Starter Type 3

[alanElec]

I think all of you in the electrical fraternity would agree with me when I say that a 45 KW Pump or AHU needs a suitable starter such as Star/delta or VFD to reduce starting current.
However, my company thinks different.They say and show me SLD's where they have fed 45 KW & Above AHU's,pumps(constant flow) etc with DOL starter.We work mostly for Petroleum company that follows Shell/DEP specifications.
I dnt have a clue as to how a DOL starter works perfectly for these large motors..
Can anyone throw some light?
Why is DOL preffered for AHU's,Pumps etc.?

 

[MikiBg]

It isn't preferred, it's allowed, if rest of installation won't be suffering because of it. If your CB won't trip during DOL start, and your contactor can cope with starting current, while voltage drop doesn't impact other load proper function, then you can apply DOL theoretically regardless of rated power. Basically, it depends on place of installation (how rigid is your grid upstream) and allowable voltage drop. Besides, some manufacturers advice DOL to be avoided if working machine isn't designed to bear with DOL starting torque on motor shaft.

 

[waross]

DOL is the cheapest and simplest.
When should you use reduced voltage start?
A/ Voltage drop and disturbances to other customers on the circuit. Above a certain size the utility may demand reduced current start so as to avoid excessive voltage drop to other customers on the circuit. A residential distribution circuit may be feeding a group of homes with a 25 KVA or 50 KVA transformer. There would not be enough capacity to run let alone start a 45 kW motor. The transformer would have to be uograded to 75 KVA or 100 KVA. even then starting a 45 kW motor would cause objectionable voltage drops at adjacent homes. Most utilities would demand a reduced voltage start for the motor.
BUT
In a petro-chemical plant the electrical houses and MCCs are typically fed from 500 KVA, 1000 KVA or larger transformers. A 45 kW motor may easily be started DOL without an objectionable voltage drop.
B/ Inertia. industrial motors are typically suitable for a maximum number of seconds of starting duty per hour. Motors driving high inertia loads may be overheated by the starting current before the load is up to speed. Reduced current start must be used to avoid early failure of the motor. Most pumps and many but not all compressors may be considered low inertia loads for starting purposes.
C/ Reduced starting torque. Most loads will take the starting torque but there are exceptions. A more common issue is large belt drives that won't take DOL starting torque. I have seen high inertia applications in the 200 HP and 300 HP size where a slight mis-adjustment in the drive belts would result in the belts slipping and burning. A DOL start of these motors would probably result in the belts being destroyed in a few starts as well as the motor being overheated.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Bill - On your point B a reduction in starting current doesn't reduce the heating of the motor. The I^2*t of the motor can actually increase with a reduced current.

The motor accelerates rapidly with DOL starting so you also have to watch for torque shock in loads with slack in the drive system.

 

[rcw retired EE]

A 45 kW (50HP in North America) is a small motor for most industrial facilities. When I worked in refineries, we would not look at reduced voltage starting until about 500 HP unless it was a special situation, like a fire pump at the end of a mile long wharf where feeder voltage drop was an issue. I don't remember using any reduced voltage starters except for fire pumps. (Larger motors > 500HP were usually higher voltage 2300 V - 13.8 kV).

A chart in my old IEEE Red Book indicates a 50 HP motor can start on a 75 KVA transformer with about 80% voltage drop on starting. If your transformer is bigger, it may be OK.

 

[davidbeach]

I think you mean a 20% drop (to 80%) rather than an 80% drop. Dropping down to a mere 20% of normal voltage doesn't sound like a reasonable start. ;-)

 

[alanElec]

Well here we have one 1600KVA transformer feeding one 540 KW Chiller and two 45 KW Chilled Water Pumps plus some minor lighting load..

 

[DRWeig]

You'll certainly be fine with the pumps DOL. Probably you can do the chiller DOL as well, but that needs calculating. Your transformer is big.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[jraef]

Bill - On your point B a reduction in starting current doesn't reduce the heating of the motor. The I^2*t of the motor can actually increase with a reduced current.

The increase is of concern only if the reduction is torque allows the motor to stop accelerating. I have seen two very exhaustive studies on this, both coming to the conclusion that there is no appreciable difference in overall motor heating between Reduced Voltage starting and Across-the-Line (DOL) starting as long as the motor torque in RV starting is sufficient to maintain the load in a constant state of acceleration. Yes, RV starting makes it take longer, but less of the energy going into the motor is wasted as heat. The area of the energy curve is actually the same, just a different shape.

The issue not mentioned in regards to pump starting is that in many cases, there is a significant benefit in being able to soft STOP a pump motor and reduce the negative effects of water hammer. That is not so directly related to the motor, but it is significant to the pumping system as a whole.


"Will work for (the memory of) salami"

 

[ScottyUK]

jraef,

Would you be able to share those studies? They sound interesting. My understanding was that Lionel's point was generally correct although there are probably specific combinations of motor and load and soft start where there is little difference. I'm more than happy to be wrong - it's all part of learning.

 

[alanElec]

DRweig,
The Chiller comes with inbuilt star/delta starter of each compressor motor(4nos in total).It also has a switch fused disconnector.

 

[alanElec]

Talking about this chiller,its FLA is 1003 A at 415 V..My incoming panel breaker size and busbar rating is 2500A. I have two chillers of FLA 1003A ,two pumps of FLA 81 A,one AHU of FLA 12 A,a Lighting DB 16 A..Can you guys pls tell me weather the oncomer breaker size is correct?

 

[jraef]

ScottyUK,
The first one I ever read was ironically published years ago by my current employer, but long befor I worked here. However I cannot yet locate it, likely because we recently reorganized our data storage systems and some archived papers have become difficult to find now. I will keep searching though.

IIRC, I was directed to that paper by someone in this very forum years ago when this exact subject was brought up before, and I believe I took the same position as LionelHutz at the time. Live and learn...

"Will work for (the memory of) salami"

 

[DRWeig]

alansam, I didn't notice that you have TWO chillers. My bad!

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[LionelHutz]

Ummm, I posted a soft-starter won't decrease the heating but that it can increase the heating. As in, a soft-starter will never make the heating better but it can make it worse.

 

[KllrWolf]

In fluid filled submersible motors, DOL is the preferred by the motor, as it uses the speed to get the fluid wedge required for the bearings. If the motor is not spun up fast enough, you start to damage them.a rough guideline is for the motor to be up to speed in about 4 seconds (or less). Our submersible line with roller type bearings do not have this restriction due to the bearing difference.

 

[jraef]

jraef,

Would you be able to share those studies? They sound interesting. My understanding was that Lionel's point was generally correct although there are probably specific combinations of motor and load and soft start where there is little difference. I'm more than happy to be wrong - it's all part of learning. smile

ScottyUK,

I was unable to find direct reference to the studies behind this, but I did find the results stated in a white paper.

https://rockwellautomation.custhelp.com/ci/fattach/get/14793/1213391936

The relevant section is starting on page 2



"Will work for (the memory of) salami"