Rating of Soft Starters !! 3

[HarryDampers]

Can anyone point me to any papers on the rating of soft starters according to IEC or similar ?

It seems to be a bit of a mine field with some manufacturers offering 5 times current for 30 seconds , 10 times per hour ...........and some offering 3 times for 10 seconds , 6 times per hour (usually these are the starters with built in bypass contactors)

Obviously we are not comparing like with like .

 

[Marke]

Hello HarryDampers
Long time no see!! - A very good question!!

I have a small amount of information at

http://www.lmphotonics.com/sstart.htm

and

http://www.lmphotonics.com/forum/viewthread.php?tid=177

but you are correct, there is an IEC rating but this does not specify how the units must be rated, it is a format that uniformly tells the customer the ratings the manufacturer puts on the unit. This is further complicated by the fact that there is no standard way that the manufacturers derive these ratings.
The constraints are thermal, just like an electromechanical contact. The correct rating is based on the junction temperature of the Thyristor, ensuring that under the stated conditions, the temperature will not exceed a nominated temperature (which should not exceed the rating of the device). Many units have been traditionally rated on the basis of fuse curves that have an I2t rating equal to that of the SCR. This is totally incorrect as the overload curve of the fuse is not matched to the thermal curve of the thyristor and heatsink.
If the manufacturer is doing their job correctly, they should be able to tell you the device rating under your starting conditions. Some can provide software to enable you to match their starter to your load.

Best regards

Mark Empson

http://www.lmphotonics.com

 

[GGOSS]

Hello Harry,

The format for specifying ratings for soft starters to IEC is well covered by the standard IEC60947-4-2 or the Australian/New Zealand equivelant, AS3947-4-2.

You will note the standard does not actually specify levels for starting current, starting time, on-load factor, starts per hour etc., rather the format in which the manufacturer must provide the information.

Most manufacturers and/or suppliers of soft start equipment have access to software that enables them to review the ratings of their respective products in accordance with 'user specified' information relating to operational duty. Needless to say, this also enables them to conduct a more 'apples for apples' comparison of ratings against opposition product.

The link provided by Marke is certainly worth a look.

Regards,
GGOSS
PS: Good to see you posting again!

 

[jbartos]

Suggestion: Visit

http://www.schneiderelectric.com.tr/ftp/products/pcp/ats/ats46_characteristics_en.pdf

for current adjustment:
2 to 7 times the motor nominal (full load or rated) current,
and
5 times the starter current limit
The compliance with any industry standard may or may not be found in the product literature easily.

 

[jraef]

HarryDampers,
The IEC reference made by Marke and GGOSS above is the closest thing available to industry standardization, and as they said, it only standardizes the data format, not the attainment of the data itself. I feel that this is a need that must be addressed in the marketplace by people such as yourself asking intellegent questions and wanting to know more. If you as a specifier demand clear ratings and verified test data, we as manufacturers must (eventually) respond. Unfortunately the majority of users make buying decisions on comparatively less important issues such as size and initial cost.

As a result, manufacturers are resorting to the cheaper/smaller/prettier demands and as you have noticed, users are not always getting what they thought they were (or what they used to get). There are now starters on the market with no heat sinks or starters with 2 sets of SCRs and a piece of bus bar for the 3rd leg. They all work, but leave a lot to be desired and are totally unsuitable in many applications. On the other hand, if you are starting a centrifugal pump that MUST be at full speed in 10 seconds and will do so at only 300% current, a heavy duty 500%/60 second design is somewhat of a waste.

Bottom line: if you want to make sure the soft starter is NEVER the limiting factor in the success of your application, or you want to be able to use it on any other application later without worries, buy the heaviest duty unit you can afford. If you are sure that the soft starter will be dedicated to one specific installation and the conditions will never change, determine the minimum requirements and buy to siut it.



Quando Omni Flunkus Moritati

 

[Farkel]

If you want a good soft starter, look at the components that go into it. The SCR voltage and continuous RMS FLA is a good start. Look at

http://www.eaco.com,

for one of the manufacturers information.

 

[HarryDampers]

Thanks for the input guys and Hi to Marke and Mr Goss .

BTW Marke I could not follow your links ........something to do with bandwith being exceeded.

What I was hoping for was a nice article , clearly written with all the pros and cons laid out .

I have a copy of AS 3947 in front of me as I write ........It is a dog of a document and can surely only be intended for manufacturers .......It seems to have been designed to confuse the hell out of anyone that wants help in specifying starters .

 

[GGOSS]

Hello HarryDampers,

Unfortunately LMPhotonics has been down for a couple of days now. Not sure what's going on there but I'm hoping all will resolved soon.

