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Soft Starters vs. DOL Starters

02/16/2010 3:17 AM

Dear Sirs,

I have a 120 KW AC motor that is being started / run by using DOL Starter.

I want to change the starter by a Soft Starter cum Energy Saver.

Can someone suggest me the following -

1. Advantages of using a soft starter

2.Effect on Life of the motor

3. Tentative Power saving (approx)

Regards,

Rooney

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#1

Re: Soft Starters vs DOL Starters

02/16/2010 4:20 AM

1- Reduce the risk of mechanical shock on the axis of rotation for each of the engine and pregnancy as well as the gears or conveyor belts from the motor to pregnancy, which increases the life.

2- Decrease the starting current to a value which motor's coils withstand.

3- Maintaining the stability of the electrical network's voltage because the high starting current lead to a reduction in network's voltage, which cause problems for the rest of the loads.

4 - Reduce the risk from high starting current to begin distribution in a transformer, which is running on the motor.


5 - Saving energy during periods of start-up and soft starters can save energy during the motor working.

Estimations:

Motor size Estimated Saving

Kw Hp % rated Kw

less than 5 7.5 10

22.5 30 6.5

55 75 3.5

110 150 2.5

More than 110 150 1.5


6 - Use a smaller cross-sectional area of the cables between the network and related to the motor.


7 - Reduce the hydraulic shocks on each of the pumps & the pipelines that carry liquids.


8 - Rarely a soft starter needs to the maintenance because it does not contain moving parts which reduces the cost of maintenance was done for contactors when you use the Star / Delta starting method.

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#5
In reply to #1

Re: Soft Starters vs DOL Starters

02/16/2010 11:02 PM

Dear sir,

Thank you very much for your expert advice. It has been of great support.

I require your help to clear the following doubt. If the motor size is bw 7.5 to 30 HP what would be the % savings.

The saving % to be take in on the rated HP / KW or the motor starting or running power consumption.

Regards,

Rooney

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#2

Re: Soft Starters vs DOL Starters

02/16/2010 7:54 AM

While industrial electric motors are wonderful machines, their starting moment is a true disaster. It is very powerful, it is immediate, it has literally a shock effect. So a gradual acceleration from zero to working speed, over a suitable time period, is very desirable, to the motor itself and to whatever the motor is connected to... it considerably extends the life span of your transmission system, such as bearings, couplings, reduction gears, pulleys or drive belts or whatever. The starting current will also decrease dramatically. But there is one thing that you should also know. The Soft Drive feature must be adapted to your motor and system. Discuss with Soft drive suppliers.

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#3
In reply to #2

Re: Soft Starters vs DOL Starters

02/16/2010 10:07 AM

It is already been answered. Here are some additional information:

1. Advantages of using a soft starter

Another advantage of soft starter is the soft stopping. If the motor load needs the soft stopping (such as the pumps and conveyor belts loaded with fragile products), it helps to stop the system gently.

However, the main purpose of soft starter is

(1) to reduce the motor starting current and hence to reduce the voltage dip in the electrical system during motor starting,

(2) to control the starting torque to reduce the mechanical impact on the load system. In this case, the soft starter setting should be such that the motor can generate enough starting torque to be accelerated.

2. Effect on Life of the motor

Smooth starting and stopping reduces the mechanical impact of the motor and loads. Less starting current reduces the electrical impact on stator and rotor. This increases the life of the motor.

3. Tentative Power saving (approx)

In fact, soft starter doesn't save energy, rather it has its own losses and additional auxiliary consumption for fan motors (used for medium and large size soft starters). To avoid this loss, soft starters are many times bypassed during the running period with a bypass contactor.

For additional help, see

http://library.abb.com/global/scot/scot209.nsf/veritydisplay/2985284834bcff7fc1256f3a00274038/$file/1sfc132002m0201.pdf

http://www.rockwellautomation.com.au/applications/gs/ap/GSAU.nsf/files/ACDrives_and_SoftStarters_ApplicationGuide.pdf/$file/ACDrives_and_SoftStarters_ApplicationGuide.pdf

- MS

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#20
In reply to #3

Re: Soft Starters vs DOL Starters

06/05/2020 12:23 PM

10.5 years later, I doubt the original poster cares any more...

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#4

Re: Soft Starters vs DOL Starters

02/16/2010 2:01 PM

Good stuff so far, but I will reiterate that energy savings is tentative at best, most likely non-existent. Buy the soft starters for the other reasons listed, but don't waste a lot of effort on convincing someone of the "energy savings".The biggest problems with it are the claims they make are exaggerated and they almost always fail to tell you that if you have to bypass the soft starter, which in 75% of the times you do, then the energy savings are nil.

