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NG transmission pumping, constant torque, constant load?
Question:
On electric drive large NG compressors, inlet pressure 70 psi, outlet about 1100 constant torque, what about electrical load, especially the reactive component, is it dynamic or relatively static?
Answer:
According to your information...that is 3 stage rotating...I am fairly sure that you are describing a centrifugal booster pump which is variable torque by nature. I'm including a description of variable vs. constant torque that I found on the web. Also, the question that you are asking "what about electrical load, especially the reactive component, is it dynamic or relatively static?" That's hard to answer without more info. Motor and pump nameplate data would be helpful. Also is this compressor run at differing speeds or constant speed?
If it is a centrifugal compressor, and you are using the variable frequency drive only as a soft start, it behaves as a variable torque load.
If you use the VFD drive for flow control, there are areas of it being constant horsepower.
However, if you are using a reciprocating compressor, it is constant torque, especially in starting. 0.01% speed is still 100% torque (unless you have unloaded all stages of the machine). What is your flow control scheme? Recips behave differently than centrifugals.
Be sure to work with the compressor packager to confirm type. Recips are notoriously high torque loads and in conjunction with a variable frequency drive will require careful sizing considerations. The torque effort curves often require 200-300% overload capacity because of the torque pulsations and will be continuous. Most VFD's will offer 150% overload in heavy load for limited time, 60 seconds/over time? And at 175-200% will trip immediately. Also look at ambient with VFD control, 40* C normal. Above this there will be additional derate to accomodate heat transfer from the drive. This could cause oversizing the VFD 2x or more.
I suggest that you discuss this with the packager, compressor and variable frequency drive vendor(s).
On electric drive large NG compressors, inlet pressure 70 psi, outlet about 1100 constant torque, what about electrical load, especially the reactive component, is it dynamic or relatively static?
Answer:
According to your information...that is 3 stage rotating...I am fairly sure that you are describing a centrifugal booster pump which is variable torque by nature. I'm including a description of variable vs. constant torque that I found on the web. Also, the question that you are asking "what about electrical load, especially the reactive component, is it dynamic or relatively static?" That's hard to answer without more info. Motor and pump nameplate data would be helpful. Also is this compressor run at differing speeds or constant speed?
If it is a centrifugal compressor, and you are using the variable frequency drive only as a soft start, it behaves as a variable torque load.
If you use the VFD drive for flow control, there are areas of it being constant horsepower.
However, if you are using a reciprocating compressor, it is constant torque, especially in starting. 0.01% speed is still 100% torque (unless you have unloaded all stages of the machine). What is your flow control scheme? Recips behave differently than centrifugals.
Be sure to work with the compressor packager to confirm type. Recips are notoriously high torque loads and in conjunction with a variable frequency drive will require careful sizing considerations. The torque effort curves often require 200-300% overload capacity because of the torque pulsations and will be continuous. Most VFD's will offer 150% overload in heavy load for limited time, 60 seconds/over time? And at 175-200% will trip immediately. Also look at ambient with VFD control, 40* C normal. Above this there will be additional derate to accomodate heat transfer from the drive. This could cause oversizing the VFD 2x or more.
I suggest that you discuss this with the packager, compressor and variable frequency drive vendor(s).
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