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
