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Copper Rotors Help Meet Unusual Requirements
Specialty Motor Manufacturer Finds Efficiency Is Good, but There's More to the Story “We use copper rotor bars in most of our AC motors,” says Kapadia, “because in those cases, you just can’t meet the specs without it. However, until recently, most of our rotors were hand-fabricated, and some still are. Today, we can show you examples of fabricated motors in which we would definitely have used die-cast copper rotors if they had been available at the time, and we can also show you cases where only by using die-cast copper rotors could we meet the customer’s requirements. 
To understand Kapadia’s points, we have to look at a few of the ways Welco uses copper. Figure 1 shows the die-cast copper rotor from a recent design. Notice that the laminations are skewed, an important point we’ll get to in a moment. Here, the customer needed high power in a small package. What he got from Welco included a rotor not much larger than a Wisconsin bratwurst, yet the motor in which it spins turns out an astounding 8 hp. In another instance, Welco supplied motors for a state-of-the-art commercial airliner that pack 14 hp into only 12 lb of active components (stator and rotor), a job in which Welco was challenged to take out one-half ounce of weight…and did. Welco routinely doubles the horsepower in a given (conventional) volume. “We make a fabricated motor in which we put 100 hp in a NEMA 180 frame” says Kapadia “Of course, it’s not just the copper, but other design features, as well, that make this possible, But, at 30,000 rpm, you can forget about aluminum, and, at 100 hp, your primary problem is how are you going to cool that thing? Die-cast copper is one part of the answer.” In still another application, a customer asked for, and got, a 250-hp, 60-Hz motor running at 3,600 rpm with 97.13% efficiency. The NEMA Premium efficiency for this type of motor is 96.5%. According to Kapadia, a die-cast copper rotor would have improved efficiency even more. “And,” he recalls, “you could put your hand on the motor and not tell it’s running at 3,600 rpm and 250 hp.” Welco also designed and made a 211-hp, NEMA 320-frame motor that could accelerate from zero to 8,000 rpm, stop, and reverse itself to the same speed in just one second. “My jaw dropped when I saw it,” says the obviously proud motor designer. “That particular motor had a fabricated copper rotor, but it’s one of those which, if we had to do it today, would be a good candidate for die-cast copper.” Copper Reduces Noise and Weight  Why die-cast copper? Kapadia explains that “In these lightweight, high-speed, low-noise, and/or variable frequency motors, you have to design-in rigidity because you can run into dynamic effects. In this regard, one of the advantages we see with die-cast rotors has to do with the rotor bars themselves. In these motors, we have to skew either the rotor bars or the stator to reduce noise. That’s especially important in 60-Hz submarine motors where so-called structure-borne noise can produce an audio signature that an enemy can identify. Sometimes we skew the stator, but that makes it difficult and expensive to wind. On the other hand, if we skew the rotor bars we face the problem of fitting the bars through the slots. If there’s an error in just one slot position, it accumulates, and soon the bars won’t get through the stack. You can reduce the bars’ thickness — and we can do that — but then we reduce current-carrying capacity and, consequently, efficiency, and we also have a loose fit that’s going to cause vibration. We can work around the vibration problem with vacuum pressure impregnation (VPI), but that’s a secondary operation and costs money. The better solution is to fill the slots entirely with cast copper, and that’s what we’ve begun to do in some cases. Die-casting the conductor bars actually solves both problems — larger bar size for higher efficiency and rigidity for cost-effective noise reduction — plus it simplifies manufacturing.” MORE >> |
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