written by John Richard-Reed President When it comes to new or replacement motors, a common mistake that creates a lot of headaches is OVERALL SIZE of the motor. Such a basic common-sense factor as available space for where the motor is installed gets overlooked routinely. Our own industry may be the cause for this mistake of assuming a motor will fit. Why? One Frame Fits All – Nope! Decades ago, our industry did both a good and bad thing. The industry group (NEMA) set up a size standard called "Frame Size". As can be seen in the dimension drawings from all manufacturers, this standardization was very specific in MOST dimensions. The output shaft diameter (U), the shaft height (D), the mounting feet bolt-hole pattern are all standard no matter which manufacturer. These are standardized by the NEMA frame call-out. This accomplished the very good goal of making motors from many different manufacturers very interchangeable. The standard also helped keep the various manufacturers competitive with each other and helped improve ready availability. The inadvertent bad result was the general assumption that ALL the dimensions for the same frame size were standardized; if you specified the same frame for a motor you were replacing, it would fit.... Wrong! For example: 200Hp – 1200 RPM – TEFC – 449TZ Frame Overall Motor Length ("C" on the drawing below) Teco Westinghouse Overall Length 54.37 inches Brook Crompton Overall Length 75 inches !!!! As in this example, even though the frame sizes that we all assume specify standard dimensions are exactly the same (449TZ), this frame size does not guarantee that one motor will fit into the same tight space that another motor had been in. One is almost 2 feet longer than the other. Basically, it is safe to say there is no standard for overall motor sizes no matter what the frame size. The same lack of standard applies to overall width and height. What to do… oh what to do? - If replacing an existing motor:
- Do
measure the overall space available where the motor goes - Do
ask for a dimension print when fitting a motor to a tight space - Don’t
just limit yourself to the exact brand and model you’re replacing because you limit yourself in price, availability, efficiency, service factor. - If designing or specifying a new installation:
- Do
leave as much space for the motor as possible (more on this latter) - Do
look at the overall dimensions of various brands of motors in the rating your application needs to determine the minimum overall space needed (Tip: Premium Efficiency, 1.25 Service Factor, Brook Cromptons all tend to be larger in Overall dimensions – allow for space required by these designs if possible). - The more space you allow for the motor, the more options (efficiency, prices, brands, stock availability) you will allow yourself.
IF Size Matters, WHY No Standard in the Standards? One of the primary means to increase electrical usage efficiency, service factor, operating temperature, overall quality and longevity is to design a motor with more active materials. A bigger cast iron frame can help a motor run cooler and increase life and service factor. Longer rotors and stator cores may help increase efficiency. Conversely, in applications where the motor runs only occasionally and the above issues are not a concern, but initial motor cost is, then a smaller overall dimension may fit the bill. Final Notes on Overall Space - Allow for extra space around the motor:
- Helps with cooling – longer life
- Helps with maintenance
- Helps with removal and installation
- Ensures you will have many options (price, availability, efficiency, brands)
- Don’t forget about the size of the Terminal Box (Conn. Box, Peckerhead). They are not all the same size either. Some manufacturers try to give big terminal boxes for ease of connection and safety – but that makes for a larger overall width.
- Dimension Drawing Tips:
- "C" is the overall length of motor
- "P" is the overall width of motor but does not include the terminal box
- "O" is the overall height of motor but does not include the lifting eye.
Overall Size Matters! Take a few minutes with a measuring tape.
|
