MOTOR SELECTION Electric motors should be selected to satisfy the requirements of the machines on which they are applied without exceeding rated electric motor temperature
IMPORTANT MOTOR AND LOAD PARAMETERS Mechanical Power Rating Expressed in either horsepower or watts Horsepower= Torque x Speed Constant The slower the motor, the more torque it must produce Slower motors need stronger components thus larger, heavier and more expensive than faster motors of equivalent HP
Current Full-load ampere Amount of amperes drawn under full-load Also known as name-plate amperes Used in determining the size of overload sensing elements for the motor circuit
Current Locked Rotor Current Current the motor draws under starting condition Aka Starting inrush current Service Factor Amperes Current the motor will draw when subjected to a percentage of overload equal to the service factor on the motor s nameplate
Code Letter NEMA code letters assigned to motors for calculating the locked rotor current Based upon the kilovolt- amperes per horsepower per nameplate horsepower.
Code Letter 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.
Code Letter LR Current (single-phase motors) = Code letter value x hp x 1000 Rated voltage LR Current (three-phase motors) = Code letter value x hp x 577 Rated voltage
Design Letter NEMA standard motor designs for AC motors: A, B, C, and D Denotes the motor s performance characteristics relating to torque, starting current, and slip. B is the most common design. relatively high starting torque with reasonable starting currents.
Efficiency 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, stator and rotor resistance loss, mechanical losses and stray losses
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. To be considered energy-efficient, a motor s performance must equal or exceed the nominal full-load efficiency values provided by NEMA
Frame Size Motors come in various frame sizes to match the requirements of the application. Frame size gets larger with increasing horsepower or with decreasing speeds. NEMA prescribes standard frame sizes for certain dimensions of standard motors.
Frequency This is the frequency of the line power supply for which an AC motor is designed to operate. Electric motors in North America : 60-Hz power Rest of the world: 50 Hz.
Frequency It is important to make sure equipment designed to operate on 50 Hz is properly designed or converted to provide good service life at 60 Hz. Example: three-phase change in frequency from 50 to 60 Hz can result in a 20 percent increase in rotor rpm.
Full-Load Speed Represents the approximate speed at which the motor will run when it is supplying full rated torque or horsepower. Example: a typical four-pole motor running on 60 Hz might have a nameplate rating of 1,725 rpm at full load, while its synchronous speed is 1,800 rpm.
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.
Load Requirements 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.
Load Requirements 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.
Load Requirements Constant-horsepower loads Constant-horsepower loads require high torque at low speeds and low torque at high speeds, which results in constant horsepower 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
Load Requirements 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.