The 3-phase induction motor can be connected in star and delta depending on the supply voltage. In a three-phase motor, there are six individual windings, two for each phase. The internal construction and connections of the coil within the motor is determined during motor manufacturing.
Three-phase motor connection configurations can be categorized in two ways. One is Star and the other Delta.
In the high-voltage configuration, the two windings of each phase are connected in series with each other. The highest value of the supply voltage is shared equally between the winding and the nominal current passes through each winding.
In the low-voltage configuration, the two windings of each phase are connected parallel to each other. Thereby the lower value of the supply voltage is equally shared between the windings and the nominal current passes through each winding.
Note that the low voltage connection must necessarily draw twice the amount of current from the source as the high voltage connection. Most motors indicate two values for voltage and current on their nameplates.
3-phase motors have three independent winding. The stator of the motor holds all three winding in the stator slots. These windings are displaced 120 degrees electrically from each other. It is powered by a three-phase alternating current (ac) system.
Asynchronous 3-phase motors can be found in two classes.
- Those with wound rotor belong to the class first
- Class second is one with the rotor in short circuit or also known as the squirrel cage rotor. This is due to its shape similar to a cage.
Three-Phase (3Ø) motors
It is possible to connect any three-phase winding circuit in a star or delta configuration. In the star connection, all the ends of the coil are connected to a coon point and are powered by the other free ends.
The delta connection, on the other hand, connects each end of the coil to the beginning of the next phase, allowing the system to be fed through the connection points.
In the star connection, the current flowing through each phase coincides with the line current, while the voltage applied to each phase is (1/Sqrt of 3) less than the line voltage.
On the other hand, in the delta connection, the intensity that runs through each phase is (Root of 3) less than the line intensity. Whereas the voltage to which each phase is subjected coincides with line voltage.
We will discuss both the configuration of star and delta and how to connect a 3-phase motor in Star and Delta.
The Star-Delta motor connection
The stator of an electric motor has three windings, each with two ends. The first winding or winding has ends called U and X or U1 and U2. The seconds winding V and Y or also called V1 and V2. Third winding calls its ends W and Z or also W1 and W2.
1. Star connection
To make a star connection we will connect the ends W2, U2, and V2 of the windings. As indicated in the diagram of the following image, which corresponds to the terminals of an electric motor.
At each end of the windings called U1, V1, and W1, the lines L1, L2, and L3 of the three-phase power supply will be connected. The representation in an electrical diagram of a star connection is as follows.
2. Delta connection
In order to make a delta connection, we connect the ends of coil U1 to the end W2, coil V1 to U2 and finally, the end of coil W1 to V2. The following image shows the delta connection scheme in an electric motor.
The representation in an electrical diagram would correspond to the following image.
We can connect the Electrical motors in star or delta configuration depending on the input voltage we have, it is also possible to start the motor in star-delta. For this, we will need the delta motor voltage rating to be equal to the supply voltage of the engine.
For example, an electric motor whose nameplate indicates 690/400 volts can be connected in star-delta if we have a supply voltage of 400 volts. Whereas for a 400/230-volt motor, the supply voltage must be 230 volts.
In this way, we will not use as much power at the start, so lapse of some time we can change over from star to delta connection and increase the power of the motor. This process is usually done in motors with a power greater than 7. KW depends on the force or torque that the motor has at startup.