I spent a lot of time some years ago reproducing AS3947-4-2 for the purpose of educting our own staff and industry in general. If you are keen to know more, please provide an email address to which I can forward the document or sections relating specifically to starter ratings.

Regards,
GGOSS

 

[HarryDampers]

GGOSS , I would love to have a look at what you have done .

send it to me at freo2001au@yahoo.com.au

thanks .

Harry

 

[jbartos]

Suggestion: The soft start is not intended for the full voltage across the motor terminal even though the softstarter manufacturer may provide the softstart output capable of the full voltage across the motor terminal start. Then, the softstarter purpose is only for motor starts that allow the softstarter to start the motor softly. However, if all motor starts require full voltage across the motor terminal to start the motor, then a proper justification of the softstarter is needed. Also, the softstarters have some harmonic content on their outputs. A harmonic filter on the softstarter output may be needed to suppress the harmonics.

 

[GGOSS]

jbartos,

People install soft starters for 2 reasons:

1. To reduce electrical stress.
2. To reduce mechanical stress.

Therefore even when the full voltage is required to accelerate the load to rated speed a correctly sized soft starter will provide mechanical benefits not achieveable with other forms of motor starting.

On the subject of soft starters and harmonics all I can say is that I have been heavilly involved with soft starters for more than 20 years now and cannot recall a single site where harmonics have caused an issue. Harmonic distortion eminating from soft starters are low in magnitude, they are also transitional ie present during soft starting & soft stopping unless of-course the start also provides control during run eg energy savings, phase rebalancing etc etc.

Regards,
GGOSS

 

[HarryDampers]

[blue]The soft start is not intended for the full voltage across the motor terminal even though the softstarter manufacturer may provide the softstart output capable of the full voltage across the motor terminal start[/blue]

I am not too sure that I understand you correctly JBartos .........some Soft Starters most definitely DO allow you to start the Motor DOL , technically it is not difficult as long as your thyristors are rated for it.

This can be a useful feature for a situation where one or more of the Thyristors has failed .Obviously , the major purpose of a Soft Starter is to provide a soft start ........so this would for "emergency" use only .

Nevertheless , it is a "nice to have"

 

[GGOSS]

Hello HarryDampers,

You may be interested to know LMPhotonics is back up an running. Try the link suggested by Marke for more details on starter ratings.

Regards,
GGOSS
PS: Did you receive my email?

 

[GGOSS]

Hello HarryDampers,

I would suggest you try the Tech Notes section of the AuCom web site

http://www.aucom.co.nz

for further information. Should this together with information collected from

http://www.lmphotonics.com

be insufficient for you, I would be happy to provide more detail on request.

Regards,
GGOSS

 

[Marke]

Hello HarryDampers

I am not sure that you actually got the answers that you were after.
As an end user, the ratings must often seem to be very confusing and the relativity between different units is very difficult to understand.
Perhaps, as a soft starter designer, I can offer my comments. If we look at applications, there are a number of different requirments that need to be met when applying a soft starter to an application. The botttom line though, is that the motor must produce enough torque for sufficient time to accellerate the machine to full speed. To do this, the starter must be rated to supply sufficient current for the required time.
If we step back and look at the total package, we have a driven load, a motor, a starter and the supply. The driven load determines the required starting torque and time. The motor converts electrical energy to mechanical energy and determines the start current required. The soft starter controls the start voltage or current and the energy comes from the supply (provided that it is strong enough).
Many easy to start machines will require a start torque of around 40%. These machines can be off load conveyors, centrifugal pumps, bypassed screw compressors etc. Other machines may require 100% torque or more.
Having studied a very wide range of motors available in New Zealand, I have found that the ability of the motor to develop torque under high slip conditions varies dramatically, but the average current to develop 40% torque for small to medium sized motors is in the order of 300%. Bad motors may require 450% current. As the motor size increases, the current required to develop 40% torque increases, so for very large machines, the required current may be more like 400 - 450% for a light duty start. For heavy starts and high inertia loads, the start torque may be 100% or higher resulting in average start currents for small machines in the order of 450 - 550% and of course higher for large machines.
Low inertia machines may start in 10 - 15 seconds while high inertia machines may require 40 - 60 seconds. Yes, there is a wide varience and it is very difficult for a brochure to cover all of these combinations.
Some manufacturers try to give some sensible options by giving light duty, medium duty and heavy duty start ratings. Other manufacturers prefer to quote the ratings under optimum start conditions to make their product seem more attractive. You can find ratings at 200% current for 5 seconds, and the only machine that you will start is an open shaft motor!! apply this rating to a real machine and you are overloading the starter, but to the uninitiated, it appeared to be the cheapest option.
I believe that you need to asses your application, determine the start current / time that you require and then go to the market to find starters rated for those conditions. If not in the brochure, then go to the suppliers. If they can not help, then try another brand.

