Chuck Yung 
EASA Technical Support Specialist 

Due to economies of scale, the use of cast iron is a popular choice in the manufacturing of NEMA and IEC frames. Cast iron is robust and easily machined. It is dimensionally stable and transfers heat well. However, foundry work is an energy-intense process, not well-suited to limited production runs. 

For larger electric motors, which are manufactured in smaller quanti­ties, the frame is more often fabri­cated from steel. You may have heard various nicknames for this type of construction: “shoe-box,” “skeleton frame,” “bathtubs” or other terms. Those who work on these motors regularly know that the frame requires care in handling, especially those fit­ted with sleeve bearings. 

Consequences of twisting 
If the manufacturer provided 4 lifting ears, one per corner, we should use all 4 of them every time we handle the motor. (See Figure 1) It only takes one improper lift, one careless person, to twist the frame. That means if the customer or a pre­vious repairer failed to rig the cables evenly, we may face the consequenc­es of their actions. 

What are those consequences? If the frame is twisted: 

• The feet are no longer co-planar. That causes difficulty in shimming the motor during align­ment. 
• The airgap will likely be affected, increasing twice-line-frequency vi­bration and perhaps even causing electrical noise. 
• The magnetic unbalance can also contribute to circulating currents if the winding has parallel circuits. Believe it or not, that may even contribute to shaft currents. 
• The flat fits – where the brackets bolt to the frame – are no longer parallel. When the end brackets are no longer square, the bearing housings are no longer in align­ment. That reduces bearing life. 

Now that I have your attention, let’s talk about some of the special handling issues we need to be aware of to avoid problems. Each time a large motor is rigged to be lifted, we should use all the lifting points. It may seem logical to assume that the 4 lifting ears are for picking up the assembled motor, and that a stator by itself is significantly lighter in weight. So there is a temptation to lift the sta­tor at two diagonally opposite corners. Doing so increases the possibility of creating one or more of the problems listed above. 

Other areas of distortion 
There are other areas where a frame might be distorted. One is the burnout oven; the frame should be supported flat on all feet during the burnout process. Steel should not be affected by temperature unless it reaches annealing temperatures approximately 1100°F (600°C) for common alloys – which further underscores the importance of using a controlled pyrolysis burnout oven. The water injection system used in modern burnout ovens displaces oxygen to quench combustion. The phase shift from liquid to gas also removes a tremendous amount of energy, controlling oven tempera­tures. Oxygen-rich ovens may support combustion and result in excessive burnout temperatures. 

Frame support 
Another area for concern is how the frame is supported during the as­sembly process. If you place a heavy frame on timbers that are not parallel, the frame may twist to conform to the support. Insert the rotor and bolt on the end brackets, and we have a chance of actually keeping the frame in a distorted condition. Here is an analogy that carpenters will relate to: When framing a wall, the carpen­ter will lay out the top plate, studs and bottom plate, and nail them all together. A savvy carpenter will then measure the diagonal distances. He knows that if the wall is square, the diagonals will measure exactly the same. If not, it is a simple matter to move it until the diagonals are equal. 

Then the carpenter temporarily nails a brace diagonally to 2-3 studs to hold the wall square. That newly assembled motor is similar to the stud wall. The shaft and brackets stiffen the frame and perform the same function as the diagonal on the stud wall. If the frame is twisted when the motor is assembled, it will stay twisted. 

Effect of poor lifting practices 
What if you carefully follow all these steps, the motor operates smoothly during the in-shop test run, and then the customer complains about vibration or alignment difficulty?That should be an alert that they might not have rigged the motor properly during the installation phase. An inexperi­enced rigger might look only at the lifting capacity of the cables. A good rigger considers the possible effect of poor lifting practices. 

A quote attributed to Archimedes is: “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.” The principles of leverage are well known. They pertain to lifting as well. The use of longer cables, with a smaller interior angle, exerts less force than shorter cables with their necessarily larger interior angle. A popular rule is to not use cables that result in an angle of 45 degrees or greater. Shorted cables exert more force on the frame. 

So lift properly, making sure your customer is aware of the problems they can create by poor rigging meth­ods and use these tips to trouble-shoot problem motors. If the motor has a hood, i.e., a “top hat” or “dog house,” make certain that your customer is aware of safe lifting requirements. 

Unless the hood is removed to lift the motor, a spreader bar must be used to prevent damage to the hood. We know of cases where a motor was lifted by the hood and of course broke the lifting devices intended for the hood only, resulting in catastrophic damage to the motor. 

Categories: AC motor enclosures, DC motor enclosures, Frame

Tags: Motor enclosures

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