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  1. #61
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    without knowing the pwm ripple current of the phase perfect its impossible to know if they were actually stressing the capacitors substantially beyond what the manufacture anticipated.

    if the capacitors were only rated at 240vac then they were certainly over stressed imo. but i would imagine that 370vac rated capacitors would have ample margin to handle the extra ripple current. so its interesting to me that they have replaced them with 480v rated capacitors.

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    Well, they've obviously worked fine for most of us for years. But some have had issues, mostly with many machines tied to them and higher temperatures. wheelieking71 mentioned with the new 480V caps his unit no longer has the bacon in a frying pan sound during peak ramp up/down. I know my PT-355 does it, I look forward to seeing if that goes away. Not that it seems to matter a lot either way since it does it thousands of times in a day with the lathe but eh, bigger is usually better.
    I know I never heard a buzz out of my old blue 10hp unit when it starts a 7.5hp lathe. Those older units seemed sturdier, higher pitch is more annoying to the eat but doesn't bother as many things it seems, but also weighed a lot more. I recently had to get a new microwave(unrelated) but it no longer picks up the old blue unit like the previous microwave did.

    They also somewhat recently came out with 480V in/out PP designs, but I don't know what those use for filter cap and if the bigger 480V ones came about out of that design too or if they use an even higher voltage type.

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    For 480 volt applications, I would think the caps would at a minimum be rated 720 volts or higher. I might be cheaper to add a transformer to a 240 design. However, my 480 volt rpc has been chugging along for nearly 50 years with 600 volt GE oil filled caps.

    Tom

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    Quote Originally Posted by johansen View Post
    without knowing the pwm ripple current of the phase perfect its impossible to know if they were actually stressing the capacitors substantially beyond what the manufacture anticipated.

    if the capacitors were only rated at 240vac then they were certainly over stressed imo. but i would imagine that 370vac rated capacitors would have ample margin to handle the extra ripple current. so its interesting to me that they have replaced them with 480v rated capacitors.
    he over-stress is inferred from the fact that they recommend such a short replacement time.... There is no very good reason for that other than thermal degradation, or possible self healing issues over time reducing capacitance. Thermal degradation would be an issue of either ripple current, or possibly bad mechanical placement next to hot things.

    The voltage change would tend to reduce ripple current (heating) problems, so much of the evidence points toward the parts being pushed rather hard in the design,

    Quote Originally Posted by TDegenhart View Post
    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
    The fact that the capacitor is MADE FOR higher voltage does not necessarily mean more dissipation. It means that the capacitor must be made to HANDLE more dissipation. Big difference.

    if used AT 480 V, the current drawn will inherently be 2x that at 240V for same value capacitance. So the cap HAS TO be made to handle 4x the dissipation due to current. The surface area is not gonna do that for a reasonable sized part, so the answer is that the internal resistance is made less so that the "I^2 * R" heating is less.

    That means that when the part is used at 240V, and does not inherently draw that much current, there is considerably more reserve capability available before the dissipation is too much for the part to handle. It is my judgement that this effect is a big factor in their decision to make the change they seem to have made. I would have done that also, as it is minimally invasive as to changes to structure of the device and other parts.


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