AIM Manual

Motor Application » Three-Phase Motors (Page 40)

Variable Speed Operation

Franklin Electric’s three-phase, encapsulated submersible motors can be used with variable frequency drives (VFD) when applied within the guidelines below.

All three-phase, encapsulated submersible motors must have the VFD sized based on the motor’s nameplate maximum amps, not horsepower. The continuous rated amps of the VFD must be equal to or greater than the motor’s nameplate maximum amps or warranty will be void.

Franklin Electric’s single-phase, 2- and 3-wire, encapsulated submersible motors can only be used with the appropriate Franklin constant pressure controller.

Franklin Electric’s submersible motor Application, Installation, Maintenance (AIM) manual should be checked for the latest guidelines and can be found online at

WARNING: There is a potential shock hazard from contact with and/or touching the insulated cables connected to the variable frequency drive output anytime the motor has energy applied.

Output Filter Requirement Test:

NOTICE: An incoming power supply or line-side filter for the drive does not replace the need for additional output filters.

An output filter is required if the answer is yes to one or both of the items below:
#1 - Does the peak voltage at the motor terminals exceed 1000-volts or is the rise time of the VFD's voltage less than 2 micro-seconds? Per NEMA MG 1-2011, the rise time is defined as the time between 10% and 90% of the steady-state voltage
(i.e., DC bus voltage).

#2 - Is the motor nameplate voltage more than 379 Volts and is the cable from
drive-to-motor more than 50 ft (15.2 m)?

More than 99% of the drives applied on water well submersible motors will require the purchase of additional output filtering based on question #1.

Output filters can be expensive. However, when needed, it is required for the motor to be considered for warranty. Make sure this item is not overlooked when quoting a job.

PWM dV/dt value can be defined as: the rate at which voltage is changing with time or how fast the voltage is accelerating. This information can be supplied by the drive manufacturer or the manufacturer’s drive specification sheet. The dV/dt value cannot be measured with typical field equipment, even when using a true-RMS voltage/amperage multi-meter.

Franklin Electric has a line of VFDs that are specifically designed for Franklin application systems. These VFDs are used in the MonoDrive and SubDrive constant pressure systems. Franklin drive systems have the required additional output filtering installed; however, the SubDrive HPX does not.

Types of Output Filters:

A resistor-inductor-capacitor (RLC) filter has both a high pass filter & a low pass filter section and are considered the best practice, but a high pass reactor filter is also acceptable.

Filters should be recommended by the drive manufacturer; for the correct recommendations provide them with answers to all five of the items below.

(1) VFD model (2) Carrier frequency setting (3) Motor nameplate voltage (4) Motor nameplate max amps (5) Cable length from the drive output terminals to the motor

Input Current & Motor Overload Protection:

  • Motor input current should be set at the system’s typical operating current when running at nameplate rated voltage and frequency (Hz).
  • Motor overload protection should be set to trip at 115% of the system’s typical operating current.
  • Motor overload protection must trip equal to or faster than NEMA Class 10 motor overload curve requirements.

Motor Maximum Load Limits:

  • The system must never operate in excess of the motor nameplate maximum amps.
  • On 50 Hz motors, nameplate amps are maximum amps as these motors have a 1.0 service factor.