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Old 13th December 2007, 03:33 PM   #1
JoshK is offline JoshK  Canada
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Default ESR of motor-run caps?

I am trying to model some PSUs in PSUDII and it occurred to me that I am using the default ESR of 2R's for the caps. Does anyone know ball park ranges for ESR of motor run caps?

I have a bunch of different 40uf, 80uf and 100uf in both 370VAC and 500VAC ratings. Any guesses as to what ESR might be?

Greatly appreciated!
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Old 13th December 2007, 04:42 PM   #2
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Datasheet lists dissipation factor of less than 0.1%.

D.F. = ESR/Xc, where Xc is 1/(2*pi*f*C).

For a 40 uF cap, Xc = 66 ohms at 60Hz or 33 ohms at 120Hz (if used as PS filter).

ESR= D.F. * Xc = 66 milliohms or 33 milliohms at 120Hz.
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Old 13th December 2007, 04:44 PM   #3
JoshK is offline JoshK  Canada
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Thanks!
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Old 13th December 2007, 04:47 PM   #4
rdf is offline rdf  Canada
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Another way of looking at it is that the DF of modern polypropylene motor run caps is so low that a typical amp's internal harnessing will swamp ESR.
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Old 13th December 2007, 05:22 PM   #5
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by zigzagflux
Datasheet lists dissipation factor of less than 0.1%.

D.F. = ESR/Xc, where Xc is 1/(2*pi*f*C).
I'm not so sure about that. The implicit assumption is that all the losses come from the ESR. But we know that leakage across the dielectric causes losses...
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Old 13th December 2007, 05:31 PM   #6
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The formula is correct, and the losses for that one frequency are all lumped together and represented by the single resistor, the ESR. The simple model says nothing about where the losses come from, just what they look like at a single frequency. The nice thing about DF is that it tends to stay more constant over frequency, though it will change somewhat. I find ESR a less useful concept, though it's what one ends up using in a simulation. Also, most people don't include DA in the simple model, much less voltage linearity effects- fortunately small in quality caps, though significant in some ceramics and such.
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Old 13th December 2007, 05:38 PM   #7
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Yes, the equation looks like one of those RF (single frequency) equations where the Q of a resonant circuit can be held the same whether a series or parallel resistor is used. But a power supply isn't a single frequency application, hence my doubt as to the validity of the equation.

On a more practical note, PSUD is the exact problem that lead me to make an ESR meter. If I had some motor run capacitors, I'd measure them. They're probably quite low, <0.5 Ohm.
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Old 13th December 2007, 07:35 PM   #8
JoshK is offline JoshK  Canada
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60hz vs. 120hz? This bring up the question I don't know the answer to. I know the 60hz comes from the line freq of the mains, but what is the 120hz from, or why is the the important freq? Does it have to do with rectification?
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Old 13th December 2007, 07:58 PM   #9
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Yes, a full wave rectifier will output 120Hz (100Hz in Europe). Each half cycle of the AC is rectified, so there are 120 "pulses" each second; think of mirroring the negative-going pulses of each half-cycle to positive.
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Old 13th December 2007, 07:59 PM   #10
JoshK is offline JoshK  Canada
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ah... almost answered my own question. I guess if I thought it through fully I would have seen that.

Thanks.
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