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Run caps?
So I've got my converter running. However, I was told that by adding
run capacitors I could lower the needed Amps to drive the idler? Is this true? If so would I simply put them inline on the incoming 220 hot leads going to the idler? Or else where? Thanks, Paul |
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#2
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Run caps?
Paul,
There are different setups (balanced and unbalanced). Mine is an unbalanced, with one cap between each "real" 220V leg and the "manufactured" third leg. Getting the right sizing takes some experimentation and a voltmeter. A clamp-on ammeter is handy, too. I started with 15uF per HP, or 150uF on my 10HP idler. As I recall, my final set was 160uF from L1-L3, and 40uF from L2-3. Be very careful when adding caps. Make certain the caps are rated as run caps, and have a voltage rating above 220V. There's a LOT of good info on the web, so hit Google and search for "rotary phase converter". Dan "Paul Baygents" wrote in message ... So I've got my converter running. However, I was told that by adding run capacitors I could lower the needed Amps to drive the idler? Is this true? If so would I simply put them inline on the incoming 220 hot leads going to the idler? Or else where? Thanks, Paul |
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#3
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Run caps?
On Sun, 21 Mar 2004 19:58:30 -0800, "Paul Baygents" wrote:
So I've got my converter running. However, I was told that by adding run capacitors I could lower the needed Amps to drive the idler? Yes. Is this true? If so would I simply put them inline on the incoming 220 hot leads going to the idler? Or else where? There are two different uses for run caps in a converter. You can use them as balance caps to equalize the phase voltages, and you can use them as power factor correction caps to lower reactive current draw from your electrical service. Balance caps connect between L1 and L3, and between L2 and L3. (L3 is the "wild" or manufactured leg). Typical values are in the range of 20 to 40 microfarads per horsepower. Note that the amount of capacitance between L1 and L3 will probably be different from the amount of capacitance between L2 and L3 at the point of best balance. You have to experiment to find the exact values for your particular motor. (Paralleling a number of lower value capacitors lets you form any value you actually need.) Note, you'll want to balance the converter so the wild leg voltage is a bit high when unloaded, and sags a bit low under full load. You can't get it to be *exactly* the same as the other legs under both unloaded and fully loaded conditions. If you can balance L1 to L3 and L2 to L3 within 10% of L1 to L2 when loaded and unloaded, call it good and quit. Power factor correction caps connect between L1 and L2. The value is whatever reduces the 1 ph line current from the service entrance to the lowest value. Again experimentation will be required. Do this *after* you balance the converter, if you're going to balance it. The value won't be the same for a balanced converter as it will be for an unbalanced converter. (Usually, it'll be smaller for a balanced converter.) What the power factor correction caps do is cancel the inductive reactance due to the motor which is seen by the service entrance. This reactance can set up high circulating currents in the primary feed if it isn't canceled. These are wattless watts in the sense that the current is out of phase with voltage and doesn't represent real power being drawn from the utility, but they can cause the wires to heat up (series R loss), and require you to use a larger than normal breaker to prevent nuisance tripping. I can't supply you with exact values since those will vary depending on the particular motor you're using, ie different motors have different impedances. What you're shooting for with the power factor correction caps is to make the motor impedance R + j0. You probably can't hit that exactly, but getting close can make a big difference in the current draw. Balance caps and power factor correction caps aren't strictly necessary to make a working rotary converter. But they are nice to use because they'll give you better phase to phase voltage balance, and will reduce the converter's power factor, letting you run from a smaller breaker. Gary |
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#4
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Run caps?
Gary sez: "Power factor correction caps connect between L1 and L2. The
value is whatever reduces the 1 ph line current from the service entrance to the lowest value. Again experimentation will be required. Do this *after* you balance the converter, if you're going to balance it. The value won't be the same for a balanced converter as it will be for an unbalanced converter. (Usually, it'll be smaller for a balanced converter.)" Right on, Gary! The 2 key points in Gary's closing paragraph are "do this after you balance the converter" and "reduces the 1 ph line current from the service entrance to the lowest value" Important here is that the current from the service entrance is measured with a clamp-on type of Ammeter. Unlike the voltage measurements for balancing this is a current measurement. Bob Swinney |
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#5
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Run caps?
"Robert Swinney" wrote in message ... Gary sez: "Power factor correction caps connect between L1 and L2. The value is whatever reduces the 1 ph line current from the service entrance to the lowest value. Again experimentation will be required. Do this *after* you balance the converter, if you're going to balance it. The value won't be the same for a balanced converter as it will be for an unbalanced converter. (Usually, it'll be smaller for a balanced converter.)" Right on, Gary! The 2 key points in Gary's closing paragraph are "do this after you balance the converter" and "reduces the 1 ph line current from the service entrance to the lowest value" Important here is that the current from the service entrance is measured with a clamp-on type of Ammeter. Unlike the voltage measurements for balancing this is a current measurement. Bob Swinney I've never used/owned one of those ammeters before. Guess I need to buy one. Honey? I need a new tool! Reno, Paul |
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#6
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Run caps?
