Selection of copper versus aluminum rotors for induction motors

On squirrel-cage induction motors, there is an important choice between utilizing a lower cost die-cast or fabricated aluminum rotor versus the more expensive copper bar rotor. Utilizing the wrong rotor construction for the application can either increase costs unnecessarily or lead to catastrophic failure. This paper provides the background necessary to assist in making the proper choice. The fundamentals of rotor construction and basic information on how the induction motor works are discussed. Additionally, the effects of various materials and types of rotor construction on motor performance are analyzed.

I. Introduction

Many induction motors in service today are running critical processes in which failure at any time can be very costly. Many times these critical processes will not have a backup. As a result, a large portion of this process will shut down until the motor is repaired and put back into service. Those aware of this situation will strive to purchase a motor that will maximize reliability. As a result of the strong push for maximum reliability, it is easy to overspecify costly components not required for the specific application. These decisions will result in an unnecessarily high motor purchase price. To maximize reliability without overspending, large induction motors need to be properly matched to the specific application. With this consideration, the choice between various rotor constructions needs to be evaluated. It is generally assumed that copper bar rotors are the most reliable. In certain applications this may be true, and they can at times outperform aluminum rotors. However, many times, the applications are such that there will not be any appreciable performance benefit from the use of copper bar rotors. It will be shown later in this paper that the percentage motor cost increase can be very large on smaller machines but may not be as significant on larger machines.