Electric motors come in various shapes and sizes, with some being standardized for general-purpose applications and others intended for specific tasks. When selecting an electric motor, it is important to ensure that it satisfies the requirements of the machine on which it will be applied without exceeding the rated electric motor temperature. The following are some of the important motor and load parameters that need to be considered as part of the selection process:

  • Motor speed
  • Motor power ratings
  • Motor load torques
  • Motor losses
  • Motor efficiency

Mechanical Power Rating

The mechanical power rating of motors is expressed in either horsepower (hp) or watts (W): 1 hp = 746 W. Two important factors that determine mechanical power output are torque and speed. Torque and speed are related to horsepower by a basic formula, which states that:

Horsepower = (Torque x Speed) / Constant

where

  • Torque is expressed in lb-ft.
  • Speed is expressed in rpm.

The value of the constant depends on the units that are used for torque. For this combination, the constant is 5,252.

The slower a motor operates, the more torque it must produce to deliver the same amount of horsepower. This means that slow motors require stronger components than those of higher-speed motors of the same power rating to withstand the greater torque. As a result, slower motors are generally larger, heavier, and more expensive than faster motors with equivalent horsepower ratings.

Current

Motor current is an important consideration when sizing and protecting motor circuits. There are three main types of motor current:

  • Full-load amperes (FLA) is the amount of current the motor draws under normal operating conditions. This is the current rating listed on the motor nameplate.
  • Locked-rotor current (LRA) is the amount of current the motor draws when it is first started. This current is typically several times higher than theFLA and can last for a few seconds.
  • Service-factor amperes (SFA) is the maximum amount of current the motor can safely draw for an extended period of time. The service factor is a multiplier of the FLA that is specified on the motor nameplate.

The overload-sensing elements in a motor circuit are designed to protect the motor from damage caused by excessive current. The overload-sensing elements should be sized based on the FLA of the motor.

Here is a simplified explanation of the different types of motor current:

  • Full-load amperes (FLA): The normal operating current of the motor.
  • Locked-rotor current (LRA): The starting current of the motor, which is typically several times higher than the FLA.
  • Service-factor amperes (SFA): The maximum current the motor can safely draw for an extended period of time.

Code Letter

NEMA code letters are assigned to motors for calculating the locked-rotor current based upon the kilovolt- amperes per nameplate horsepower. Overcurrent protection devices must be set above the locked-rotor current of the motor to prevent the overcurrent protection device from opening when the rotor of the motor is starting. The letters range in alphabetical order from A to V in increasing value of locked-rotor (LR) current.

Locked-Rotor Code, kVA/hp
Locked-Rotor Code, kVA/hp
  • LR current (single-phase motors) = (Code letter value × hp × 1,000) / Rated voltage
  • LR current (three-phase motors) = (Code letter value × hp × 577) / Rated voltage

Design Letter

NEMA has defined four standard motor designs for AC motors, using the letters A, B, C, and D to meet specific requirements posed by different application loads. The design letter denotes the motor’s performance characteristics relating to torque, starting current, and slip. Design B is the most common design. It has relatively high starting torque with reasonable starting currents. The other designs are used only on fairly specialized applications.

Efficiency

Motor efficiency is the ratio of mechanical power output to the electrical power input, usually expressed as a percentage. The power input to the motor is either transferred to the shaft as power output or is lost as heat through the body of the motor. Power losses associated with the operation of a motor include:

  • Core loss, which represents the energy required to magnetize the core material (known as hysteresis) and losses owing to the creation of small electric currents that flow in the core (known as eddy currents).
  • Stator and rotor resistance losses, which represent the I2R heating loss due to current flow (I) through the resistance (R) of the stator and rotor windings, and are also known as copper losses.
  • Mechanical losses, which include friction in the motor bearings and the fan for air cooling.
  • Stray losses, which are the losses that remain after primary copper and secondary losses, core losses, and mechanical losses. The largest contributor to the stray losses is harmonic energy generated when the motor operates under load. This energy is dissipated as currents in the copper windings, harmonic flux components in the iron parts, and leakage in the laminate core.

Energy-Efficient Motors

The efficiency of electric motors ranges between 75 and 98 percent. Energy-efficient motors use less energy because they are manufactured with higher-quality materials and techniques, as illustrated in Figure 5-66. To be considered energy-efficient, a motor’s performance must equal or exceed the nominal full-load efficiency values provided by NEMA in publication MG-1.

Typical energy-efficient motor

Frame Size

Motors are available in different frame sizes to suit the needs of various applications. Typically, the frame size increases with increasing horsepower or decreasing speeds. To promote standardization in the motor industry, NEMA has established standard frame sizes for certain dimensions of standard motors. For instance, a motor with a frame size of 56 will always have a shaft height above the base of 3½ inches.

