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Thread: Phase converter voltage deviation.

  1. #11
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    No Like button here PC or taptalk, as evidenced by no like listings under the user details.

    MTW

  2. #12
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    Quote Originally Posted by icy78 View Post
    Quote......For the four stages of compressors and condenser fans, it is possible to balance them individually as a zone, by moving the capacitors from the RPC, to the load side of the contactor for each individual zone.

    Thanks MTW. That's a lot of great info.

    I I am concerned about the chiller and its various loads. I was just wanting them to address the pump, if they get that right , likely,the chiller will fall into line. However, I really like that bit about putting capacitors on the load side of the contactors. I would have to see how that is calculated. It may take somebody with a lot more knowledge of this stuff than me.
    I did see on Ronks site, they dicussed that setup, but I didn't really have time to get into it.

    Anyhow, I'm really appreciating the input from you guys.

    Sent from my SM-G930V using Tapatalk
    Problem with balancing the compressor is the load likely varies at times, whenever load varies from the level you balanced it at, the balance will be off again. The fans and pumps are more of a constant load level.
    I live for today, I'm just a day behind.

  3. #13
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    Quote Originally Posted by kwired View Post
    Problem with balancing the compressor is the load likely varies at times, whenever load varies from the level you balanced it at, the balance will be off again. The fans and pumps are more of a constant load level.
    Well there's a point too.
    Hmmm. Is a CNC RPC what should've been installed?
    Something that will automatically add or subtract capacitors based on load, per design?
    (At least that's my understanding of it.)

    Sent from my SM-G930V using Tapatalk

  4. #14
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    Wow...been in the industry 50 yrs and I have learned alot today. Have several farms in area with 1 P and use RPC. When running < 5 HP motors and running close to FLA for extended tengths of time, we have experenced short life on the motors ( say less than 5 years). I am in fact installing 3 - 7 1/2 grain aeration fans (grain bins) this spring on an exiting RPC system. Currently the RPC is used on a small grinder / mixer mill which has 2 - 7 1/2 motors plus a 3 HP Leg used when unloading truck in over head bin. The RPC and grinder mixer are 30+ yrs old with original motors still going strong, running about 2-3 days /week, 8 hrs per day year around, except during planting and harvest. The RPC is rated to start 10 HP motor and max system HP of 30. They will not be able to run both grinder/ mixer and aeration fans at same time. What problems if any do you RPC experts forcast for our expanded system?

  5. #15
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    Quote Originally Posted by icy78 View Post
    Well there's a point too.
    Hmmm. Is a CNC RPC what should've been installed?
    Something that will automatically add or subtract capacitors based on load, per design?
    (At least that's my understanding of it.)

    Sent from my SM-G930V using Tapatalk
    Rotary converter isn't as bad as a "static converter" for the most part, but both typicaly have imbalances from my experiences.

    I had someone ask me to look at an air compressor once they were having troubles tripping overload. Someone put a static phase converter on it for them - they bought a used three phase unit. The converter had enough extra capacitors to just move leads around to get different values out of them. I could get reasonable balance- until it started to approach "cut out" pressure. then it was loaded more and off balance again. Don't remember where we left things set, I know we did turn the motor overload up a little more then I would have liked to see - but I guess it is what it is. I told them when that motor burned out that they should just get a three phase motor for it. Something tells me the compressor maybe gets shelled before then anyway - they were operating it without cover on, and it sure looked to me like it needed to be on for cooling air to flow properly through the unit, on top of that it did have an oil cooling coil, but not necessarily getting proper air flow over it.
    I live for today, I'm just a day behind.

  6. #16
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    Quote Originally Posted by icy78 View Post
    Well there's a point too.
    Hmmm. Is a CNC RPC what should've been installed?
    Something that will automatically add or subtract capacitors based on load, per design?
    (At least that's my understanding of it.)
    A CNC RPC is normally one that is 2X the size of the load, and voltage balanced within 5%. And best when used with a single CNC, to help maintain that balance under varying load.

    There are several methods of constructing and balancing RPC's, some use caps from a single utility line to the manufactured leg, to resonate the voltage up to the required level. Others use caps from both supply legs to the manufactured leg.

    Many RPC builders compensate on the high end, +10%, of the line voltage, to the manufactured line, to allow for starting and running the largest motor advertised. The manufactured leg is weak compared to the utility lines, it has a lot more impedance and hence the voltage and current on the generated phase falls off as load increases. This is where it can becomes like partially single phasing. The two utility lines are taking the brunt of the load, and the manufactured line may be doing very little contribution to balance the system.

