Ingeniero

Motor Specifications

Nameplate

The nameplate of a motor provides important information
necessary for selection and application. The following drawing
illustrates the nameplate of a sample 30 horsepower AC motor.
Specifications are given for the load and operating conditions as
well as motor protection and efficiency.

Voltage and Amps

AC motors are designed to operate at standard voltages
andfrequencies. This motor is designed for use on 460 VAC
systems. Full-load current for this motor is 34.9 amps.

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RPM

Base speed is the nameplate speed, given in RPM, where
the motor develops rated horsepower at rated voltage and
frequency. It is an indication of how fast the output shaft will
turn the connected equipment when fully loaded with proper
voltage and frequency applied.

The basespeed of this motor is 1765 RPM at 60 Hz. It is known
that the synchronous speed of a 4-pole motor is 1800 RPM.
When fully loaded there will be 1.9% slip. If the connected
equipment is operating at less than full load, the output speed
(RPM) will be slightly greater than nameplate.

Service Factor

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A motor designed to operate at its nameplate horsepower
rating has a service factor of 1.0. Thismeans the motor can
operate at 100% of its rated horsepower. Some applications
may require a motor to exceed the rated horsepower. In these
cases a motor with a service factor of 1.15 can be specified. The
service factor is a multiplier that may be applied to the rated
power. A 1.15 service factor motor can be operated 15% higher
than the motor’s nameplate horsepower. The 30 HP motor with
a 1.15service factor, for example can be operated at 34.5 HP
.
It should be noted that any motor operating continuously at a
service factor greater than 1 will have a reduced life expectancy
compared to operating it at it’s rated horsepower. In addition,
performance characteristics, such as full load RPM and full load
current, will be affected.

Class Insulation

The National Electrical ManufacturersAssociation (NEMA)
has established insulation classes to meet motor temperature
requirements found in different operating environments. The
four insulation classes are A, B, F and H. Class F is commonly
,
used. Class A is seldom used. Before a motor is started, its
windings are at the temperature of the surrounding air. This is
known as ambient temperature. NEMA has standardized on
an ambienttemperature of 40° C, or 104° F within a defined
altitude range for all motor classes.

Temperature will rise in the motor as soon as it is started.
Each insulation class has a specified allowable temperature
rise. The combination of ambient temperature and allowed
temperature rise equals the maximum winding temperature
in a motor. A motor with Class F insulation, for example, has a
maximum temperaturerise of 105° C when operated at a 1.0
service factor. The maximum winding temperature is 145° C
(40° ambient plus 105° rise). A margin is allowed to provide
for a point at the center of the motor’s windings where the
temperature is higher. This is referred to as the motor’s hot
spot.

The operating temperature of a motor is important to efficient
operation and long life. Operating a motor abovethe limits of
the insulation class reduces the motor’s life expectancy. A 10° C
increase in the operating temperature can decrease the motor’s
insulation life expectancy as much as 50%.

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Motor Design

The National Electrical Manufacturers Association (NEMA) has
established standards for motor construction and performance.
NEMA design B motors are most commonly used.

Efficiency

AC motorefficiency is expressed as a percentage. It is an
indication of how much input electrical energy is converted to
output mechanical energy. The nominal efficiency of this motor
is 93.6%. The higher the percentage the more efficiently the
motor converts the incoming electrical power to mechanical
horsepower. A 30 HP motor with a 93.6% efficiency would
consume less energy than a 30 HP motor with an...