Also quite useful for find which circuit is fed by which breaker
in old houses.  Just gotta have one of those!Paul Baygents wrote: "Robert Swinney" wrote in message ... Gary sez: "Power factor correction caps connect between L1 and L2. The value is whatever reduces the 1 ph line current from the service entrance to the lowest value. Again experimentation will be required. Do this *after* you balance the converter, if you're going to balance it. The value won't be the same for a balanced converter as it will be for an unbalanced converter. (Usually, it'll be smaller for a balanced converter.)" Right on, Gary! The 2 key points in Gary's closing paragraph are "do this after you balance the converter" and "reduces the 1 ph line current from the service entrance to the lowest value" Important here is that the current from the service entrance is measured with a clamp-on type of Ammeter. Unlike the voltage measurements for balancing this is a current measurement. Bob Swinney I've never used/owned one of those ammeters before. Guess I need to buy one. Honey? I need a new tool! Reno, Paul |
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#7
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Run caps?
On Mon, 22 Mar 2004 19:16:45 -0800, "Paul Baygents"
wrote: "Robert Swinney" wrote in message ... Gary sez: "Power factor correction caps connect between L1 and L2. The value is whatever reduces the 1 ph line current from the service entrance to the lowest value. Again experimentation will be required. Do this *after* you balance the converter, if you're going to balance it. The value won't be the same for a balanced converter as it will be for an unbalanced converter. (Usually, it'll be smaller for a balanced converter.)" Right on, Gary! The 2 key points in Gary's closing paragraph are "do this after you balance the converter" and "reduces the 1 ph line current from the service entrance to the lowest value" Important here is that the current from the service entrance is measured with a clamp-on type of Ammeter. Unlike the voltage measurements for balancing this is a current measurement. Bob Swinney I've never used/owned one of those ammeters before. Guess I need to buy one. Honey? I need a new tool! Reno, Paul The Harbor Freight one is a digital. While not great...it works well enough that I keep one in my CNC repair kit. http://www.harborfreight.com/cpi/cta...emnumber=42397 Gate time is pretty slow..but the "peak hold" seems to work ok. They go on sale at 1/2 price pretty regularly. Has an ok ohms/volts meter included in it. One of the tricks with many..though not all Harbor Freights..ask for the manager, tell him you are on a very tight budget and they will often sell you the item at the last sales price. For serious stuff, I also keep an old Amprobe analog (with a needle type meter) tucked away in one of the tool boxes. Gunner The two highest achievements of the human mind are the twin concepts of "loyalty" and "duty." Whenever these twin concepts fall into disrepute -- get out of there fast! You may possibly save yourself, but it is too late to save that society. It is doomed. " Lazarus Long |
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#8
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Run caps?
I tried doing this just this past weekend when I balanced out my 7.5
hp idler. Adding the PF caps didn't decrease my line current, it increased it. I tried a multitude of run cap and then PF cap combinations while balancing my converter to run my 5hp lathe. I found my best combination for loaded balance was 120 mf between L1 and L3 and 80 mf between L2 and L3. L3 being the manufactured leg. This yielded 243v between L1 and L3 and 245v between L2 and L3. Incoming line voltage is 244v. Likewise my currents for L1 are 8.1 amps, L2 7.8amps, and L3 7.7 amps. This to me is pretty close to generating real three phase power. Unloaded L1 to L3 and L2 to L3 voltages are up around 270 volts. Idling the converter measures 3.7 amps on the lines feeding it. I've added 10 and 20 mf caps across the incoming lines and the incoming idling load increases to over 5 amps. It also increases the incoming current while loaded. On top of that it throws the balance out of the generated leg voltages and currents a bit. So in this instance it doesn't appear PF caps are going to help me any. Any thoughts as to why? "Robert Swinney" wrote in message ... Gary sez: "Power factor correction caps connect between L1 and L2. The value is whatever reduces the 1 ph line current from the service entrance to the lowest value. Again experimentation will be required. Do this *after* you balance the converter, if you're going to balance it. The value won't be the same for a balanced converter as it will be for an unbalanced converter. (Usually, it'll be smaller for a balanced converter.)" Right on, Gary! The 2 key points in Gary's closing paragraph are "do this after you balance the converter" and "reduces the 1 ph line current from the service entrance to the lowest value" Important here is that the current from the service entrance is measured with a clamp-on type of Ammeter. Unlike the voltage measurements for balancing this is a current measurement. Bob Swinney |
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#9
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Run caps?
