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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Calculating Power Factor
I'm trying to figure power factor for a large transformer-type DC power
supply. This is pretty much just for my own amusement (so I can figure power draw from current and voltage readings) so there's nothing critical here. This device runs on 208V 3-phase (as in each leg is 120V from neutral but there's no neutral connection). The 3 transformer primaries are wye connected but there's no other connection to the wye point. It draws about 18A per leg. I decided to compare current and voltage waveforms using a dual trace scope. To look at current, rather than put a small resistance shunt in one line I decided to take advantage of the small voltage drop that occurs between the breaker panel and the cutoff switch for the device. So I had one channel of the scope looking at the voltage difference between one leg at the cutoff switch and the "hot" side of a 120V outlet on the same leg. For voltage I just looked at one leg vs. neutral. I'll skip over boring details about having to float the common side of the scope in order to do this without creating a short circuit. Anyway, it seemed to work. Emphasis on "seemed." What I saw looked like voltage vs. current was out of phase by about 90°. Is that what one would reasonably expect going into a transformer? Seems kind of extreme so I wonder if my entire setup was bogus in some way. Part of the reason for the question is that in retrospect I should have probably been looking at voltage as measured from one input leg to the wye point, not neutral since that's what a transformer primary sees. But I would think the wye would be similar to neutral. |
#2
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Calculating Power Factor
Steve Kraus wrote:
I'm trying to figure power factor for a large transformer-type DC power supply. This is pretty much just for my own amusement (so I can figure power draw from current and voltage readings) so there's nothing critical here. This device runs on 208V 3-phase (as in each leg is 120V from neutral but there's no neutral connection). The 3 transformer primaries are wye connected but there's no other connection to the wye point. It draws about 18A per leg. I decided to compare current and voltage waveforms using a dual trace scope. To look at current, rather than put a small resistance shunt in one line I decided to take advantage of the small voltage drop that occurs between the breaker panel and the cutoff switch for the device. So I had one channel of the scope looking at the voltage difference between one leg at the cutoff switch and the "hot" side of a 120V outlet on the same leg. For voltage I just looked at one leg vs. neutral. I'll skip over boring details about having to float the common side of the scope in order to do this without creating a short circuit. Anyway, it seemed to work. Emphasis on "seemed." What I saw looked like voltage vs. current was out of phase by about 90°. Is that what one would reasonably expect going into a transformer? Seems kind of extreme so I wonder if my entire setup was bogus in some way. Part of the reason for the question is that in retrospect I should have probably been looking at voltage as measured from one input leg to the wye point, not neutral since that's what a transformer primary sees. But I would think the wye would be similar to neutral. what you're seeing in the xformer is normal. the xformer is returning the current phase from the current applied source which comes back out of phase with it, thus, the current you see is lagging behind from which it was created from the voltage you now see going the other way... Power Factor (F) is the difference between True power (resistive) and Apparent power (reactive)... |
#3
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Calculating Power Factor
"Steve Kraus" To look at current, rather than put a small resistance shunt in one line I decided to take advantage of the small voltage drop that occurs between the breaker panel and the cutoff switch for the device. So I had one channel of the scope looking at the voltage difference between one leg at the cutoff switch and the "hot" side of a 120V outlet on the same leg. For voltage I just looked at one leg vs. neutral. I'll skip over boring details about having to float the common side of the scope in order to do this without creating a short circuit. Anyway, it seemed to work. Emphasis on "seemed." ** Pure insanity. The scope common ( or ground) is at the same voltage as one phase. For god's sake monitor the neutral current ( using a current clamp probe) and one phase. ...... Phil |
#4
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Calculating Power Factor
For god's sake monitor the neutral current ( using a current clamp
probe) There is no neutral connection so no neutral current. But assuming you misspoke, what's the difference between sensing current on one of the legs using a current clamp probe and wiring a shunt into that line and looking at voltage across it? In effect that's what I've done except I'm taking advantage of the small voltage drop that already exists. |
#5
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Calculating Power Factor
Jamie wrote:
what you're seeing in the xformer is normal. the xformer is returning the current phase from the current applied source which comes back out of phase with it, thus, the current you see is lagging behind from which it was created from the voltage you now see going the other way... Power Factor (F) is the difference between True power (resistive) and Apparent power (reactive)... My aim here is to estimate real power draw. I've got a switch mode supply putting out about the same output and it draws about 8A per leg. (I don't know anything about PF or reactive power on that one). This transformer type rectifier is drawing 18A per leg. I know from asking around that this particular brand is known to have inefficiently wound transformers. So that accounts for part of it. But there's also the matter of taking into account reactive power & power factor. BTW, output in both cases is about 62A at about 22V. |
