How to connect a 3-phase motor in Star and Delta?

Last Updated on February 1, 2024 by Electricalvolt

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 are determined during motor manufacturing.

Three-phase motor connection configurations can be categorized in two ways. One is Star, and the other is Delta.

In the high-voltage configuration, the two windings of each phase are connected in series. The highest value of the supply voltage is shared equally between the windings, 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. The lower value of the supply voltage is thus 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 holds all three windings 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 rotors belong to the class first.
  • Class second is one with the rotor in a short circuit, also known as the squirrel cage rotor. This is due to its shape, which is 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 has ends called U and X or U1 and U2—the second winding, V and Y, is 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.

How to connect a 3-phase motor in Star

At each end of the windings, U1, V1, and W1, the lines L1, L2, and L3 of the three-phase power supply will be connected. The representation of a star connection in an electrical diagram is as follows.

star connection of 3-phase motor

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.

how to connect 3-phase motor in delta

The representation in an electrical diagram would correspond to the following image.

delta connection of 3-phase motor

Depending on the input voltage, we can connect the Electrical motors in a star or delta configuration, and it is also possible to start the motor in a 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.

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About Satyadeo Vyas

Satyadeo Vyas, M.Tech,M.B.A. is an electrical engineer and has more than 36 years of industrial experience in the operation, maintenance, and commissioning of electrical and instrumentation projects. He has good knowledge of electrical, electronics, and instrumentation engineering.

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