grdstat sez:
"... I tried doing this just this past weekend when I balanced out my 7.5 hp idler. Adding the PF caps didn't decrease my line current, it increased it. ..." Check the manufactured leg with your clamp-on ammeter with your converter unloaded. With balancing close to optimum, you should read the motor's name-plate current in the 3 rd. (manufactured) leg. This may sound strange! The current you are reading is reactive. This current is out of phase with the voltage that causes it. For example, on my 7.5 HP idler, that reading was 22 amps which is the full load rated current of a 7.5 HP, 3-phase motor. My mnufactured leg, or 3 rd. leg, current could more exactly be stated as: (22 angle 30 degrees) amps, or aprox. (19 + j11) amps. When balanced, I had 93 uFd in one leg and 167 uFd in the other leg and 40 uFd connected between line 1 and line 2. This 40 uFd (power factor correction) cap between input lines caused the (clamp-on) current measured there to dip to (become a minimum) at 3.8 amps. Input line current measured at L1 or L2 was 3.8 amps. This from a very old, very heavy, Wagner 7.5 HP, 3-phase motor. Bob Swinney |
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#11
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Run caps?
Rick sez: " Just how important is that level of balancing? I've spoken with a number of people who build converters for a living, and almost to a person have said that as long as you're within 10%, you'll be fine." The readings are from my 7.5 HP converter. I'm not clear re. your question "that level of balancing". I wonder if the original poster was incorrect about how close his voltages appeared to be - this because the amounts of balancings caps he stated should not have yielded a nearly perfect balance. I hope he will go back and look at them again. The values of cap given for my converter yielded voltages well within the 10% range. Bob Swinney |
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#12
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Run caps?
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#13
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Run caps?
Rick,
The so-called power correction cap goes across the input lines, after other balancing is done. It acts much like the final tuning of a transmitter where you dip the current. It is similar in that it involves tuning a parallel resonant circuit. BE QUIET, GARY!! Yes, it is strictly to drop the input current. On a large system, it could conceivably allow for the use of smaller wiring. Single run cap converters are fine. I ran one for years. The balanced configuration (Fitch system) allows for better voltage swings when going from idling to loaded. Bob Swinney "Rick Chamberlain" wrote in message ... In article , says... Rick sez: " Just how important is that level of balancing? I've spoken with a number of people who build converters for a living, and almost to a person have said that as long as you're within 10%, you'll be fine." The readings are from my 7.5 HP converter. I'm not clear re. your question "that level of balancing". I wonder if the original poster was incorrect about how close his voltages appeared to be - this because the amounts of balancings caps he stated should not have yielded a nearly perfect balance. I hope he will go back and look at them again. The values of cap given for my converter yielded voltages well within the 10% range. Thanks Bob. In your previous post, you mentioned a 3 separate caps - one across the incoming power lines. My 5hp converter has a single run cap and I'm still within 5-6%. I was just wondering why you added caps as you did. Was it strictlt to drop the input amps? -- Regards, Rick (Remove the HIGH SPOTS for e-mail) |
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#14
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Run caps?
I checked the idling current in the generated leg. Keep in mind this
is on a newish 7.5 hp TEFC high eff motor with max load amps of 18.8 at 230 volts. PF CAPS Gen Leg Input idle Input loaded Idle Amps current amps current amps 0 18.1 3.9 10.3 10 18.8 4.6 8.7 20 18.1 5.6 7.8 30 18.1 6.4 7.8 40 18.4 7.1 7.1 50 18.6 8.2 6.3 As I added PF caps, my unloaded idling current went up but my loaded current went down. In the last case, when I turned on the lathe, the current actually dropped from 8.2 unloaded to 6.3 with load? I imagine this has something to do with phase angles but don't fully understand. Adding the PF didn't change the leg to leg current or voltages much. The leg to leg difference is still within 2 amps. From 0 to 50 mf PF caps, the overall current magnitudes didn't change by more than 3 amps per leg. L1 to L3 that has 120 mf between them is still within 3 volts of L2 and L3 which has 80 mf between them. Someone questioned my readings earlier, I checked them again and they are indeed within 2 volts of line current. I didn't shoot for this, just stumbled on it with the 120/80 cap combination. Adding PF caps did change the voltages a bit, but not the voltage difference between them. "Robert Swinney" wrote in message ... grdstat sez: "... I tried doing this just this past weekend when I balanced out my 7.5 hp idler. Adding the PF caps didn't decrease my line current, it increased it. ..." Check the manufactured leg with your clamp-on ammeter with your converter unloaded. With balancing close to optimum, you should read the motor's name-plate current in the 3 rd. (manufactured) leg. This may sound strange! The current you are reading is reactive. This current is out of phase with the voltage that causes it. For example, on my 7.5 HP idler, that reading was 22 amps which is the full load rated current of a 7.5 HP, 3-phase motor. My mnufactured leg, or 3 rd. leg, current could more exactly be stated as: (22 angle 30 degrees) amps, or aprox. (19 + j11) amps. When balanced, I had 93 uFd in one leg and 167 uFd in the other leg and 40 uFd connected between line 1 and line 2. This 40 uFd (power factor correction) cap between input lines caused the (clamp-on) current measured there to dip to (become a minimum) at 3.8 amps. Input line current measured at L1 or L2 was 3.8 amps. This from a very old, very heavy, Wagner 7.5 HP, 3-phase motor. Bob Swinney |
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#16
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Run caps?
