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MAINTAINING
ELECTRIC MOTORS
USED FOR IRRIGATION
F. Richard Beard, Agricultural Equipment,
Structures and Electricity
Robert W. Hill, Biological & Irrigation
Engineering
August 2000
ENGR/BIE/WM/06
Electricity is a clean, economical and dependable source of power for irrigation. Electric
motors can provide years of service when properly selected, operated and maintained. This fact
sheet describes factors that affect electric motor performance and service life and describes
procedures for controlling internal motor heat.
PROPER MOTOR SELECTION
Electric motors are sized (rated) to operate under a standard set of conditions. Motors
must be selected for different applications based on nameplate ratings. The nameplate describes
the operating parameters for an electric motor and communicates this information to the user. If a
40 horsepower (HP) motor is overloaded (accidentally used to drive a load larger than 40 HP or
operated at less than rated voltage), the motor will draw excessive amperage in an attempt to
provide the necessary power to drive the load. When an overload exceeds the nameplate rating,
the motor will run hotter than its design operating temperature. This increase in temperature
deteriorates motor winding insulation and shortens motor life. Motor conductors and insulation
are not designed to power loads larger than the nameplate ratings.
FACTORS AFFECTING ELECTRIC MOTOR LIFE
The life expectancy of electric motors are based on several factors. The following
operating conditions affect motor life:
d
ambient or surrounding air temperature
d
elevation above sea level
d
ventilation
d
service factor (indicates amount of overload the motor can tolerate)
d
operating voltage
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Other factors affecting motor life include:
moisture, corrosive environment (salt water), motor
enclosure, maintenance, type of bearings, single or
three phase power, motor type and duty rating.
Under normal operating conditions the
expected life of a motor may be 5 to 10 years. The
operating life of a motor may be extended to 20 years
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