Frequency

The frequency of the line power supply for which an AC motor is designed to operate is known as the power frequency. Electric motors in North America are designed to operate on 60 Hz power, while most of the rest of the world uses 50 Hz power1. It is crucial to ensure that equipment designed to operate on 50 Hz is appropriately designed or converted to provide good service life at 60 Hz1. For instance, a three-phase change in frequency from 50 to 60 Hz can result in a 20 percent increase in rotor rpm.

Full-Load Speed

Full-load speed is the speed at which the motor runs when it is supplying its full rated torque or horsepower.

For example, a typical four-pole motor running on 60 Hz might have a nameplate rating of 1725 rpm at full load, while its synchronous speed is 1800 rpm. This means that the motor will run slightly slower than its synchronous speed at full load.

This is because the motor has to overcome various losses, such as friction and windage, in order to rotate. The difference between the synchronous speed and the full-load speed is called the slip. The slip is typically expressed as a percentage of the synchronous speed.

In the example above, the slip is 4.16% (1800 rpm – 1725 rpm) / 1800 rpm.

The slip of a motor is affected by a number of factors, including the load on the motor, the type of motor, and the design of the motor.

Load Requirements

Load requirements must be considered in selecting the correct motor for a given application. This is especially true in applications that require speed control. Important requirements a motor must meet in controlling a load are torque and horsepower in relation to speed.

Constant-torque loads—With a constant torque, the load is constant throughout the speed range. As speed increases, the torque required remains constant while the horsepower increases or decreases in proportion to the speed. Typical constant torque applications are conveyors, hoists, and traction devices. With such applications, as speed increases, the torque required remains constant while the horsepower increases or decreases in proportion to the speed. For example, a conveyor load requires about the same torque at 5 ft/min as it does at 50 ft/min. However, the horsepower requirement increases with speed.

Constant-torque load

Variable-torque loads—Variable torque is found in loads having characteristics that require low torque at low speed, and increasing values of torque as the speed is increased. Examples of loads that exhibit variable-torque characteristics are centrifugal fans, pumps, and blowers. When sizing motors for variable-torque loads, it is important to provide adequate torque and horsepower at the maximum speed.

Variable-torque load

Constant-horsepower loads—Constant-horsepower loads require high torque at low speeds and low torque at high speeds, which results in constant horse-power at any speed. One example of this type of load would be a lathe. At low speeds, the machinist takes heavy cuts, using high levels of torque. At high speeds, the operator makes finishing passes that require much less torque. Other examples are drilling and milling machines.

Constant-horsepower load

High-inertia loads—Inertia is the tendency of an object that is at rest to stay at rest or an object that is moving to keep moving. A high-inertia load is one that is hard to start. A great deal of torque is needed to get the load up and running, but less torque is required to keep it operating. High-inertia loads are usually associated with machines using flywheels to supply most of the operating energy. Applications include large fans, blowers, punch presses, and commercial washing machines.

Motor Temperature Ratings

A motor’s insulation system is essential for preventing short circuits and winding burnout. Heat is the main enemy of insulation, so it is important to be familiar with the different motor temperature ratings to ensure that the motor operates within safe limits.

Ambient temperature is the maximum safe temperature of the air surrounding the motor when it is operated continuously at full load. The standardized ambient temperature rating is 40°C (104°F), but special applications may require motors with a higher temperature capability.

Temperature rise is the amount of temperature increase that occurs in the motor windings from their non-operating (cool) condition to their temperature at full-load continuous operating condition. Temperature rise is caused by electrical and mechanical losses, and it is a characteristic of the motor design.

Hot-spot allowance is the difference between the measured temperature of the windings and the actual temperature of the hottest spot within the windings. This allowance typically ranges from 5° to 15°C, depending on the type of motor construction.

The sum of the temperature rise, hot-spot allowance, and ambient temperature must not exceed the temperature rating of the insulation. Insulation class is designated by letter according to the temperature it can withstand without serious deterioration of its insulating properties.

Duty Cycle

The duty cycle of a motor refers to the duration of time it is expected to operate under full load. There are two types of motor ratings according to duty: continuous duty and intermittent duty. Continuous-duty rated motors are designed to run continuously without any damage or reduction in life of the motor. General-purpose motors are usually rated for continuous duty. Intermittent-duty motors, on the other hand, are rated for short operating periods and then must be allowed to stop and cool before restarting. Crane motors and hoists are examples of motors that are often rated for intermittent duty.

Torque

Motor torque is the twisting force exerted by the shaft of a motor. The torque/speed curve of Figure 5-70 shows how a motor’s torque production varies throughout the different phases of its operation.

Locked-rotor torque (LRT), also called starting torque, is produced by a motor when it is initially energized at full voltage. It is the amount of torque available to overcome the inertia of a motor at stand-still. Many loads require a higher torque to start them moving than to keep them moving.