    With small loads placed on a large converter, as a purchased unit, typically the manufactured voltage and current will be high while the utility legs are more normal. In this region you have one phase leg that is running hotter than it should, especially if the utility voltage is in the high range to start with. The closer you are to that +/- 10% specification on the supply, the quicker you can exceed the limit with the manufactured leg.

    A single large load on the other hand, can be enough to pull down on the manufactured leg just enough to get the balance near the sweet spot. Where the phase angle, the voltage and currents are as symmetrical as possible. There is only the one sweet spot, per balance job, above and below that point, you move further away from symmetry. Whenever the load varies, the balance point will vary.

    If you run a 3Φ RPC idler with minimal balance caps, it will never have a high voltage and current on the manufactured line. Therefore you wont get much power from the manufactured line, but it won't overheat either. Then the question is, in this region, do you have enough power at the motor to serve the load. Oftentimes the motor could be oversized, to compensate for a weak leg and still provide enough load power to satisfy the load.

    In the multi motor situation, you need to consider the characteristics of the load, what loads come on and stay on first, then what is the typical way that the loads will be used, and what variation the system will have in use. Only after considering the the real world load application, what would be the best way to go about balancing for a mid point sweet spot.

    Switching caps in and out of the RPC circuit while running can be a bad idea, some builders do it, but it can produce some serious voltage spikes in the system during the switching. If you want or need to switch them in and out, it's best if you can do it before you start the system and apply load. One builder spent years designing, building, testing and patenting an automatic solid state switcher control, but discontinued it because in order to suppress the spikes to prevent problems it proved non economical.

    That's why I suggested splitting them up with the loads, if that is even possible, depending on system design and function. Switching them with the loads will have lesser spikes when combined with the inductance of the load motor. When placed close to the load, most of the current through the capacitor is reactive, and flows between the cap and the motor windings, not down the supply leads. This would affect the system balance less, when the load was switched in and out. Switching caps with the load will make additional inrush current on the contactor or starter that is supplying it, so that needs to be considered as well.

    With the chiller in question, it really depends on what process it serves and how is it designed to run. Does all the fans and pumps run continuously, and the compressors cycle online as required. Or does the entire system cycle on and off as the load controller call for, and the fans are switched with the compressors they serve. It would be a different balancing approach depending on either case.

    I have done a utility supplied 3Φ chiller with 4 zones, where each compressor was variable with a VFD for each zone and one for the circulator pump, fans were switched across the line. I would think it could be possible to obtain a manufactured unit with oversize drives that could do the phase conversion as well as the motor control. More expensive on the purchase, but a lot more workable in the field with 1Φ supply.

    If you look back at the numbers you gave in the first post, I think you can see what I'm alluding to. But you need to keep track of the numbers in a chart form so that you keep the appropriate values and phases in the proper order, for comparison, a column for unloaded conditions and loaded conditions. Without that you will loose track quickly of what your test condition readings mean.

    MTW

  7. #17
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    Yet another option in some cases would be to have a multiple idler system. You switch on more idlers, and parallel them as the system requirements dictate. Another way to keep relative balance with more diverse load conditions. Say a small and a large unit, for little loads to prevent over voltage and excess heating, and a big unit for when you bring on major loads. The utility lines will already be in phase, so long as you wire up the supply phase the same on each, then you just need to get the rotation in sync with phasing of the manufactured leg. These can be switched in and out without too much disturbance on a running system.

    Rotary RPC's are the ones that give a bit more variance capability in the output , due to the manufactured phase supplied by the back EMF the motor rotor provides to the unpowered idler winding. But there are many variations of static converters as well, some better than others, and some even better than a RPC. Some designs go back about a hundred years, others are new solid state design that keep the balance tighter than utility supplied 3Φ power. There is a use case for each. It really depends on the load, the design and of course the budget.

    PhasePerfect
    is the solid state design and is also gangable (parallel) for more output than standard units. Nice units that can run any type of load, and stay in complete symmetry under any condition of load, but hit hard in the pocket.

    A 60 Year old induction converter design that uses a step up transformer and capacitor bank, can also be effectively used on loads that are steady, and only need one balance point. Many times used for oilfield service, these units can deliver full HP and torque for a fixed load, and require no spinning idler. Just transformer losses until the load is started. ADD-A-PHASE is a manufacturer of these type units.

