19
Application and Selection Issues
The term application not only refers to the end use of
the product but also the parameters which affect the
selection of the correct submersible motor and pump
products. The primary considerations involved with the
selection of submersible motors are discussed as
follows:
• The Insulation System. The insulation system is
the key to long motor life. The life of the insulation
system is affected by three major factors: Load,
Duty Cycle, and Temperature Rise. The load of a
motor is described in horsepower or kilowatts and is
defined as the work required to perform a function.
The load created by pumps is a result of the rotation
of impeller(s) to create a pressure forcing fluid
through a system. The duty cycle is the time period,
which the motor is operating. It is continuous or in-
termittent. Temperature rise is the difference be-
tween the operating temperature of the windings
and the temperature of the medium to cool the mo-
tor. The rise of the motor is directly affected by the
load and duty cycle. Extra load in the form of a ser-
vice factor increases the temperature rise of the
winding.
The total temperature must never exceed the maximum
capacity of the insulation system. Submersible motors
used for water well service normally employ class “F”
insulation (150°F rise), but are designed for a class A
temperature rise (60°F).
• Cooling. Submersible motors are no different than
conventional motors, in that the heat generated
within the motor must be dissipated. The tempera-
ture rise within the motor is limited to a value which
when added to the temperature of the external cool-
ing medium does not exceed the maximum temper-
ature capacity of the insulation system. The ability
to dissipate the heat depends on two factors: (1)
The temperature of the cooling medium (ambient)
and (2) the rate of cooling medium flow past the mo-
tor external surfaces. Excess ambient temperature
and reduced flow rate both require derating of the
load capability of the unit. The derating of the load
reduces the temperature rise of the winding within
the limits set by the heat dissipation capacity of the
cooling medium.
• Materials of Construction. Submersible pumps
and motors are also selected based on the chemical
and physical make-up of the water in which they will
be submerged. Sea water applications require spe-
cialized construction due to the corrosive water en-
countered. A standard motor will not survive highly
corrosive water submergence, while a specially de-
signed motor will.
• Design Factors. Other factors, which affect sub-
mersible motor selection, are voltage, depth of in-
stallation, thrust and controls. It is necessary that
the voltage and frequency variations be within the
limits set in NEMA MGI-18 (submersible motors for
deep well pumps). The maximum recommended
depth for most submersible motors relates to 290
psi on the unit (approx. 2000 feet). The thrust deliv-
ered by the pump must be less than the capacity of
the thrust bearing of the submersible motor. Con-
trols must be quick trip, ambient compensated type
to quickly pull an improperly applied or defective
motor off the line so that no damage occurs.
Submersible construction and design for 4"and 6" sizes
are covered by NEMA standards.
Selection
MS Motors
MS.book Page 19 Wednesday, January 26, 2005 3:25 PM
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