Soft starters reject heat, about 4.5W per running load amp. So on your 120kW motor for example (assuming 380V and probably 185A?), a soft starter without bypass will reject over 800W of heat. If you put that in an enclosure that must be sealed to protect against dust and moisture, as is the case in most industrial environments, then the starter will cook itself in short order. Hence the bypass contactor; it shunts power around the SCRs once the acceleration is complete. But for the "energy saver" circuit to function, the SCRs must be in full control of the motor voltage so once the bypass contactor closes you lose that. If you go with a ventilated enclosure, the air space must be relatively clean and dry which implies an air conditioned space in many circumstances. If it is air conditioned, the energy saved by the Nola circuit is MORE than lost by the increase in heating load on the air conditioning system.

As far as the exaggerated claims, this has been discussed in here many times and despite the people who continue to foist their exaggerations on us (some of which will undoubtedly chime in against me now), the claims are not what they want you to believe. Many claim 40-50% energy savings. But you can only "save" on energy that is wasted and these things only work when the motor is virtually unloaded, at least 50% of full load! Any higher loading and the device has to turn off or risk stalling the motor. So first off, if the motor is unloaded why is it running? That is the prime caveat to this issue that is often ignored.

But let's assume for argument's sake that for some reason you have a motor that needs to operate for long periods of time with almost no load. The ONLY AC motor energy that is wasted is in the form of losses in the motor itself. Those losses are very well documented by the motor manufacturers now and in most cases are NEVER more than 15%; look at the motor's 1/2 load efficiency numbers if you don't believe me. Most newer energy efficient motors are now as high as 90% efficient at 1/2 load. So even if you have an old inefficient motor where the losses are at worst only 15% at 1/2 load, how can you possibly save 50% of the energy?

Now let's break that down further. Of the 15% (if even that high) in total losses, about 4% are iron losses (magnetic hysteresis and eddy current), 4% are copper losses (wire resistance I2R), and the remaining is split between friction (bearings), windage (cooling fans) and sound. Of those 5 elements of losses, the only part that is reduced as the voltage is reduced (which is what the Nola circuit does) are the iron losses. Reducing the voltage can possibly reduce those losses by 50%. So their claims of "up to 50% energy savings" is a scam. All they can do is reduce some of the losses by 50%. But when you factor that out to the actual energy, it is 50% of 4% of 50% of the total motor full load, which ends up as 1% of the motor full load power AT BEST on the most inefficient motors. So apply that then to your 120kW motor, that is 1.2kW. Not really that insignificant, but it is a far cry from what they claim. In addition if your motor losses are only 10% total, the total that can be saved is only .6% of full, so now it's only 750W.

And don't forget, that ONLY counts if your application meets the other criteria stated above; not bypassed, ventilated enclosure into a clean dry but non-conditioned air space.

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#6

Re: Soft Starters vs. DOL Starters

02/16/2010 11:18 PM

Dear All,

True stuff posted, apart from this i would like to add one thing, By using soft starter for heavy motors reduces KVA demand also. There is a nominal saving of energy during starting. reducing starting current and cable losses. saving cost by avoiding extra curreny carrying cables and switch gears.

Thanks

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#7

Re: Soft Starters vs. DOL Starters

02/16/2010 11:30 PM

Hi Rooney. While the answers given are very precise and very accurate, to which nothing can be added, I must ask why? With such a small motor what is its application? You might find that the cost of a soft start unit not worth the effort if you study the cost of purchase, installation, and expected extended run life of the motor/system. Yes that would be offset by the power savings & maintainence savings on the system, but recouping your install cost may take a very long time.

However you could consider a small VSD (variable speed drive - variable frequency drive). As the "guest" stated, starting the motor and bringing it up to speed over a period of time is much better for the whole system. This is exactly what a VSD does, soft start and speed control all in one.

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#13
In reply to #7

Re: Soft Starters vs. DOL Starters

02/17/2010 9:46 PM

Hello !

It a nice input.

But still if we compare, a VFD with a Soft starter, a VFD gives more benefits.

But can you tell what would be the %age savings through a VFD.

I need to take my management in confidence as regards the financials - savings, investment and the payback period.

