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Thread: Phase converter voltage deviation.

  1. #1
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    Phase converter voltage deviation.

    Hello first time posting here, that I can remember at least. Looong post.
    I am an hvacr Tech and I have a question on a 3-phase Chiller and pump being powered by a roto phase. What I've always understood as far as supplied voltage and voltage under load, I should be within or less than 2% deviation. I usually get this by adding the three voltages dividing by 3 and then calculating how far off the farthest one is from that. So what I have on this phase converter is 275 254 + 240 volts when the 208-230 3 phase pump motor starts at 3 horsepower. All phase converter capacitors connected. I will read 9.8 amps 7.7 and 7.7. F L A is 8.4.

    Now my understanding is that never should I have a motor running with any leg over FLA , but the phase converter manufacturer says that is fine, the motor supplier says that is fine. However if I read information from Fluke or US Motors, or another phase converter manufacturer, they all say that is not o.k.

    So the phase converter manufacturer said to disconnect a few of the Run capacitors , to drop the voltage , and that would steady out the amps and bring them lower. The amps did not drop they actually went to 5, 8, 9.8. At new voltages of 235, 235, 246. 3% deviation.
    The chiller itself, (when running one of the 4 compressors) the amps on it run 30-27 and 13 at 3% voltage deviation.
    Another thing that I have read is a 3% voltage deviation can result in 50% reduction of motor life due to insulation over-heat.
    Both amp draws deviate by 40%! 10% is allowable.

    I'll bet you want more specific equipment info but I don't have that right now. Site is 3 hours away.
    I could get more from others tho.


    I'd love to know what you guys say about this. Maybe just more generic input about deviation.
    Thanks

    PS....the chiller startup manual calls for less than 2% voltage deviation.

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    First off, understand that people who have a horse in the race will tell you whatever it takes to go away. The RPC supplier was likely selected by the HVAC equipment supplier, so they agree because it's in their collective best interest to agree and to obfuscate the issue until you give up and go away. Then they get to sell replacement equipment once their warranty expires.

    Your instincts are correct, a small voltage imbalance creates a more sever current imbalance, and a current imbalance results in excessive heating of the motor windings. When the currents don't match, the rotor generates what are called "negative sequence currents" that counter rotate and create negative torque. It's much less than the normal torque, so it has little effect on that, but what it does is become heat in the motor. So if you look at motor heat produced PER UNIT of current, it means that at a given amount of current in the motor, the windings run hotter. Overload protection of the motor is going to be based strictly on the amount of current actually flowing, but is ASSUMING that it is relatively balanced. So when it isn't, the OL relay may not trip, but the motor insulation is being damaged by that extra heat, hence the issue of reduced motor life; Heat x Time = Failure.

    But the other question is, how was the motor sized? Because if the chiller mfr purposely over sized the motor KNOWING that this would happen because it was going to be run on an RPC, it may be fine with it. So for example if the chiller mfr put in a 5HP motor and re-labeled it as a 3HP motor, it can run like this forever.
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    Note also that to some extent the three sets of windings in the motor share a common heat sink and heat dissipation mechanism. That allows the motor to tolerate a small excess over FLA on one winding (without even having to take advantage of any Service Factor the motor might have.)
    But this allowable excess is nowhere near allowing you to act as if the total heat production from the three winding can still be tolerated if two windings are "under budget." One reason for this is that the increase of the temperature of the center of the winding over the outer parts of the winding will not be reduced just because the other windings are lightly loaded.

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    Thankyou guys!
    Can you explain what you mean, Goldigger, by 2 phases being under budget?

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    Quote Originally Posted by icy78 View Post
    Thankyou guys!
    Can you explain what you mean, Goldigger, by 2 phases being under budget?

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    Less than FLA
    If you don't think too good, don't think too much.

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    icy78,

    You didn't mention the size of your RPC (rotary phase converter) or the size of your 4 compressors, only the 3HP circulator pump.

    Right now, your worried about the circulator current balance. When your 4 compressors and condenser fans start coming on line, your problem may go in the opposite direction.

    A RPC can only be accurately balanced at one particular load, it's the nature of how they are constructed. The capacitors are selected to resonate the voltage on the generated leg. Usually selected for the maximum rating of the RPC output. At outputs below maximum, the voltage will be higher, and the voltage balance will become a bigger spread. At the full load output, the voltage should be relatively well balanced.

    Running small motors on large RPC's can cause unbalanced operation and excessive heating. Most RPC manufacturers will list what is the smallest motor that can be run, on a particular model, to prevent excessive heating.

    With a single large load, RPC's can perform well. With multiple smaller loads that come and go, things can get more complicated. If I understand correctly, you have one circulator pump motor, four hermetic compressors, and several condenser fans, all staged to come on line as the load requires. So your balance will be all over the place, depending on the load conditions. High voltages and motor currents when lightly loaded and low voltages and currents at the maximum loads.

    A couple of suggestions that can help mitigate the problem. If the circulator runs continuously you might consider swapping out the pump for a single phase model, so that it runs under all conditions of chiller loading without overheating. 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.

    In this way, each compressor and associated fans, can be balanced as an individual unit, and will have minimal effect on the overall system balance when it is switched in and out. Its more complicated and time consuming but can be a benefit to system reliability.

    When removing caps from the RPC, to use for zone balancing, can affect the starting of the RPC idler. Both start and run capacitors are paralleled (additive) in the RPC for the phase shift required to get the RPC idler started, at which time the start caps are switched off. This loss of start capacitance, by removing the run capacitance, can be compensated for by adding additional start capacitance, that gets switched off.

    As to the unbalance operation and affects. It will never be perfect, you want to shoot for the happy medium, of the typical running load. Any time you run over the nameplate FLA, you are potentially overheating one phase winding, and reducing lifespan. Once you get over the service factor rating, if any, then your getting in the quick degradation area.

    In comparing the voltage and current symmetries, I would use the difference between the utility supplied phase and the highest manufactured phase reading. Comparing the average to the highest reading, is not the difference that the motor is working with.

    MTW

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    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.

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    BTW. How do I quote, or "like" others posts?

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    Quote Originally Posted by icy78 View Post
    BTW. How do I quote, or "like" others posts?

    Sent from my SM-G930V using Tapatalk
    In the bottom right there is a box that says Reply With Quote, click that instead of Post Reply. You can quote multiple posts by clicking the icon that looks like a speech bubble on any additional post you want and finish with same Reply With Quote on the last post you want to include.
    If you don't think too good, don't think too much.

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    Quote Originally Posted by ActionDave View Post
    In the bottom right there is a box that says Reply With Quote, click that instead of Post Reply. You can quote multiple posts by clicking the icon that looks like a speech bubble on any additional post you want and finish with same Reply With Quote on the last post you want to include.
    Thankyou.
    I'm on Tapatalk so it must be different. I tap on the post to highlight it and on the top of the page I tap the arrow.
    I don't see a way to " like" the post tho. Maybe not an option on this forum?

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