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  1. aussiejohn2

    Arc Flash Fed by Generators-What's The FCT

    Waross has it right: it depends. Decrement curve is often (always?) drawn for constant excitation. If you have PMG driven excitation system that can sustain high levels of field forcing, output will be sustained until (/unless) over excitation limiter or rotor thermal protection kicks in.
  2. aussiejohn2

    NYC Wind Turbine Failure

    Paul Gipe: In 1961, Louis Vadot described the limitations of the [fixed pitch] Darrieus turbines in a United Nations report on wind power. "Finally", he said, "it is very difficult to design and build a practical furling device to protect these machines in high winds". With horizontal axis...
  3. aussiejohn2

    Disconnection time 120 volts vs 150 volts

    Mbrooke, IEC 479 is worth a read if you have access (even though you have to follow the rules of your jurisdiction). It has a fair bit of the physiology including charts showing the ("consensus") time-current boundaries for various physiological outcomes. I don't have it on this PC but...
  4. aussiejohn2

    Excel 2016 name box missing

    Win 10 also caused graphics issues for me in Excel 2016 (no other apps) - name box was ok but formula bar would vanish, charts & their formatting dialogues went haywire, and sometimes it would get stuck in a loop of refreshing/redrawing. Update of graphics drivers at the time (and everything...
  5. aussiejohn2

    Time Current Curves Stop at 6 cycles

    Mbrooke, Depends on voltage and equipment, but one easy (and depressingly common) way to get bolted faults is through operator error by leaving earths on when re-energising after outages. Three phase cable faults can also be pretty close to bolted. John.
  6. aussiejohn2

    Connector/receptacle/plug suitable for motor power connection

    Paul, Even outside the hazardous area stuff, I think that some of the IEC 60309 switched-sockets have a threaded "bushing"(to prevent accidently pulling out) and are mechanically interlocked (so they turn off if you do pull them off). 60309-4, perhaps. I would expect there would be an...
  7. aussiejohn2

    OHEW under phase conductors

    Scotty, It's a stretch, but one rare example is where people want to have a MV SWER (single wire earth return) system, but the dirt is too poor for the achievable earthing to be safe with the required load current. Supplementing the earth return with a neutral (even a small one) can help. The...
  8. aussiejohn2

    480 volt Fused Switchgear

    All, In principle fuses can have some of the capability often ascribed only to CBs. * You can have remote indication, either by striker pins or by monitoring the voltage across them. * Striker pins allow for ganged operation. There are even medium voltage multi-shot reclosers using fuses...
  9. aussiejohn2

    OHEW under phase conductors

    Power, That would be my guess. I tend to call these "underslung" EWs. John.
  10. aussiejohn2

    Single core vs Three core, armored vs unarmored, 11 KV, 95 sq copper cable

    Muthu, It is best to cleat single cores to keep them together under fault current forces. Three core means one less thing to buy and remember. However availability is also a factor especially as you need such a short length. Three core seems more common (so more likely available without a...
  11. aussiejohn2

    Halbach array plus steel extension

    Hi SpaceRanch, Free finite simulator femm would let you simulate. KJ is a great site by the way, one of my stickies. A lot of their images look to have been made in femm. John.
  12. aussiejohn2

    Low Voltage THD But High Current THD

    The nonlinear load acts (to good approximation) as a harmonic current source. Those harmonic currents flow all the way back to the system generator neutral point(s) (except for the triplens which may stop at a star-delta neutral). The harmonic voltages are a result of the volt drop from these...
  13. aussiejohn2

    Low Voltage THD But High Current THD

    Muthu, Currents no (except for trapping of triplens in a delta), but voltages yes (voltage divider between the transformer impedance and the system impedance).
  14. aussiejohn2

    Selection of Motor Overcurrent relays:

    Hi NCTHAI, 1.2 seconds is a relatively long time for the voltage to decay. Is it measured or calculated? You can estimate the open circuit time constant from the rotor circuit parameters - search of this site for motor bus transfer (or fast transfer) should turn up old posts where electricpete...
  15. aussiejohn2

    Monitoring solenoid health

    David, I suppose anything is possible but the ones I have seen the brake is applied by the springs and the solenoid releases it so it is fail-safe against the likely failure modes, and "TCS" would be effective for finding open circuit wiring or solenoid. J.
  16. aussiejohn2

    PV generation hourly simulation

    HOMER, too...although it no longer seems to be free? For spreadsheet you can start with the equation of time to determine sun angle relative to panels & sunrise and set times for direct light. Refinements include consideration of light strength varying with earth-sun distance through the year...
  17. aussiejohn2

    How does a fused switch in an MCC protect a motor if only one phase is blown during a GROUND FAULT ?

    All, I have seen the consequences of "single" (actually double) phasing in an industrial facility (paper) that lost a lot of motors. Event was due to false opening of one phase on the utility side. Their incoming transformer was Yy so the single phasing was reflected through to them. However I...
  18. aussiejohn2

    Electric Motor Heater

    Hi rhatcher, My experience in industrial, mining, oil and gas, and utility is different. Rectifier output is only ever measured with a *true RMS* meter on AC range. True rms hea^X^X^X meters measure the rms value (regardless of waveform), either by physically integrating and averaging in a...
  19. aussiejohn2

    Earth pit?

    Hi Farale, Consider two limiting cases: 1. If the rods are very (infinitely) far apart then you will measure the sum of the two rods i.e. 40 ohms. Neglecting the lead resistance is not entirely unreasonable, since you could use superconducting leads or (easier) use a four-terminal measurement...
  20. aussiejohn2

    When starting three 200HP motors at the same time is that equivalent to starting one 600HP motor ?

    Hi BDN, > So how do you calculate from that data the starting time? Same as at 100% voltage. Net torque (motor minus load) and rotational inertia tells you the angular acceleration. Integrating this tells you angular velocity. Time to reach steady-state speed is the starting time. Since...

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