lower voltage, because it can handle 4x the dissipation of the original part, no matter if that is counted as dielectric or resistive dissipation..

    The issue is that PF and motor run capacitors do not routinely offer the ESR as a spec, although in some cases they do. This makes it difficult to select a capacitor which will be subjected to significant harmonic current, for a number of reasons. .You are not often given the spec for current, you are not often given an esr spec, you are not generally given a spec for impedance vs frequency, you are not usually given the self-resonant frequency, the usual electronic specifications are simply less often given for a power systems capacitor.

    it can be challenging to develop the specs and ensure that they are adhered to. It is better to find a manufacturer who will agree to produce parts to your own specifications. PP may have done that. Or they may have found a higher voltage versu=ion of the same part, and called it good, given that the original parts "nearly" worked reliably.... they lasted at least 3 years in general.

    Either approach is workable, I tend to prefer getting the manufacturer to make or recommend parts that will mee the spec you need.

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    Just to muddy things up further, For a given power level, you would use less capacity at the higher voltage in most cases.

    Bill

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    Quote Originally Posted by 9100 View Post
    Just to muddy things up further, For a given power level, you would use less capacity at the higher voltage in most cases.

    Bill
    Yes, but that is not the point.... a certain capacitance is wanted, and they have switched to using a higher voltage part of the same capacitance, using it at the lower voltage. There is no attempt to match VA, etc, it is a straight-up matter of a certain value capacitor needed and used. As T Degenhart points out, the voltage rating does not affect how the part works, but as I noted, it does improve the maximum ratings of the part

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    I guess the issue that I have is the assumption that increasing the voltage rating linearly increases the current handling and hence, the power dissipation.

    I don't think any of us knows enough about capacitor manufacture to accurately predict the effect of voltage rating has on the power dissipation. Increasing the voltage rating means either or both increasing the thickness or/and type of dielectric. How this changes the ESR is unknown.

    As an example from McMaster-Carr, 7602K71 and 7602K83 370v and 440v round caps. Both 3.75 high by 1.75 dia and 2 dia respective.

    Percent increase in voltage and hence current 19%. Increase in surface area, 15% more. The higher rated cap should by this run hotter. Probably not, change in construction probably compensates.

    However, using that extra capacity for harmonic currents may negate added heating capability because the harmonics will cause more heating that line current.

    When its all said and done, just pay your money and take your chances.

    Tom


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