Star-to-delta converters are often used to start a 3 phase induction motor. When the motor is turned on, the stator coils are connected in a "star" shape. This is changed to a "delta" shape when the motor reaches 3/4 of its full speed and the stator coils have built up enough "back electromagnetic force" (emf). The 3-phase induction motor starter circuit shown here has two main benefits: it stops a single phase from happening and it automatically changes from star to delta. It can only be used with motors that are rated for connection in a delta configuration at the given line voltage and have both ends of each of the three stator windings available separately. Starter circuit for a 3-phase induction motor Figure shows the automatic star-to-delta converter's circuit, which is made up of a single-phase preventer and a timer. How induction motors work The AC induction motor, which is also known as the squirrel cage motor, has a simple rotor that looks like a cage and a stator with three windings. The AC line current in the

stator creates a changing field that causes a current to flow in the rotor. This current interacts with the field and makes the motor turn. The number of poles in the stator windings and the frequency of the AC input voltage determine the AC motor's base speed. When you put weight on a motor, it slips the same amount as the weight.

To step down the 3-phase supply, three single-phase transformers are used. Transformers X1, X2, and X3 take the phases R, Y, and B and step them down. This gives the secondary output of 12V at 300 mA. A full-wave rectifier and a capacitor are used to change the direction of the current coming out of the transformer. Relays RL1, RL2, and RL3 are controlled by the three 12V DC supplies. When all three phases are present, the 12V DC supply from the R phase is sent through the contacts of relays RL1 and RL2 to the coil of relay RL3 and the timer circuit. As a result, relay RL3 energizes. The timer NE555 (IC1), which is set up as a monostable multivibrator, is also set off at the same time. Its length of time is set by capacitor C4, resistor R1, and setting VR1. With VR1, you can set how long it takes for the motor to reach 3/4 of its full speed. The negative pulse that triggers IC1 comes from the combination of resistors VR1, R1, and C4. Through resistor R2, the output of pin 3 of the timer is linked to the base of transistor T2. So, transistor T2 is driven to saturation, and relay RL4 is turned on (indicated by glowing of LED2). So, when the power is turned on, relays RL3 and RL4 turn on (if all three phases are present) to connect the stator windings in a star shape. If you follow the connections, you'll see that the R phase is connected to the R1 end of the R windings, the Y phase is connected to the Y1 end of the Y windings, and the B phase is connected to the B1 terminal of the B stator windings. To make a "star connection," the other ends of all the stator windings (R2, Y2, and B2) are connected.