Induction motors requires less accessories & less maintenance. That’s why Induction motors are most widely used motors in Industries.
For motor controlling; start & stop; some means should be there. like DOL starter, Star-delta starter, VFD & autotransformer starters.
One more important thing about the Induction motors is; high initial current. This high initial current can be multiple times the rated current.
In small motors, Initial high current is not big concern. But for large size induction motors, Initial high current is dangerous !
There are multiple methods to limit the high initial current such as :
- Star-Delta starter
- VFD (Variable Frequency Drive)
- Autotransformer starters
This post is all about Star-Delta Starter. Further, Configuration & circuit diagram of the starter is discussed.
Star delta Starter
Induction motors have two connections one is ‘STAR’ & another one is ‘DELTA’. In STAR connection, Motor consumes 1/3rd current in comparison with the current consumption in Delta.
Since star connection consume less current that’s why to limit high initial current in star-delta starter, Motor is connected in star connection first at the time of starting.
After some time when motor reaches its 80% of rated speed; Connection get shifted to Delta connections. Both star & delta winding configurations are discussed below.
Star-Delta Connections
Connections of induction motor can be done in two ways; one is Star connection and another one is Delta connection. Every Induction motor has 3 coils in which each coil has two open ends, That means there are total 6 open ends. By connecting these open ends of the coils in a particular manner, A circuit can be created in ‘Star’ or ‘Delta’.
Star connections
In star connection, One end of all the 3 coils are shorted so that one junction get formed, that junction forms mid point of the winding which is technically a ‘Neutral’.
In Star connection, every coil get a phase voltage(Line-Neutral). If star connections are connected with 3 phase 415Volt supply then each coil get a voltage around 240Volts.
Delta connections
In Delta connections, All the 3 coils are connected in a series manner & 3 terminals are brought out in such a wat that each coil get a line voltage(Line-Line).
It means each coil getting a voltage of 415 Volts when connections are connected with 3 phase 415Volt supply.
Both the winding configurations are discussed through a GIF below:
Next in this post, For better explanation, Start–Delta starter for 75 HP motor is been explained.
Star-Delta starter for 75Hp Induction motor
As discussed above; In this starter; motor first runs in ‘Star connection’ while after some time connection get shifted in Delta. All the six terminals of the motors are been brought inside the electrical panel. There are total three contactors : Main contactor, Star contactor & Delta contactor.
Motor’s three terminals are directly connected to the main contactor while remaining three terminals are parallelly connected to star & delta contactors.
Rated current range for 75 HP motor is; 100 to 115 Amps. Since current is very high, Heavy contactors are needed accordingly. Since starter is star-Delta, load current got divided in half that is 55 amps if motor’s current rating is 110 amps.
Starters are one time investment specially when we are dealing in big motors. Contactors must be heavy enough so that it bear the fault current. In this 75HP induction motor starter, Contactors been designed of double the size of continuous current(55Amps) flowing through the contactor. Here we are using ML-6, 110Amps contactors made by L&T.
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Components of Star delta Starter (75HP Induction Motor)
- 3pole;200 Amps MCCB make L&T.
- 3 contactors ML-6, coil voltage 415volts & current carrying capacity is 110 Amps.
- Overload relay ranges from 45 to 75 make L&T.
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- GIC make star-delta timer ranges from 3 sec. To 120 sec. One more best thing about this timer is, A pause time between star- Delta contactor switching can be varied. Pause time ranges from 60ms to 150ms. (NOTE- Pause time must not be less than 50ms.)
- Analog ammeter ranges from 0 to 200Amps. CT ratio 200/5.
- 35mm2 copper wire for power wiring. & 1mm2 copper wire for control wiring.
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Connections of Star-Delta Starter
In star-delta starter, Basic function is; Start the motor in star connection until motor reaches its 80% of rated speed Than switch the connection to Delta. At starting, ‘Main contactor + STAR contactor‘ works after that ‘Main contactor + DELTA contactor‘ works.
Important points for the circuit:
- Since coil voltage of contactors & timer is 415 volts. That’s why direct looping of one of the phase to the coils of all contactors & timer. Another 2nd phase is used in interlocking.
- 2nd Control phase should pass through the control MCB & NC terminals of thermal overload relay, First.
- There are NO & NC points for start & stop respectively.
- At start when start button get pressed, timer must be operated through the NC of main contactor.
- Parallelly with timer, STAR contactor must be operated through in series connection of NC of timer’s relay & NC of Delta contactor.
- Main contactor , at starting, should be operated after STAR contactor.
- Delta contactor should be operated through NO of timer’s relay plus in series with NC terminal of star contactor.
- Delta contactor should be operated after when motor reaches minimum of its 80% of rated speed.
- Star contactor should be operated through the NC contact of Delta contactor. Similarly, Delta contactor should be operated through the NC of Star contactor.
- There should be time delay of minimum 50msec in between star & Delta contactor switching.
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Circuit diagram of Star-Delta Starter
By considering all the 8 important points discussed above, A circuit diagram represented below:
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Connections for ON-OFF Remote
This starter is made for 1.5 Ton Drop forging Hammer. Panel is placed far away from Hammer That’s why On/Off remote box must be near to the hammer.
This On/Off remote contains 1NO block & 1NC block. For remote wiring, 1(mm^2) 3 core cable is used.
Actually there are two On-Off remotes are working parallelly. One is made on starter panel while another one near to the hammer.
NC contact is used for switch off function
NO contact is used for switch On function.
Both remotes NC’s are connected in series. Both remotes NO’s are connected in parallel.
NC contact is used for switch off function
NO contact is used for switch On function.
Both remotes NC’s are connected in series. Both remotes NO’s are connected in parallel.
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Post Summary
- Induction motors are favored in industries due to their low maintenance and fewer accessories.
- They require effective control measures to manage starting and stopping, utilizing methods like DOL, Star-Delta, and VFD starters.
- A significant concern for larger induction motors is their high initial current, which can be mitigated by some of the starters. The Star-Delta starter is one of them.
- Star-Delta starter connects the motor in a Star configuration at startup to reduce current, switching to Delta as the motor reaches 80% speed.
- The post details the components and connections of a Star-Delta starter designed for a 75 HP motor, emphasizing the importance of heavy-duty contactors and timers for safe operation.
Hope ! You like the post. If you have any doubt, suggestion or query please do comments.