AIM Manual

Motor Application » Three-Phase Motors (Page 41)

Variable Speed Operation (Continued)

Motor Operating Hertz, Cooling Requirements, and Underload Settings:

  • Standard practice for large VFD installations is to limit the operation to 60 Hz max. Operating at greater than 60 Hz requires special system design considerations.
  • The motor must never operate below 30 Hz. This is the minimum speed required to provide correct bearing lubrication.
  • The motor’s operating speed must always operate so the minimum water flow requirements of 0.5 ft/sec for 6-inch & 8-inch motors and 0.25 ft/sec for 4-inch motors is supplied.
  • The motor underload protection is normally set to trip at 80% of the system’s typical operating current. However, the underload trip point must be selected so that minimum flow requirements are always met.

Starting and Stopping Ramp Settings:

  • The motor must reach or pass the 30 Hz operating speed within 1 second of the motor being energized. If this does not occur, the motor bearings will be damaged and the motor life reduced.
  • The best stopping method is to turn power off followed by a natural coast to stop.
  • A controlled stop from 30 Hz to 0 Hz is allowed if the time does not exceed 1 second.

Drive Carrier Frequency:

  • The carrier frequency is set in the field. The drive typically has a selectable rangebetween 2k and 12k Hz. The higher the carrier wave frequency setting, the greater the voltage spikes; the lower the carrier wave frequency setting, the rougher/poorer the shape of the power curve.
  • The carrier frequency should be set within the range of 4k to 5k Hz for encapsulated submersible motors.

Application Function Setting:

  • If the VFD has a setting of centrifugal pump or propeller fan it should be used.
  • Centrifugal pumps and fans have similar load characteristics.

VFD Frequency of Starts:

  • Keeping the starts per day within the recommended numbers shown in the frequency of starts section of the AIM manual provides the best system life. However, since in-rush current is typically reduced when used with a properly configured VFD, large 3-phase submersible motors can be started more frequently. In all cases a minimum of 7 minutes must be allowed between a power off and the next restart attempt or consecutive restart attempts.

NEMA MG1 Above Ground Motor Standard Comments:

  • Franklin Electric encapsulated submersible motors are not declared inverter duty motors by NEMA MG1 standards. The reason is NEMA MG1 standard part 31 does not include a section covering encapsulated winding designs.
  • Franklin submersible motors can be used with VFDs without problems or warranty concerns providing Franklin's Application, Installation, Maintenance (AIM) manual guidelines are followed. See Franklin's on-line AIM manual for the latest guidelines.