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Experiment_22_The_Capacitor-Start_Motor - Experiment 22 The...

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22 1 Experiment 22 The Capacitor-Start Motor OBJECTIVE To measure the starting and operating characteristics of the capacitor-start motor. To compare its starting and running performance with the split-phase motor. DISCUSSION When the split-phase rotating field was described, it was stated that the different resistance- reactance ratio of the two windings was designed to give the difference in time phase of the currents in the windings necessary to produce a rotating magnetic field. In two-phase machines, where the windings are identical but displaced in space by 90°, the ideal time phase displacement of the winding currents is 90°. For both two-phase and split-phase motors the toque developed at starting can be calculated using the relationship: ܶ ൌ ݇ܫ ܫ sin ሺߙሻ (1) where k is a machine constant, I 1 and I 2 are the currents in the windings, and α is the angle between the currents. Because of the small magnitude of α in the split-phase machine the developed toque is relatively low. It is possible to increase α by adding capacitance in series with the auxiliary winding. If too much capacitance is added, the impedance of the winding is increased to the point that there is an unacceptable reduction in the current which more than offsets the benefit gained from increasing α . The optimum value of α is that where the product of the sine of α and the auxiliary winding current is a maximum.
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