Motor Operating Hertz, Cooling
Requirements & Underload
- 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
- 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
- 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 range between 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
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 seven minutes
must be allowed between a power off and the next
restart attempt or consecutive restart attempts.
NEMA MG1 Above Ground Motor
- 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
- Franklin submersible motors can be used with VFDs
without problems or warranty concerns providing the
guidelines are followed. See Franklin’s on-line
Application Installation Maintenance (AIM) manual for
the latest requirements.