5HP Compressor Creates Too Much Voltage Drop...Power Factor Correction Capacitor????
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    Default 5HP Compressor Creates Too Much Voltage Drop...Power Factor Correction Capacitor????

    EDIT: The motor is actually a 10 HP motor!!!

    I have a 10 HP 230VAC 3-phase compressor that is creating a pretty big voltage drop at the circuit panel. One problem is that my building has 120VAC or 208VAC 3-phase power supply that usually reads 214-216 VAC depending on what legs you probe. Because of lower voltage, the compressor is pulling more amps than the nameplate says it should. Just before the pressure switch turns off the motor it is pulling about 28 Amps and it is listed at 24 Amps on the nameplate, but that doesn't really bother me as I have a 30 Amp breaker at the panel for safety. The wiring from the panel to the compressor is all 8 gauge so the wire is sized correctly for the job.

    When the compressor turns on, the voltage drops at the main lugs of the panel from 214-216 VAC down to 200-202 VAC for about 3-5 seconds then it jumps back up to normal. This wouldn't bother me except for the fact that my Cosmec Conquest 510 CNC Router is connected to the same panel and every once in a while the voltage drop will cause the CNC to go into Watchdog Mode due to the voltage drop. It doesn't specifically say it was due to a voltage drop, but I have concluded that is the issue. I have been at the control panel enough times, at the exact moment when it goes into Watchdog Mode and that is precisely when the compressor turns on. Like I said earlier, it isn't every time, or every 3 times, but the only time this error happens is when the compressor has just turned on (like within 1 second of the compressor turning on).

    I can't EASILY isolate the compressor from the CNC without running 200+ feet of 8/3 MC from another panel in the warehouse. That is a pretty good chunk of change for the wiring, plus the rental of a scissor lift, and labor time involved with the install. I was doing some research and I found this at Grainger, and thought it might be a good fix for this problem....
    WEG Power Factor Correction Capacitor,3.00 KVAR,240VAC Voltage,7.9" Width,4.8" Depth,15.8" Height - 16Y128'|'BCWTC300V29B4-N - Grainger

    The price is right on this product if it will do what I need, but I was hoping maybe you guys could confirm this would work, or had a better solution, or at least the right solution for around the same price point????

    Thanks,
    Zac

    EDIT: The motor is actually a 10 HP motor!!!
    Last edited by Zac Penn; 10-07-2019 at 08:35 AM.

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    That's a lot of money for a metal box with 3, 100uf capacitors in it. (or whatever the number is) and it will only offset about 8 amps. you need to offset the 100+ amps the compressor consumes at startup, and there is no cheap way to do that.

    For 300$ you can find a vfd to soft start the compressor ($265 but who knows how long it will last), or reconfigure the compressor to run continuously but rather bypass the output (solenoid valve to open the discharge line to atmosphere?). or buy a new compressor that can unload the valves.

    Another option is investigate how much air you actually need and if a 3 hp motor running continuously can deliver that much air, then buy a high efficiency 3 hp continuous duty motor and the right sized pulley to drive the compressor. such a motor might pull half as many starting amps as what you have now.


    Line amps for a 5 hp 240v motor are 13-15 amps, a little more on 208. 28 amps on shutoff is close to burn out unless the motor is designed specifically for "air compressor duty" as i suspect it is.


    you can also use a buck boost transformer to deliver 230 volts to your cnc router, and this might be the cheapest option.

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    The wire size to the compressor is not as important in this case as the size of wire to the panel as this is where the voltage drop is measured. I agree the shut down current sounds high but the start up current must be even higher since that is when the voltage drops.

    Are you sure that your unloader is working? If not this would account for the high current at start up. Does it have the same issue when the air pressure is at 0 psi in the tank?

    Bill

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    Sorry for the confusion, but the motor is actually 10 HP, not 5. I don't know why i can't remember that? I always tell people it is 5hp on accident. I guess deep inside, I feel like 10 HP is just too much for a compressor LOL.

    I think the Un-Loader is working properly, because you can hear a distinct sound change in the compressor after about 2 seconds of the motor turning on. It spins freely, the noise changes, then it starts compressing until about 140 PSI, then the compressing stops as the motor and flywheel spin free and decelerate.

    If the unloader wasn't working, wouldn't the motor stop spinning after maybe a few revolutions due to the compression forces inside the compressor?
    Also, wouldn't it not have that sound change in the beginning of operation?
    I am new to large industrial compressors and equipment, so you will have to bear with me a little bit.

