Excessive amp draw on screw compressor upon start
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  1. #1
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    Default Excessive amp draw on screw compressor upon start

    Friend ordered a screw compressor with a single phase 7.5 hp motor. When he starts it it dims the lights to his whole acreage. He is 60 ft from the pole so voltage drop is not an issue. What could he do to get a softer start. When I have had this problem with piston compressor’s, I have put a small buffer tank between the compressor head and tank. This allows the motor to get up to speed with less of a load. Would this work on a screw??

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    In my experience any compressor should start 100% unloaded. I would guess there is something wrong with your friends setup.

    Stuart

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    If this is a single phase motor after you confirm it is starting unloaded check the motor capacitors, and then the centrifugal switch that puts them in the circuit to make sure they are also working. A 7.5 hp motor draws a huge amount on startup even just to get a large pulley spinning before loading. If he's not on a solid 60A line starting may get dramatic.

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    There is a lot of mass in the rotors of most screws,and in some brands the makers crib by spinning the rotor unit at 3 or 4 times motor revs,to cut down size and so cost .

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    You say "it dims the lights at the whole place", then say "voltage drop is not the issue"..... When in fact, light dimming is EXACTLY caused by voltage drop.

    He might well have the right wire size to the compressor, but if the primary or transformer is not up to the challenge, you will have issues. That compressor can likely pull 200A at startup. I commonly see POCOs provide small pole pigs as they know most people will not need the full capacity. See what the current is on the line before the compressor starts. You might be just asking too much. If things seem sized right on the secondary and you confirm the voltage drop, the primary can be at issue. That becomes an issue for the POCO. You should be able to pull your rated service amps within the required voltage tolerances.

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    I'm 80 feet from the pole and a 200 amp surge drops my supply voltage to 160 volts. I can't start anything larger than that or the contactors drop out.

    Pretty sure our line service is so terrible that it can't even provide the current required for a short circuit detecting breaker to notice a dead short.

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    Thanks for the good responses. It is my understanding that a split phase motor has more starting torque than a three Phase motor. In this case if the screw had a three phase motor, the starting problems would be greater. Right???

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    The 3ph 7.5hp motor likely had a star delta starter,and the start current was not much greater than FL .Although most suppliers allow 10 hp DOL,its not often used.

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    7.5 hp is simply not that huge a motor, a decent supply should start it without drama

    My first shop was run off one service of a 2 family, the services had been run when we purchased the house. The line from the pole had never been updated when the services were added to replace the original single 60 amp service.

    Check the size of the main wire run, my utility upgraded it for free.

    If he has 3 phase available, I would think it would start better.

    I would have thought a screw compressor would start easier than a piston, but maybe I am wrong

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    Yes I thought a screw compressor would be an easier start.

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    7-1/2HP 230V single phase would be about 40A FLC, so starting current could be up to 240A. What size is his service drop from the utility? If it's a residential / light commercial 100A 1 phase service, the transformer on the pole could be as small as 15kVA but not likely larger than 25kVA. Even a 200A service usually doesn't get more than a 37.5kVA pole pig.

    If (as is most likely) the Locked Rotor kVA Code on the motor nameplate is a G, that's 5.6 to 6.3 kVA / HP, making the starting kVA anywhere from 42 to 47kVA. So to avoid a significant voltage drop, he would need a 50kVA service transformer, and most utilities will not give that to you unless you specifically ask (and pay) for it.

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    Lets not forget that pole pigs are typically conservatively rated and should handle a pretty good overload for a motor start. However, if sized WAY under what is needed, or if the primary (overhead hig voltage wire) just cannot handle it, there will be an issue. I know in my area, if we had this issue, I would complain and the service or transformer would get upgraded free. I don't think it's right to have a "150A service" but if you actually load to 150A, the voltage drop is pathetic. They need to fix it....

    Get the meters out and start testing!

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    So long as its wired properly, right size breaker, and not tripping, I wouldn't worry much about it. Pretty normal for loads that size to dim lights for a second on little 240V/1Ph services.
    My 7.5hp screw comp is on a 20Hp PP fed by 125A breaker, it starts unloaded, still dims the lights a tiny bit in the cnc mill running on the same PP and did so on a 400A/240V service the pole transformer is only 40-50' away and I'm the only one on it.

    Almost every house around here with ducted heat pumps have the lights dim every time that thing starts up, and they often tie a dozen homes on one little 50kva transformer at the pole... life goes on.

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