Soft Starter vs. Close Transition Star Delta Starter

JRaef ,Thank you very much , i will post a question before i start to this project.

My Dear Friend,

U said u have 3 compressors . U should draw a piping layout and show all valves dampers and any air storage tank etc and elect diag with ratings from tranf PDboard starters and motors. specify Motor nameplate current and actual current when it is running

Since u have a soft start starter for Rotary Air compressor there should not be any problem if u keep your inlet damper closed either manually or automatically till full speed is achieved , but above details will help to analyse

If u can u can ask from vendor or ask him to modify to a V/F constnt starter with characteristic of Torque/Speed characteristic of compressor with inlet damper closed all your worries will be over n u will have a good time. Hope, I am not giving a Strong Bitter Medicine. I dont know where u r in globe n who is ur supplier. In India we r starting such equipments with proper matching i.e. copmp,mptor,starter,transf

Pointwise explanation of ur doubts.. hope u r elect worm or u have an elct g friend to cook for u

Q I have Centrifugal air compressor is driven by 522kw 2P 415v 3ph 50Hz induction motor which is connect to main switch board 4000A LV system.

A You may be having an inlet damper or close valve y/n

Q My problem is starting current problems as below :

When i start with Soft starter (1000A Rating) with delta connection at motor and starting time drag to 40sec,i have try many setting with the soft starter technical specialist but cant reach to rated speed and starting current as high to 4500A and trip the incoming 1250A ACB for soft starter.

SOFT START CHARACTERISTIC IS NOT CORRECT... yS COMMENT

A Soft starter reduces voltage but frq is same , same job as Y D starter. Any motor takes 3 to 6 times rated current when is started with 100 percent voltage. So if connected in D is getting L to L voltage which is 1.7 times of L to N and motor will draw more current. Refer to P = ... relation of v I pf etc from textbook

Same motor when in Y gets lower voltage, but torque developed is less, sorry refer a T = formula, to accelerate and takes more time.

So, try following and tell me the result....

Caution: You must allow a time of 5-10 minutes bet successive adjustments to take care of your motor

U must close the inlet damper so that air ir not sucked and it is only base mass under rotation. Confirm outlet is open so no warming of compr will be observed.

U must have a timer to change from Y to D. Initially keep it at 5 to 7 seconds.

Have a Tacho/RPM meter and adjust timer when u get 60 percent rated speed.

On full speed Open inlet damper thereafter

Q Since the soft starter cant perform this job and i changed to close loop star delta starter. During start in star connection the line current at 1700A then drag for 35 sec and switch over to delta running current drop to 500A and slowly increase as the inlet valve of the compressor is open ...

THIS IS WHAT SHOULD NOT BE ... ys COMMENT

...and also outlet discharge valve.

My question is why star delta starter is perform better job in my this centrifugal compressor.

A. U have not specified ur positon, owner, operator, elect engr, contractor, comp supplier. Can u buy a V/F controller with an auto control of inlet outlet ?

Check for availabilty n call me on +91 94 224 754 11 specify ur country to guide

Q I have 3 units of 522kw 415V 3Ph 50Hz compressor in my plant and connected to one 2500Kva transformer feeder it is correct design ?

A (522 kw X 3 X 0.8 load factor )/pf 0.8 is always safe for 2500Kva but u have to be carefull about current during start

WITHBEST REGARDS AND APPRECIATION OF YOUR DESIRE TO GO TO ROOT OF PROBLEM , A THING BECOMMING A RARE COMMODITY

Dear YS Elect Solar Consultant,

Thank you very much of your very very good explaination of my post.I am from Malaysia and im worked in Glass production factory with little E&I back ground.I am one of technical in charge in this company.

Compressor maker : Elliott USA ,Model : DA330,Centrifugal type,

Motor : 522Kw,415V,50Hz,2980rpm,Name Plate : 847AMP , Actual running : 820 to 840A

Compressor is equiped with MicroP controller ,

Yes there are inlet and outlet damper which is fully control by MicroProcessor controller.

i set the Y/D timer at the 35sec is optimun..if shorter than this the chang over delta main current is 2000A for few second then drop to 500A and slowly increase as the inlet valve of the compressor is open ..and chanel to plant air network which is the air demand is 500M³ / MIN ,(link together with 10 unit Atlas Copco GA-160w)

Thank you very much

U said

Thank you very much

Is the compressor startng, have u tried with lower timer and inlet closed

Dear YS,

Yes compressor is running.Timer cant be lower anymore due to high surge cuurent during in delta running.

Compressor start with inlet fully close (Butterfly valve but with 4 12mm dia hole at the valve plate)

COngratulations and wish you success in your job Do call +91 9422475411 or mail yskulky@rediffmail.com n visit India . My address yskulkarni 12, monidan apptt tamtalao vasa city 401 201 maharashtra state india

by

I think that the soft starter is to small for this motor.

• When the motor is connected in star, it takes 1700 Ampères.
• A motor connected in star delivers only 1/3 of its original shaft torque.

• Your soft starter (1000A rating) allows your motor to take maximum 1000 Ampères,
• This 1000A is much lower than the 1700A in star, so the starting torque will also be much lower then the 1/3 of the original shaft torque.

This torque will probably be not enough to start your motor.

Solution increase the size of the softstarter.

Dear rudy,

Thanks for your reply,during i used soft starter to start the compressor,motor connection was in Delta. Loading current at 1000A but of course the voltage during the starting is lower than 240v. Even i hv prolong up to 60sec and switch to 300v also tripped.

You must keep a few things in mind.

Your compressor needs a minimum amount of torque to start.

So your motor must capable to deliver this minimum torque.

When you use a star / delta starter your starting torque is 1/3 of the torque when the motor is running in delta.

As I understand it, you can start the compressor with a star/delta starter, and you can not start the compressor with the soft starter. This means that the motor torque with the softstarter is too small.

When the softstarter gives more current to the motor, the motor will have a bigger starting torque.

But you can not start the compressor because the softstarter does not allow an increased motor current to run.

The only thing I can imagine in this case is that the softstart is at its maximum current.

This means that the actual softstart current is to low for your application.

This means that you have to look for a bigger model of softstarter.