Gary Coffman wrote in message . ..
On 23 Mar 2004 06:53:03 -0800, (gradstdnt) wrote: I tried doing this just this past weekend when I balanced out my 7.5 hp idler. Adding the PF caps didn't decrease my line current, it increased it. I tried a multitude of run cap and then PF cap combinations while balancing my converter to run my 5hp lathe. I found my best combination for loaded balance was 120 mf between L1 and L3 and 80 mf between L2 and L3. L3 being the manufactured leg. This yielded 243v between L1 and L3 and 245v between L2 and L3. Incoming line voltage is 244v. Likewise my currents for L1 are 8.1 amps, L2 7.8amps, and L3 7.7 amps. This to me is pretty close to generating real three phase power. Unloaded L1 to L3 and L2 to L3 voltages are up around 270 volts. That's not good. You're using too much tuning capacitance. You want the unloaded voltage balance to be within 10%, and the loaded balance within 10%. You have the loaded balance within 1%, but the unloaded voltage is too high. You could overstress circuitry connected to the wild leg when operating lightly loaded. Let the voltage swing above and below nominal by about the same amount when going from unloaded to loaded. Then you'll find that a power correction cap will reduce the primary current. Gary Well, as it is I'm just a tad over 10% on the high side at 275 volts unloaded. Converter runs very smooth and quiet when the lathe is running. Haven't tried running lighter loads yet. When idling unloaded, it does make a bit more noise. I'm also have a feeling that it will run a bit warmer when idling unloaded for a period of time. I tried a configuration with 90 and 60 mf run caps. Runs a but quiter unloaded but doesn't change much when loaded. My loaded voltages decreased as expected, currents for each leg are not as nicely balanced. The current to power the converter while under load went up. By adding a 10 mf PF cap, I was able to decrease my unloaded idling current to it's lowest value out of all configurations. So your statement above is quite correct. My loaded current was still higher then when nicely balanced under load. The idling current in the third leg also went from 18 amps down to 13 amps. So according to another poster, I'm not blanced as well as I could/should be??? I'm up in the air on where I should settle on balance. Having things nicely balanced under load or slighty out of balance both unloaded and loaded. Pros and cons of each? I have a 2 hp mill drill and a 1/2 hp pedistal grinder to power as well. I realize that they will not be balanced under load as my lathe, the biggest motor to power. Thanks again for everyones input and expertise on the subject. |
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#17
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Run caps?
gradstdnt sez:
". . .currents for each leg are not as nicely balanced. . . . " You should not be measuring current in the 3 legs when attempting voltage balance. It will totally confuse you! The current flowing in a 'rotary phase converter / parallel load' is very complex. Some of the current looks like it should be flowing in 2 directions, against itself. Of course, this doesn't actually happen, but the reactive currents combine, add, and subtract in the 3 legs in ways that make them an extremely unreliable indicator of balance. Stick with voltage balancing techniques and leave the clamp-on ammeter for the final power factor adjustments. Bob Swinney "gradstdnt" wrote in message om... Gary Coffman wrote in message . .. On 23 Mar 2004 06:53:03 -0800, (gradstdnt) wrote: I tried doing this just this past weekend when I balanced out my 7.5 hp idler. Adding the PF caps didn't decrease my line current, it increased it. I tried a multitude of run cap and then PF cap combinations while balancing my converter to run my 5hp lathe. I found my best combination for loaded balance was 120 mf between L1 and L3 and 80 mf between L2 and L3. L3 being the manufactured leg. This yielded 243v between L1 and L3 and 245v between L2 and L3. Incoming line voltage is 244v. Likewise my currents for L1 are 8.1 amps, L2 7.8amps, and L3 7.7 amps. This to me is pretty close to generating real three phase power. Unloaded L1 to L3 and L2 to L3 voltages are up around 270 volts. That's not good. You're using too much tuning capacitance. You want the unloaded voltage balance to be within 10%, and the loaded balance within 10%. You have the loaded balance within 1%, but the unloaded voltage is too high. You could overstress circuitry connected to the wild leg when operating lightly loaded. Let the voltage swing above and below nominal by about the same amount when going from unloaded to loaded. Then you'll find that a power correction cap will reduce the primary current. Gary Well, as it is I'm just a tad over 10% on the high side at 275 volts unloaded. Converter runs very smooth and quiet when the lathe is running. Haven't tried running lighter loads yet. When idling unloaded, it does make a bit more noise. I'm also have a feeling that it will run a bit warmer when idling unloaded for a period of time. I tried a configuration with 90 and 60 mf run caps. Runs a but quiter unloaded but doesn't change much when loaded. My loaded voltages decreased as expected, The current to power the converter while under load went up. By adding a 10 mf PF cap, I was able to decrease my unloaded idling current to it's lowest value out of all configurations. So your statement above is quite correct. My loaded current was still higher then when nicely balanced under load. The idling current in the third leg also went from 18 amps down to 13 amps. So according to another poster, I'm not blanced as well as I could/should be??? I'm up in the air on where I should settle on balance. Having things nicely balanced under load or slighty out of balance both unloaded and loaded. Pros and cons of each? I have a 2 hp mill drill and a 1/2 hp pedistal grinder to power as well. I realize that they will not be balanced under load as my lathe, the biggest motor to power. Thanks again for everyones input and expertise on the subject. |
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#18
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Run caps?
