AIM Manual

Motor Application » Three-Phase Motors (Page 41)

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.

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.

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.

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

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.

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.