## Sunday, October 10, 2010

### The Star-Delta 3-phase Motor Starting Method

The Star Delta starting method is a motor starting mechanism that minimizes the large amount of starting current that motors draw in. The Star Delta, as the name suggests basically involves feeding the motor with 1/sq.root3 (58%) of the full load current until it attains speed then applying the full load current. This method is commonly referred to as "Soft Starting" the motor, For this to work the whole set-up requires 3 contactor i.e The Star Contactor, The Delta Contactor & The Main Contactor.However for the motor to be started in Star Delta, its internal connection at the terminal box has to be wired in Delta-giving it capability of receiving the full-load current at any instant.

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.
Advantages of this method is low or reduced cost as compared to other methods...

The Concept
For me to be able to explain this method properly we shall need to digrams  for the Star Delta:
(i)Power Circuit Diagram
(ii)Control Circuit Diagram

Power Circuit Diagram
The Power Circuit enables us to see the flow of power and interconnection of phases to and fro the motor.

Initial States:
KM3 (Main Contactor)-NC
KM2 (Delta Contactor)-NO
KM1(Star Contactor)-NC

When the power is fed into the circuit, KM3 allows current to flow to the motor since it is NC (Normally Closed).Current flows into the motor and out to the KM1 which is the star-connected starter.After a specified period defined by the clock delay (Usually 5sec) the KM2 (Delta) Closes and KM1 opens to allow the motor to receive the full load current and run at delta.

Control Circuit Diagram
The Control Circuit enables us to see the connection of the starters that are going to start the motors.For small power motors, the starters are fed with Single-Phase (120 or 250V) but as the power rating of the motors increases, 3phase Starters are required.

By using the control circuit diagram one can easily connect the terminals of the contactor easily.

An Example
I will take a motor with some characteristics then we shall examine its values of current & voltage in star and Delta.
Initial Conditions:
Power Rating: 120kW
No.of Poles: 4
Voltage: 380V
Connection(Terminal Box): Delta
3phase 50Hz
Running Conditions (Delta):
Power Intake: 120kW
Voltage: 380V
Current: 210A
Efficiency: 93.9
p.f: 0.87
Speed: 1491rpm
Torque: 769Nm
Starting Conditions (Star):
Power Intake: 120kW
Voltage: 380V
Current: 212A
Efficiency: 93.9
p.f: 0.86
Speed: 1469rpm
Torque: 780Nm

1. nice post :)

2. AnonymousMay 24, 2011

coooooooooooooooool

3. really very basic....

4. Superb...

5. star connection rong

1. AnonymousJuly 12, 2012

6. actual connection u2,v2,w2
but u do u2,w2,v2

1. does it matter how these u v w connections are? NO...if the motor turns wrong way then just swap 2 wires

2. dhinka ka chika

7. Thanks for that, let me do some consultation. Thanks anyway.
u2,v2,w2

8. AnonymousJune 17, 2011

the conection for motor is U1,V1,W1 and W2,U2,V2.. As long is in delta during running, in star U1,V1,W1..and loop wire contactor star function.

9. AnonymousJuly 17, 2011

Thanks

10. AnonymousJuly 18, 2011

Jesus loves people that don't withold good info. God bless u. AnawaJesus loves people that don't withold good info. God bless u. Anawa

11. hello
i want to know that when we can start the three phase motor on only one contact relay than why
to use the delta and star contactor and why these contactor connect like this
, what is the u1,v1,w1and from the other side u2,v2and w2 ?

12. The reason why we start a motor using Star or Delta is simply to reduce the start-up current. A motor can draw up to 5X its running current and this will result to affecting all other devices on the same line.Also circuit breakers will open citing a fault.

13. LOOKING FOR A ELECTRICAL POWER PROJECT TO DO SHARE IDEAS

14. M1 & M2 are a.c 3 phase conveyor drive motors from a warehouse to the dispatch.Draw a schematic diagram for the control cct for the 2 motors.Use interlocks to control the two motors such that M2 can not be started while M1 is off or tripped.M1 must not start running when M2 has tripped.M1 has a remote start and stop buttons and an indicator lamp for the run/on condition.M2 to show when it has tripped by am alarm/indicator lamp and the run/on condition by am indicator lamp.

15. The star delta motor connection on terminals should be u1v2, v1w2, w1u2 respectively.

16. good so nice post.simple and basic
Habibullah awan rescue 1122

17. nice one my dear brother.thanks for promoting electrical by sharing knwledge.lol

18. power circuit diagram is very nice and simpliy understandable thanks lot.

19. thanks for giving the circuit
but above as you have mention control i have doubt in star and delta control .
In above

20. Where is the doubt?

21. very nice

22. AnonymousJune 28, 2012

it helped me alot thanks

23. cute

24. its nice to know star delta cct with power and control cct. it will be very usefull for many electrical technicians..... thanks for this post

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31. nice description HELP FULL..

32. thanks Sir.
got a job interview, and this is a must known topic..
thanks again..

33. Adam please is that ur connection correct, I thougth is.v1-v2. W1-w2. V1-v2.? Please I really need to know. From my book here am seeing a diffrent connctionm

34. gud but delta connection is wrong baby....