Selecting a Soft Starter for a Screw Compressor

A solid state soft starter is ALWAYS a better choice than Star-Delta. A Star-Delta starter does not really reduce the starting current. It just trades one big hit for 2 smaller ones, and the second one can often be worse than just starting DOL if the timing is not perfect. The only advantage of star-delta is that it is the cheapest form of "reduced voltage" starting... as long as you ignore the fact that it is not really doing anything for you that is.

On top of that, when you transition from star to delta, there is a transition spike of torque that will often damage your mechanical components. This is especially true in screw compressors by the way, because they start unloaded and there is very little inertia to keep the motor spinning during the open transition. That means the motor will slow during that moment, and when reconnected it can still be a generator that is now out of phase with the line frequency. When reconnected in delta, that our-of-synch generator creates a massive spike that has been known to twist the shafts right off of the motor or compressor.

Thank you for the info. We too are looking at solid state or delta for our three new screws. I'm going to look into more info so I can let my boss know to pick a soft start. I hate what we have now just a big wham and they are on.

Thank you "reaf ", i have done some more excersize and find out soft starter it the best option as i am not looking from cost point of view . but one thing is trubling me is that screw compressor vendor is insisting that star-delta can give starting current lower than soft starter , i am not convinced but i don't find way to prove righ now .can you provide some logical approach.

thanks once again at right time.

regards

veer

He is absolutely incorrect about that. There is nothing that a Y-Delta starter can do that cannot be equaled or improved upon by a soft starter. If the motor will start with a soft starter, that means that it can accelerate with 33% torque, because that is all you can possibly get from a motor connected in the Y pattern. That is incredibly simple for a soft starter to accomplish and the line current draw will be identical between the two methods. If the motor CANNOT fully accelerate with 33% torque with the Y-Delta starter, then when it transitions to Delta the current will be HIGHER than if they had used a soft starter, because it will revert back to being the same current as DOL, maybe even WORSE!

Your compressor supplier is ignorant.

Hello,

I use to use electronic soft starts until I had to do some diesel powered screw compressors.

I used mechanical soft start on diesel powered screw compressors of up to 200HP.

Very economical compared to electronic controls and I have since used DODGE Flexidyne soft start couplers for all my soft start needs. Over 25 years without a single failure, (the oldest units are Saudi Arabia.) Check them out!

http://www.dodge-pt.com/products/pt_components/flexidyne/flexidyne.html

The technical superiority of soft starters over conventional starters for screw compressors and large capacity motors is well explained and the extra economical burden of buying them is also pointed out in previous replies .Well explained.

An engineer with experience will further consider the quality and competence of your organisation before making final selection. It means assess the quality of available power supply , frequency of power interruption and cost of maintenance of your organisation ,weather it can afford to employ technically competent engineers ,afford to keep sufficient inventory of spare PCBs and components for ready replacement, and permissible down time of compressors.Power supply quality means stability of voltage , frequency,harmonics etc. If these are positive go for soft starters otherwise if it is negative go for conventional starters. Reason if those are negative the soft starters are prone to frequent breakdown . Conventional starters can withstand these poor quality power supply and will not come under frequent breakdown and even if they come under breakdown can easily be repaired by ordinary electricians and spare parts are readily available in market and down time will be low .your vendor might have considered all these otherwise why should they recommend low value item .So think well before recommending to your boss later you may land up in problem.

Best of luck and good night

Mechanical soft start couplers do not care about how good or bad your electrical supply is.

And failure is almost non existent.

Think outside the box and be a hero.