In many practical applications the 3-phase AC electric motors are started in star/delta (Y/D) configuration. In this configuration, the motor is started in star connection and after a few seconds it's switched in delta connection. When running in star connection the motor consumes lower current per phase then running in delta connection, as expected. In some applications where the motor runs at great mechanical load, the starting must be in star/delta configuration since the currents at start will be very high, and the circuit will brake down. The 3-phase AC motors that are predicted to work in star/delta configuration have six plugs for connection of the AC voltage. The plugs are usually marked as U, V, W and X, Y, Z. Every plug is actually one end of the coil of the motor, and in this case of 3-phase electric motor, there are three coils. The plug U is the start and the plug X is the end of the first coil. Plugs V and Y are the start and the end of the second coil, plugs W and Z are the start and the end of the third coil, respectively. The star/delta connection of the 3-phase motor is shown on Picture 1. As we can see from the picture, the contactor c2 is 3-phase switch for turning on/off the motor, contactor c1 forms star connection of motor coils and contactor c3 forms delta connection. When the motor runs in star connection, the contactor c1 is switched on and the c3 is off, the phases from the power line R, S and T are leaded to its plugs U, V and W, and the plugs X, Y and Z are connected togather through the closed contacts of the contactor c1. This way, the motor coils are connected in star. From the other side, when the contactor c1 is off and c3 is on, the motor runs in delta connection. The phase R is leaded to the plug U, and the phase S is leaded to the plug X, thus the first coil of the motor is connected between phases R and S. If we follow the rest of the lines of the circuit we will see that the second coil is connected between phases S and T, and the third coil is connected between the phases T and R. This way, the three coils of the motor forms a triangle, or they are connected in delta configuration. Just to mention that in both cases, star and delta, the contactor c2 is always on.
Picture 1: Star / Delta Connection of 3-phase AC motor
Now, let's see the control circuit for star/delta start of the 3-phase motor. The control circuit is shown on Picture 2. With the b1 switch we can turn on or off the motor. When b1 is in position 1 (ON), we need to press the taster switch b2 in order to start the motor running. For that short time interval when b2 is pressed down, the contactor c1 will be turned on through the closed contact of Td1 and the closed contact of contactor c3. In the very same moment, the contactor c2 will be powered up and turned on through the pressed b2 and the working contact of c1 which is now closed since the c1 is on. After b2 is turned off, the both c1 and c2 will be powered on through the b1 and working contacts of c2, which are now closed. At this moment, the motor is powered up and starts in the star connection. The Td1 is the time delay relay, which is powered on as long as contactor c1 is on too. The contact of this time relay will be switched in the other position after the time that is set on it, starting from the moment when it is powered on. The time can be set in defined range, depending of the type of the relay. For example, it can be set in range from 3 to 12 seconds. So, for example, if the Td1 is set to 5 seconds, that means that after 5 seconds from starting the circuit with pressing the switch b2, the Td1 contact will switch to the other position. That will switch off the contactor c1 and switched on the contactor c3 through the normally closed contact of c1. After c3 is switched on, it remains on through its own working contact, the Td1 contact is switched back to the starting position, but both c1 and Td1 will remained off, since the normally closed contact of c3 will be opened while c3 is on. At this moment, the motor is switched from star to delta connection and it will remain running in delta connection until the switch b1 is turned off. The time that is set in the Td1 is actually the time of running the motor in star connection. The amount of the time needed for running in star before switching to delta connection is variable and it depends of various applications, the power of the motor, and so on. In cases when the motor need to achieve a high mechanical moment for starting the system for which is used, the star mode of operation is usually longer, and it can last for 10 seconds or more. Finally, the h1 and h2 are the signal lamps for state of the motor power circuit. When the motor is started and running, the signal lamp h1 is turned on, and when motor is switched off, the lamp h1 is turned off. From the other side, the lamp h2 will be turned on only if the motor current protection circuit e4 is turned on in case when over current will flow through the motor coils. Therefore, the signal light of h1 can be green color, and the signal light of h2 can be red color, since this is alarm for broken circuit.
Picture 2: Control circuit for Star / Delta Start of 3-phase AC motor
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