On Wed, 24 Mar 2004 20:03:28 -0800, "Robert Swinney" wrote:
gradstdnt sez: ". . .currents for each leg are not as nicely balanced. . . . " You should not be measuring current in the 3 legs when attempting voltage balance. It will totally confuse you! The current flowing in a 'rotary phase converter / parallel load' is very complex. Some of the current looks like it should be flowing in 2 directions, against itself. Of course, this doesn't actually happen, but the reactive currents combine, add, and subtract in the 3 legs in ways that make them an extremely unreliable indicator of balance. Stick with voltage balancing techniques and leave the clamp-on ammeter for the final power factor adjustments. Bob's right. Balance the phase voltages, not the currents. The only time you want to measure currents is when tuning out power factor on the primary. Gary |
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#19
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Run caps?
Gary Coffman wrote in message . ..
On Wed, 24 Mar 2004 20:03:28 -0800, "Robert Swinney" wrote: gradstdnt sez: ". . .currents for each leg are not as nicely balanced. . . . " You should not be measuring current in the 3 legs when attempting voltage balance. It will totally confuse you! The current flowing in a 'rotary phase converter / parallel load' is very complex. Some of the current looks like it should be flowing in 2 directions, against itself. Of course, this doesn't actually happen, but the reactive currents combine, add, and subtract in the 3 legs in ways that make them an extremely unreliable indicator of balance. Stick with voltage balancing techniques and leave the clamp-on ammeter for the final power factor adjustments. Bob's right. Balance the phase voltages, not the currents. The only time you want to measure currents is when tuning out power factor on the primary. Gary I would like to clarify my three phase amp measurements. I was not measuring currents to/from the converter. I was masuring currents in each leg to the load motor while under power. I have measured both and they are different. My initial thought was to better understand how much current I was providing in the generated leg. I measured the other legs for reference and found the results interesting. When the voltages were blanced to within 1%, the currents were also nicely balanced. I thought this was the goal of balancing but others have pointed out issues with idling currents.By removing capacitance, thus unbalancing the loaded system, the currents in each leg to the load motor were also thrown out of balance. One let got hotter, as two others got weaker. In general, any further inbalance I have in the converter decreases the current provided in the generated leg. Thanks again for the input. I will work with balancing the converter for my min and max loads. This should provide me with the boundary of my operating conditions. I can then better evaluate the tradeoff between loaded and unloaded voltage balance. |
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#21
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Run caps?
Note that fully loaded means the load motor is mechanically fully loaded to its nameplate hp. Under light loads, a load motor will be a very reactive load, which can lead to some wildly misleading amp clamp readings when driven by a reactive source like a rotary converter. Correct. The load motor has a low power factor when unloaded or lightly loaded, and the power factor improves as the mechanical load on the motor increases. Nothing says you can't compensate for this by adding a PFC cap at the load motor. |
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#22
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In article , Gary Coffman says...
To really know what's going on, you need to use a dual trace scope setup to sum vector voltage and vector current in each circuit mesh. A *floating* dual trace scope. Current can be measured with a sampling resistor. That'll tell you how much actual power is being supplied by each phase. You want those to balance. It would be a surprising coincidence if that occurs at the point where the amp clamp meter readings are equal. Not *that* suprising. I suspect that once a fitch-type tuning is done, this will be true. Ie, the phase-to-phase voltages at the load motor will agree at the same point that the real currents (measured not by just an amp-clamp) come into agreement. I think I would have to invoke some kind of symmetry argument as a first hand-waving approach. The main thing that you want to worry about when tuning a rotary with ordinary test equipment is to achieve rough voltage balance phase to phase over the operating range of the converter. To do that, you want the voltage deviations from equality to swing roughly symmetrically about nominal as you go from no load to fully loaded, while at the same time deviating no more than 10% of nominal in either direction. This becomes easier and easier to do when the idler motor impedance is low. An argument in favor of oversizing the idler motor - a lot of the finessing issues simply go away. My converter had no tuning or pf correction, yet the phase to phase is well inside ten percent over the entire operating range. Jim ================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================== |
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#23
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Run caps?
