When replacing the electric motor that powers a pump, conveyor, or shop equipment, it is important to ensure that the new electric motor will fit into the same space as the old motor. Using a standardized frame rating system for electric motors will ensure that the new motor will have the same mounting dimension as the old motor. Without a standardized frame rating system for electric motors, you would have to measure each dimension of the electric motor by hand to ensure that the new motor will fit into the same spot as the old motor, an inefficient process.
The frame designation printed on every electric motor contain information regarding the size of the electric motor. The first two digit of the frame indicate the height of the shaft centerline of the motor from the mounting surface of the motor. The third digit within the frame number indicates the length of the electric motor body within that specific height of the motor.
How to Choose the Right Electric Motor
Each electric motor also features a letter suffix to the frame number that indicates whether the motor is a moddern, standardized frame or an older series of electric motors with different dimension for the shaft of the motor. By understanding these specifications of the electric motor that is to be replaced, it is possible to ensure that the replacement motor feature the same dimensions. Frame size is related to the horsepower of the electric motor because larger electric motors generate more heat, and greater amounts of surface area are necessary to allow that heat to dissipate.
Furthermore, it is also necessary for larger motors for the motor shafts to be able to handle the increased amount of torque that motors with higher horsepower ratings generate. Small pumps may utilize electric motors with frames as small as 56, but motors with 20 horsepower may require electric motors with frames of 256 to allow for the dissipation of heat and the shaft to handle the create torque. Charts that display frame sizes and the number of horsepower of those motors can be utilized to quickly determine the horsepower ratings of motors of specific frame size.
These charts assist in the decision-making process prior to reviewing other specifications of the electric motors. Another important specification for electric motors is the enclosure type. Motors with open drip-proof enclosures can be used in dry indoor location only; the motors will pull dust and other contaminant into the electric motor if placed in a dusty shop or outdoor location.
Motors with totally enclosed fancooled enclosures prevent water and dust from entering the motor; these motors are best used in food processing plant or chemical plants where exposure to water or chemicals may occur. You can review the options for enclosures in the reference material to ensure that the motor will withstand the same environment as the old motor. Efficiency is another specification for electric motors.
Motors that use standard efficiency may work in electric motors to perform the same tasks as the old motors, but standard efficiency motors will cost more to operate over time than premium efficiency motors. Premium efficiency motors will reduce the cost of electricity for the motor and will run cooler than motors of standard efficiency. Cool operation of motors reduces the rate at which the insulation within the motor begins to break down and fail.
A chart that compares the efficiency of standard and premium motors of different sizes and hours of operation each year allow electric motor technicians to determine if the higher initial cost of premium motors is cost effective over time. The insulation class of motors relates to the amount of heat that the electric motor windings can take before the insulation begins to break down. Most electric motors use insulation of class F for the motor windings.
The motors are often rated to a temperature rise that is less than the insulation class of the motor; the motor will run more cool than the insulation class rating. This ability of motors to run cooler than their insulation class ratings is beneficial in that the extra heat tolerance extends the life of the motor. Another specification for motors is the service factor for the electric motor.
The service factor is a safety factor that is built into electric motors to allow the motor to exceed its rated load for short periods of time. However, you should not run electric motors above their nameplate ratings for any length of time as this will shorten the life of the motor. Furthermore, excessive heat create by running motors above the nameplate load will cause the motors bearing or windings to fail.
Electric motors should be sized to run at between 75 and 100 percent of their rated load under normal operation. When replacing electric motors, the first thing that should be examined on the electric motor that is being replaced is its nameplate. Information that should be noted on the nameplate includes the frame size, enclosure type, voltage, electric motor speed, and the service factor of the motor.
If the electric motor is of the older series, a cross-reference chart can be used to find an electric motor of a modern frame size that feature the same diameter shaft. Motors that use a variable-frequency-drive will require the purchase of an electric motor that is rated for inverter use to handle the voltage spike that these drives create. Finally, you should ensure the shaft alignment of motors after installation to avoid bearing damage that may develop months after installation of the electric motor.
A grid within the specifications for electric motors displays the relationship between the horsepower of motors, the speed of the motors, and the frame size of the motors. Motors that have higher speeds will generate less torque with the same horsepower; hence, motors of higher speeds will have smaller frames than motors with the same horsepower but lower speed. This grid can be used to determine if the electric motor that is to be purchased fits into the same location as the old motor before using a tape measure to determine specifications.
You can choose the correct electric motor by matching each factor of the application (type of motor, load, environment) to the capabilities that is built into electric motors. For instance, frame size ensures that the motor will physically fit into the motor location, enclosure type will protect the motor from the external environment, efficiency ratings will lower the cost of operation of the motor, and the insulation class sets the thermal limit for the motor. When each of these factors is correctly matched to the motor, the motor will run reliably.
If each of these factors is incorrectly match to the motor, the motor may overheat, make noise, or fail prematurely. This standardized rating system for electric motors exists to ensure that each electric motor that is purchased and installed will be correctly match to the requirements of the application in which it is to be used.