    I kind of like the VFD idea but I don't want to waste money if you think it would only last a couple years.

    I was hoping the capacitor idea would provide just enough boost to keep the voltage drop in check during that initial 2 seconds.
    This is what I found on Grainger on how to select the right capacitor...
    screen-shot-2019-10-07-8.55.16-am.jpg

    If I wanted to be conservative and bump up to the 5 Kvar version it isn't much more expensive...
    WEG Power Factor Correction Capacitor,5.00 KVAR,240VAC Voltage,7.9" Width,4.8" Depth,15.8" Height - 16Y129'|'BCWTC500V29B4-N - Grainger

    But you guys don't seem to think it will cure my voltage drop issue at the panel???

    There is nothing I can do about the size of the feed wires going into my 200 Amp panel, but they seem like they were size properly. My electrician runs things conservatively so I am pretty sure he sized those wires right. If they were undersized, wouldn't it take much longer for the voltage drop to rise back to 214-216?

    Thanks for your help.

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    Do you know how far away your pole transformer is? Also what size it is?

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    Inspect all connections for corrosion and loose fitting wires. Kirchhoff is the reason why.

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    As stated in an earlier post, the power factor correction will only reduce the current a very small part of the start up current of approximately 150 amps. Power factor correction is usually sized for running conditions not start up.

    Also as Johanson suggested a buck boost on the CNC would probably solve the issue. This is a rock solid solution as far as reliability is concerned.

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    Okay I will start looking at buck boosters but I have more questions

    The buck booster is specifically chosen to take a supply voltage and boost it to a desired voltage. IE 214VAC to 230VAC

    If the compressor turns on and creates a voltage drop from 214 down to 202, isn't the output voltage from the buck booster going to drop at the same level? IE: 230 down to 218 VAC???

    Granted that is a much more desirable voltage for my CNC machine, but I don't know if it is the actual change in voltage that is causing my error on the CNC or if it is the actual lack of voltage.

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    There are active line conditioners that are basically a multi-tap transformer with a microprocessor control that changes the taps to create step up and down as load changes.

    Have a couple for data centers and they work great.

    A 5 hp one may exist someplace, Google active line conditioners

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    Quote Originally Posted by Zac Penn View Post
    If the compressor turns on and creates a voltage drop from 214 down to 202, isn't the output voltage from the buck booster going to drop at the same level? IE: 230 down to 218 VAC???

    Granted that is a much more desirable voltage for my CNC machine, but I don't know if it is the actual change in voltage that is causing my error on the CNC or if it is the actual lack of voltage.
    correct, and we don't know either.


    your cnc router may be safe with 240vac which means you'll have even more margin.. that is find a 16/32v buck boost transformer and it will add 27 volts, or 32 volts, depending on how you wire it.

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    Can you also turn the pressure switch cut off point down 10-20 psi ?

    Or

    If you really need the high pressure, how about flow ? can you put a slightly smaller
    pulley on the motor ?

    Or an adjustable one, tune it using the amp meter.

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    Default Staying above the lower voltage limit, during the voltage drop.

    My solution, given the service you have would be three things.

    Reduce the startup load. Change the motor pulley, to reduce pump speed and load.

    The motor is drawing over nameplate FLA, at the end of the pump cycle, because it's designed that way. All reciprocating pump manufacturers setup their units that way. To overload the motor at the high pressure end of the cycle, because overloading it for a partial cycle and then letting it rest won't normally overheat the motor. You get better output CFM that way.

    Set a belt ratio so that it can run continuously, at or below FLA, at the high pressure end of the cycle. This will also reduce the startup load.

    Next would be to install a continuous run unloader system. Use this mode when running the CNC.
    Motor stays running, head unloaders hold open when at set point.

    Use the Start/Stop mode when the CNC is not running for normal operation.

    The third thing, would be to install a boost transformer on the CNC feeder. Configure this to boost your highest utility voltage, normally in the middle of the night, to be at the high end of what the CNC can handle. That way when the voltage drops, it won't drop below the critical level of the CNC requirements. Your currently operating at the low end of the voltage, and it can't tolerate any more drop.

    SAF Ω

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    Quote Originally Posted by SAF View Post
    My solution, given the service you have would be three things.

    Reduce the startup load. Change the motor pulley, to reduce pump speed and load.

    The motor is drawing over nameplate FLA, at the end of the pump cycle, because it's designed that way. All reciprocating pump manufacturers setup their units that way. To overload the motor at the high pressure end of the cycle, because overloading it for a partial cycle and then letting it rest won't normally overheat the motor. You get better output CFM that way.

