Starting Current for Star Delta and DOL Starter

As standard in motor starting current, the DOL starter will start the motor with 7.5 time of the rated motor full load current or FLC. Where, the Star Delta Starter will start the motor with 1/3 of the motor full load current of that DOL starter.

For 4 KW motor with DOL motor starter, your starting current is 49.1 Amp and your running current is 6.547 Amp. MCB or MCCB will be 15 Amp at 35KA.

For the Star Delta of 12KW motor, the starting current is about 52.17 Amp (1/3 of the motor with DOL) and your running current is 20.868 Amp. MCB or MCCB will be 35 Amp / 35KA

<...MCB or MCCB will be 15 Amp at 35KA...MCB or MCCB will be 35 Amp / 35KA...>

The rating of the circuit protection device, the ambient temperature, the circuit conductor size and the method of installation of the proposed cables are interrelated under BS7671, and nothing has been stated about the cables in this thread!

Hello mago,

I would get a circuit breaker for the current in the 12 kw load. If you have a Wye Delta starter you will soft start your load (btw is it a motor?) so the current draw won't be as high as with a DOL unit. Size your breaker for the normal load (most CB's have a time element anyway for LRA). For your DOL unit the LRA (locked rotor amps) is your starting current, you should find it on the data plate if it's a motor. Again it's very short time duration so most CB's won't see it. I calculate your 12kw current at:

I = 12000/415 x sq rt 3 or 16.69 amps (3 phase pwr = I x V x sq rt 3). Since the load is in kilowatts the pf is already considered (KW = KVA X pf). Good luck.

hughes838

Any motor as far as I know will depend on the type of lock rotor where the starting current be calculated it is found in every nameplate and depend whether it is an induction squirel cage or wound rotor type:

a) Induction squirel cage type rotor: 250%-300% of FLA as per NEC and NEMA Standard on Code Letter

b) Wound rotor type rotor : 125%-250% of FLA as per NEC and NEMA Standard on Code Letter

This is where your starting current based intended for Circuit breaker is calculated and the interrupting capacity depend on the type of load continuos or intermitent duty.

Under normal feed ( 415V delta) the nominal (rated) current has been given as 21A, with a starting current of (assume) 600%, that is 126A.

When starting in star (Y, wye), the current is 1/3, thus 7A and the starting current (assumed also 600%) will be 42A.

Since under normal start (delta) the breaker shall withstand the starting process (select 60A for a short-time start with 126A), in the case of a star start you can select 30A. Be careful that after the starting period you have to reverse the connection from start to delta thus 21A(I assume the motor has been manufactured for delta operation), otherwise the torque will be also only 33% of the rated torque.

Please see also for additional clarifications

http://library.abb.com/global/scot/scot235.nsf/veritydisplay/b065a29b8762f021ca2574bc0024c057/$File/1VGA677022%20-%20ZVC-SD%20Star%20Delta%20Motor%20Starter.pdf

Dear Mr. Tomad,

The principle of 3phase in star connection the rated ampere is 21A since it is already along the line current (DOL) only the voltage is reduce 415V/sqrt 3=240V per phase but in delta the voltage (DOL) 415V the current is reduce per phase( I = I rated /sqrt 3) 21A/sqrt 3 = 12.12A( assume of power factor of 80%). The starting current are based on the lock rotor type of motor that this size of motor is of wound type with High torque. Or to proved what you are trying to convey why dont you experiment in your own way or simulate the same. To see is to believe, lucky are those who can't see but can visualize.

Thank you.

hi friends,

you required the imformation about the starting current.

your calculation is right for resistive connected load. but if load is not purly resistive then we have to go with different calculation.

for example if load is inductive and capacitive then we have to calculate initial current with the help of transient analysis with intial theorem.

for load of motor we have to calculate different way, because induction motor take intial starting current more than 5 to 7 time it's rated value.

so friend starting current is depend on load, according to connected load its vary so which kind of load you connected according to that we can calculate the starting current.

ON URGENT BASIS ,I WANT TO START A SUBMERSIBLE PUMP OF CAPACITY 50KW WITHOUT STARTER IS IT POSSIBLE?

ANY IDEAS TO START IT?

YOU WANT TO START IT WITHOUT A STARTER, AND YOU WANT TO KNOW HOW TO START IT????

First learn to type without the Caps Lock key on, it makes your post difficult to read. Humans read words not only by the letters but by the shapes. Using all caps forces us to slow down and read each letter individually. I get bored easily.

A "Starter" is a device for starting, stopping and protecting a motor. How would you start it without starting it? Would you connect the wires to it while they are live? Good luck with that and make sure your wife knows where to find the money stash when you are dead.

If you would plan on using a manual switch to disconnect the motor and turn it on when needed, that essentially becomes part of a "starter", albeit manual. But that does nothing to protect it.

You could add fuses as protection, but unless you get the absolute perfect fuse, you are going to find it difficult to provide adequate protection AND the ability to avoid nuisance clearing of the fuse from the high starting current of an AC motor. The same would hold true for a circuit breaker.

So now if you increase the breaker or fuse size to allow starting without nuisance clearing, then you are too high to protect the windings of the motor.

So if you then add a Motor Overload Relay, it will provide the proper running protection for the motor as well.

But an Overload Relay is not a disconnecting device, and cannot be easily tied into a disconnect or circuit breaker without special trip release mechanisms. So if you add these special trip release mechanisms to a circuit breaker or disconnect switch, guess what you have?

A MANUAL MOTOR STARTER!!!

Now if you get tired of standing there manually connecting this device to start your motor, you use an electromagnetic coil that can be operated remotely sop you can so sit in the office and eat donuts while you push a button to make the motor start.

NOW YOU HAVE AN AUTOMATIC MAGNETIC MOTOR STARTER!!

Did you mean start this 67 hp pump w/o a wye-delta starter or just any starter? If you're trying to save money a frequency controller is out too, so that leaves you with your only two possibilities: DOL (direct accross the line) magnetic starter (safest if it's 3 phase) or use your supply breaker (it better have the capacity for LRA) to turn it on and off. I hope your situation is temporary and your final installation will be according to NEC (or similar). Good luck!

hughes838

The protocols for doing this sort of thing can be found in a range of national electrical standards.

If applicable, the one used in the UK is British Standard 7671.