Variable Speed Submersible Pump Operation, Inverter Drives |
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Franklin three-phase submersible motors are operable
from variable frequency inverter drives when applied
within guidelines below. These guidelines are based
on present Franklin information for inverter drives, lab
tests and actual installations, and must be followed
for warranty to apply. Franklin two-wire and three-wire
single-phase submersible motors are not recommended
for variable speed operation.
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| WARNING: There is a potential shock hazard from
contact with insulated cables from a PWM drive to
the motor. This hazard is due to high frequency
voltage content of a PWM drive output.
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Load Capability: Pump load should not exceed motor nameplate service factor amps at rated voltage and frequency.
Frequency Range: Continuous between 30 Hz and rated frequency (50 or 60 Hz). Operations above rated frequency require special considerations, consult factory for details.
Volts/Hz: Use motor nameplate volts and frequency for the drive base settings. Many drives have means to increase effi ciency at reduced pump speeds by lowering motor voltage. This is the preferred operating mode.
Voltage Rise-time or dV/dt: Limit the peak voltage to the motor to 1000 V and keep the rise-time greater than 2 µsec. Alternately stated: keep dV/dt < 500 V/µsec. See Filters or Reactors below.
Motor Current Limits: Load no higher than motor nameplate service factor amps. For 50 Hz ratings, nameplate maximum amps are rated amps. See Overload Protection below.
Motor Overload Protection: Protection in the drive (or separately furnished) must be set to trip within 10 seconds at 5 times motor maximum nameplate amps in any line, and ultimately trip within 115% of nameplate maximum amps in any line.
Subtrol-Plus and SubMonitor: Franklin’s Subtrol-Plus and SubMonitor protection systems ARE NOT USABLE on VFD installations.
Start and Stop: One second maximum ramp-up and ramp-down times between stopped and 30 Hz Stopping by coast-down is preferable.
Successive Starts: Allow 60 seconds before restarting.
Filters or Reactors: Required if all three of the following conditions are met: (1) Voltage is 380 or greater and (2) Drive uses IGBT or BJT switches (rise-times < 2 µsec) and (3) Cable from drive to motor is more than 50 ft (15.2 m) A low-pass filter is preferable. Filters or reactors should be selected in conjunction with the drive manufacturer and must be specifically designed for VFD operation.
Cable Lengths: Per Franklin’s cable tables unless a reactor is used. If a long cable is used with a reactor, additional voltage drop will occur between the VFD and the motor. To compensate, set the VFD output voltage higher than the motor rating in proportion to the reactor impedance (102% voltage for 2% impedance, etc.).
Motor Cooling Flow: For installations that are variable-flow, variable-pressure, minimum flow rates must be maintained at nameplate frequency. In variable-flow, constant pressure installations, minimum flow rates must be maintained at the lowest flow condition. Franklin’s minimum flow requirements for 4” motors : 0.25 ft/s (7.26 cm/sec) and for 6” and 8” motors: 0.5 ft/s (15.24 cm/sec).
Carrier Frequency: Applicable to PWM drives only. These drives often allow selection of the carrier frequency. Use a carrier frequency at the low end of the available range.
Miscellaneous: Franklin Electric three-phase motors are not declared “Inverter Duty” motors per NEMA MG1 standards. The reason is Part 31 of NEMA standard MG1 does not include a section covering encapsulated winding designs. However, Franklin submersible motors can be used with VFDs without problems and/or warranty concerns provided these guidelines are followed.
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