    Then at the bottom of the heap are start capacitor only static converters. The lowest cost unit with just a start cap, cap starts the 3Φ load motor, then switches off the cap and runs it in single phase mode. Reduced HP and torque, but acceptable for small individual loads that start unloaded and can operate the load with less than 2/3 of nameplate. Such as drills and mills, used in a home shop. There are many builders of this low cost/performance units.

    One step up are static cap units, that include start and run caps. Both caps used for starting, then the start cap switches off, and just the run caps remain connected to the load. Better than the previous model type, is not running just on 1Φ power, so it has better HP output. But still not generally suited for demanding type loads and full HP output.

    And lastly there is the Steelman method, it uses a 12 lead Wye Load motor and re-configures the windings in a different way for operation from 1Φ power, with a start and run cap. With these unit more than nameplate torque can be achieved if set up properly.

    MTW
    Last edited by MTW; 03-13-18 at 12:46 AM.

  8. #18
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    Quote Originally Posted by MTW View Post
    Yet another option in some cases would be to have a multiple idler system. You switch on more idlers, and parallel them as the system requirements dictate. Another way to keep relative balance with more diverse load conditions. Say a small and a large unit, for little loads to prevent over voltage and excess heating, and a big unit for when you bring on major loads. The utility lines will already be in phase, so long as you wire up the supply phase the same on each, then you just need to get the rotation in sync with phasing of the manufactured leg. These can be switched in and out without too much disturbance on a running system.

    Rotary RPC's are the ones that give a bit more variance capability in the output , due to the manufactured phase supplied by the back EMF the motor rotor provides to the unpowered idler winding. But there are many variations of static converters as well, some better than others, and some even better than a RPC. Some designs go back about a hundred years, others are new solid state design that keep the balance tighter than utility supplied 3Φ power. There is a use case for each. It really depends on the load, the design and of course the budget.

    PhasePerfect
    is the solid state design and is also gangable (parallel) for more output than standard units. Nice units that can run any type of load, and stay in complete symmetry under any condition of load, but hit hard in the pocket.

    A 60 Year old induction converter design that uses a step up transformer and capacitor bank, can also be effectively used on loads that are steady, and only need one balance point. Many times used for oilfield service, these units can deliver full HP and torque for a fixed load, and require no spinning idler. Just transformer losses until the load is started. ADD-A-PHASE is a manufacturer of these type units.

    Then at the bottom of the heap are start capacitor only static converters. The lowest cost unit with just a start cap, cap starts the 3Φ load motor, then switches off the cap and runs it in single phase mode. Reduced HP and torque, but acceptable for small individual loads that start unloaded and can operate the load with less than 2/3 of nameplate. Such as drills and mills, used in a home shop. There are many builders of this low cost/performance units.

    One step up are static cap units, that include start and run caps. Both caps used for starting, then the start cap switches off, and just the run caps remain connected to the load. Better than the previous model type, is not running just on 1Φ power, so it has better HP output. But still not generally suited for demanding type loads and full HP output.

    And lastly there is the Steelman method, it uses a 12 lead Wye Load motor and re-configures the windings in a different way for operation from 1Φ power, with a start and run cap. With these unit more than nameplate torque can be achieved if set up properly.

    MTW
    Quote Originally Posted by MTW View Post
    A CNC RPC is normally one that is 2X the size of the load, and voltage balanced within 5%. And best when used with a single CNC, to help maintain that balance under varying load.

    There are several methods of constructing and balancing RPC's, some use caps from a single utility line to the manufactured leg, to resonate the voltage up to the required level. Others use caps from both supply legs to the manufactured leg.

    Many RPC builders compensate on the high end, +10%, of the line voltage, to the manufactured line, to allow for starting and running the largest motor advertised. The manufactured leg is weak compared to the utility lines, it has a lot more impedance and hence the voltage and current on the generated phase falls off as load increases. This is where it can becomes like partially single phasing. The two utility lines are taking the brunt of the load, and the manufactured line may be doing very little contribution to balance the system.

    With small loads placed on a large converter, as a purchased unit, typically the manufactured voltage and current will be high while the utility legs are more normal. In this region you have one phase leg that is running hotter than it should, especially if the utility voltage is in the high range to start with. The closer you are to that +/- 10% specification on the supply, the quicker you can exceed the limit with the manufactured leg.

    A single large load on the other hand, can be enough to pull down on the manufactured leg just enough to get the balance near the sweet spot. Where the phase angle, the voltage and currents are as symmetrical as possible. There is only the one sweet spot, per balance job, above and below that point, you move further away from symmetry. Whenever the load varies, the balance point will vary.

