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Reduced stopping time with a soft starter

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#1 BigTrev1


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Posted 12 September 2005 - 09:41 AM

I have an application where the load (industrial grinding machine) is presently started by a soft starter. We normally free wheel stop, but now want to consider safety and try and stop the grinder more quickly. I have done some investigation and am considering "plug braking" by reversing the motor phasing and starting the soft starter again. I guess I will need a zero speed detector of some sort to prevent the motor load from ramping up in the reverse direction.

I don't need to stop the grinder as quickly as it starts so was considering using a lower current limit when "plug braking". Does the motor produce the same torque/current vs speed profile when doing this? Is is possible to work out what the stopping time is for a given current limit? I have all the load details such as motor info, gear ratio and load inertia.

Also, the soft starter has DC injection braking on it. The maximum current that it can inject is 100% for 60secs. I don't know if the 100% relates to the soft starter size or the motor size. Will 100% current be sufficient to help stop the motor? I read another thread on DC injection and this helps explain what goes on in the motor but I was wondering is there any formula for working out the actual stop time? As mentioned above we don't have to stop it as quickly as it starts. (it starts in 43 secs with 350% FLC. I hope to stop it in 180secs)

Our site safety comittee needs me to state some figures. If they approve then I will have to make it stop in the time I give.

Any help would be appreciated.

#2 jraef


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Posted 12 September 2005 - 04:23 PM

Welcome to the forum BigTrev1,
Responses in order of your many (excellent) questions:

Yes, you will probably need Zero Speed detection. Definately if you are doing soft plug reversing only, plus it is a good idea as a backup even if you use DCIB. Your soft starter may have it built-in to the DCIB system for you, but usually that will not be usable for the plug reversing.

Yes, the torque/speed curve is the same, except of course at a current limit, it is lower than Across the Line (DOL).

Yes it is possible to work out the stopping time at a particular current limit if you know all the load information. It is the same as the starting time from a dead stop under the same circumstances. So if you determine what your starting time would be at for instance, 200% current limit, that will be your stopping time using soft plug reversing. One thing to be careful of here however. Your motor will already be warm from running, and the overload protection curve will essentially be a lot shorter than it would on a cold start. So be prepared for the likelyhood that you cannot extend your stopping time too long without causing an overload of the motor.

The "100% current" term applied to DCI braking systems built in to soft starters is more marketing spin than engineering reality. What they mean is that they can only deliver current up to the limits of the SCRs, so the 100% means "of the SCR maximum current". The usefullness of that current in DCIB is not what you may think. Most stand-alone DCIBs must be capable of 200-400% of motor FLA to be successful. If the soft starter is rated for that in startup, then OK. If the soft starter is marginal, that "100%" is just as marginal. That said, the fact that your soft starter already has it, and that you don't care how long it takes, means that you should at least try it first before you go to the soft plug reversing expense. Just make sure that the soft starter still maintains OL protection during DCI braking.

Calculating stopping time of a DCIB in advance is difficult because it also deends on the method of controlling the DC. Some are current controlled, some are voltage controlled, some are full wave, some are 1/2 wave. Hard to know which one without knowing the manufacturer and model. Even then, it is not easy to predetermine the stopping time. There are numerous papers written on how to do it, none of them have been accurate in my experience. I recently did one where the formula said it would take 3 minutes at 200% current, it came to a stop in 45 seconds! You just can't tell for sure until you try.

Although you didn't ask this, an excellent option for you if the DCIB built in to the soft starter is insufficient t do the job, is for you to use a combination of the two. Soft plug reverse it to a near stop, then finish the job with the DCIB. You could use a simple timer to determine when to shut down the starter and deploy the DCIB by trial and error method. This assumes that your DCIB in the soft starter has zero speed detection built-in as well. If not, you must add it, which, if you use one with an intermediary setpoint, you can use to shut down the starter at a specific speed instead of relying on the timer.

I'm concerned that your safety committee must see hard numbers. Electronic braking should NEVER be considered as a primary safety system. Don't forget; if power fails so does the brake! Electronic braking is mainly used to increase productivity, and as a braking choice only to save wear and tear on the true primary safety braking device, a mechanical system that employs in a fail-safe manner.
"He's not dead, he's just pinin' for the fjords!"

#3 marke


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Posted 12 September 2005 - 08:04 PM

Hello BigTrev1

In addition to the ex cellent reply from jraef, I would like to add the following:
1] Some of the later generation soft starters include the logic to determine when the motor reaches zero speed, both in plug braking mode, and in DCI mode. It may be possible to avoid the use of a rotation sensor depending on the particular soft starter that you are using.
2] The DC Injected braking on soft starters comes in a couple of formats, either you need to add a shorting contactor across two windings, or you reduce the injection level, or the injection only begins after the speed has fallen well below synchronous speed. If you apply DCI from a soft starter close to synchronous speed with a high brake current, there can be enough line frequency component present for the motor to continue to rotate at close to synchronous speed. The shorting contactor smooths the rotor current and reduces the line frequency component. Once again, this is dependent on the actual soft starter used.

Best regards,

#4 BigTrev1


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Posted 14 September 2005 - 08:57 AM

Thanks for your help jraef and marke

You mention some good points that I ahdn't given sufficient consideration to, namely the issue of safety when relying on electrical only means. I'll look further into this.

Here's another idea I had to try and determine how long it would take to stop with DC injection. If I work out the ratio of inertia of the load to the motor rotor inertia, then apply this amount of inertia (ratiometrically) to a smaller motor on the work bench, I could try the effectiveness of the DC braking with a test soft starter/motor combo. Do you think this would give me an idea as to how long it would take on the bigger motor (provided the soft starter is the size relative to the motor size)?

If this doesn't work then I think I'll go for the plug braking option and maybe combine the DC injection towards the end as you suggets jraef.

Thanks for the ideas

#5 mariomaggi


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Posted 18 October 2006 - 12:10 PM

One year later ....

Do you think this would give me an idea as to how long it would take on the bigger motor (provided the soft starter is the size relative to the motor size)?

No, there is no proportionality on small and big combinations.
The first parameter to consider is the load energy to be dissipated. You have to consider that almost all energy will be accumulated on the rotor during DCIB!

Il the energy is high (due to hogh speed and high inertia load), about the same quantity of energy will be dissipated only in the rotor mass. Therefore, knowing the rotor mass, you can calculate the peak temperature reached in the rotor. It is easy to find very high temperatures, over the maximum temperature accettable by bearings.


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