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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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Phase Converter cycling with compressor to save money.
I finally got my 15 hp phase converter working today using capacitors
and a momentary push button. I plan on using it with a 10 hp 3 phase air compressor. However just at idle it is pulling 20 amps. I figure I can use an additional pressure switch to have the converter turn on before the compressor does. But what do I need to have it turn off after the compressor cycles off? Thanks Steve |
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gtslabs wrote:
I finally got my 15 hp phase converter working today using capacitors and a momentary push button. I plan on using it with a 10 hp 3 phase air compressor. However just at idle it is pulling 20 amps. I figure I can use an additional pressure switch to have the converter turn on before the compressor does. But what do I need to have it turn off after the compressor cycles off? How are you measuring this current? Be aware that there are two kinds of current - real and imaginary - and you pay for the former but not the latter. You may well be able to cut that apparent power down by a factor of 4 or 5 using some run caps between L1 and L2. Your converter looks very inductive, if you put some capacitance across it it may look more resistive. Sorry for the EE terminology, I don't know any better. Grant |
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I am using electrolytic caps for the startup. I think I read somewhere
NOT to use them for run capacitors. Tuning it was my next step. However I only use this converter for this compressor and it is not needed on all day, mainly when I am sand blasting. I was uning a tong type meter. |
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gtslabs wrote:
I am using electrolytic caps for the startup. I think I read somewhere NOT to use them for run capacitors. Tuning it was my next step. However I only use this converter for this compressor and it is not needed on all day, mainly when I am sand blasting. I was uning a tong type meter. This has nothing to do with start capacitance or tuning caps or balancing current between legs, nothing whatever. I'm talking about power factor correction capacitors. Here is what Gary Coffman posted once on this subject: "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." |
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"gtslabs" wrote in message oups.com... I finally got my 15 hp phase converter working today using capacitors and a momentary push button. I plan on using it with a 10 hp 3 phase air compressor. However just at idle it is pulling 20 amps. I figure I can use an additional pressure switch to have the converter turn on before the compressor does. But what do I need to have it turn off after the compressor cycles off? Thanks Steve Steve Is there any reason you wouldnt want to spin-up the idler with the seperate pressure switch and turn it off with the compressor's high pressure cut off switch?? I dont know what contacts are available on the two pressure switches you are using. But, if I knew what you had I am confident that even I could make something with cheap relays to control your motor contactors Jerry |
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Thanks I see the difference now. I have a case of ~130 uF 240V
electrolytic caps. I know these are not for continious use but only start-up. However I would like to use these to find the correct capacitance needed then buy the oil run type caps. How long is safe to run these caps in a continious mode for testing? |
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Yes, this is what I was thinking would work. My pressure switches are
double pole or L1, L2, M1, M2. I had trouble figuring how to wire these with 3 phase wires. I have one leg going directly to the motor and the other 2 going thru the switch. Perhaps this is not the correct wiring but I could not easily find a 3 pole pressure switch at graingers. |
#8
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Electrolytic "start" caps are not suitable for use as running caps. They
will blow up if left in circuit form more than a few seconds. You can't use them for run "test caps" as you suggest - they will blow! With a 15 HP idler motor you will need around 450 microfarads of run capacitance permanently connected from one side of the input line to the "manufactured phase". This will result in an "unbalanced" but satisfactory rotary phase converter. Start by putting in 350 microfarads, or so, and add capacitance while metering the manufactured phase. Continue to add capacitance until the manuf. leg voltage rises to approximately 10% greater than the input (240 ?) volt input line voltage. Be very careful - you are working with lethal voltages! Bob Swinney S "gtslabs" wrote in message ups.com... Thanks I see the difference now. I have a case of ~130 uF 240V electrolytic caps. I know these are not for continious use but only start-up. However I would like to use these to find the correct capacitance needed then buy the oil run type caps. How long is safe to run these caps in a continious mode for testing? |
#9
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In article . com,
gtslabs wrote: Thanks I see the difference now. I have a case of ~130 uF 240V electrolytic caps. I know these are not for continious use but only start-up. However I would like to use these to find the correct capacitance needed then buy the oil run type caps. How long is safe to run these caps in a continious mode for testing? Hmm ... perhaps two seconds *once*, if you are lucky. Be sure to wear a face shield, as they are very likely to explode in your face. You really want AC rated oil filled caps for the task. Remember -- you don't need to buy all the sizes of caps. You can switch in combinations in parallel to cover a lot of values. For example, a set consisting of: 1 ea 10 uf 2 ea 20 uf 1 ea 40 uf can give you every value from 0 to 100 uf in steps of 10 uf. Add in a 5 uf and you have more resolution to fine tune things, if you need that, but I suspect that the 10uf steps would get you close enough. Add a 100 uf cap, and you can double your range. If you were going to be doing this frequently, I would put them all in a box with a toggle switch in series with each cap, so you could change values while things are running. And remember -- measure the current between the breaker box and the caps, not between the caps and the idler motor. 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 --- |
#10
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In article .com,
gtslabs wrote: Yes, this is what I was thinking would work. My pressure switches are double pole or L1, L2, M1, M2. I had trouble figuring how to wire these with 3 phase wires. I have one leg going directly to the motor and the other 2 going thru the switch. Perhaps this is not the correct wiring but I could not easily find a 3 pole pressure switch at graingers. The three phase should go through a motor starter contactor, with heaters to detect over-current when one of the three phases is lost. So -- your pressure switch controls the coil of the contactor, and the contactor switches the motor. 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 --- |