Motor torque/speed curve

Pull-up torque (PUT) is the minimum torque generated by a motor as it accelerates from stands-till to operating speed. If a motor is properly sized to the load, pull-up torque is brief. If a motor’s pull-up torque is less than that required by its application load, the motor will overheat and stall. Some motors do not have a value of pull-up torque because the lowest point on the torque/speed curve may occur at the locked-rotor point. In this case pull-up torque is the same as the locked-rotor torque.

Breakdown torque (BDT), also called pull-out torque, is the maximum amount of torque a motor can attain without stalling. Typical induction motor breakdown torque varies from 200 to 300 percent of full-load torque. High breakdown torque is necessary for applications that may undergo frequent overloading. One such application is a conveyor belt. Often, conveyor belts have more product placed upon them than their rating allows. High breakdown torque enables the conveyor to continue operating under these conditions without causing heat damage to the motor.

Full-load torque (FLT) is produced by a motor functioning at a rated speed and horsepower. The operating life is significantly diminished in motors continually run at levels exceeding full-load torque.

Motor Enclosures

Motor enclosures are designed to provide adequate protection, depending on the environment in which the motor has to operate. The selection of the proper enclosure is vital to the successful safe operation of a motor. Using a motor enclosure inappropriate for the application can significantly affect motor performance and life. The two general classifications of motor enclosures are open and totally enclosed, examples of which are shown in Figure 5-71. An open motor has ventilating openings, which permit passage of external air over and around the motor windings. A totally enclosed motor is constructed to prevent the free exchange of air between the inside and outside of the frame, but not sufficiently enclosed to be termed airtight.

Motor enclosures

Open and totally enclosed categories are further broken down by enclosure design, type of insulation, and/or cooling method. The most common of these types are:

  • Open drip-proof (ODP) motors are open motors in which all ventilating openings are so constructed that drops of liquid or solid particles falling on the motor at any angle from 0 to 15 degrees from vertical cannot enter the machine. This is the most common type and is designed for use in nonhazardous, relatively clean, industrial areas.
  • Totally enclosed, fan-cooled (TEFC) motors are enclosed motors equipped for external cooling by means of a fan integral with the motor, but external to the enclosed parts. They are designed for use in extremely wet, dirty, or dusty areas.
  • Totally enclosed, nonventilated (TENV) motors are enclosed motors generally limited to small sizes (usually under 5 hp) where the motor surface area is large enough to radiate and convey the heat to the outside air without an external fan or airflow. They are particularly effective in textile applications where a fan would regularly clog with lint.
  • Hazardous location motors are designed with enclosures suitable for environments in which explosive or ignitable vapors or dusts are present, or are likely to become present. These special motors are required to ensure that any internal fault in the motor will not ignite the vapor or dust. Every motor approved for hazardous locations carries a UL nameplate that indicates the motor is approved for that duty. This label identifies the motor as having been designed for operation in Class I or Class II locations. The class identifies the physical characteristics of the hazardous materials present at the location where the motor will be used. The two most common hazardous location motors are Class I explosion-proof and Class II dust ignition resistant.

Metric Motors

When you need a replacement for a metric (IEC) motor installed on imported equipment, the most practical way to proceed is to get an exact metric replacement motor. When direct replacements are not available, the following may need to be considered:

  • Metric motors (Figure 5-73) are rated in kilowatts (kW) rather than horsepower (hp). To convert from kilowatts to horsepower, multiply the kW rating of the motor by 1.34. For example, a 2 kW metric motor would be equal to approximately 2.7 hp and the closest NEMA equivalent would be 3 hp.
  • Metric motors may be rated for 50 rather than 60 Hz speed. The following table shows a comparison of 50 and 60 Hz induction motor speeds.
Speed, rpm
  • NEMA and IEC standards both use letter codes to indicate specific mechanical dimensions, plus number codes for general frame size. IEC motor frame sizes are given in metric dimensions, making it impossible to get complete interchangeability with NEMA frame sizes.
  • Although there is some correlation between NEMA and IEC motor enclosures, it is not always possible to show a direct cross-reference from one standard to the other. Like NEMA, IEC has designations indicating the protection provided by a motor’s enclosure. However, where the NEMA designation is in words, such as open drip proof or totally enclosed fan-cooled, IEC uses a two-digit index of protection (IP) designation. The first digit indicates how well protected the motor is against the entry of solid objects; the second digit refers to water entry.
  • IEC winding insulation classes parallel those of NEMA, and in all but very rare cases use the same letter designations.
  • NEMA and IEC duty cycle ratings are different. Where NEMA commonly designates continuous or intermittent duty, IEC uses eight duty cycle designations.
  • CE is an acronym for the French phrase Conformité Européene and is similar to the UL or CSA marks of North America. However, unlike UL (Underwriters Laboratories) or CSA (Canadian Standards Association), which require indepen-dent laboratory testing, the motor manufacturer through “self- certifying” can apply the CE mark that its products are designed to the appropriate standards.

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