Clearly, the mass increases if the rotor bar area is increased,the resistance and the torque decrease. In addition, dependingon how the slot size is modified, the reactance may increaseor decrease with the same effects on the torque. Therefore, thestall time increases but so does the acceleration time becausethe torque margin between the load and the available motortorque decreases, and the risk of a hang up below synchronousspeed grows. The lower resistance means higher inrush andextra stator heating during the acceleration period from boththe increased stator current and the longer acceleration time.The second impact of the higher stator current is a shorterstator stall time.If the rotor bar length is increased, then the torque increasesas well as the bar mass, but there is only a small amountby which the bar length can be increased beyond the rotorcore before mechanical and other complications take effect.The increase in torque shortens the stall time, and this couldoffset the gain in stall time created by the greater bar mass.In addition, if there is more resistance, the rotor losses willbe higher and the rotor cage rise will be greater, which itselfcauses more resistance and shortens hot stall time. The netresult is a marginal gain before the laws of “diminishingreturns” start to flourish.

T JFig. 1. Typical rotor slot and bar shapes.If the mass of the bar is increased by increasing the areawhile the resistivity of the bars is also increased by changingthe material, one would expect to get a longer stall time.Unfortunately, (3a) does not show that if the resistivity ofthe material is greater or if the slot, is deeper, the effectiveresistance related to the inductive effect (skin effect) of theslot-leakage flux on the current distribution in the rotor barswill be higher. The combined effect of the greater area andhigher resistance is generally a shorter stall time if the startingtorque is the same or greater than that of the smaller lowerresistant bar. The only time a material change will consistentlyresult in improved stall times is a switch from an aluminumcage to a copper alloy cage of the same resistivity.Myth: All stall times are calculated similarly, and motorswith the same stall time have the same thermal and startingcapacities.The assumption is that all stall times are calculated in thesame manner and are equally conservative. Similar machinesin the same application with a design by different manufacturershave been found to perform differently and to havedifferent rates of failure or success. Recent developments incalculating rotor bar life analysis [ 11, [2] confirm that the stalltime calculation is not as simple as just shown but is a rathercomplex calculation affected by the slot design, the stator slotpitch and the material properties of the rotor bar [SI.

Fig. 1 shows a number of the more popular shapes developedfor the bars used in the cages of squirrel-cage inductionmachines. The shapes are designed to produce the differenttorque and inrush (starting current) characteristics that arerequested for the many applications in which induction motorsare found.Fig. 2 shows the corresponding shape of the speed-torquecharacteristic that might be expected for each bar. A generaldescription of each bar shape and the corresponding speedtorquecurve will help to understand some of the significantpoints that will emerge during the course of this paper.Fig. l(a) is commonly known as the inverted T bar, whichproduces high starting torque and low inrush. It has, duringstartup, the highest output torque per ampere of any of the barsin Fig. 1, but it also has a very flat speed-torque profile, as seen