AC Motor Types
Introduction
AC motors can be divided into two major categories-asynchronous and
synchronous. The induction motor is probably the most common type of
asynchronous motor (meaning speed is dependent on slip). When reviewing
induction motors, there are two rating designations-NEMA and IEC.
Another type of asynchronous motor is the wound rotor motor. This type
of motor has controllable speed and torque because of the addition of a
secondary resistance in the rotor circuit. A third type of popular asynchronous
motor is the single-phase motor. The single-phase AC motor will not
be covered because of their limited use in industrial applications when
connected with variable-frequency drives.
The synchronous motor is inherently a constant-speed motor, when operated
directly across the line. This type of motor operates in synchronism
with the line frequency. Two types of synchronous motors are non-excited
and DC-excited.
The basic principles of AC induction motors have been previously covered.
In this section, attention will be given to motor designations, ratings, and
designs.
Standard AC Induction Motors (NEMA and IEC)
NEMA frame motors are in widespread use throughout U.S. industry. This
motor design was developed before the 1950s and has well served many
types of fixed-speed applications. In 1952 and 1964, NEMA evaluated
standard frame sizes and re-rated the frame standards. The result was
smaller diameter motor frames (e.g., original 326 frame prior to 1952, to a
284U frame in 1952, to a 256T frame in 1964). As the re-rating took place,
the frame sizes (numbers) were reduced, as was the amount of iron in the
stator. With less iron in the stator, less overload capability is realized compared
with the "U" frame or the original frame size.
However, with smaller-diameter frames comes more efficiency and faster
response to changes in magnetic flux. Figure 3-36 indicates the construction
of a standard AC induction motor. All the major motor components
are identified.
It should be noted that all standard motors include a small rectangular slot,
cut lengthwise in the shaft, called a keyway or keyseat. This slot includes a
tapered-cut rectangular piece of steel, call a key. The key is inserted into
the keyway and pressure-fit snugly to mechanically connect the shaft and
coupler or connection device, such as a pulley or gear.
As seen in Figure 3-36, the induction motor is a fairly simple device. However,
precision engineering is required to create small tolerances and air
gaps that will allow maximum efficiency and torque generation.
The AC induction motor (polyphase induction motor) can be divided into
five classifications, according to NEMA. The speed/torque characteristics
for each classification have been presented in an earlier section. A brief
description of each classification will be presented here, followed by a
comparison to IEC frame motors.
- NEMA design A: This type of motor has a high breakdown
torque characteristic, compared with NEMA design B motors.
These motors are normally designed for specific use, with a slip
characteristic usually less than 5%.
- NEMA design B: This type of motor is designed for general-purpose
use and accounts for the largest share of induction motors
sold. The typical slip for a design B motor is 3-5% or less.
- NEMA design C: This type of motor has a high starting torque,
with a relatively normal starting current and low slip. The type of
load applied to a design C is one where breakaway loads are high
upon start. The loads, however, would be normally run at the rated
point, with very little demand for overload.
- NEMA design D: This type of motor has a high starting torque,
high slip, but also low full load speed. Because of its high slip (5-
13%), the speed can easily fluctuate because of changes in load.
- NEMA design E: This type of motor is known for high efficiency
and is used mainly where the starting torque requirements are low.
Fans and centrifugal pumps make up the bulk of applications using
this type of motor.
Figure 3-37 indicates the NEMA designs and compares design with rated
starting current and speed.
As shown in Figure 3-37, though design E may have the highest efficiency,
it also has the highest starting current about 800%. This fact must be
reviewed when sizing the proper overload heater elements. Most standard
induction motors have closer to a 600% starting current rating.
Though NEMA motors are rated in horsepower, there are times when a
motor is specified on the basis of its frame size. NEMA supplies standard
frame designations, up to the 445T frame. Above that rating, motor manufacturers
can supply their own standards and designate the motor rating as
exceeding the NEMA ratings.
There are standard frame sizes of motors and are based on a given horsepower
or base speed. NEMA designates a foot to centerline dimension as an
indication of the frame size. There is also a designator for frame diameter.
Figure 3-38 indicates an AC induction motor, with an indication of frame
size.
Using a 324 T frame motor as an example, the motor designer designates
the shaft centerline distance to the foot at 8 inches. To figure any shaft
centerline distance to foot, divide the first two digits of the frame number
by 4 (32 ÷ 4 = 8 inches). With this information, an application engineer
can design a machine with the motor dimensions in mind. This also assists
in comparing one motor with one from another manufacturer. All motor
dimensions are standard.
Since motor dimensions are standard, so too are motor nameplate ratings.
As with DC motors, AC motor nameplates contain all the necessary information
to effectively apply the motor. Figure 3-39 is an example of a typical
AC motor nameplate.
- Frame: Indicating the frame rating per specific horsepower, given
the rated voltage and frequency (example: 256T frame).
- HP: Available horsepower at the designated voltage and frequency
ratings.