Jim sez:
"... This becomes easier and easier to do when the idler motor impedance is low. An argument in favor of oversizing the idler motor - a lot of the finessing issues simply go away. My converter had no tuning or pf correction, yet the phase to phase is well inside ten percent over the entire operating range. ..." Yep! Lower is better. As per Jim's experience, with a large [idler : load] HP ratio there is less need for "balancing" components. Of course, the down side is all that pulling required on the rope starter, not to mention wire size requirements... Did'ja ever see Jim's right arm? IMO, key to understanding this and most other aspects of rotary phase converters is to get past thinking of them as generators. Think more in terms of parallel networks. Take an idler motor and load motor schematic -- visualize folding the drawing such that the load overlays the idler. BAM! Parallel circuit. Right? Even the (optional) balancing capacitance is hung off on the outside. Reference material says the very early single-phase motors were really 3-phase motors operated with the 3 rd leg open. Sheds some new light on "static" phase converters doesn't it? Bob Swinney "jim rozen" wrote in message ... In article , Gary Coffman says... To really know what's going on, you need to use a dual trace scope setup to sum vector voltage and vector current in each circuit mesh. A *floating* dual trace scope. Current can be measured with a sampling resistor. That'll tell you how much actual power is being supplied by each phase. You want those to balance. It would be a surprising coincidence if that occurs at the point where the amp clamp meter readings are equal. Not *that* suprising. I suspect that once a fitch-type tuning is done, this will be true. Ie, the phase-to-phase voltages at the load motor will agree at the same point that the real currents (measured not by just an amp-clamp) come into agreement. I think I would have to invoke some kind of symmetry argument as a first hand-waving approach. The main thing that you want to worry about when tuning a rotary with ordinary test equipment is to achieve rough voltage balance phase to phase over the operating range of the converter. To do that, you want the voltage deviations from equality to swing roughly symmetrically about nominal as you go from no load to fully loaded, while at the same time deviating no more than 10% of nominal in either direction. Jim ================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================== |
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#24
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In article , Robert Swinney says...
Yep! Lower is better. As per Jim's experience, with a large [idler : load] HP ratio there is less need for "balancing" components. Of course, the down side is all that pulling required on the rope starter, not to mention wire size requirements... Did'ja ever see Jim's right arm? "I'm popeye the sailor man...." But seriously the 'right arm' is mechanical, for the few who have not seen the setup befo http://www.metalworking.com/DropBox/_2000_retired_files/Conv.jpg I always was worried about what would happen if the rope got caught back up in a pull-start type, I would never do it that way. Jim ================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================== |
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#25
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Run caps?
.... IMO, key to understanding this and most other aspects of rotary phase converters is to get past thinking of them as generators ... Fitch tried an RPC as a generator [ * ] and it didn't work. Which perhaps should come as no surprise. If the idler was a "wound rotor, separately excited" machine, it would have worked, however. [ * ] By applying power to the star-connected 240 volt windings, and trying to withdraw power from the 480 volt windings, which would normally be connected in parallel with the other windings in 240 volt mode. |
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#26
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Run caps?
On 26 Mar 2004 05:46:46 -0800, jim rozen wrote:
In article , Gary Coffman says... To really know what's going on, you need to use a dual trace scope setup to sum vector voltage and vector current in each circuit mesh. A *floating* dual trace scope. Current can be measured with a sampling resistor. Or one that has isolated differential preamps, like one of mine does. Ordinary scopes use unbalanced inputs, with one side tied to chassis. Trying to measure phase voltages and currents with one made that way will give you a *hot* chassis, which is a serious shock hazard. That'll tell you how much actual power is being supplied by each phase. You want those to balance. It would be a surprising coincidence if that occurs at the point where the amp clamp meter readings are equal. Not *that* suprising. I suspect that once a fitch-type tuning is done, this will be true. Ie, the phase-to-phase voltages at the load motor will agree at the same point that the real currents (measured not by just an amp-clamp) come into agreement. I think I would have to invoke some kind of symmetry argument as a first hand-waving approach. Lots of hand waving. :-) The main thing that you want to worry about when tuning a rotary with ordinary test equipment is to achieve rough voltage balance phase to phase over the operating range of the converter. To do that, you want the voltage deviations from equality to swing roughly symmetrically about nominal as you go from no load to fully loaded, while at the same time deviating no more than 10% of nominal in either direction. This becomes easier and easier to do when the idler motor impedance is low. An argument in favor of oversizing the idler motor - a lot of the finessing issues simply go away. My converter had no tuning or pf correction, yet the phase to phase is well inside ten percent over the entire operating range. Yes, that's right. My 20 hp converter barely notices when I turn on a 2 hp load motor. It pays a bit more attention when I fire up a 7.5 hp load motor. Gary |
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#27
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Run caps?
In article , Gary Coffman says...