There still is a major varience between the actual ratings applied by different manufacturers. I have reverse engineered soft starters and determined the ratings that I would apply and in many cases, I believe that the starters are overrated and this shows up by early failure in the field when working hard.
The method of calculating the ratings is not covered by a standard, just the method of expressing the ratings. This is also true of contactors, with some rated for 500,000 operations and others, 3,000,000 operations. It is difficult to ensure that you are comparing like for like.

I would suggest that to correctly apply, select a soft starter, you must first engineer the starting application, if possible selecting the correct motor for the job, and then apply the starter that you believe is fit for the purpose based on the manufacturers ratings and your confidence in that manufacturer / sales outlet.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[GGOSS]

A few people here have made reference to IEC 60947-4-2 (and various derivatives thereof) as this standard outlines the format in which soft starter ratings information is to be presented, either on the product or in the manufacturers supporting literature. The following is a summary of the information sent to the originator of this thread via email.

Ratings format outlined by the standard for soft starters intended for Continuous (non-bypassed) operation

Rating Amps: AC53a: X - Tx: F - S

Ratings format outlined by the standard for soft starters intended for Bypassed operation

Rating Amps: AC53b: X - Tx: Off-time

Where;
AC53a = Soft starters intended for starting of squirrel cage motors, starter rated for continuous operation.
AC53b = Soft starters intended
X = Overload current (starting current)
Tx = Overload time (starting time)
F = On-load factor
S = Number of operating cycles per hour
Off-time = Time between starts

Table 4 of the standard provides suggested values of X & Tx. As the starter is intended for use with a motor, the suggested values are in accordance with motor overload protection trip classes 10A, 10, 20 & 30. The standard provides preferred values for F & S in addition to providing guidance on the minimum ambient temperature at which the ratings be calculated.

Although the above would initially appear complete, there are some 'deficiencies' as implied by both Marke & Jraef through their own posts.

1. The ratings format described by the standard is used by manufacturuers to calculate ratings for their products. They/their marketing people are free to select values of X, Tx, F & S which provide the best possible rating for their respective products.

2. The values of X, Tx, F & S adopted by many manufacturers do not reflect that of typical of industrial applications. Also, as these values adopted are different from manufacturer to manufacturer, it is extremely difficult for the end client to evaluate and/or compare the ratings of products of different manufacture. I believe this was the primary concern of the originator of this thread.

3. The prescribed ratings format does not allow for additional SCR heating arising from controlled stopping. It should be noted controlled stopping can result in overload currents and times similar to that of the starting condition.

4. The only way for an end client to be certain he/she is receiving a product that is suitable for the intended application it to advise all potential suppliers of his/her prefered values of X, Tx, F & S together with any other application/installation specific details that may influence ratings. With that information available the supplier is obligated to recalculate ratings and offer a product accordingly.

For further information, or to assist with clarification of the above, please feel free to visit the Tech Notes section of

http://www.aucom.co.nz,

relevant papers include;

a) How to read AC53 soft starters utilisation codes
b) Calculating required starting current for new or existing applications

Hope this helps.

Regards,
GGOSS

 

[jraef]

Nicely put GGOSS.



Quando Omni Flunkus Moritati

 

[jbartos]

Suggestion to GGOSS July 14, 2003 posting: Do you happen to have specific numbers available to those relationships posted above as an example?

 

[GGOSS]

Hello jbartos,

Would I be correct in assuming your question relates to specific values (numbers) for X, Tx, F & S? If so, the answer is that the standard does not specify values, rather it leaves it up to the manufacturers to use their own discretion.

Unfortunately this makes it very difficult for consumers, particularly when they want to compare products of different manufacture. Therefore I would advise all prospective soft start users to advise potential supliers of their exacting requirements ie relating to X, Tx, F & S.
Those who adopt this policy will at least be assured that the products offered are suitably rated for the intended application. The decision to purchase one soft starter over another can then be made on the basis of features, available pre and post sale tech support, product availablility, pricing etc etc.

If the above does not answer your question, please clarify and I will add accordingly.

Regards,
GGOSS
PS: many customers experience difficulty in specifying values for X & Tx becuase these are directly influenced by the motor/driven machine combination. To further simply the selection process, we have produced a list of the more common industrial applications and allocated values of X & Tx based on experienced gained over 20 plus years. I would assume this is similar to the practice adopted by Fairford.