Grundfos MS 4000, Руководство по эксплуатации продукта

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18
Selection
MS Motors
Most electrical utilities guarantees voltages to a +/-5
percent standard; for "480" service voltage will be
between 456 V and 504 V at the meter; for "240"
service, the voltages must be between 228 V and
252 V.
If a motor is damaged as a result of over or under
voltage outside the service limits, the utility may be
liable for damages.
Because motors will operate cooler with higher volt-
ages, reasonable over voltage levels rarely causes
problems. There are only small variations in power
factor and efficiency near rated conditions, volt- amps
for a particular load can be assumed constant over the
range of voltage guarantee by the utility.
The maximum continuous load sustained by a motor is
indicated by the service factor. A motor with a service
factor of 1.15 can maintain a 115% overload; provided
voltages are at the rated level and well balanced and
the insulation system can be maintained at or below
rated temperature. The actual motor load percentage
can be calculated using the formula listed below:
where; Em = motor efficiency
IHp = Input Horsepower
Motor design and economic criteria have forced motor
manufactures to build less service factor (SF) into
motors.
The SF allows the motor to provide power under
optimal conditions at the nameplate rated power times
the SF. At rated conditions, (ie. 100 Hp motor with a SF
of 1.15 is designed to provide 115 HP under continuos
load).
A 1982 survey of motor manufacturers showed six of
seven respondents recommending loading at 100
percent of rated power or less while only one still
suggests loading up to SF rating.
For this reason, it is recommended that motor loading
not exceed 100% of the nameplate horsepower rating.
It is best to consider the SF as a contingency against
over loading as a result of low voltage, current imbal-
ance and/or adverse ambient conditions.
Motor Efficiency
An electric motor operates at a relatively constant effi-
ciency and speed over a wide range of loadings.
Efficiency does not change significantly with age of the
motor or the load applied to it.
Motor efficiency is practically constant at motor loads
between 50 and 100%.
Reducing motor size for the sake of energy conserva-
tion, as a result of efficiency increases associated with
loading the motor closer to full magnetic saturation
(100% load) is not recommended.
As a general rule, a bigger motor that is underloaded
(down to 50 percent) is more efficient than a fully
loaded smaller motor driving the same load. Submers-
ible pump motors will have slightly lower efficiencies
than surface motor as a result of the compact design
requirements and the need for internal cooling/lubri-
cating fluid.
Most submersible motors have an efficiency stamped
on the nameplate. The average or nominal efficiency
values associated with "canned/ hermetically sealed"
type submersible motors are listed in the Electrical Data
Section.
% Motor Load
EM IHp
×
Rated HP
--------------------------
100
×
=
MS.book Page 18 Wednesday, January 26, 2005 3:25 PM