Due to their durability and simplicity, three-phase induction motors are the workhorse of modern industry. You may have noticed that a starting method known as star-delta is employed to start an induction motor with a high power rating. In this lesson, we'll talk about the requirement for a star/delta starter and how it works in practice. A thorough understanding of induction motor performance is essential to comprehend why a star/delta starter connection is required. A stator and a rotor are the two primary components of the induction motor.
The stator of a three-phase induction motor receives input power when it is linked to a three-phase power source, which causes the stator to generate a revolving magnetic field. The RMF is responsible for turning the rotor.
According to electromagnetic induction, this rotating magnetic field will cause the rotor bars to generate electricity. So the rotor is like current carrying bars immersed in a magnetic field this will induce an electromagnetic force according to Lorentz force law and the rotor will rotate in the same direction as RMF.
Instead of a direct connection, electromagnetic induction induces electricity on the rotor bars. It follows that transformer action is what makes the induction motor work, the rotor bars serve as the secondary and the stator coil as the primary. Consider the rotor RMF interaction when the motor first starts the rotor speed is 0 when the induction motor starts, then it gradually increases.
This implies that the rotating magnetic will rapidly cut the rotor bars. A strong EMF and current will be induced on the bars at first as the rotor speed increases; however, as the rotor speed increases, the rate of flux cut will decrease, causing the current to fall to its usual amount. Since the rotor and stator currents are coupled as a result of transformer action, the stator current at startup must likewise be large. As a result, when the motor begins, the stator coils will consume a significant quantity of current. When a heavy rating induction motor starts, this will result in a significant voltage drop in the cables and have an impact on other devices.
How does one overcome this problem?
One of the most popular solutions for this is the star delta connection, which has a design that leaves the coil's ends free. There are two ways we can join the free end of each coil: either as a star with the end terminals connected together
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| Star Connection |
Or as delta, with the one coil, is connected to the start of the next coil. This is clearly depicted in the schematic.
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| Delta Connection |
The trick to overcoming the high starting current is simple: just connect the coils in star at the starting. Reduced input voltage results in reduced current at the stator coil. When we apply a three-phase voltage across a star connection, each coil will receive a significantly lower voltage than when we apply a voltage across a delta configuration. All of the coil terminals are fed into the motor's junction box, where they are arranged in the way shown below to make disconnection necessary. Input power is provided to coil terminal one. Simply connect the second terminals together to establish a connection in a star.
After the rotor reaches a considerable speed, the stator current will be normal so that we can switch to delta configuration.
However, such a manual rearrangement of terminals is practically difficult in practice, in order to achieve this we use: Contactors, Timer relay
The Star Delta starting method is a motor starting mechanism that minimizes the large starting current that motors draw in. The Star-Delta, as the name suggests basically involves feeding the motor with 1/√3 (58%) of the full load current until it attains speed and then applying the full load current.
It is required three contactors i.e., the Star Contactor, the Delta Contactor, and the Main Contractor. However for the motor to be started in Star Delta, its internal connection at the terminal box has to be wired to Delta-giving it capable of receiving the full-load current at any instant.
When the power is fed into the circuit, Main Contactor allows the current to flow to the motor. Current flows into the motor and out to the Star Contactor, the star-connected starter. After a specified period defined by the clock delay (usually 5 sec) the Delta Contactor Closes and Star Contactor opens to allow the motor to receive the full load current and run at delta.
Traditionally, in many regions, there was a requirement that all motor connections be fitted with a reduced voltage starter for motors greater than 4KW (5HP). This was to curb the high inrush of starting currents associated with starting induction motors.
The star and delta contactors are mechanically interlocked i.e. if one of them is closed the other cannot close. This is done to avoid a dead short circuit in case both the contactors close simultaneously. Electrical interlocking has also been provided, by using contactors control contacts.
An advantage of this method could be low or reduced cost as compared to other methods.
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