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;
* 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-increasing energy 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 on the 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.
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 with two 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 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 a higher 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.