Selection of dol, star-delta and soft starter for motor

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Abhi

Abhi

I m an Engineer
Location
Jharkhand, India
Occupation
Electrical & Instrumentation Engineer
Upto what kilowatt of power a dol starter or a star delta starter or a soft starter is choosen for motors. Please help. There are various ranges available on internet, so which one is correct.

Thanks
 
That is mostly determined by your utility, as to the largest size motor they will allow starting DOL., by HP,volts and phase. I had one star delta starter 40 years ago and it is gone, thankfully. Anymore I prefer VFDs, as with softstarts if the service is undersize the SS will not work well or will trip the main.
The utility sets the max DOL based on its distribution system, so if you are way out on the end of line a large motor will cause excessive volt drop
If you are in an industrial location, then its an internal design issue
Remember this is a site on the US NEC and your codes may be different
 
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Yes, there is no universally correct rule on where reduced voltage starting is required. I have seen it as low as 25HP @480V, as high as 100HP and have seen plenty of utilities allow a variance to start even up to 500HP DOL. It is always determined by the serving utility.
 
Abhi said:
Upto what kilowatt of power a dol starter or a star delta starter or a soft starter is choosen for motors. Please help. There are various ranges available on internet, so which one is correct.

Thanks
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As others have stated it usually depends on what the utility allows, or you can get away with.

IEC style motors are often started with wye delta starters because there is less strain on the motor, and the equipment it is driving. It can also reduce contactor sizes and over current protection device ratings.

In the US we didn't really used to worry about such things much, although there is a fair amount of equipment that is made in Europe or Japan that is heavily influenced by these ideas that are imported to the US.

I don't know if it's my imagination, or just my little piece of the world, but I have not used a soft starter in at least 5 years. The cost of VFDs has come down to the point where it is often cost-effective for various reasons to use VFDs as opposed to soft starters, especially on smaller motors.

The thing is with a VFD you don't have to figure out exact gear ratios, or allow for gear ratios to be changed in the field to accommodate field conditions. This can often reduce the complexity of the mechanical part of a machine, and hasten the design work because you don't have to worry about those details so much. And again VFDs take a lot of stress off the motors and the rest of the equipment so you can make the equipment not quite as heavy duty, but still be solid.
 
Abhi said:
Upto what kilowatt of power a dol starter or a star delta starter or a soft starter is choosen for motors. Please help. There are various ranges available on internet, so which one is correct.

Thanks
Click to expand...
Consider the motor to be run at constant speed, no need to vary speed
 
Abhi said:
Upto what kilowatt of power a dol starter or a star delta starter or a soft starter is choosen for motors. Please help. There are various ranges available on internet, so which one is correct.

Thanks
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Forget wye delta. It’s just another soft starter and these days the electronic one is better and cheaper.

That leaves two choices. Soft starting Is chosen for several application specific reasons:
1. You are above the size where contactors are available. So it’s either soft starting or using a circuit breaker which except in very low duty applications is not a great way to do it.
2. You have some kind of current limitation, either breaker, transformer, or generator. A soft start is a trade off which allows for smaller equipment.
3. You need “slow roll” for maintenance/tooling reasons. Some soft starts can do low speeds in either direction (cycloconverter mode) but is much less expensive than a VFD.
4. Reduced mechanical torque requirements and/or extended mechanical life. Kurt psi’s can be severely reduced (the “snap” that bends shafts at startup). This results in cheaper, smaller mechanical equipment and/or longer life. This is much more important above 250 kw.
5. Greatly increased contactor life from lots of starts/stops. Contactors have a stated electrical and mechanical life. Electrical life is often 1/3rd of mechanical life and far less in AC4 applications. In a bypass configuration life is typically 20 years irrespective of load.
6. Water hammer issues. This is for both soft starting and soft stopping.
7. Braking. A soft start can generate DC injection braking at very high torques very inexpensively.
8. You need/want communications, electronic overloads, current/power feedback, or rotor protection (above 150 kw) which you get for “free”.
9. Motor heaters when offline using the stator to increase insulation life.

Down to around 25 kw soft starts still exist but they are less common. For all the reasons above realistically unless you need a VFD, DOL is a cost decision.

One way you can look at it is what us it worth to get close to full life (20 years) out if your insulation assuming bearings hind up. It’s the cost of a rewind, installation labor, and lost production downtime.
 