Regards,

Rooney

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#14
In reply to #13

Re: Soft Starters vs. DOL Starters

02/17/2010 10:01 PM

Rooney any % savings would have to be calculated by you. I do not have cost figures of equipment to be purchased, cost of power per unit, install costs, projected long term savings, etc. The % savings you may be talking about, re, in your original question, can only be studied by you and management. for me to give an accurate answer would need careful study of the facts, in-depth knowledge of your company expenditure limits and then the final answer might not sit well with your idea.

I suggest that you get someone who knows what you want, knows your limits cost-wise and get in touch with a VSD supplier, who do this kind of thing as part of their business.

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#16
In reply to #14

Re: Soft Starters vs. DOL Starters

02/18/2010 12:13 AM

Dear Brich,

Thank you very much.

Actually i wated to have an idea with respect to the savings comparison in %age that could be achieved by a VFD vis a vis a soft starter.

If that general comparison in not available , it is ok.

Thanks

Rooney

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#18
In reply to #16

Re: Soft Starters vs. DOL Starters

02/18/2010 2:02 AM

Hi Rooney

You are welcome, it is not that general comparisons are not available, it is because any comparison made at this point would be way way off what you'd need to know due to the lack of exact details which are not available in this forum.

If you contact a VSD supplier, in fact contact several, tell them exactly what you require, they will offer you a complete service as they want to make the sale. As they are "in country" they will have "local" knowledge, understand your particular difficulties and requirements and give you the complete package, which could include purchase costs, saving comparisons, installation costs and a service contract.

Then it is not a general comparison, but exact the cost of purchase and projected overall cost savings.

What would be good is if you do go ahead and get a supplier, buy the VSD, get it installed, let us all know. We all give our advice freely, but rarely get any feedback.

I hope it all works out OK for you.

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#8

Re: Soft Starters vs. DOL Starters

02/16/2010 11:32 PM

The advantage of using soft starter over DOL mainly the requirement of transformer /Generator capacity minimisation since the starting current can be minimised in soft starters.

Secondly it has another advantage that the starting current can be minimised by lengthening starting duration which is variable in soft starter while it is being programmed.

CVS Murthy

09866550981

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#9

Re: Soft Starters vs. DOL Starters

02/16/2010 11:33 PM

The life of the motor also will be better as by using soft starter the torque on the motor is less.

Energy als will be saved.

CVS murthy

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#10

Re: Soft Starters vs. DOL Starters

02/17/2010 12:15 AM

Hello,

Firstly using a DOL for a high rating motor is not recommended so it is good that you are planning to switch over.

The next point that I wish to make is about soft starter. Soft starters do not inherently save energy. There are some devices which have got soft start + energy saving features. Moreover soft starters are known to introduce the problem of harmonics due to the usage of solid state devices. So ironically they could result in loading of cables and equipments and add on to losses rather than preventing them.

Answering your point number 1 - advantage of using soft starter is just reduced mechanical stress. Which could be achieved by a reduced voltage starter like Auto Transformer Starter (with Korndorfer closed transition technology).

Life of motor will be improved due to reduced stress. But soft starter do not guarantee 100% removal of transient which is possible in the previously mentioned ATS - which definitely improves the life of motor to a great extent.

Hope you find this useful.

Regards,

RRV

www.econserve.in

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#12
In reply to #10

Re: Soft Starters vs. DOL Starters

02/17/2010 4:00 PM

Soft starters only introduce harmonics during acceleration, roughly 30 seconds worth at each start. This is considered a "transient event" when determining harmonic content and is not a factor in mitigation strategies, cable heating etc. DOL starting creates a lot more power disturbance than a solid state soft starter, Star-Delta is even worse than DOL.

Autotransformer starters, Korndorfer or not, do NOT really reduce mechanical stress as much as people think. There is always a transition spike in torque, so all you are really doing is trading one big hit for two smaller ones. A Solid State starter has no transition, it is a smooth application of power and is truly gentle on the mechanical systems.

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#11

Re: Soft Starters vs. DOL Starters

02/17/2010 11:50 AM

Hi.

Buy a VFD, it's the better option for energy savings, plus you can control the speed of the motor, the aceleration and desceleration curves, among other things, i sugest Yaskawa

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#15

Re: Soft Starters vs. DOL Starters

02/17/2010 10:46 PM

In my experience, we have used soft starters in applications where the motor is required to start under full load (without having the benefit of redesigning the drive system). Specifically, a loaded conveyor belt.

The soft starter allowed us to produce starting torque without extended periods of near locked rotor current on the motor, which had been the problem. In other words, we were able to control the current going through the motor at a level that would not trip the overloads or instantaneous overcurrent device while the belt came up to speed (which could have been over 30 seconds).

