DS

1 Introduction Star-delta (wye-delta) starting continues to be used for switching on 3-phase induction motors. When this type of connection method is used, the starting current is reduced to 1/3 of the current required for direct switching. When switching from a star (wye) circuit to a delta circuit, transients can occur in the motor, which, further intensified by an unfavorable constellation of line frequency and inductor field, lead to the development of larger current peaks than is the case if the stopped motor is directly switched into a delta connection. In an unfavorable case, the following problems could occur: • Short-circuit devices could trip, • The delta contactor could become welded or be subject to high contact erosion, • The motor could be subject to extreme dynamic overloading. A preferred circuitry configuration for star-delta (wye-delta) starters is described in this Functional Example. When the main circuit is connected in a favorable manner, it is possible to reduce ...

This circuit can be used to construct an attractive Christmas Star. When we switch on this circuit, the brightness of lamp L1 gradually increases. When it reaches the maximum brightness level, the brightness starts decreasing gradually. And when it reaches the minimum brightness level, it again increases automatically. This cycle repeats. The increase and decrease of brightness of bulb L1 depends on the charging and discharging of capacitor C3. When the output of IC1 is high, capacitor C3 starts discharging and consequently the brightness of lamp L1 decreases. IC2 is an opto-isolator whereas IC1 is configured as an astable multivibrator. The frequency of IC1 can be changed by varying the value of resistor R2 or the value of capacitor C1. Remember that when you vary the frequency of IC1, you should also vary the values of resistors R3 and R4 correspondingly for better performance. The minimum brightness level of lamp L1 can be changed by adjusting potentiometer VR1. If the brightness of...