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Chapter 5—Electrical Network 5–38 kW are routinely started across line voltage in generating stations, where a starting current of 5 to 10 times the rated current can easily be supplied. The motor itself will not be damaged by these high currents unless they are sustained long enough to overheat the motor. There are three basic ways to accomplish reduced-voltage starting. These are illustrated in Fig. 20. 1. Line resistance or reactance starting uses series resistances or reactances in each line to provide a voltage drop and reduce the voltage at the motor terminals. After a suitable time delay these components are removed in one or more steps. The notation L 1 , L 2 , and L 3 refers to the three phases of the incoming power line. 2. Autotransformer starting uses tapped autotransformers to reduce the motor voltage. These taps normally provide between 50 and 80 percent of rated voltage. 3. Wye-delta starting is used when the motor is designed for delta operation but has both ends of each phase winding available external to the motor. The phase windings are reconnected by contactors into a wye circuit for starting. Once the motor is running, it is changed back to its normal delta configuration. This technique reduces the voltage seen by each phase by the factor √ 3. Complete circuits would include push button start and stop, fuses, and undervoltage protection as well as other features to meet electrical codes. In each case a triple-pole switch is moved to the start position until the motor has accelerated the load to almost full speed, and then rapidly thrown to the run position, so that the motor is connected directly across the line. The autotransformer has the same characteristic as a two winding transformer in that the input and output apparent power in kVA have to be the same, except for any transformer losses. Fig. 21 shows an autotransformer supplying power to an induction motor. V L and I L are the voltage and current supplied by the line and V 1 and I 1 are the voltage and current delivered to the motor. For an ideal transformer V L I L = V 1 I 1 (70) Transformer operation requires that V 1 be less than V L , which means I L will be less than I 1 . Because of the nature of the load, when V 1 is reduced below its rated value, I 1 will also be reduced. The starting current supplied by the line is therefore reduced by both the autotransformer action and by the reduced motor voltage, which makes autotransformer starting rather popular on limited capacity lines. Example An autotransformer starting system is used to reduce the voltage to the motor of the examples in the previous section to 0.6 of its rated value. Find the motor starting current I 1 , the line current input I L to the autotransformer, the total motor losses at start, and the total starting torque. Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
Chapter 5—Electrical Network 5–39 Figure 20: Starting methods for induction motors: (a) line resistance or reactance starting; (b) autotransformer starting; (c) wye-delta starting. Figure 21: Circuit for autotransformer start of induction motor. The applied voltage to the motor is V 1 =0.6(127) = 76.2 V/phase The input starting current is also 0.6 of its value from the previous example on motor starting current. I 1 =0.6(155.58/ − 57.92 o )=93.35/ − 57.92 o A Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
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