Industrial AC motors 1/3 to 1000 HP.
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POWER SOURCES: The common power source in the United States is
alternating current, 60 hertz. The power supply may be either single phase or three phase.
Generally speaking, single phase is used on electric motors that are less than 1
horsepower. Motors 1 horsepower and larger should be supplied with three phase power if it
is available. Generally available power supply voltages for single phase are 120 volt and
240 volt. 120 volts is generally used on motors no larger than 1/3 horsepower. Three phase
voltages generally available are 208 volt, 240 volt, 480 volt and 600 volt. Since there is
a voltage drop between the power source and the electric motor, the single phase motors
are rated either 115 volt or 230 volt. Three phase motors are rated 200 volt, 230 volt,
460 volt and 575 volt. Since it is rather easy to build a motor suitable for use on two
voltages as long as they are in the ratio of 2-to-1, it is common to specify single phase
motors 115/230 volt. It is also common to specify three phase motors 230/460 volt.
Occasionally three phase motors are designated 200-230/460 volt. This means that the motor
can be operated on 200 volts when it is connected for 230 volts. When the 230 volt motors
operated on 200 volts, there is always a sacrifice in torque capability and usually a
sacrifice in overload capability.
RATINGS: Motors are rated in horsepower and RPM. The application
always determines the horsepower load requirement. The RPM is determined by the load and
the drive system which is used to connect the motor to the load. In addition to the
horsepower and RPM, consideration should also be given to duty cycle, torques, ambient
temperature and service factor. Horsepower ratings of motors have been standardized by
NEMA. Figure 1, which is an excerpt from the NEMA Standards, lists the horsepower ratings
available. The most commonly used motors are squirrel cage induction motors. The
synchronous speed of a motor is determined by the power supply and the number of poles
built into the structure of the winding. With a 60 cycle power supply the synchronous
speeds available are 3600 RPM, 1800 RPM, 1200 RPM and 900 RPM. Induction motors develop
their torque by operating at a speed which is slightly less than synchronous speed.
Typically they will operate at 3500 RPM, 1750 RPM, 1160 RPM or 875 RPM.
AMBIENT: The ambient conditions are important in selecting the type
of motor. Ambient temperature is the temperature of the air surrounding the motor. Special
lubricants and insulation may be necessary for either very high or very low ambient
temperature. High moisture, humidity, and/or corrosive environments also must be
considered when specifying a motor with specific applications. Standard motors are
designed to operate in an ambient temperature of up to 40 degrees C (104 degrees F) and
are supplied with standard high temperature grease . At altitudes greater than 3300 feet,
the lower density of the air reduces the motors cooling ability so altitude as well as the
ambient temperature must be taken into consideration. Refer to the paragraph dealing with
service factors for more information on higher altitude applications.
TORQUE: The turning force which a motor develops is known as torque.
The amount of torque necessary to start a load (starting torque) is usually different from
the torque required to keep the load moving (full load torque). Loads which have a high
breakaway friction or that require extra torque for acceleration, should have a motor
specified to have high starting torque. NEMA specifies design letters to indicate the
torque, slip and starting characteristics of three phase induction motors. They are as
follows: Design A motors are similar to Design B motors except that starting currents are
not limited for Design A motors by NEMA. Design B is the general purpose design used for
industrial motors. This design has low starting current and normal torques and slip
(approximately 3%) which can be used for many types of industrial loads. Design C motors
have high starting torque, low starting current, and also have low slip. This design is
good for hard to start loads. Design D have very high starting torque, high slip, and low
starting current. Design D motors are available in 5 to 8% slip and 8 to 13% slip.
SERVICE FACTOR: The service factor shown on the motor nameplate
indicates the amount of continuous overload the motor can be subjected to, under nameplate
conditions, without damaging the motor. When the voltage and frequency are at the same
values as shown on the motor nameplate, the motor may be overloaded up to the horsepower
indicated by multiplying the rated horsepower by the service factor. When operated at
service factor load, the motor may have an efficiency, power factor, and speed slightly
different from those shown on the nameplate. Service factor can also be used to determine
if a motor can be operated continuously at altitudes higher than 3300 feet satisfactorily.
At altitudes greater than 3300 feet, the lower density of air reduces the motor's cooling
ability thereby causing the temperature of the motor to be higher. This higher temperature
is compensated for by reducing the effective service factor to 1.0 on motors nameplated
with a 1.15 service factor or greater. If the motor is operated outdoors at higher
altitudes. it's sometimes possible to use full horsepower and full service factor since
ambient temperatures are usually lower at those altitudes.