- Voltage, phase, and frequency: Designations for the rated voltage,
phase, and frequency in hertz. Many industrial motors contain
a dual-voltage rating. This means that they can be connected to two
different voltage lines. The operating voltage is designated by either
jumper strips or wire configurations that are completed in the conduit
box. Typically, NEMA frame motors are rated for 60-Hz operation.
- FL Amps: Current rating of the motor, listed in amperes. Some
nameplates indicate current rating as FLA (full load amps). This
would indicate that the motor would draw the stated amps under
rated voltage, frequency, and load. If the motor is a dual-voltage
motor, two values of amps would be listed. The first value would
coincide with the first value of voltage stated. The second value
would coincide with the second voltage value listed. (Example: A
230/460V motor may indicate nameplate amps of 68/34 amps. The
motor would draw twice the amps on the 230-V connection, compared
with the 460-V connection.)
- rpm: This is the motor speed in rpm at base speed. Base speed is
indicated as rated voltage, frequency, and load. Due to less slip, an
unloaded motor speed would rise from this speed to close to synchronous
speed.
- Design and insulation class: The design class would indicate the
NEMA designation for A, B, C, D, or E. Typically, the insulation
class would indicate the temperature capability of the stator winding
insulation. For example, a common designation of Class B
insulation would allow for a maximum temperature rise of 130°C
(266°F). A Class H insulation would allow for a maximum temperature
rise of 180°C (356°F). Temperature rise means the amount of
temperature increase, above the normal ambient rating of 40°C
(104°F).
An additional classification is now being included with motors that of the
electrical strength of the stator winding insulation (referred to a dielectric
strength). AC motors applied to variable-frequency drives run the risk of
possible insulation damage from the power conversion technology in the
drive. Voltage stress beyond the rating can cause microscopic pin holes in
the insulation, which could result in an open phase and eventual motor
failure. Motors designated as inverterduty have the electrical insulation
strength to avoid failure due to drive technology issues.
NEMA MG-1, Part 31 standards indicate that motors operated on 600 V or
less drives should be capable of withstanding peak voltage of 1600 V.
Motor cable length and drive carrier (switch) frequency also play a part in
the possible damage to a motor's insulation strength. Motors with a 1200-V or 1000-V insulation strength should not be applied to AC drives unless
additional precautions are taken. Special drive output filters will reduce
the effects of high-peak voltages and lower the risk of insulation failure.
The motor manufacturer should always be consulted when questions arise
regarding the insulation strength of the windings. The manufacturer can
make recommendations as to additional safeguards that may be needed to
increase motor life when connected to a drive.
- Duty and S.F. (Service Factor): Most standard AC motors list
duty as "continuous" or "intermittent." The service factor of the motor is the multiplier or additional safe power loading above the
rating. Small fractional horsepower motors may carry a service factor
of 1.4, while larger integral horsepower motors may list only
1.15 service factor. For example, a 1.15 S.F. would indicate a
motor's capability of 15% additional horsepower output, above the
rating. A 1.4 S.F. would indicate 40% additional horsepower output.
- Efficiency and Ambient: Many motors may list a designation of
premium efficiency. In addition, an actual number may be referenced,
such as 89.5. The efficiency is closely tied with the NEMA
classification, such as design A, B, C, etc. The motor manufacturer
will acquire the rating from an independent testing agency. The
ambient temperature is the maximum normal operating temperature,
below the amount indicated in the temperature insulation
class.
Not all AC motors contain every piece of data listed above. But all motor
nameplates would indicate the most important information, such as voltage,
frequency, amps, and rpm. This information is required by an AC
drive, in order for the drive to match internal diagnostics with the motor
data.
Some motor nameplates indicate a wiring diagram for the dual voltage
windings; others have a sticker or label inside the conduit box, stating the
wiring connections. Some of the new motors manufactured today indicate
the dielectric strength of the insulation or mounting design.
AC Motor Types
Introduction
AC motors can be divided into two major categories-asynchronous and
synchronous. The induction motor is probably the most common type of
asynchronous motor (meaning speed is dependent on slip). When reviewing
induction motors, there are two rating designations-NEMA and IEC.
Another type of asynchronous motor is the wound rotor motor. This type
of motor has controllable speed and torque because of the addition of a
secondary resistance in the rotor circuit. A third type of popular asynchronous
motor is the single-phase motor. The single-phase AC motor will not
be covered because of their limited use in industrial applications when
connected with variable-frequency drives.
The synchronous motor is inherently a constant-speed motor, when operated
directly across the line. This type of motor operates in synchronism
with the line frequency. Two types of synchronous motors are non-excited
and DC-excited.
The basic principles of AC induction motors have been previously covered.
In this section, attention will be given to motor designations, ratings, and
designs.
Standard AC Induction Motors (NEMA and IEC)
NEMA frame motors are in widespread use throughout U.S. industry. This
motor...
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