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Ac Motors: Rotor Design

01 January 2005

While the construction of the rotor of an ac induction motor may be relatively simple, its design is quite complex. In this article Rob Easthope discusses some of the basic rotor designs and the effect they can have on a motor's characteristics

GThe typical squirrel cage rotor comprises a cylinder made up of round laminations pressed onto the motor shaft, and a number of short-circuited windings. The rotor windings are made up of rotor bars passed through the rotor, from one end to the other, around the surface of the rotor. The bars protrude beyond the rotor and are connected together by a shorting ring at each end. The bars are usually made of copper, aluminium or brass. The position relative to the surface of the rotor, shape, cross sectional area and material of the bars determine the rotor characteristics. Copper rotor bars offer some significant advantages, especially on larger motors. Copper rotor bars have a much higher thermal margin. Copper also provides a lower coefficient of expansion, this is important in rotor bars, as the higher the coefficient of expansion the more creep will be present. The movement caused by creep eventually leads to fatigue failure in rotor material due to thermal expansion and contraction. Copper also has a higher tensile strength and is thus better able to withstand high centrifugal force and repeated hammering of the current-induced forces during each start. Aluminium rotor bars are usually used on smaller motors. It is possible to produce the rotor cages by die casting methods, using aluminium as the rotor bar and end ring material. Often, fan blades are integrally cast as part of the end ring to aid cooling. This manufacturing technique gives a big cost advantage over the separate copper rotor bars that have to be brazed to the end rings. The cross sectional area of the rotor bars and the type of material used for their construction will affect the characteristic of the motor. A bar with a large cross sectional area will have low resistance, while a bar of small cross sectional area will have a higher resistance. The type of material used will also affect the resistance; copper having less resistance than brass. Thus a copper bar will have a low resistance compared to a brass bar of equal proportions Typical torque-speed and current-speed characteristics of a four pole motor. The torque-speed and current-speed characteristics will typically change if the rotor resistance is increased. As a result of the increase in bar resistance, the rotor losses are increased and the motor efficiency is reduced. The full load speed is also reduced but, on the positive side, the run up torque is increased. Thus providing more torque for less current at start up. The position or depth of the rotor bars will also change the characteristics of the motor. Positioning the bar deeper into the rotor will increase its effective resistance. Hence two bars of the same material and of equal dimensions could exhibit different effective resistances, dependent on their position relative to the surface of the rotor. The rotor of the deep-bar cage employs heavy cross-section copper rectangular bars designed so that the current will be forced up toward the top of the bar during starting, thus increasing the effective resistance of the rotor. Consider the typical torque-speed and current-speed characteristics of a four-pole motor with a bar depth of two to three times the width of the rotor bar, and the torque-speed and current-speed characteristics of a motor with a deep bar rotor, the rotor depth being typically four or more times the width of the rotor bar. Although the maximum speed is the same, the maximum torque developed by the deep bar rotor is reduced; however, more torque for less current is available during run up. Deep-bar rotors are suitable as universal drive motors for a wide range of uses, and they are often used where


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