A *floating* dual trace scope. Current can be measured with a sampling resistor. Or one that has isolated differential preamps, like one of mine does. Ordinary scopes use unbalanced inputs, with one side tied to chassis. Trying to measure phase voltages and currents with one made that way will give you a *hot* chassis, which is a serious shock hazard. The plug-ins for the 7000 series tek scops *look* like they're isolated ground. They have the bnc shell isolated from the front panel with a teflon insulator. But this is to minimize internal ground loops. The low side is indeed tied to power common. But the appearance has fooled many a grad student.... Jim ================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================== |
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#28
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Run caps?
Yep! Just goes to show you don't always get what you think you paid for.
Grad students are like the rest of us. They are lulled into a false sense of security by the old gremlin, "it has always worked that way". A plain scope works just fine for above ground voltage and current measurments -- just be sure the scope is powered through an isolation transformer. All good shops have one . . . . . don't they?? Bob Swinney PS: A good way to display AC current is with a current transformer. Wrap a heavy wire, 1 turn around the whole winding core of a little transformer, say, a small 110 - 12 V power transformer. Connect that wire in series with the current leg you wish to measure. You will get a small signal, calibrateable if desired, across the secondary terminals of the transformer (another one across the primary also) which can be read on your scope. |
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#29
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Run caps?
In article ,
jim rozen wrote: In article , Gary Coffman says... A *floating* dual trace scope. Current can be measured with a sampling resistor. Or one that has isolated differential preamps, like one of mine does. Ordinary scopes use unbalanced inputs, with one side tied to chassis. Trying to measure phase voltages and currents with one made that way will give you a *hot* chassis, which is a serious shock hazard. The plug-ins for the 7000 series tek scops *look* like they're isolated ground. They have the bnc shell isolated from the front panel with a teflon insulator. But this is to minimize internal ground loops. The low side is indeed tied to power common. But the appearance has fooled many a grad student.... The teflon insulation is also to isolate a ring surrounding the outer shell, to couple in information on what the attenuation of the probe is -- to switch readout scale values. (Some of the probes have a 1X/10X switch on them.) There are some which *are* truly differential -- but they use two BNCs for a single channel of differential input, and have a long-range offset adjustment to allow you to tune out DC offsets. Unfortunately, one of the nicest has an interesting plastic gear inside to couple a switch to the knob on the front panel, and the plastic has gotten brittle over time. I've seen two of those with the plastic disintergating. I own one of them. I would love to see a drawing of the gear's dimensions, with the hopes that I could make a replacement. The plug-in is too nice to consign to the trash bin. It is a barrel (perhaps 3/4" long), with a hex hole through it, and a set of gear teeth only about 1/8" long in the middle of it. Enjoy, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#31
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Run caps?
In article , DoN. Nichols says...
Unfortunately, one of the nicest has an interesting plastic gear inside to couple a switch to the knob on the front panel, and the plastic has gotten brittle over time. I've seen two of those with the plastic disintergating. I own one of them. I would love to see a drawing of the gear's dimensions, with the hopes that I could make a replacement. The plug-in is too nice to consign to the trash bin. It is a barrel (perhaps 3/4" long), with a hex hole through it, and a set of gear teeth only about 1/8" long in the middle of it. Don, what is the model number of that particular plug-in? Jim ================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================== |
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#32
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Run caps?
In article ,
Gary Coffman wrote: On 28 Mar 2004 00:00:38 -0500, (DoN. Nichols) wrote: There are some which *are* truly differential -- but they use two BNCs for a single channel of differential input, and have a long-range offset adjustment to allow you to tune out DC offsets. Or in the case of the differential plugins I have, two banana jacks per channel. Older ones -- fully tube-based, I suspect. Unfortunately, one of the nicest has an interesting plastic gear inside to couple a switch to the knob on the front panel, and the plastic has gotten brittle over time. I've seen two of those with the [ ... ] Dean Kidd is about the best source for obsolete Tek parts. Thanks! I've found a snail-mail address for him with a Google search, so I'll be trying to contact him soon. Again, thanks, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#33
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Run caps?
In article ,
jim rozen wrote: In article , DoN. Nichols says... Unfortunately, one of the nicest has an interesting plastic gear inside to couple a switch to the knob on the front panel, and the plastic has gotten brittle over time. I've seen two of those with the plastic disintergating. I own one of them. I would love to see a drawing of the gear's dimensions, with the hopes that I could make a replacement. The plug-in is too nice to consign to the trash bin. It is a barrel (perhaps 3/4" long), with a hex hole through it, and a set of gear teeth only about 1/8" long in the middle of it. Don, what is the model number of that particular plug-in? O.K. Found it! I seem to have misremembered the dimensions of the plastic gear. But it is in a 7A13 plug-in. The "Fine Comparison Voltage" is two concentric knobs. The inner one turns a 10-turn pot, and the three LSDs of a Veeder-Root style counter on the front panel. (Lots of small plastic gears for that part.) The outer one turns the MSD of the Veeder-Root counter, and it *should* also turn a 10-position switch, but the gear collar is all gone (It took me a while to remember/rediscover just what was missing and how it should work.) All of the other gears are fine, but that one is long gone. The metal hub (with setscrews) which it should be on has four ridges to give a bit more grip to the gear so it can rotate the switch (which takes a respectable amount of torque). The whole thing lives under a bent aluminum shield in the upper-right hand corner of (just behind) the front panel. Wonderful construction -- but the materials were not up to the stresses. All of the gears are a brown plastic -- a bit lighter than Phenolic or Bakelite.. I'm wondering whether I can make some replacements from Delrin, and how well they would hold up. (I first have to figure the tooth pitch and the count.) Thanks, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#35
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Run caps?
