Depending on the supply voltage, the 3-phase induction motor can be connected in star or delta connections. There are six separate windings in a three-phase motor, two for each phase. During motor manufacture, the internal construction and connections of the coil within the motor are specified.
There are two types of three-phase motor connection arrangements:
- Star &
The two windings of each phase are connected in series in the high-voltage configuration. The maximum supply voltage is shared equally among the windings, and the nominal current flows through each winding.
The two windings of each phase are linked in parallel in the low-voltage configuration. As a result, the lower value of the supply voltage is shared equally among the windings, and the nominal current flows through each winding.
It should be noted that the low voltage connection must draw twice as much current from the source as the high voltage connection. On their nameplates, most motors show two voltage and current values.
Three independent windings are used in three-phase motors. All three windings are held in the stator slots by the motor’s stator. These windings are electrically separated by 120 degrees. A three-phase alternating current (ac) system powers motor.
A star connection has four wires whereas a delta connection has three wires.
A method of starting mechanism for a three-phase alternating current induction motor is the star-delta connection. A starter is typically used to start a motor smoothly. The motor is started by reducing the starting current with a star-delta starter.
Asynchronous 3 Phase Motor:
There are two types of asynchronous 3-phase motors.
- Those with wound rotors fall within the first category.
- The second class is one with a shorted rotor, commonly known as a squirrel cage rotor. This is because of its cage-like shape.
Three-phase (3-phase) motors:
Any three-phase winding circuit can be connected in a star or delta configuration. The star connection connects all of the coil’s ends to a coon point and is powered by the other free ends.
In delta connection, connects each end of the coil to the start of the following phase, allowing the system to be transferred through the connection points.
The current flowing through each phase coincides with the line current in the star connection, while the voltage supplied to each phase is (1/√3) less than the line voltage.
In the delta connection, however, the intensity that goes through each phase is (√3) less than the line intensity. Whereas the voltage applied to each phase coincides with the line voltage.
This section will go over both the configuration of Star and Delta as well as how to connect a 3-phase motor in Star and Delta.
The Star-Delta motor connection:
An electric motor’s stator has three windings, each with two ends. U1 and U2 are the endpoints of the first winding. V1 and V2 are the second windings. W1 and W2 are the endpoints of the third winding.
1). Star Connection:
To form a star connection, join the windings’ ends W2, U2, and V2 as seen in the below diagram, which corresponds to the terminals of an electric motor.
The lines L1, L2, and L3 of the three-phase power supply will be connected to each end of the windings U1, V1, and W1, as shown.
The following points should be noted in a balanced Star-connected network:
- Line voltages are √3 times of phase voltages.
- Line currents and phase currents are equal.
- Line voltages are separated by 120 degrees.
- Line voltages are always 30 degrees ahead of respective phase voltages.
2). Delta Connection:
Connect the ends of coil U1 to the ends of coil W2, coil V1 to U2, and finally the ends of coil W1 to V2 to form a delta connection. The image below shows the delta connection arrangement in an electric motor.
Electrical motors are usually connected in a star or delta configuration depending on the input voltage, but it is also feasible to start the motor in a star-delta configuration. For this, the delta motor voltage rating must be identical to the engine’s supply voltage.
The different ends of the three-phase windings are linked together in a delta connection.
Because only one phase is included between any pair of line outers in delta, the potential difference between the line outers is referred to as the line voltage and is equal to the phase voltage.
VL – Line Voltage and
VP – Phase Voltage
The following points are noteworthy in the case of the delta connection system.
- Line voltages and phase voltages are equal.
- Line currents lag behind phase currents by 30 degrees.
- Line currents are separated by 120 degrees.
- Line currents are √3 times phase currents.
When should utilise Star and Delta?
- When more current and a neutral point are required, the star connection is utilised, but when high voltage is required, the delta connection is used.
- In general, the star connection with neutral is used for distribution and the delta connection for transmission.
What happens if a delta-connected motor is connected to a star connection?
Because line voltage is√3 times phase voltage and line and phase currents are equivalent, a star connection is typically utilised for higher voltage ratings and lower current ratings. Because line and phase voltages are identical and line current is √3 times phase current, the delta connection is utilised for lower voltage and higher current ratings. The preceding two statements assume a balanced three-phase power supply and equal power output for both the star and the delta.
If reconnect the delta-connected three-phase induction motor as a star, the phase voltage across the winding will be 1/√3, or approximately 58% of the value it was previously. Because torque is related to voltage squared, it will become 1/3 (or) 33.33% of the value it was previously, and a load with a higher torque requirement will stop.
Also, the line current will be √3 or 1.732 times higher, resulting in higher line loss and, in certain situations, insulation failure. Because the load’s power is the same as the torque, the speed becomes three times the rated value, which may cause severe damage to the load. Over-speeding may cause rotor damage.
As a result, under normal working conditions, the star or delta connection should not be modified.
Which of the three-phase connections, star or delta, has the greater power handling capacity?
The majority of motors are intended to run constantly in delta connection. In a delta connection, they deliver rated power. They are operated in start connection during start-up to reduce starting current. When the motor is nearing rated speed, the connection is changed to Delta since the delta connection has better power delivering capacity.
Motors with lesser power ratings are only designed to work in a star connection. These motors only have three connections.
Why is there no neutral wire in a delta connection?
The phasor sum of all line currents is the current in the neutral wire. When all currents and power factors are equal in a balanced system, the phasor sum of all line currents equals 0A. So, in a balanced system, there is no need for a neutral wire.