Chilled Water Pump Starter 2

[alanElec]

I have 45 KW Chilled water constant flow pump.It was earlier proposed to start it by DOL.But now due to limitation in incomer breaker size and transformer capacity we have to go for another method of starting.Which method can I use so that I can keep starting current to the least possible value.Is it possible to have full load current as starting current by any means of starting..say like VFD?

 

[dpc]

An adjustable frequency drive will provide the lowest possible starting current, at the highest cost. You can probably limit the starting current to the motor FLA is this is a centrifugal pump of some kind.

A soft starter will be less expensive, but starting current will be higher than an AFD.

 

[squeeky]

I agree on the above. Modern soft starters have current limiting capability from 100 OEL300%. This will extend the start up time if set to OEL100% so the pump must be started with closed valve. This will be current ramp as supposedly time or voltage. You will have to watch the heating of the motor if you require frequent starts. If the SS has a thermistor input, I would recommend using it. Also look at the trip class and set it according to your motor supplier data sheet.

 

[X49]

The upstream breaker should not be an issue if it is sized to code.

The upstream transformer will drop voltage during the motor start. The extent of the voltage drop can be calculated with the transformer impedance, X/R, and motor starting current.

A voltage drop of 20 or 25% should not be an issue for most loads on the same bus, and the motor should start just fine if it is a centrifugal pump. If the voltage drop is expected to be much higher than this and you have sensitive loads on the bus, consider a soft starter. In a centrifugal pump application, it should reduce your startup current by half. A VFD is much more expensive and would normally be overkill for this type of application.

What is your transfomer size?

 

[alanElec]

The transformer size is 1600 KVA.With rated amps 2100 Amps...The breaker is 2500A 4P,drawout motorized acb with micrologic trip unit with O/L,S/C & E/F protection-100 KA/1sec,make:schneider elctric,Model:NW25 H2+Micrologic 6.0 A..The locked rotor starting time of pump is 10 sec as per motor data sheet..
I have the following loads connected to Panel:
1.Two chillers of max load current 1003 A.Their starting current is 969 as the 4 compressors in it have inbuilt star/delta starters and are started in sequence.
2.Three pumps of FLA 81 and Locked Rotor Amps of 555,out of which only two will be working.
3.One lighting DB
4.One 5.5 KW AHU
5.One 20 KW heater

 

[crshears]

Hmmm...what kind of motor on the chilled water pump? Buried star point, or are all winding connections brought out to the connection box? If the latter, could make this pump star/delta start similar to the compressors... perhaps avoiding the need for a closed-valve start.

CR

 

[LionelHutz]

I find it hard to believe you'd need to limit the starting current of a 81A motor to FLA to allow it to start on a 1600kVA transformer, even with other loads already running on it.

Also, I highly doubt the motor will take 10 seconds to start full-voltage. That is the motor limit, not what it will actually take. It would probably take more like 1 to 2 seconds to start a centrifugal pump full-voltage.

 

[X49]

I see from your other thread that you're concerned about the total load on your transformer / breaker. Soft starting will not help with this. The max loading on your transformer is generally determined by adding all loads, plus 25% of the largest motor FLA. You must apply this 25% factor whether or not the motor is soft started.

Soft starting will not reduce the average load / heating of your transformer. It will reduce starting current, but starting time will be increased accordingly, and total heating will be approximately the same. The only benefit to soft starting would be to reduce voltage dip on your bus during starting. With such a large transformer, this is not an issue.

Also, Lionel is correct regarding the starting time.

 

[alanElec]

Thanks Lionel & X49..
When I have both Chillers,one pump n minor load running Total FLA is 2145..at this time if the pump starts ,the total current will be 2145+555=2700A..Im just wondering if this could be a problem for the transformer..Also can the 2500 A Breaker withstand this without tripping?
Motor Data sheet of Pump attached..you can check that too

 

[ScottyUK]

alansam,

I'm reading this with increasing concern that you're out of your depth here. It's commendable that you're trying to learn and trying to move this job forward, but you need some guidance from a more senior engineer at your employer. You need more help than we can realistically offer over the internet.

The transformer won't be bothered about a light overload for a few seconds. There's enough thermal mass there to ensure that the temperature barely moves in the timeframe of a motor start.

The 2500A breaker will have a thermal or perhaps thermal-magnetic characteristic, so it will tolerate increasingly severe overcurrent conditions for progressively shorter periods of time. If the protection is set properly there should be no problem with tripping at 2700A during a motor start.

 

[alanElec]

Thanks a lot ScottyUK..It has always been like this for me..I cant find a new job due to lack of experience...and fed up asking senior engineers for guidance..they wont help as they don't want to take responsibility..

 

[alanElec]

ScottyUk,
I have done my bit of searching & researching..I have checked the time current characteristic of the breaker..from that it should hold even higher currents..i also send a technical query to schneider regarding this..But the consultant has advised us to use starters of the pump hence absolving them of any responsibility in future..Hence it is my responsibilty to make sure..hence posted a query here too..Its all a learning experience..

 

[LionelHutz]

So, at 2700A how long does it take the breaker to trip?

 

[squeeky]

When you speak to Schnieder, you will need to tell them which relay you are using, i.e., 2.0 up to 6.0P. I presume you will be using the micro logic units.

 

[alanElec]

Yess..Micrologic 6.0 A...Schneider actually got the local agent to respond..He said would send a detail response tomorrow.