    If you run a 3Φ RPC idler with minimal balance caps, it will never have a high voltage and current on the manufactured line. Therefore you wont get much power from the manufactured line, but it won't overheat either. Then the question is, in this region, do you have enough power at the motor to serve the load. Oftentimes the motor could be oversized, to compensate for a weak leg and still provide enough load power to satisfy the load.

    In the multi motor situation, you need to consider the characteristics of the load, what loads come on and stay on first, then what is the typical way that the loads will be used, and what variation the system will have in use. Only after considering the the real world load application, what would be the best way to go about balancing for a mid point sweet spot.

    Switching caps in and out of the RPC circuit while running can be a bad idea, some builders do it, but it can produce some serious voltage spikes in the system during the switching. If you want or need to switch them in and out, it's best if you can do it before you start the system and apply load. One builder spent years designing, building, testing and patenting an automatic solid state switcher control, but discontinued it because in order to suppress the spikes to prevent problems it proved non economical.

    That's why I suggested splitting them up with the loads, if that is even possible, depending on system design and function. Switching them with the loads will have lesser spikes when combined with the inductance of the load motor. When placed close to the load, most of the current through the capacitor is reactive, and flows between the cap and the motor windings, not down the supply leads. This would affect the system balance less, when the load was switched in and out. Switching caps with the load will make additional inrush current on the contactor or starter that is supplying it, so that needs to be considered as well.

    With the chiller in question, it really depends on what process it serves and how is it designed to run. Does all the fans and pumps run continuously, and the compressors cycle online as required. Or does the entire system cycle on and off as the load controller call for, and the fans are switched with the compressors they serve. It would be a different balancing approach depending on either case.

    I have done a utility supplied 3Φ chiller with 4 zones, where each compressor was variable with a VFD for each zone and one for the circulator pump, fans were switched across the line. I would think it could be possible to obtain a manufactured unit with oversize drives that could do the phase conversion as well as the motor control. More expensive on the purchase, but a lot more workable in the field with 1Φ supply.

    If you look back at the numbers you gave in the first post, I think you can see what I'm alluding to. But you need to keep track of the numbers in a chart form so that you keep the appropriate values and phases in the proper order, for comparison, a column for unloaded conditions and loaded conditions. Without that you will loose track quickly of what your test condition readings mean.

    MTW
    MTW, I don't know how to respond to that wealth of information. But allow me to say :
    THANKYOU!
    For your time and knowledge.


    Sent from my SM-G930V using Tapatalk

  9. #19
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    Mar 2018
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    US
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    9
    Update....
    The factory tech came out to verify voltages and amperage is and give his yay or nay blessing. He had at that time a 6% deviation on voltage. After discussion with the factory they said that that was fine it just can't go to 10%. I don't understand that. Two of the compressor amp readings recorded by him were approximately 30 26 + 14 and 30, 27 + 11. To me that's just insane but the factory said that was okay. Anybody want to comment on that? I'm a little confused by it, especially since the manual spells out 2%. Only thing that I can think of is that they are confident that it will last beyond the one-year warranty and after that it's not their problem anymore. Thats the only thing that makes sense to me. I don't agree with it but it makes sense.

    Sent from my SM-G930V using Tapatalk

  10. #20
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    Icy78,

    For the part that you cant believe, and don't understand, go back and read post #2. Jeff spelled it out pretty succinctly.

    The chiller specifications calls the voltage deviation very tightly to cover their behind, an ideal condition not always achievable in the field, the tight spec makes it your problem if something should go pear shaped.

    You would never be able to hold that, with any rotary converter, with the type of variable load you are running. The only converter that could hold that spec in your situation, would be the Phase Perfect mentioned earlier.

    As you see, the voltage deviation is the least of the concern. The current deviation is normally multiples of the voltage differences. This is the major problem, the current is what does the real work. The main thing to watch for is that none of the line currents run at above the marked FLA rating, of each respective motor. If the currents remain at or lower than the FLA, then burnout is not too likely. As Jeff mentioned, with non symmetrical currents, you might get some circulating currents producing heat, but that's not usually a major factor.

    Then the next consideration is, do you have enough torque to start and run the load with the reduced voltages and currents. It appears that you do, cause it runs OK. Not as strong as it could be if well balanced, but good enough to get er done. The heat of summer will be the acid test to see how well she hold's up to extreme loading. Having hermetic compressors as the main loads is beneficial, as the refrigerant helps keep the temps of the windings in check.

    MTW

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