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The DC Test for Stator Resistance



                The rotor resistance R2  plays an extremely critical role in the operation of an induction motor. Among other things, R1 determines the shape of the torque-speed curve, determining the speed at which the pull out torque occurs. A standard motor test called the blocked-rotor test can be used to determine the total motor circuit resistance. However, this test finds only the total resistance. To find the rotor resistance R1  accurately, it is necessary to know R1 so that it can be subtracted from the total.


                There is a test for R1 independent of R2, X1 and X2. This test is called the dc resistanse  test, Basically, a dc voltage is applied to the stator windings of an induction motor. Because the current is dc, there is no induced voltage in the rotor circuit and no resulting rotor current . Also, the reactance of the motor is zero at direct current. Therefore, the only quantity limiting current now in the motor is the stator resistance, and that resistance can be determined. The basic circuit for the dc test is shown in the figure. This figure shows a dc power supply connected to two of the three terminals of a V-connected induction motor.



               To perform the test, the current in the stator windings is adjusted to the rated value, and the voltage between the terminals is measured. The current in the stator windings is adjusted to the rated value in an attempt to heat the windings to the same temperature they would have during normal operation .The current in flows through two of the windings, so the total resistance in the current path is 2R1. Therefore,



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               With this value of  the stator copper losses at no load may be determined, and the rotational losses may be found as the difference between the input power at no load and the stator copper losses. The value of  calculated in this fashion is not completely accurate, since it neglects the skin effect that occurs when an ac voltage is applied to the windings.


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