Engineering Data

VFD Operation of NORTHAM Rewindable 3 phase Submersible Motors

All three-phase, rewindable 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.

Output Filter Requirement Test:

An output filter is required if the answer is yes to one or both of the items below:

• 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? The rise time is defined as the time between 10% and 90% of the steady-state voltage
  (i.e., DC bus voltage).
• Is the motor nameplate voltage more than 379 Volts and is the cable from drive-to-motor more than 50 ft (15.2 m)?

NOTICE: 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 during installation.

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.

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.

REQUIRED ITEMS FOR PROPER VFD FILTER SIZING

• VFD model
• Carrier frequency setting
• Motor nameplate voltage
• Motor nameplate max amps
• 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

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 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 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 NORTHAM Submersible Motor catalogue 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.