Three-Phase induction motors are the workhorse of modern industry due to their robustness and simplicity.
You might have seen that in order to start a high power rating induction motor a starting technique called [Star-Delta] is used.
In this video we will understand why a star double starter is needed and how it is accomplished practically.
To understand why a star delta connection is needed [a] proper [understanding] of induction motor operation is imperative.
The induction Motor has two main parts a stator and rotor.
The input power is given to the stator
When connected to a three-Phase power supply the stator of a three-Phase induction motor produces a rotating magnetic field.
The RMF is the one which makes the rotor turn.
Let's simplify this winding to understand the connection
This is the simplest coil arrangement to produce an RMF. We have three coil sets as shown
let's Denote it with U
V and w
The ends of each coil are represented as [one] and [two] respectively
When excited by a three-Phase ac power supply this simple coil configuration will produce a two pole rotating Magnetic field.
This rotating Magnetic field will induce electricity in the rotor bars according to electromagnetic induction
So the ruler 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 the RMS
This is how the induction motor works.
Electricity is induced on the rotor bars due to electromagnetic induction rather than direct connection
So it is clear that the induction motor works due to transformer action
The stator coil acts like the primary of a transformer and the rotor bar as the secondary
Here comes a really interesting problem think of the rotor RMF interaction at the starting of the motor
When the induction motor starts the rotor speed is zero and slowly increases
This means the rotating Magnetic field will cut the rotor bars at a very high rate
As the rotor Gain speed the rate of Flux cut will decrease
So a high Emf and current will be induced on the bars at the start
As the rotor speed increases the current will lower to the normal value
At the start stator current also must be high since the rotor and stator current are coupled due to transformer action
Due to this the stator coils will draw a huge amount of current when the motor starts
This will produce a large voltage drop in the line and affect other devices when a heavy rating induction motor starts
How does one overcome this problem?
Star Delta Connection is one of the most commonly used solutions for this
You can see in the current design that all the ends of the coils are free
We could connect one free end of each of the coils in two different ways
Either as star where the end terminals are together
Or as Delta, with the one coil is connected to the start of the next coil. This is clearly depicted in the schematic.
When we apply a three-Phase voltage across a star connection each coil will receive a much reduced voltage
Whereas if one applies a three-Phase voltage across the Delta configuration we will get the same applied voltage
So the trick to overcome the high start and current is simple just connect the coils in star at the starting
Reduced input voltage means reduced current at the stator coil
To necessitate disconnection all the terminals of the coils are fed to the junction box of the motor they are arranged in the following manner.
input power is given to the terminal one of the coils.
To connect in star what we have to do is just connect the second terminals together
After the rotor reaches a considerable speed, the stator current will be normal so that we can switch to Delta configuration.
To switch to Delta you just have to connect the terminals as shown
However such a manual rearrangement of terminals is practically difficult in practice in order to achieve this, we use contactors.
Use of contactors are simple, just draw three cables from the second end of the terminal
If these three cables are connected together it is clear that it will result in a star connection
This is exactly what a contactor does
When this contactor is activated that will result in a star connection
To achieve Delta connection the second terminal cables are branched and one more contactor is introduced
The second Branch is connected to the input supply as shown
So if the second contactor is closed it will result in a delta configuration.
The exact timing of contactor action is determined by a timer
We hope you developed a clear insight it to start all the connections
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