#11
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"gtslabs" wrote in message oups.com... Yes, this is what I was thinking would work. My pressure switches are double pole or L1, L2, M1, M2. I had trouble figuring how to wire these with 3 phase wires. I have one leg going directly to the motor and the other 2 going thru the switch. Perhaps this is not the correct wiring but I could not easily find a 3 pole pressure switch at graingers. Steve I would have the idler switched thru a contactor and the compressor switched thru a "Motor Stater", which has heaters in it to shut it down when/if damaging current flows to the motor. With the contactors carrying the load current, the pressure switches need carry only single phase control current. If you agree about using contactors but dont have the contactors, I could be talked into sending you some 3 phase contactors. You'll be lucky if I have a motor start contactor with the correct heaters. But, I do have some contactors I'd send, if you want to use them. I also have an overload supply of Alan Bradley industrial ON-OFF buttons if you'd like to be able to enable/disable this system with push buttons. Jerry |
#12
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Bob My experience indicates that the improvement in HP delivering capacity of the idler that results from "tuning" is so small that it probably isnt worth the expense for powering an air compressor. But, what do I know?? How much do you suppose the capacity would cost for balancing that 15 HP idler? How much more performance would be realized as result of tuning? Jerry "Robert Swinney" wrote in message ... Electrolytic "start" caps are not suitable for use as running caps. They will blow up if left in circuit form more than a few seconds. You can't use them for run "test caps" as you suggest - they will blow! With a 15 HP idler motor you will need around 450 microfarads of run capacitance permanently connected from one side of the input line to the "manufactured phase". This will result in an "unbalanced" but satisfactory rotary phase converter. Start by putting in 350 microfarads, or so, and add capacitance while metering the manufactured phase. Continue to add capacitance until the manuf. leg voltage rises to approximately 10% greater than the input (240 ?) volt input line voltage. Be very careful - you are working with lethal voltages! Bob Swinney S "gtslabs" wrote in message ups.com... Thanks I see the difference now. I have a case of ~130 uF 240V electrolytic caps. I know these are not for continious use but only start-up. However I would like to use these to find the correct capacitance needed then buy the oil run type caps. How long is safe to run these caps in a continious mode for testing? |
#13
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Jerry sez:
" My experience indicates that the improvement in HP delivering capacity of the idler that results from "tuning" is so small that it probably isnt worth the expense for powering an air compressor. But, what do I know??" Hey, Jerry! You da Man with the dyno .... ! If Steve is concerned with the starting load of an air compressor he may want (need ? ) the load extra starting capability of a voltage balanced RPC. He did mention that he wanted to balance for whatever reason. OTOH, "balancing" may not be that attractive considering the cost of oil-filled caps. Bob Swinney |
#14
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I checked the voltage on the 3 legs without any run capacitors and I
got 220,220,240 so this is within 10% and I may not need any balancing capacitors after all at idle. I have seven 250V 108-130uF start capacitors in parallel. How do I determine the required resistance / watts to put a bleed resistor across them? If Watts=VI then 240*37 = 8.88KW and the resistance of R=V/I gives me 6.5 ohms. Does this sound about right? |
#15
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The purpose of a bleed resistor is to .. well, errrr, "bleed" of any
residual charge from a capacitor bank thus eliminating possible shock hazard to the operator. Sufficient protection would be afforded if the charge could be bled off in, say 30 seconds. The universal time constant is given as: T = RxC; transposing gives R = T/C. A cap is said to be fully charged or discharged after 5 time constants. Therefore the desired RC time constant of each cap and a bleeder resistor is 6 seconds. Assuming the value of each of the 7 caps is 120 microfarad, the resistor value is figured as R = T/C, or R = (6 / 0.000120) = 50000 ohms. The nearest size standard value is 56 K. Figure the power dissipated by each resistor as (E squared) / R; or P = (240 squared) / 56K = aprox. 1 watt. You would need a 56K, 2 Watt bleeder resistor across each of the 7 caps. Another way to do it would be place a single 56K, 10 Watt resistor across the cap bank. All this represents a power loss of around 7 watts if the bleeder resistor(s) is in circuit constantly. The bleeder could be applied via a N.C. set of relay contacts only after shut down, if desired, rather than leaving it in circuit constantly. Bob Swinney |
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Correction: The resistor placed across the entire cap bank would have to be
aprox. 7K ohms, not 56K ohms. Power dissipation of the resistor would be (240 squared) / 7K = 8.3 watts. It would have to be a 10 Watt resistor. Sorry, Bob Swinney "Robert Swinney" wrote in message ... The purpose of a bleed resistor is to .. well, errrr, "bleed" of any residual charge from a capacitor bank thus eliminating possible shock hazard to the operator. Sufficient protection would be afforded if the charge could be bled off in, say 30 seconds. The universal time constant is given as: T = RxC; transposing gives R = T/C. A cap is said to be fully charged or discharged after 5 time constants. Therefore the desired RC time constant of each cap and a bleeder resistor is 6 seconds. Assuming the value of each of the 7 caps is 120 microfarad, the resistor value is figured as R = T/C, or R = (6 / 0.000120) = 50000 ohms. The nearest size standard value is 56 K. Figure the power dissipated by each resistor as (E squared) / R; or P = (240 squared) / 56K = aprox. 1 watt. You would need a 56K, 2 Watt bleeder resistor across each of the 7 caps. Another way to do it would be place a single 56K, 10 Watt resistor across the cap bank. All this represents a power loss of around 7 watts if the bleeder resistor(s) is in circuit constantly. The bleeder could be applied via a N.C. set of relay contacts only after shut down, if desired, rather than leaving it in circuit constantly. Bob Swinney |
#17
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Correction: The resistor placed across the entire cap bank would have to be
aprox. 7K ohms, not 56K ohms. Power dissipation of the resistor would be (240 squared) / 7K = 8.3 watts. It would have to be a 10 Watt resistor. It would probably be much easier to find a 10k 10 watt resister that is close enough for this job. |
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