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The motor shall be designed for operation in a 40 degrees C 105 degrees F
ambient temperature and all temperature rises shall be above this ambient
temperature. The rated horsepower of the motor shall be not less than 110
percent of the determined maximum load requirement of the pump. Motors
shall have a service factor of 1.0 or shall be applied using a service
factor of 1.0 if standard service factor is greater than 1.0. The
temperature rise above the ambient temperature for continuous rated
full-load conditions and for the class of insulation used shall not exceed
the values given in NEMA MG 1, paragraph 12.42 or paragraph 20.40.
2.1.2
Operating Characteristics
2.1.2.1
Torques
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NOTE: The "Operating Characteristics" specified are
to limit the locked-rotor current to a value
sufficiently low to permit full-voltage starting.
Manufacturer's standard is to limit locked-rotor
current to 600 percent of design full-load current.
However, if local conditions are such that lower
starting current is desirable, the locked-rotor
current may be specified not to exceed 500 percent
of the design full-load current. When 500 percent
is specified, the breakdown torque of 150 percent of
full-load torque will be used. The designer will
note that these percentages are for design full-load
current at rated power factor and will vary based on
actual load and supply conditions. Designer should
consult local utility for actual requirements for
limitations on inrush currents. When inrush current
cannot be sufficiently limited by motor design, a
reduced voltage starter will be used.
When reduced voltage starting is required, closed
transition autotransformer type reduced voltage
starters should be used. These starters provide the
most flexibility during installation since both
input voltage and inrush current may be adjusted.
This specification is not meant to limit the
selection of a reduced voltage starter to only
autotransformer type. The use of a wye-delta type
starter may be appropriate in certain situations.
The reduced starter cost for use of other than
autotransformer type starters must be weighed
against the increased cost in motor designs; however
each design should be evaluated for the most
suitable type starter. All reduced voltage starters
will reduce the motor starting torque, so the
designer should evaluate the load characteristics to
ensure that motor torque will be sufficient under
all starting conditions. If reduced-kVA starters
are required, EM 1110-2-3105 should be consulted for
further guidance and possible pump design
considerations.
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SECTION 16221A
Page 10
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