DUTY CYCLE: A motor should be rated continuous duty if it operates
at full load for 60 minutes or more in any 24 hour period. If the motor operates less than
60 minutes, it may be given an intermittent duty rating or a short time rating. In either
case, the time designated is that time which will elapse before the motor reaches full
MOUNTINGS: The most popular motor-mounting style for industry is
rigid base. This basic mounting is readily available in all frame sizes and enclosures. It
is adaptable to direct-connected loads and to belt or chain driven loads, The most common
industrial motor will be arranged with the output shaft, base and conduit box located as
in Figure 4 and Figure 5. This is called the F-1 mounting position. Other mounting
configurations can be found in NEMA publication MGI-4.03. Other popular mountings for NEMA
rated motors are resilient mount, C-face mount and D-flange mount. Resilient mounted
motors are normally used on fan applications or where vibration isolation is desirable.
They are generally available up through 2 horsepower. C-face motors are designed to have
something mounted to them such as a gearbox or a pump. C-face motors have a pilot
concentric with the shaft and have threaded holes in the face. D flange motors are
designed to be mounted to something else. They have a pilot concentric with the shaft and
clearance holes in the flange. Refer to Figure 9.
ENCLOSURES: The two most common types of enclosures for electric
motors are open drip proof (ODP) and totally enclosed fan cooled (TEFC). The open drip
proof motor allows a free exchange of air from the surroundings to the inside of the
motor. See Figure 5. The totally enclosed fan cooled motor (Figure 4) limits exchange of
ambient air to the inside of the motor, thus keeping dirt and water out of the motor.
Other types are totally enclosed non-ventilated, totally enclosed air over, and explosion
proof. Selection is determined by the environment.
INSULATION: The common insulation classes used in electric motors at
the present time are Class B, Class F and Class H. The present frame size assignments are
based on Class B insulation. It is the predominant class of insulation used in motor
manufacturing today. Based on a 40 degrees C ambient temperature, the Class B insulation
is suitable for 80 degrees C rise by resistance. Class F insulation is suitable for 105
degrees C rise by resistance. Class H insulation is suitable for 125 degrees C rise by
resistance. Use of Class F insulation or Class H insulation can increase the service
factor or the ability to withstand high ambient temperature conditions.
FRAMES AND DIMENSIONS: Standard frame size assignments based on
horsepower and speed are shown in Figure 1. Standardized frame sizes throughout the
industry result in common dimensions for shaft diameter. shaft height, shaft length, bolt
hole spacing and location. In addition to standard foot mount dimensions, there are
standardized dimensions for C-face, D-flange, P base, JM and JP pump mounts and west coast
EFFICIENCY: The efficiency of an electric motor is the usable
horsepower that you get out of the motor as a percent of the power that goes into the
motor. Unused energy is converted to heat in the motor. The user pays for the energy that
goes into the motor but only gets benefit from the output of the motor. The difference,
the losses, are consumed and paid for with no benefit received Energy efficiency is always
important since the losses are paid for whenever the motor is running. Energy efficiency
is particularly important if power costs are high or if the motor operates for long
periods of time. Smaller motors are generally less efficient than larger motors. Motors
operated at less than half load are usually inefficient. The Figure 3 will allow you to
make an easy determination of whether or not the higher cost for a premium efficiency
motor can be quickly recovered.
V-BELT APPLICATION: Other than direct coupling, v-belt drives are
the most common and easiest way of connecting a motor to the load. Standard bearings in
the electric motor will handle the radial load imposed by a v-belt drive provided the
v-belt drive is designed in accordance with Figure 6.
MULTISPEED MOTORS: Multispeed motors are motors that are
reconnectable so that they can be operated at more than one speed. Multispeed motors can
be either single winding or two winding. The single winding motors are reconnectable in
such a way that they can provide two speeds that are in the ratio of 2-to-1. Two winding
motors have two separate windings that can be wound for any number of poles so that speed
ratios other than 2-to-1 can be obtained. Ratios greater than 4-to-1 are usually
impractical due to the size and weight of the motor. Single phase multispeed motors that
operate with a centrifugal switch are usually impractical. Power output of multispeed
motors can be proportioned for each different speed. These motors are designed with output
horsepower capacity in accordance with one of the following load characteristics.
Variable-torque motors have a speed torque characteristic that varies as the square of the
speed. For example, an 1800/900 RPM motor that develops 10 HP at 1800 RPM produces 2.5 HP
at 900 RPM. Since some loads such as centrifugal pumps, fans, and blowers have a torque
requirement that varies as the square of the speed, this motor design is adequate for
them. Constant-torque motors can develop the same torque at each speed, thus power output
varies directly with speed. For example, a two speed motor rate at 10 HP at 1800 RPM would
produce 5 HP at 900 RPM. These motors are used in applications with constant torque
requirements such as mixers, conveyors, and positive displacement compressors.
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