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  • Users: nhcf
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  1. nhcf

    Starting Large 480 VAC motors DOL with Switchgear Breaker

    We are looking at project specs which are specifying new 480 VAC switchgear that will control several larger HP (125 - 300 HP) motors directly across the line using the switchgear breakers and not via an MCC with motor starters/contactors. I am more familiar with motor starter construction -...
  2. nhcf

    Distribution system voltage drop (480 VAC) during motor start

    Is there an IEEE or similar standard for recommended max voltage drop on a 480 V distribution system during a motor start? I'm evaluating options for reduced voltage/soft start/VFD start for an existing installation - we can figure out what the transient voltage is, but I'm not sure of what...
  3. nhcf

    Diesel Standby Generator - Life Expectancy

    Can anyone point to a good reference to evaluate life expectancy of diesel engine-driven generator in an industrial, standby application? We have a 750 kW unit. I have seen some reference suggesting 10000 - 20000 hours, depending on quality of maintenance, frequency of operation, loading...
  4. nhcf

    SIL2 motor interlocks

    We have an application which requires SIL2 motor interlocks - to prevent 2 pumps running simultaneously. My experience with SIL systems is limited to process instruments, not necessarily motor starters - is there such a thing as SIL-rated motor contactors/starters?
  5. nhcf

    Generator Reactances

    We have a standby generator - 480/277 V. The vendor has provided p.u. reactance data for a 139/240 configuration. I should be able to convert the p.u. to ohms and back to 480, correct? I am uncertain as to how the windings are reconfigured in the 480 application - is it as simple as doubling...
  6. nhcf

    UPS Output Voltage

    APC SmartUPS UPS System has an output voltage of 208 VAC. Its modular battery system provides battery packs at 192 VDC. How does the inverter produce a 208 VAC sine wave from 192 VDC voltage?
  7. nhcf

    Parallel Generator Grounding Configuration

    We are looking at a project (conceptual design) which will include four 4160V, 2KW synch generators tied to grid and to in-house loads. Regarding grounding, various references, including Blackburn's book, discuss options for grounding, including grounding each generator or installing a common...
  8. nhcf

    API 521 Noise Calc, Figure 18

    Section 5.8.10.3 provides calculations for estimating noise from a discharge pipe. The equation uses "L", sound pressure level, which is taken from Figure 18, and based on a pressure ratio = absolute pressure upstream of relief valve/absolute pressure downstream. The figure only shows ratios up...
  9. nhcf

    Noisy RTDs on VFD controlled motor

    A plant is reporting noisy temperature signals from motor RTDs when the motor is operating. Motor is 100HP and VFD controlled. The RTDs and associated cabling are in separate conduits/boxes from any motor conductors/cables. Motors are supplied with Belden shielded VFD cable, which appears to...
  10. nhcf

    Static Electricity in Natural Gas Piping

    I have an application a pressure relief valve vents into a vessel which is vented to atmosphere. During initial opening of the relief, there is the potential that the vessel will contain both oxygen (ambient air) and natural gas. Both the piping and the vessel are steel and bonded together and...
  11. nhcf

    VFD Incorrect RPM Setting

    What is the impact of an incorrect motor speed setting in a VFD? Today I noted that a 2 pole, 3555 rpm motor was connected to a VFD configured for 4 pole 1780 rpm. Application is a 75 HP centrifugal blower, open loop application. The motors / drives have probably been running this way for 3+...
  12. nhcf

    6-lead motor connections

    If a single voltage, 3-phase, 3-lead motor is designed as delta winding but connected in the field as star - what is the impact to the motor? Lower torque? What if vice versa - motor designed as wye but connected at field as delta. Seems this would put 1.73x design voltage onto each winding.
  13. nhcf

    CHP Grounding

    I observed an existing installation - 480/277 VAC wye generator, 1KW, operating in parallel with facility distribution/utility. It appeared the neutral in the generator was bonded to ground. It also appeared (based on site drawings) that utility transformer N was also bonded. Generally only...
  14. nhcf

    Standby Generator - Allowable Frequency Dip - Motor Starting

    The general spec/default for max frequency dip on motor starting in gen sizing programs is 10%. I have application where software is telling me approx. 13% freq. dip when starting a large VFD driven motor. What are the potential impacts of the higher frequency dip in a standby application?
  15. nhcf

    Hard-wired 2oo3 Relay Logic

    I am looking to implement 2oo3 relay logic to shutdown process equipment. It will tie into an existing hard-wired master control circuit that is normally energized, de-energize to trip. Three output channels from an existing PLC, through interposing relays that are hardwired in 2oo3...
  16. nhcf

    Transformer Rating

    What does transformer designation OA/FA-T indicate? This is from a database and not from the actual transformer nameplate. Not sure what the 'T' signifies.
  17. nhcf

    Motor Failure - Reciprocating compressor on VFD

    Does anyone have good reference material on application of VFD on reciprocating compressor loads. Existing facility experienced a motor insulation failure on a 150 HP 480 VAC compressor. It is not clear how much engineering was applied to the initial conversion of the compressor to VFD...
  18. nhcf

    Resistive Heater - VFD

    Has anyone used a VFD to control power output to a resistive heating system? The system is currently ON/OFF control (with a contactor), approx. 50 KW, 480 VAC. A client has purchased a VFD to allow varying control to the resistive heating element. I am more familiar with SCR control in this...
  19. nhcf

    Soft Starter Voltage Drop

    A very basic question on effect of a RVSS on upstream power distribution. If an across-the-line starter draws 600% FLC, resulting in a voltage drop of 5% on upstream distribution - if we use RVSS and limit start current to 400% - does this mean voltage drop is reduced by 1/3? I am trying to...
  20. nhcf

    Overhead Electric Line Failure - Clearances

    I have a design requirement for process storage tank that it shall not be located where exposed to failure of overhead electric power lines operating at more than 600V. Is there a rule of thumb regarding clearance distances from an OH distribution or transmission line where the tank would no...

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