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Three-phase
submersible pump motors typically demand very well balanced
three-phase power in order to operate safely. The pump motor must be
small enough to fit in a well casing deep underground, so space is
at a premium. Oversizing the motor to keep it lightly loaded and
help it withstand voltage imbalance is expensive and may not be
physically possible. Consequently these motors usually operate at
maximum capacity and are very sensitive to voltage imbalance.
 Submersible
motor manufacturers like Franklin Electric may even void the
warranty of their motors if specified voltage balance is not
maintained by the power supply to the motor.
Static
phase converters should never be used to power submersible pumps
because of their poor voltage balance.
There are special static converters that incorporate auto
transformers which allow the voltage balance to be tuned to a load.
These can be used with submersible pumps as |
long as the load on the pump does not change. Even slight load changes result in significant voltage
imbalance with these converters.
Rotary
converters can be used to power submersible pumps with certain
restrictions. The converter is usually oversized by a factor
of two or three times
to help maintain voltage balance and to provide current for
starting. Most pump motor manufacturers will not allow the motor to
be operated into its service factor on these types of converters
because of the potential for motor damage from voltage imbalance.
Rotary converters also present a challenge in this type of
application because the pump almost always operates on an automatic
signal from a pressure or float switch. In this case, the rotary
converter must be left running, which wastes electricity, or
configured to turn on automatically before the pump, then turn on
the pump after a few seconds delay.
Variable
frequency drives (VFD) can be an effective phase converter for
submersible pumps with some restrictions. If a drive with a
three-phase input is used, |
it must be derated by one half
(use a 20 HP drive on a 10 HP motor).
And because the
output of the drive is not a sine wave voltage, if the distance
between the drive and the
pump motor is too great, dangerous voltage rise can occur. Filters
on the output of the drive can be added to mitigate the harmonics,
allowing longer lead lengths for deep well applications. Because
the drive is connected directly to the pump motor leads, there is no
need for a three-phase pump panel to start and protect the motor.
The drive will also start the motor by slowly ramping up the motor
speed, eliminating line disturbances on the single-phase line.
Digital
phase converters are a good choice for operating submersible
pumps because they maintain perfect voltage balance under any load
condition and with sine wave voltage, lead length between the
converter and the pump motor is not an issue.
Digital phase converters can also be configured to start and protect
the pump, eliminating the need for a three-phase pump panel.
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