#6
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Calculating Power Factor
"Steve Kraus" For god's sake monitor the neutral current ( using a current clamp probe) There is no neutral connection so no neutral current. ** Then monitor one of the phases and the voltage of that same phase. But assuming you misspoke, what's the difference between sensing current on one of the legs using a current clamp probe and wiring a shunt into that line and looking at voltage across it? ** The common connection is LIVE in your case - IDIOT !! In effect that's what I've done except I'm taking advantage of the small voltage drop that already exists. ** You are so dangerously * STUPID * you have no clue as to what PF even is. Hint: In the example you are discussing, phase angle barely comes into it. ..... Phil |
#7
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Calculating Power Factor
Steve Kraus wrote:
Jamie wrote: what you're seeing in the xformer is normal. the xformer is returning the current phase from the current applied source which comes back out of phase with it, thus, the current you see is lagging behind from which it was created from the voltage you now see going the other way... Power Factor (F) is the difference between True power (resistive) and Apparent power (reactive)... My aim here is to estimate real power draw. I've got a switch mode supply putting out about the same output and it draws about 8A per leg. (I don't know anything about PF or reactive power on that one). This transformer type rectifier is drawing 18A per leg. I know from asking around that this particular brand is known to have inefficiently wound transformers. So that accounts for part of it. But there's also the matter of taking into account reactive power & power factor. BTW, output in both cases is about 62A at about 22V. It's possible it has a saturated core transformer. What this does is causes the input side to exert the rated operating current at all times but this type of design is best at keeping the transformer at a safe operating Q to help prevent unwanted harmonics and oscillations in the circuit that can take place where capacitors are in the coil design. You see this in microwave ovens due to the cap in the hV circuit.. with out the saturated transformer it can hit a resonance that can become destructive. This isn't a problem if you plan on using the supply to it's fullest, as far as efficiency goes.. I think what you're looking for his is efficiency measurements, not PF.. Just load the supply to its max and measure the input side and compare the wattage to the output side. You will see that more energy is going to be used on the input side.. |
#8
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Calculating Power Factor
Phil Allison wrote:
** The common connection is LIVE in your case - IDIOT !! I'm aware of that and dealt with it carefully and appropriately. It's not particularly difficult not to contact anything live for the few seconds I was looking at the traces. Scope front? Plastic. Scope knobs? Plastic. It's not rocket science. You've never worked around something live? Well good for you. I'm sure I don't have to point out that the voltage, while certainly dangerous, is less than in other parts of the world. But by all means please continue with your name calling. |
#9
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Calculating Power Factor
"Steve Kraus is a Krackpot " Phil Allison wrote: ** The common connection is LIVE in your case - IDIOT !! I'm aware of that and dealt with it carefully and appropriately. ** Like hell you did !!!! Only a COMPLETE IDIOT does what you did. But by all means please continue with your name calling. ** Listen here pal - you are one ****ing arrogant, pig ignorant, snipping, trolling pile of vile autistic dung. And them are all your GOOD points. .... Phil |
#10
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Calculating Power Factor
Jamie wrote:
This isn't a problem if you plan on using the supply to it's fullest, as far as efficiency goes.. No, it won't need to go much more than it is now. These are xenon arc projection lamp supplies. We have sufficient light output at 55A but as the bulbs age it's necessary to raise the current to keep the arc from wandering on the electrode tips. If I have to go much over 65A (70A is max on these bulbs) it will be time to replace the bulbs but that time is coming very soon on the basis of hours alone. Fortunately for me the bulbs last years. I was just kind of curious what the difference in power draw was and realized simple current & voltage measurements that I would take might be meaningless when going into a transformer. I do know the difference is real since the conduit going to the transformer type supply gets slightly warm after many hours but the one going into the switch-mode supply doesn't seem to rise detectably above ambient so I tend to believe the 8A figure on that one. If I'd known the transformer type of this particular brand was so inefficent I would not have bought it 3 years ago. The original 20 year old equipment (two of that same brand) failed about 6 months apart in 2007. The first I replaced with a new one of the same make though of larger capacity. When the 2nd old one gave up the ghost I decided to go with the switch-mode supply. Electricity is a tiny part of operating costs but still, just on principle...8A vs. 18A* for the same output? It seems rather amazing. *not counting reactive power issues Oh, in case anyone cares, I'm measuring current with a Fluke / LEM LH2015 true RMS AC/DC clamp meter. |
