guidelines motor application designs
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May 1, 1996
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Guidelines for motor application designs.Robert J. Lawrie |Electrical Construction and Maintenance
Effective selection, application, operation, and maintenance of modern motors require a strong working
knowledge of rotating-machine basics as well as an in-depth awareness of the latest technical
developments.Modern motor application designs have become more complex than ever before because of
the emergence of premium efficiency (PE) motors. The Energy Policy Act of 1992 (EPACT) requires that the
Effective selection, application, operation, and
maintenance of modern motors require a strong working
knowledge of rotating-machine basics as well as an in-
depth awareness of the latest technical developments.
Modern motor application designs have become more
complex than ever before because of the emergence of
premium efficiency (PE) motors. The Energy Policy Act of
1992 (EPACT) requires that the most frequently used
motors - squirrel-cage induction motors - no longer be
manufactured after October 24, 1997. As a result, you must give a careful look to any
application that calls for the use of such motors because their characteristics are different
from those of standard induction motors.
Not all induction motors are affected. Specifically, the law applies to general-purpose,
T-frame, single-speed, foot-mounted, polyphase NEMA Design A and B, continuous
rated, 230/460V, 60 Hz motors in sizes from 1 to 200 hp. These motors are used in 70%
to 80% of all motor applications.
Although synchronous, wound-rotor, single-phase, DC motors, and special motors are
widely used, squirrel-cage induction motors are the focus of this report.
Motor application
The primary concern when designing a motor circuit is the application at hand, and the
type of motor to do the job. A great many factors are involved when selecting a motor;
these include horsepower, torque, speed, frequency, load variations, efficiency, and
numerous installation considerations such as environment, enclosures, and mounting.
Also important are the type of drive, motor starting method, and available voltage.
Special concerns. It's important to emphasize early on in your design process that special
care be taken where PE motors are involved. You should be sure of the following items:
* The application warrants a PE motor, at least until standard motors are no longer
available;
* Depending on load, the appropriate NEMA design letter (A, B, C, D, or E) is best for the
job;
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* Locked-rotor, breakdown torque, and starting current (which can be particularly high
for the newest NEMA Design E motor) are double checked;
* The proper size and type of motor starter are used.
You should also check that the slightly higher speed of most PE motors will not affect the
application. This is particularly important where adjustable frequency drives (AFDs) may
be incorporated, and the load may be a pump or fan.
Efficiency considerations
The operating efficiency of a motor has become a major factor because of ever-increasingenergy costs. More than half of the typical industrial user's power costs is energy
consumed by motors. This makes it essential that operating costs be considered versus
initial costs when selecting a motor.
High-efficiency motors are available having substantially lower losses than standard lines.
In recent years, most major manufacturers have standardized on the term "premium
efficiency" to define their most efficient motors. These newer motors have improved steel,
laminations and insulations, more copper, and rotor fin designs that provide more
cooling.
Efficiency of a motor is determined by a standard test called for by NEMA in its standard
MG-1-1993, Revision 1, Part 12.58. The test technique, called IEEE 112A-Method B,
provides a consistent efficiency measurement standard for those who use it. Also, CSA
Standard C390 may be used. The resultant efficiency is stamped on the motor nameplate;
this nameplate value is a nominal or average efficiency of the motor. In addition, a
minimum efficiency value may be determined and used in investment payback
calculations to obtain a conservative estimate.
The cost of a higher-efficiency motor is usually higher than a standard motor, depending
upon the quality of its design. If the motor runs continuously or at least 16 hrs per day or
more, this extra cost is usually well justified and will be returned in one to two years. In
some instances, even an 8-hr operation may result in reduced total costs that will justify
the initial premium paid for a high-efficiency motor.
Motor selection parameters
Horsepower. A fundamental first step in selecting an induction motor is to determine
[TABULAR DATA FOR TABLE 1 OMITTED] its horsepower rating so that it will drive the
load. Sometimes, this is as simple as obtaining the specifications from the nameplate onthe driven load. Possibly the rated load can be obtained from the supplier or from other
similar loads. The horsepower requirements can also be calculated from known data, or
possibly the load can be tested and the required power measured. Ideally, the motor
should be sized so the load is 75% to 95% of its rated full load. This assures high
efficiency. As a final resort, try driving the load at rated load and voltage with a motor that
appears to be about the right rating. Measure the input current and temperature rise of
the motor. This will tell if your test motor is too small or too large, and then using
common sense, the proper size motor can be determined.
Torque and speed. The hp rating of a motor also depends upon the motor rated-load
output characteristics of torque and speed. For a particular application, the motor must
have a rated-load torque to drive the machine at the required speed. However, there are
three other torque characteristics, as shown in Fig. 1, on page 80, that must be
considered:
* Locked-rotor or starting torque;
* Pull-up torque; and
* Breakdown torque.
The motor must have sufficient starting and pull-up torque to bring the driven machine to
operating speeds, and it must be able to overcome peak loads (breakdown torque) without
stalling.
You should review the Fig. 1 curve and understand it because NEMA has available
standard curves, as shown in Fig. 2, to which all NEMA-design motors must adhere. This
will enable you to effectively select the right motor for the job at hand.
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For example, if a Design B motor is used to drive a load that needs a high starting torque,
the motor may overheat during starting and trip out prior to reaching, operating speed.
