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  1. podobing

    NEC Substation Clearances

    Bacon4Life, Thanks for the response. I have been told that NEC 90.5(C) (Explanatory Material) says "Explanatory material, such as references to other standards, references to related sections of this Code, or information related to a Code rule, is included in this Code in the form of...
  2. podobing

    NEC Substation Clearances

    Hello, We are installing a customer-owned 25 kV to 4.16 kV substation. This is a skid mounted system, and the only exposed energized parts are the tips of the high side transformer bushings. The substation will be outdoors, and will be protected by a 7 foot high fence with a locked gate. Table...
  3. podobing

    Wiring in hazardous location - NEC Std. - On-Skid of Moto-Compressor

    From the picture, it is hard to tell if they are explosion proof seals just below the block heater starter box. They look small. If they are explosion proof, then the remainder of the conduit fittings are in a Class I, Division 2 environment. You will need another seal off fitting within 10...
  4. podobing

    Static Wire vs. Neutral Conductor

    Mbrooke, I am not sure if the conductor is steel or not. I hope to find out next week. bacon4life, Is alumoweld suitable for use as a current carrying conductor? I am not a lineman. Thanks for the education on wire types. PS- Good name - makes me hungry! waross, The wire is run high all of...
  5. podobing

    Static Wire vs. Neutral Conductor

    Mbrooke, I assume that you mean a (galvanized) steel conductor would not be legal. Would the conductor have to be adequately insulated (5 kV?) from the structure? Podobing
  6. podobing

    Static Wire vs. Neutral Conductor

    Hello, I work for an electrical contractor, primarily in the mining and gas industries. A customer installs and maintains their own 12,470 volt power distribution system. They use wooden poles with a cross arm that holds the three phases. There is also a single wire at the top of the pole...
  7. podobing

    Which Voltage to choose (and why)

    Guys, I like the responses so far. EdStainless - In a previous life, I have seen large HP VFD's used at both 480 volts and 4160 volts. I worked in the mines, and in the mine itself, 480 volts was used for both soft start and vfd applications for conveyor belt (for example), regardless of HP...
  8. podobing

    Which Voltage to choose (and why)

    Hello, We are in the very early stages of doing the electrical engineering on a pump project. The pumps will be used to pump water over a long distance (not much lift) for a gas processing operation. The people that engineered the hydraulics say that they need three 600 horsepower motors to do...
  9. podobing

    Intermittent vibration on large electric motor

    Guys, Sorry for taking so long to respond, but I have been gathering information. When I changed the power system analyzer to look for voltage imbalance (thank you for that recommendation), the voltage imbalance "spikes" corresponded with the motor vibrations. We re-located the analyzer to...
  10. podobing

    PT Secondary Fuse

    Charz, I have seen the PT system that you describe used for ground fault detection in medium voltage (4160 volt) systems that are ungrounded. The neutral of the PT primary bank must be grounded or else you cannot detect a ground fault. As you said, the main reason for grounding the secondary...
  11. podobing

    Intermittent vibration on large electric motor

    Guys, I have been back to the site, and have some new information, but nothing concrete yet. The motor, shaft, and bearings are all horizontal. The couplers used are grid-pack couplers. The system RPM is 890, and all bearings are grease lubricated. The power system analyzer was changed from...
  12. podobing

    Intermittent vibration on large electric motor

    Guys, Thanks for all of the responses and interest in this problem. To address some of the questions that were asked: Itsmoked: The motor starter is about 25 feet from the motor. It is an across the line starter, and it is the only load on the 4 kV bus. Waross: Other loads operated by...
  13. podobing

    Intermittent vibration on large electric motor

    The monitor triggers at 10 percent (sag or swell) on any one phase. It also monitors frequency, and I believe that the tolerance is plus or minus 0.2 Hertz.
  14. podobing

    Intermittent vibration on large electric motor

    Hello, Underground coal mines use large electric motors to operate their ventilation fans. The number of fans required depends on the size and depth of the coal mine. One fan, in particular is a 2000 HP, 4kV, 890 rpm motor that is started directly across the line. The motor has two vibration...
  15. podobing

    DC Shunt Motor Question

    Bill, I never thought of using AC to do the measurement. I was focused on DC, and once I get focused, it's like putting blinders on a horse. What you suggest makes a lot of sense. I'll have to give that a try. Thanks, Dave
  16. podobing

    HIGHER SERVICE FACTOR OF A MOTOR

    V, I think that a higher service factor indicates that the motor is mechanically able to run at 115% or 125% of nameplate horsepower. That would mean that the bearings and shaft are properly sized to operate at these horsepower levels without any adverse effects on their life. Aside from...
  17. podobing

    DC Shunt Motor Question

    Guys, Thank you all for the information, as it is all good information to have and use as necessary. But does anyone have a feel for how much inductance is in the field winding? In my mind, this information is what is needed to select the proper dc contactor, or the proper discharge resistor...
  18. podobing

    DC Shunt Motor Question

    Bill, The diode is fine, and was installed as a means to protect the contact tips. The system is working properly now (with the diode), but we would like to find a properly rated contactor to do the job without the diode (for political reasons). The original contactor was never rated for this...
  19. podobing

    DC Shunt Motor Question

    Hello All, We have an elevator application that uses a low speed (55 rpm), 100 HP DC shunt wound motor. We have a lot of problems with contactors in the shunt field circuit that fail. These contactors are being used for required testing purposes. The contact tips burn severely when the...
  20. podobing

    Parallel conductors feeding a three phase motor

    Chuck, There is no other source of power readily available. This area is on top of coal storage silos, and the cable(s) that were damaged run in cable tray on top of the inclined conveyor that feeds the first silo. The problem with changing the entire cable is working with a cable tray that...

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