#11
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Calculating Power Factor
On 11/14/2010 9:02 PM, Phil Allison wrote:
Only a COMPLETE IDIOT does what you did. Along with several idiots at Xerox. While i was working on a switching power supply, we had reason to measure the input (live) AC line. Tectronix to the rescue. Apparently they knew there were a few insane people that would want to float a scope for live AC line measurements and they had a special isolation box that the scope plugged into for just that occasion. Of course, some fool removed it while we were at lunch. Ya know, it's amazing the fwipzzzt noise the ground lead on an expensive scope probe makes when it goes up in flames when you turn the power on. Jeff |
#12
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Calculating Power Factor
"Steve Kraus = IMBECILE " Electricity is a tiny part of operating costs but still, just on principle...8A vs. 18A* for the same output? It seems rather amazing. ** No one tiny bit - if you have a clue about PSUs. *not counting reactive power issues ** Already told ya - there IS no reactive power issue. Oh, in case anyone cares, I'm measuring current with a Fluke / LEM LH2015 true RMS AC/DC clamp meter. ** Then you already know the VA figure. But to find true power in this example - you need a wattmeter. No if or buts. ..... Phil |
#13
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Calculating Power Factor
"Jeffrey Angus" Phil Allison wrote: Only a COMPLETE IDIOT does what you did. Along with several idiots at Xerox. While i was working on a switching power supply, we had reason to measure the input (live) AC line. Tectronix to the rescue. Apparently they knew there were a few insane people that would want to float a scope for live AC line measurements and they had a special isolation box that the scope plugged into for just that occasion. Of course, some fool removed it while we were at lunch. Ya know, it's amazing the fwipzzzt noise the ground lead on an expensive scope probe makes when it goes up in flames when you turn the power on. ** ROTFL ...... Phil |
#14
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Calculating Power Factor
On Sun, 14 Nov 2010 21:22:33 -0600, Jeffrey Angus
wrote: On 11/14/2010 9:02 PM, Phil Allison wrote: Only a COMPLETE IDIOT does what you did. Along with several idiots at Xerox. While i was working on a switching power supply, we had reason to measure the input (live) AC line. Tectronix to the rescue. Apparently they knew there were a few insane people that would want to float a scope for live AC line measurements and they had a special isolation box that the scope plugged into for just that occasion. Of course, some fool removed it while we were at lunch. Ya know, it's amazing the fwipzzzt noise the ground lead on an expensive scope probe makes when it goes up in flames when you turn the power on. Jeff Nicely done. I usually had a sign hanging on the test mess during lunch. Something like "Touch anything and you'll die". However, ty-wrapping the power cords to both the instrument and the power strip worked somewhat better. I also placed mouse traps in strategic locations until my boss demanded that I stop (after one got him). So, why didn't you use two probes, with the scope set to differential? To get the phase, you would need a 4 trace scope, but those are common enough. I believe that differential is also the recommended method as it keeps the scope case at ground level and takes care of the common mode noise on the line. The isolation xformer method puts one side of the power line on the case of the instrument, which is not pleasant if you touch both the instrument case and a nearby ground. Remember.... you have but one life to give to your profession. Incidentally, I always kept my old broken scope probes. The intermittent probes were especially prized. I would destroy one about every 2 months. I would mark them with a tiny red nail polish dot, and leave them connected in a conspicuous location, while I hid my working scope probes. When they would predictably disappear, I would just smile. I also had a #2 Phillips screwdriver with a self stripping plastic handle. I think it was borrowed (stolen) at least 3 times, and then returned. I still have it somewhere. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#15
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Calculating Power Factor
On Sun, 14 Nov 2010 18:44:27 -0600, Steve Kraus
wrote: I'm trying to figure power factor for a large transformer-type DC power supply. This is pretty much just for my own amusement (so I can figure power draw from current and voltage readings) so there's nothing critical here. If all you want are the results, the common Kill-A-Watt power meter has a PF measurement feature. $25-$40 everywhere. http://www.p3international.com/products/special/P4400/P4400-CE.html I have 4 of these (various models). If you want to do it safely with a scope, it really requires a 4 channel scope (or 2 channels with two differential inputs per channel). Put a small value non-inductive resistor in series with the power line. One pair of diffential input probes go across the resistor for the current waveform. The other pair of probes go across the line for the voltage waveform. With a switcher, you're going to have a non-sinusodial current waveform. Therefore, you can't just use the phase angle as the power factor and will need to measure the Apparent Power and the RMS powers. http://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/nonsinusoidal_power.pdf There's also quite a bit of work done on power factor correction these days which probably includes some measurement techniques. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#16