In article , DoN. Nichols says...
O.K. Found it! I seem to have misremembered the dimensions of the plastic gear. But it is in a 7A13 plug-in. Arghh, somebody tossed out the entire stock of scrap 7000 series plug-ins in the last cleanup! It may take me a while to find a 7a13 but I will do my best don. Jim ================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ================================================== |
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#36
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Tek Scopes (was Run caps?)
In article ,
Gary Coffman wrote: On 30 Mar 2004 23:20:56 -0500, (DoN. Nichols) wrote: In article , Gary Coffman wrote: [ ... ] Dean Kidd is about the best source for obsolete Tek parts. Thanks! I've found a snail-mail address for him with a Google search, so I'll be trying to contact him soon. Dean was in charge of designing the 453A, the best scope Tek ever made (I have 3 of them). While I have two of the 454. Same package, higher bandwidth (150 MHz vs 50 MHz) and nuvistors on the front ends, instead of FETs. (We had some of each where I worked, and as we worked with high voltage, every so often an input FET on a 453 would get zapped, and they were hard to find. The 454s never suffered from that.) He bought the entire set of spares for it when he retired. He also got parts for other models as Tek was emptying the warehouse. If anyone has the part you need, or at least has the best chance of knowing where to get it, he's the guy. Great, Thanks, DoN. P.S. Does he have an e-mail address? It wasn't in the information which I found. If so, you had probably better send it in private e-mail, or at least break it up so the spammers won't harvest it from the newsgroup, since he doesn't post here at all No sense exposing him to the garbage if he isn't already getting it. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#37
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Run caps?
In article ,
jim rozen wrote: In article , DoN. Nichols says... O.K. Found it! I seem to have misremembered the dimensions of the plastic gear. But it is in a 7A13 plug-in. Arghh, somebody tossed out the entire stock of scrap 7000 series plug-ins in the last cleanup! It may take me a while to find a 7a13 but I will do my best don. I'm not too sure whether it would be worth the effort. The gear seems to be the most likely part to fail on the whole thing, so it is probably the reason for scrapping them. But -- just in case ... Check whether the outer knob rotates fairly smoothly or has a serious detent. If it is smooth, then the gear is probably gone in that one, too. And If the side panels are still good, one of mine is rather dented and torn a bit. I think that it is the right one (as seen from the front) which is the one with located holes for test points and tuning access. Thanks much, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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#38
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Tek Scopes (was Run caps?)
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#39
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Tek Scopes (was Run caps?)
DoN. Nichols wrote:
In article , Gary Coffman wrote: On 30 Mar 2004 23:20:56 -0500, (DoN. Nichols) wrote: In article , Gary Coffman wrote: [ ... ] Dean Kidd is about the best source for obsolete Tek parts. Thanks! I've found a snail-mail address for him with a Google search, so I'll be trying to contact him soon. Dean was in charge of designing the 453A, the best scope Tek ever made (I have 3 of them). Then you've never owned a 485, my personal favorite. While I have two of the 454. Same package, higher bandwidth (150 MHz vs 50 MHz) and nuvistors on the front ends, instead of FETs. (We had some of each where I worked, and as we worked with high voltage, every so often an input FET on a 453 would get zapped, and they were hard to find. The 454s never suffered from that.) He bought the entire set of spares for it when he retired. He also got parts for other models as Tek was emptying the warehouse. If anyone has the part you need, or at least has the best chance of knowing where to get it, he's the guy. Great, Thanks, DoN. P.S. Does he have an e-mail address? It wasn't in the information which I found. If so, you had probably better send it in private e-mail, or at least break it up so the spammers won't harvest it from the newsgroup, since he doesn't post here at all No sense exposing him to the garbage if he isn't already getting it. |
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#40
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Tek Scopes (was Run caps?)
"Jim Stewart" wrote in message
... Then you've never owned a 485, my personal favorite. My favoirite is a 475, but only because it's the second of two scopes I've owned in my life. :^) That reminds me, Mark - do you have any more probes? Tim (P.S. thanks again -- "I have misplaced my pants." - Homer Simpson | Electronics, - - - - - - - - - - - - - - - - - - - - - - --+ Metalcasting and Games: http://webpages.charter.net/dawill/tmoranwms |
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