Aluminium rotor bar in squirrel cage induction motors 2

[Zhk1983]

Dear All

Can anyone tell me disadvantages of using aluminium material instead of copper in rotor of squirrel cage induction motor.
Cost is one factor any other technical factor we should look.

 

[3DDave]

There's actually a thread for electric motors. Anyway, Google can help also:

https://w3.siemens.com/powerdistrib...Center-Whitepaper-Aluminium-versus-Copper.pdf

 

[waross]

Aluminum is much easier to cast.
It is much easier to use special bar profiles with cast in place aluminum.
Aluminum is lighter, hence less inertia and easier starting.
Aluminum bars are generally cast in place.
Cast in place bars are generally tighter and less likely to develop movement than inserted copper bars.
Copper bars are generally inserted and then an end ring is braised or welded in place.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Along those same lines as I've heard it;

Aluminum is better when you want lower rotor inertia and higher efficiency (thus less rotor heat), but aluminum is subject to the end ring connections breaking under shock loading. Copper is better when the motor is going to be subject to high torque shocks due to sudden changes in the load, because the brazed end ring connections are stronger, but the added weight of the copper means that more of the available torque is consumed in rotating the rotor mass itself rather than the load. So for example aluminum would be better for a pump where you want faster acceleration to speed but there is virtually no chance of any immediate change in loading. Copper is better for something like a rock crusher where there can be a sudden change in load causing a torque shock that puts added stress on the rotor bars and end ring connections. You will find that most "crusher duty" motors will have copper rotor bars. You can use copper for anything as long as you accept the additional mass, but you should not use aluminum for shocks loads like crushers or flywheel machines such as punch presses.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Zhk1983]

Dear All

Thanks for your replies . As i have heard the coefficient of linear expansion of aluminium is more than copper , hence the chances of fatigue failure is more in case of aluminium during operation .It is correct.We are using fabricated aluminium bar and short circuit ring rotor.

 

[electricpete]

Here is info from Westinghouse. They want you to believe copper is more reliable.

http://www.joliet-equipment.com/pdf/rotor_construction.pdf

Here is info from GE. They say the OEM is best suited to evaluate the particular choice of rotor material/construction for a specified application.

http://apps.geindustrial.com/publibrary/checkout/e-GET-M1005?TNR=White Papers|e-GET-M1005|generic

Here is info from Siemens. Seems like a pretty detailed and balanced view to me. Figure 2 is typical fabricated aluminum construction. Figure 3 is typical fabricated copper construction

https://www.industry.usa.siemens.co...ation/Documents/copper-vs-aluminum-rotors.pdf

They summarize the difference as follows:

While there are many similarities of AlBar [aluminum fabricated] to CuBar [copper fabricated] construction, there are two notable differences: the end connector of an AlBar rotor is welded to the rotor bars (as opposed to brazed), and, the end connector of an AlBar rotor clamps the rotor punchings (as opposed to end heads). It should be noted that AlBar rotors can also be built with a construction method similar to CuBar rotors, but this method is more expensive and not as common.

You are right that aluminum certainly has a higher coefficient of thermal expansion. Whether that translates into an inherent weakness I'm not sure (I'd think the design attempts to account for the material properties). In older days it was in vogue to specify copper rotors but my general understanding is that these days there is not a significant difference in reliability with respect to bar to end ring joints. We have had a problem in each type of construction. Aluminum rotor is more susceptible to melting from short term abuse such as many-starts-in-short interval or motor not tripped quick enough during locked-rotor condition, but standard protection should prevent those scenarios. If you do end up having to repair the rotor, the copper technology is typically easier for a repair shop to repair. For one thing they are more familiar with the copper brazing because it's similar to what they do in the stator endwinding connections, and for another they are more familiar with copper rotor repair overall simply because it is a more common rotor type. Aluminum fabricated is more specialized and harder to repair.



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(2B)+(2B)' ?