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Calculating Power Factor
"Jeff Liebermann" With a switcher, you're going to have a non-sinusodial current waveform. ** Same for a transformer based PSU too. Therefore, you can't just use the phase angle as the power factor and will need to measure the Apparent Power and the RMS powers. ** PF = true power / VA No phase angles in sight ...... ..... Phil |
#17
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Calculating Power Factor
On Mon, 15 Nov 2010 16:24:55 +1100, "Phil Allison"
wrote: With a switcher, you're going to have a non-sinusodial current waveform. ** Same for a transformer based PSU too. True. Once you get away from a pure sine wave current waveform, the phase angle method doesn't work. See the PG&E article I cited. Therefore, you can't just use the phase angle as the power factor and will need to measure the Apparent Power and the RMS powers. ** PF = true power / VA No phase angles in sight ...... True. I just hate to agree with you, but I'll make an exception this one time. But don't worry... it probably won't happen again. Handy buzzword translator: RMS power = True power Apparent power = VA power. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#18
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Calculating Power Factor
Jeff Liebermann wrote:
If all you want are the results, the common Kill-A-Watt power meter has a PF measurement feature. $25-$40 everywhere. http://www.p3international.com/products/special/P4400/P4400-CE.html I have 4 of these (various models). Thanks Jeff; that looks like a useful gadget. I may have to get one. But I don't think it's applicable for my situtation which is 208V / 3-phase. Actually there could be a way to use it for 3-phase. Assuming it measured current on just the hot leg one could run one leg at a time through the meter with the neutral side of the meter connected to the power line neutral OR, even better, connecting it to the Y point of the power supply. The meter would only be exposed to 120V. Basically, it would be imagining this as 3 separate single phase loads. Then just sum the results. But the meter is only good for 15A and my supply is pulling 18 and up. |
#19
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Calculating Power Factor
I enjoyed this thread, especially the pranks.
Anyway, I too had worked on a few high power (1000 W) ARC lamp supplies. The last had photofeedback unit that controlled intensity. The intensity got lowered from a fixed level from the feedback unit. The unit had a shutter, so the setpoints were remembered when the lamp was off. This photofeedback unit was designed in ahigh tech fashion. The light was sampled with fiber optics at the last of the optical path. The sensor temperature was controlled via a thermoelectric block. |
#20
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Calculating Power Factor
Jamie t wrote:
Steve Kraus wrote: Jamie wrote: what you're seeing in the xformer is normal. the xformer is returning the current phase from the current applied source which comes back out of phase with it, thus, the current you see is lagging behind from which it was created from the voltage you now see going the other way... Power Factor (F) is the difference between True power (resistive) and Apparent power (reactive)... My aim here is to estimate real power draw. I've got a switch mode supply putting out about the same output and it draws about 8A per leg. (I don't know anything about PF or reactive power on that one). This transformer type rectifier is drawing 18A per leg. I know from asking around that this particular brand is known to have inefficiently wound transformers. So that accounts for part of it. But there's also the matter of taking into account reactive power & power factor. BTW, output in both cases is about 62A at about 22V. It's possible it has a saturated core transformer. What this does is causes the input side to exert the rated operating current at all times but this type of design is best at keeping the transformer at a safe operating Q to help prevent unwanted harmonics and oscillations in the circuit that can take place where capacitors are in the coil design. You see this in microwave ovens due to the cap in the hV circuit.. with out the saturated transformer it can hit a resonance that can become destructive. dumb question time. What does the magnetron behave like in a circuit? I realize it's basically a giant rectifier tube, but I'm not wise on tube characteristics and behavior. I imagine there's some bizarre resistive component, but that's all I can come up with. |
#21
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Calculating Power Factor
"Cydrome Leader" What does the magnetron behave like in a circuit? I realize it's basically a giant rectifier tube, but I'm not wise on tube characteristics and behavior. I imagine there's some bizarre resistive component, but that's all I can come up with. ** Suffice to say that the load presented by a typical microwave oven to the AC supply is close to sine wave current and pretty much in phase. Resonance is definitely involved. ..... Phil |
#22
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Calculating Power Factor
Phil Allison wrote:
"Cydrome Leader" What does the magnetron behave like in a circuit? I realize it's basically a giant rectifier tube, but I'm not wise on tube characteristics and behavior. I imagine there's some bizarre resistive component, but that's all I can come up with. ** Suffice to say that the load presented by a typical microwave oven to the AC supply is close to sine wave current and pretty much in phase. Resonance is definitely involved. .... Phil I just measured mine to see if an emergency generator would like it. Measured .95 on a kill a watt. |
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