Secondarily, it does greatly reduce the mechanical shock on a rotor AND and the stator helping to prevent premature motor failure.

Additionally, it MAY save you some $$ in your power bill if you are constantly starting the motor during peak times when the utility will nail you for peak draw (even if it's only 10 seconds long).

As a point of disagreement, since most soft starters close across the line after the programmed starting time, it offers no special features or benefits once you are up and running and you certainly CANNOT reduce the cable size for the motor. If you have an 80 FLA motor, you will still need a cable rated for 100A at 75 degree C rise.

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#17

Re: Soft Starters vs. DOL Starters

02/18/2010 1:10 AM

Soft staters are like the usual Star/Delta Stater, Auto Transformer stater, rotor Resistance, their function is to reduce current in rush during motor starting that is usually accompanied with voltage instability and distorbance of the supply system.

The soft starters are very good noisless systems that make use of solid state devices.

The changing over time remain the same as preset and not detered by any external influnce.

The accompaning protection of the soft stater qurantees the motor for a longer life as issues of low voltages and single phasing, etc. are taking care of.

But repairs can not be garanteed as only special skilled personnel can achieve that.

where as the electromechanical contactors used for same controllers as indicated above can be repaired easily in the factory.

Calculate ur power in is star, calculate ur power in delta see which of them is smaller. The smaller the most recommended

Or u can calculate ur current for star and delta use them to calculate ur power saving.

Dickson

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#19

Re: Soft Starters vs. DOL Starters

12/18/2019 2:56 AM

I have a grey beard.. it's true... and I hope you are mostly newly graduated engineers, because you do not fill me with confidence. There is much more to this story.. and its kind of complicated because it probably needs a mechanical engineer to explain it (I am electrical/electronic).Torque.... turning force. What is the nature of your load. Torque is proportional to current. Soft starters limit voltage.. reduce current.. try starting a loaded rock crusher with a soft starter and watching the guys spend half a day digging it out. DOL delivers maximum torque.. your drawing x 10 FLA current for a reason baby! Oh and be careful of new motors... new high efficiency motors draw way more than 10 times FLA. VSD's if intelligent can draw 100% rated Torque at zero speed.. which is nice. But who knows what a LRS is and why it might be desirable?

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#25
In reply to #19

Re: Soft Starters vs. DOL Starters

12/22/2020 2:54 AM

Dig those TLAs, Man!

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#21

Re: Soft Starters vs. DOL Starters

06/15/2020 4:21 AM

The difference is how they act while getting to full speed. When comparing the voltage between a Soft Starter and a DOL Starter, we can see that there is a direct inrush of voltage on the DOL where the Soft Starter takes more time to get up to full voltage. The current is more regulated using a soft starter

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#22
In reply to #21

Re: Soft Starters vs. DOL Starters

12/20/2020 4:52 AM

Starting motor torque is proportional to the square of the voltage limited by the current regulation by the softstarter. A few people said that you have to pay for that detail because a lot of people threw money at its installation and the situation that all possible settings can't start the machine or I can't get it to nominal speed .

Good luck and think twice before pulling for your wallet

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#23

Re: Soft Starters vs. DOL Starters

12/21/2020 6:00 AM

Why does the <...want...> precede the evaluation and assessment?

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#24

Re: Soft Starters vs. DOL Starters

12/21/2020 1:05 PM

A1) An advantage might be the reduction in voltage dip elsewhere on the supply network during starting.

A1) An advantage might be that whatever it is driving starts more slowly.

A2) it has the potential to increase it.

A3) it’s negligible-to-negative.

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#26

Re: Soft Starters vs. DOL Starters

12/28/2020 5:35 AM

what is the driven machine?

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#27

Re: Soft Starters vs. DOL Starters

12/29/2020 8:19 PM

One other advantage that was not mentioned, depending on the soft starter you using, you can increase the number of starts per hour. From memory a 120kw 4 pole S1 motor is good for 4/5 starts per hour. A decent soft starter will allow you to increase this amount significantly.

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#28
In reply to #27

Re: Soft Starters vs. DOL Starters

12/30/2020 5:14 PM

One other advantage that was not mentioned, depending on the soft starter you using, you can increase the number of starts per hour.

Commonly held myth...

The amount of energy it takes to accelerate a motor from a dead stop to full speed is finite, there is no way to change that. The only thing a soft starter does for you is to stretch out the curve; instead of it being of short duration with a high peak, it is a lower peak and longer duration. But the heat expressed into the motor windings is always the same.

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