    Set a belt ratio so that it can run continuously, at or below FLA, at the high pressure end of the cycle. This will also reduce the startup load.

    Next would be to install a continuous run unloader system. Use this mode when running the CNC.
    Motor stays running, head unloaders hold open when at set point.

    Use the Start/Stop mode when the CNC is not running for normal operation.

    The third thing, would be to install a boost transformer on the CNC feeder. Configure this to boost your highest utility voltage, normally in the middle of the night, to be at the high end of what the CNC can handle. That way when the voltage drops, it won't drop below the critical level of the CNC requirements. Your currently operating at the low end of the voltage, and it can't tolerate any more drop.

    SAF Ω
    Thank you so much for your thoughtful suggestions. I hadn't even thought about a continuous unloader for when the CNC is running. That is a great idea. I am already burning like 100 amps when everything is running, so I might as well just let the motor keep spinning on the compressor if the other suggestions don't fix my problem.

    Now comes the point where I call myself dumb LOL...
    It turns out my CNC machine already has transformers on it, but they are STEPPING DOWN, not up. I guess the other shop had too high of line voltage so they stepped the voltage down. I feel like an idiot for not testing the voltage at the CNC power switch until now.
    My panel this morning was showing between 210 - 213 VAC between legs.
    The main switch terminals on the CNC were showing between 199 - 201 VAC. I didn't test those terminals, this morning, when the compressor turned on, but I am sure it was dropping 8+ volts which definitely put it out of the 220VAC +/- 10% range that the CNC Manual dictates, which would be 198 VAC.

    Here are the pictures of my panel to transformer setup and the wiring diagram for each...
    mvimg_20191010_102317.jpg
    mvimg_20191010_102329.jpg

    I noticed that only two of the phases were being stepped down as the 3rd phase was still reading 124VAC at the switch terminal. The other two phases were showing 109 or 110 VAC. Can someone based on those pictures show me which wires to use inside each transformer to STEP UP the voltage on those two phases?

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    its common to use two transformers instead of 3 for a couple reasons.

    you may be able to simply pull the wires off the H1 and H4 connection and swap them, on both transformers.

    are the H2 and H3 connections jumpered together?
    Last edited by johansen; 10-10-2019 at 04:37 PM.

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    If you removed the Buck transformers you would get 16 more volts to the CNC, than you have now.
    You stated that you get 8 volts of drop when your compressors starts. With 16 more volts from the Buck transformer removal, you might not have any problem.

    Before you connect the transformer bank in additive polarity, you need to test that middle of the night voltage as suggested before. If the voltage becomes too high during periods, it can be much worse than when it gets too low.

    Another pointer from your photo, is that you need to add a dedicated ground wire from the loadcenter to your CNC panel. Plastic and flexible metal conduit, is not a viable low impedance grounding path.

    For information on Buck/Boost transformers, get yourself a copy of an Acme Transformer catalog. There is plenty of information on selecting and connecting transformer banks in there.

    SAF Ω

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    Seems to me you need a soft start box for your compressor.

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    I'll repeat what a few others have already noted- if you don't need it, shed some of your compressor load. If you're regulating the pressure down substantially, reduce your compressor's upper cutoff. If you don't need the volume, and the compressor pump doesn't require a high minimum speed, change your pulley ratio to provide more leverage, so lesser load. If it's a 'real' industrial machine (I suspect it is) then change it to constant-speed motor with pressure-pilot unloader to eliminate startups. If not, consider changing the compressor motor drive system over to a VFD, then under-drive the compressor head considerable, overspeed the motor and set up a startup curve that will minimize inrush, while still giving considerable amount of power at the compressor head. One could even put a cutout solenoid on the unloader so that you bring the machine up to speed completely unloaded, then snap it in once at full speed.

    If the CNC is running buck, then bring it back up a tap or two, but what I'd be most concerned about is that you're challenged by an overabundance of Ohms... and that the voltage coming TO you is fine but you're asking for more CURRENT than your facility's transmission system is able to carry.

    Could be something as simple as a corroded and insufficiently-tightened conductor. Call a buddy at the local power company, have them come in at night for a beer, bring his thermograph camera, and take a look at the machine cabinets, your electrical panels, transformer connections, etc... and then start up all the machines. Enjoy your beers for a couple fifteen minutes, and take another look, and see what changed the most... then FIX it...


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