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How To Reduce Starting Current Required By Soft Starter To Start A 220 Hp Motor To Rotate A 12 Ton Flywheel


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#1 PANKAJ GUPTA

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Posted 04 November 2006 - 12:44 PM

TO ROTATE A 12 TON FLYWHEEL I HAVE USED A 220 HP 3 PHASE MOTOR WITH A SOFT STARTER [TELEMECANIQUE (ATS 48C41 Q) A BRAND OF SCHNEIDER]. IN MY APPLICATION THE RUNNING CURRENT WILL BE NOT MORE 100 AMPS, BUT THE STARTING CURRENT THAT IS REQUIRED IS 4 TIMES THE RATED CURRENT (IN 289) ON THE MOTOR, FOR THIS 550 KVA GENSET IS REQUIRED, MY PROBLEM IS I CANNOT GO ABOVE 300 KVA GENSET AS MY RUNNING CURRENT WILL NEVER GO ABOVE THIS RATING, AND IF I USE 550KVA GENSET IT WILL NOT BE COST EFFECTIVE. THEREFORE I WANT TO RUN THE EQUIPMENT WTIH 250 RO 300 KVA GENSET.

HOW CAN I REDUCE THE INITIAL CURRENT REQUIRED ?

CAN SOME ONE SUGGEST ME A WAY ?

IS THEIR A SOFT STARTER THAT WORKS ON STAR DELTA PRINCIPAL ?

IS THEIR A SOFT STARTER WITH 6 WIRE OUT PUT ?


#2 jOmega

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Posted 04 November 2006 - 01:16 PM

The simplest answer would be to consider using a Varible Frequency Drive (VFD).

This will allow you to accelerate the motor (and flywheel) either under current limit control or with an acceleration ramp time set long enough to keep the current from exceeding 100 % motor FLA.

Have done this in many such applications. Works Good!

With Genset though, you need to also be concerned about Harmonics on the mains from the Genset. The harmonic distortion has been known to disrupt the voltage regulator circuitry of the Genset. Some means of Harmonic mitigation will be required to prevent this from being a problem. This can be in the form of 5% line reactors or 5-7 harmonic trap filters. A harmonic analysis will identify the correct solution.

Good luck.


#3 jraef

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Posted 04 November 2006 - 05:35 PM

I agree. The problem you will run in to with ANY soft starter is that if you reduce the current to be able to use a smaller generator, you have reduced the torque so low that the motor may not accelerate. In your case I would guess this is an almost certainty. I have NEVER seen a loaded motor accelerate to full speed without the current getting to at least 200% FLA (In) at some point in the process and those that have, were vary lightly loaded motors. If you are at 400% FLA with your soft starter now, that is probably what it needs in order to create enough accelerating torque; any lower and you run the risk of stalling. The brand of soft starter has very little to do with that, it is a physics issue.

One experiment you could easily do with what you have now is to set your current limit lower to see if it is possible. If the motor stalls or overloads, don't waste any more time on that avenue. If the SOFT STARTER trips out on Over Temperature but you determined that the motor was still accelerating when it did, then there is a remote possibility that you can do it with lower current and a better rated soft starter. To be honest though, that is a very remote possibility.

If that doesn't work, then you have fewer choices available. One is a VFD as JOmega mentioned. Another popular choice with large flywheel loads is a magnetic drive system, essentially a magnetic clutch. The only other one I know of would be to use a Pony Motor; a smaller motor used to partially accelerate the flywheel, then finish it with the soft starter. That is still a risk however. The VFD or Mag-Drive would be the lowest risk of failure for you.



By the way, typing in all capital letters makes your posting very difficult to read. Humans read not only by looking at letters, but also by looking at the shapes of words. When you use all caps, the shapes blend into one another and force the readers brain to read each letter to make words, which slows down the reading process and subliminally frustrates the reader. Most people who use forums and email consider all caps to be a form of YELLING!
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#4 kens

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Posted 04 November 2006 - 07:17 PM

You asked about 6 Wire soft starters. Yes you can connect certain soft starters in an 'inside delta' configeration but this simply allows the use of a smaller starter, it does not affect the current required. I agree with jomega and jraef that the use of a VSD would allow a small genset to be used. Pay attention to jomegas point about harmonics.
Ken

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#5 jOmega

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Posted 04 November 2006 - 10:48 PM

PANKAJ GUPTA, et.al.,

One other important consideration ...... How do you plan on stopping the motor with the flywheel load ?

Could you share any of the application mechanical detail with us ?

Here's what would be important to know:
  1. Motor kW
  2. Motor Frame Size
  3. Rated Motor RPM
  4. Inertia of Flywheel
  5. Min. Time to Accel Flywheel _____ seconds to what ______ RPM
  6. Time for Deceleration of Flywheel ______ seconds to what ______ RPM (if other than zero)

If you decide to pursue the VFD solution, it will not be inexpensive, particularly if you also have to add a dynamic or regenerative braking control and harmonic filtering to it. I would offer that had the company spent a few more $$ to purchase a Genset with a power rating that would support DOL or Soft-Start acceleration of the motor/flywheel , it would have cost them less $$$ overall.

Seems there is an 'old' saying: $0.01 wise ; Kg foolish ......

But, from such experiences, hopefully, comes future wisdom.

Kind regards,

#6 PANKAJ GUPTA

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Posted 07 November 2006 - 11:54 AM

Dear sir,

First of all let me thank you all to have shown interest to my problem and your valuable suggestions.

We do not intend to have a 300 kva genset against a 550 kva genset because of the purchase cost of the same, my running load requirenment can be fulfilled by a 300 kva genset. By taking 550 kva genset the running cost to the customer is more therefore we want to use 300 kva genset.

Motor KW = 160
Motor rated RPM = 750
Rotar RPM = 560 (reduction by pully)
Estimated time to reach full RPM = 40 secs

My equipment is a shredder to shredder ferrous scrap, the inside rotar is approx 12 tons.
i have used hydraulic cylinders to lift the rotar during maintainence. i can use this hydraulic power to give my rotar the initial start through a clutch system and the rotar can be brought to 100 RPM with the hydraulic power, now on reaching 100 rpm if i start the motor through the soft starter, will it work with the limited current available (ie. on 300 kva genset).

#7 jraef

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Posted 07 November 2006 - 03:55 PM

It will not work to limit the starting current, but it will lower the amount of time that the high current is necessary, which may have the effect you are looking for. The only way to be sure is to run a simulation using the new lower delta-speed for your acceleration time calculations. In other words, your motor speed at the beginning of ELECTRICAL acceleration is now only 617RPM (750RPM minus the 100RPM from the hydraulic pony motor, adjusted for the belt reduction to 133RPM at the electric motor). Now when you recalculate your acceleration based on this and a lower current limit from a soft starter, it may fall withing the transient reaction time capability of your smaller genset.
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