Jraef said:
Yes, there is no universally correct rule on where reduced voltage starting is required. I have seen it as low as 25HP @480V, as high as 100HP and have seen plenty of utilities allow a variance to start even up to 500HP DOL. It is always determined by the serving utility.
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I've worked on Size 5 DOL starters. I can't imagine a Size 7. You would need earplugs when it closed :)
 
paulengr said:
Forget wye delta. It’s just another soft starter and these days the electronic one is better and cheaper.
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Wye delta is a pretty cheap way of implementing lower voltage starting. I have not seen any cases where it is more expensive than a semiconductor soft start. The semiconductor software needs a bypass contactor anyway.
 
Actually on the big 1200A starters they used a DC coil and it closes fairly gently.

CONTACTOR GH - 450A...1200A AC-3 (700A/2000A AC-1)

www.ghisalba.com www.ghisalba.com

This brand is not sold in the US (as far as I know) but was brand-labeled by Siemens, A-B and Telemecanique at that time. I worked on the Siemens one, they didn’t even bother to change the name. It had two coils, I thought it was a bad design and sure enough, one coil went bad right away. But to change the coils, you had to do it from the bottom, which meant removing it from the back panel. It weighed 112lbs! Needless to say, I was not happy.
 
petersonra said:
Wye delta is a pretty cheap way of implementing lower voltage starting. I have not seen any cases where it is more expensive than a semiconductor soft start. The semiconductor software needs a bypass contactor anyway.
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The software doesn’t care.

On the largest sizes over about 1000-1300 A contractors aren’t even available.

In a continuous duty soft starter the SCRs are fully rated and never shut off.

In a bypass design they can be undersized. They only fire long enough for the motor to come up to speed. Since it does not see full inductive loads either the contactor can also be undersized. It is AC-1 duty. It can also be applied in an inside delta configuration which is a hybrid…it takes the place of the delta and wye contactors allowing a smaller soft starter.

Soft starters tend to be close to or beat costs if we offer it as an alternative when either the main or run contactor are damaged. It’s somewhat cheating because on a new install we would offer IEC instead of OEM NEMA.

But using say this one: EMX4i-0184B-V5-C1-H
List price $2835.


In contactors we need two 100A contactors, $288, and the wye, at 50 A, $214, and the overload, $187, same manufacturer. We are up to about $1,000. We also need a delta-wye timer/controller relay, and it needs 6 leads compared to just 3 plus all the wiring at the contactors adding hours to installs, and higher maintenance costs. At best it’s a wash cost wise without considering motor and mechanical life.

As the size decreases it tilts towards wye delta or just DOL while at higher currents the advantages definitely outweigh the cost which is almost a wash. There is a reason few manufacturers offer small soft starts like ABB.
 
Wye-Delta adds a risk of a "transition spike" of the voltage and/or torque. I have seen the torque spike shear off the shaft of a 500HP Toshiba (meaning not cheap junk) motor on a refrigeration compressor. That can be mitigated by a "closed transition Wye-Delta" starter, but now you have 4 contactors and a resistor bank, in which case a Solid State starter is ALWAYS less expensive.
 
Jraef said:
Wye-Delta adds a risk of a "transition spike" of the voltage and/or torque. I have seen the torque spike shear off the shaft of a 500HP Toshiba (meaning not cheap junk) motor on a refrigeration compressor. That can be mitigated by a "closed transition Wye-Delta" starter, but now you have 4 contactors and a resistor bank, in which case a Solid State starter is ALWAYS less expensive.
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Yep and more than once I’ve had to clean up after one turns to slag. They really make a mess.

Only soft starter I’ve ever had problems with was basically a very marginal situation in the first place and we tried a 2 phase soft starter. Still not sure if it was just an application issue or 2 phase soft starters are just a bad idea.
 
Jraef said:
Wye-Delta adds a risk of a "transition spike" of the voltage and/or torque. I have seen the torque spike shear off the shaft of a 500HP Toshiba (meaning not cheap junk) motor on a refrigeration compressor. That can be mitigated by a "closed transition Wye-Delta" starter, but now you have 4 contactors and a resistor bank, in which case a Solid State starter is ALWAYS less expensive.
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We're talking about a really unusual application there. Most applications it just doesn't matter.
 
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