When this happens, the operator may decide to defeat the motor protection, causing the
motor to burn out. Or, someone may decide to install a larger motor, which will cost more
initially and, because it's oversized, will operate inefficiently.
The differences of the curves shown in Fig. 2 are due primarily to the differences in rotor
resistance and reactance introduced during design. The curves for any specific design also
vary according to motor size. Output torque values drop as rated hp increases at any given
synchronous speed.
The Design B motor is perhaps industry's workhorse for general-purpose across-the-line
starting duty. It has a "normal" or relatively high starting torque for accelerating high-
inertia loads, and can handle short-duration overloads to 200% full-load torque or more
before reaching the breakdown point.
Where the load duty-cycle has a peak in excess of the Design B breakdown torque, a
Design A motor may be used. This type has a starting torque very close to the Design B
motor but develops a higher breakdown torque and will have a higher starting current.
The Design C motor is characterized by a low starting current and high starting torque.
It's suitable for loads requiring a high starting torque and rather rapid accelerating loads,
such as conveyors and compressors.
For extremely heavy starting conditions, the Design D motor is available.
Characteristics of the Design E motor have just been introduced by NEMA, and related
data is available in the 1996 NEC. This high-efficiency motor is just coming onto the
market and appears to be best suited for fan or pump applications because its breakdown
torque is somewhat lower than Design B, and it has high starting current. Characteristics
of NEMA design motors and their appropriate applications are shown in Table 1, on page
82.
Load variations. Where the load varies with time, a horsepower-versus-time curve will
help you determine the peak horsepower required. Calculation of the root-mean-square
horsepower will indicate the proper motor rating from a heating standpoint. In case of
extremely large variations in load, or where shutdown accelerating or decelerating
periods make up a large portion of the cycle, the horsepower may not give a true
indication of the equivalent continuous load. In situations like this, the motor
manufacturer should be consulted.
Where the load is maintained at a constant value for an extended period (varying from 15
min to 2 hrs, depending upon the size), the horsepower rating usually will not be less than
the constant value, regardless of other parts of the cycle. If the driven machine is to
operate at more than one speed, the horsepower required at each speed must be
determined.
Selecting the right motor and speed can sometimes avoid the necessity of using a speed-
control device. Constant-speed motors operate at a practically uniform speed during
normal operations. Induction motors are available from 514 rpm to 3600 rpm in the
smaller sizes. Synchronous speed ratings of integral-horsepower motors are given in
Table 2.
Multispeed motors are available for use on loads that can be most effectively operated at
two or more specific speeds. A multispeed motor can be of the single-winding type withtwo independent speeds or special 2-speed, single winding motor with flexible ratio of
low-to-high speed. Multispeed motors can be selected as either variable torque (for fans
and centrifugal pumps); constant torque (for conveyers, compressors, and positive-
displacement pumps); and constant horsepower (for winches and machine tools).
Where the application requires speed adjustment over a range, the DC drive, variable-
frequency AC motor drive, or mechanical speed changer can be provided.
Service Factor. Service Factor is defined as the permissible amount of overload a motor
can handle within defined temperature limits without overheating. When voltage and
frequency are maintained at nameplate rated values, the motor may be overloaded up to
the horsepower calculated by multiplying the rated horsepower by the service factor
shown on the nameplate. However, locked-rotor torque, locked-rotor current, and
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breakdown torque are unchanged. NEMA service factor values range from 1.00,1.15
(standard for open motors), and 1.25.
Insulation and temperature rise. The insulation of motor windings is subject to thermal
aging, and degradation of dielectric capability allows shorting to occur between
conductors and causes failure. There is a specific temperature rise that is permitted by
standards based upon the capabilities of the insulating material. A rule-of-thumb says
that for every 10 [degrees] C rise above the limit, insulation life is halved. The total
allowable temperature for different insulation classes (including ambient temperature
and temperature rise) are:
* Class A, 105 [degrees] C;
* Class B, 130 [degrees] C;
* Class F, 155 [degrees] C; and
* Class H, 180 [degrees] C.
Depending upon the method of measurement, size of motor, ambient temperature, etc.,
the permitted temperature rise will vary. However, the maximum temperature must not
be exceeded.
When designing a motor circuit and selecting an appropriate motor, it's normally not
necessary for you to indicate the type of insulation required. Class B insulation is
considered standard and most often will be supplied. Requirements such as a 1.15 service
factor for a totally enclosed motor will usually be met by the manufacturer by supplying ahigher grade of insulation. There are cases, however, when selecting a higher insulation
class is justified as a safety factor or to provide for some particular condition that may not
be adequately covered by the ambient temperature chosen. An encapsulated motor
includes more material over the windings, leading to higher-than-normal temperatures.
The increased temperature of an open dripproof motor with a 1.15 service factor can be
compensated for by reducing the service factor or by supplying a higher-rated insulation.
Permitted temperature rise of different insulations is based on operation of the motor at
altitudes of 3300 ft or less. When this elevation must be exceeded, there are several
alternatives. If the motor has 1.15 service factor, then it can be operated at unity factor at
altitudes up to 9000 ft in a 40 [degrees] C ambient.
Cycling of the load also affects the temperature of the windings. Standard motors are
rated for continuous duty; that is, the load is relatively constant for long periods of time.If the application requires that the motor be started and stopped often, or if the load is a
cyclical, duty-cycle information should be included in the specifications. Larger frame
sizes or higher-rated insulations may be required.
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