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VFD to use as a soft starter for 6 motor individualy
5

VFD to use as a soft starter for 6 motor individualy

VFD to use as a soft starter for 6 motor individualy

(OP)
Hi,
I want to install one VFD to be use as a soft starter in a wire drawing machine with 6 AC motors. They start individualy. 2 or more motors will never start simultaneously. My question is: ¿Is it posible to use the VFD from 0 to nominal speed and then conmutate the motor to the line voltage and then use the VFD to start the next motor and so on?


Thanks,
lukin1977

RE: VFD to use as a soft starter for 6 motor individualy

Yes, it's POSSIBLE, but I'm guessing it would be far less expensive and less complicated to simply use solid-state reduced voltage starters for each motor.

To use a single VFD, each motor has to be isolated from the VFD via a contactor and connected to the line via another contactor with proper overload protection, etc.  In addition, you have the required interlocking to make sure everything works in the proper sequence and you don't blow yourself up.  

It's a bad idea.

 

RE: VFD to use as a soft starter for 6 motor individualy

(OP)
ok, but, these motors need high starting torque from 0 speed and the soft starter reduces the starting torque. I need full torque at any speed. This is why I was thinking on the VFD
Is this right? The soft starter reduces the starting torque?

thanks

lukin1977

RE: VFD to use as a soft starter for 6 motor individualy

OK, then use 6 VFDs, it will probably still be cheaper.  What size are the motors?  

 

RE: VFD to use as a soft starter for 6 motor individualy

(OP)
I forgot the say that each motor already has a direct contactor start from main voltage supply. I need to install a soft start because the high mechanical shock on the start keeps breaking motors shafts, gears teeth, roller bearings, etc

lukin1977

RE: VFD to use as a soft starter for 6 motor individualy

(OP)
The motors are 30HP - 3x380V - 50HZ each

RE: VFD to use as a soft starter for 6 motor individualy

You could use a voltage ramp soft start with a bridging contactor, ramp up to full voltage fairly quickly and eliminate the torque "shock" but still get Full voltage torque.
Or you could use a soft starter selected for a high start current of say 500%. This will still give you a reasonable torque but reduce the shock loading.

It really depends on how much torque you need and what the motor curves are like.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

RE: VFD to use as a soft starter for 6 motor individualy

If you use one VFD for all of the motors, you need to have a time delay between opening the VFD contactor and closing the line contactor. The delay needs to be long enough to let the motor field decay, perhaps 0.5 to 1.5 seconds. Otherwise, you get a current surge and torque impulse at transfer. If the speed drops too much during the delay, you will still get a current surge and torque impulse. You also need to make sure the VFD can tolerate suddenly dropping the load by opening a contactor. I think most of VFDs are ok with that these days, but check to be sure.

This is something that has been done from time to time, but usually some other method is better.

RE: VFD to use as a soft starter for 6 motor individualy

(OP)
thanks to all

CJCPE:
I was thinking on that delay and that is exactly why I posted this thread. My doubt was how to do the conmutation from VFD to line and for this aplication it might be not possible to do it, because the drawn wire will not give me 1,5 sec to do it. It will slow too much the capstan and when I re-connect the motor to the line: BUM... shock again.

Probably the best choice would be to buy 6 VFDs. More VFDs but less contactors (30HP contactors are not cheap). Plus I can eliminate the mechanical shock when stopping the machine too. This "stop shock" is just as bad as the starting one

Marke: Do you recommend 6 VFDs or & Soft-Starters?

thanks,

lukin1977

RE: VFD to use as a soft starter for 6 motor individualy

Ouch six 30hp VFDs!

However think of the advantages.  You can slowly ramp the entire machine up to full speed taking minutes even.  You can run the machine all day at something other than full speed.  You can run it at a creep for threading, troubleshooting, maintenance, etc.  And of course no additional contactors as you've mentioned.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD to use as a soft starter for 6 motor individualy

I'm not Marke and he will answer more completely than I can, but many (most?) soft starters can also do soft stops as well as soft starts.  As long as your torque requirements during acceleration don't require current beyond what the soft starter can provide, they should be much less costly than VFDs.

But as Keith notes, with six VFDs, you can do about whatever you want with motors and maybe they will have benefits beyond just starting and stopping.  

 

RE: VFD to use as a soft starter for 6 motor individualy

(OP)
Yes. I understand. so is like this:

1st option, the best but the most expensive: 6 VDFs
2nd option: 6 soft starters, less expensive but less versatile
3rd option: 1 VFDs and contactors
4th option: 1 Soft starter and contactors, the cheapest

Where can I find cheap VFDs? haha

thanks,

lukin1977

RE: VFD to use as a soft starter for 6 motor individualy

4

Quote (CJCPE):

If you use one VFD for all of the motors, you need to have a time delay between opening the VFD contactor and closing the line contactor. The delay needs to be long enough to let the motor field decay, perhaps 0.5 to 1.5 seconds. Otherwise, you get a current surge and torque impulse at transfer. If the speed drops too much during the delay, you will still get a current surge and torque impulse.
This is indeed a problem, but there is an alternative. There are  few high-end VFDs on the market which offer what is called "synchronous transfer", wherein when you approach full speed, the VFD synchronizes it's output with the line supply phase angles so that the DOL bypass contactor can be closed without first opening the VFD output contactor. This does NOT reduce the number of contactors of course, it just eliminates that surge or speed drop on transferring the motor to DOL. I know for a fact that Siemens has this feature in their Sinamics G120 product line (because I work for a different division of Siemens and have seen it), but not in their other VFD products, so be careful. I also heard that ABB claims to have it as well but only in their ACS800 series, and probably one or two manufacturers. In general, this feature seems to be available in VFDs that can have a "line regenerative front end", even if that option is not selected. It makes sense though, because in order to use line regen, the VFD would need to be able to sense the incoming line phase angles. These are, as you might expect, the highest cost VFDs available in their respective product lines, but certainly they would be a lot less expensive than 6 individual lower-end VFDs.

In my previous job, I used to get this request a lot and worked up dozens of proposals to prove / disprove the economic viability on this scheme over the years. Here is my experience on costs:

1) It does indeed work out to be less expensive to use 1 VFD and multiple interlocked motor starters, UNLESS you must also be capable of reversing the motor starters. In that case, the 3rd contactor and relatively complex logic pushes the cost over the edge to where individual VFDs make more economic sense. One trick that a lot of people miss however is the motor overload protection. Make sure that the OL relay is in the circuit DOWNSTREAM of the common connection point of the VFD and DOL contactors, otherwise the motor starting current is not seen by the OLR and it could result in motor damage.

2) Multiple soft starters are sometimes less expensive, especially if it is very large motors, i.e. over 150HP (110kW). But this is not absolute and it depends on a lot of other mitigating factors. But certainly if you have limitations on starting power and/or you need long acceleration times, then the VFD method is a better choice.

3) In low voltage, it NEVER works out to try to do this with 1 soft starter and multiple motor starters; the soft starter costs are just too low to be worth it. Multiple LV soft starters will always end up costing less. However it DOES work out when discussing MV soft starters, because the soft starters are a lot more expensive. The caveat there however is starting duty. You usually cannot use a MV soft starter more than 2 or 3 times per hour. I know none of this was previously discussed here, I only bring it up for future reference


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RE: VFD to use as a soft starter for 6 motor individualy

Just to be clear, from my observation the VFD does not have to have regeneration, it just needs to be capable of regen as an option. With that comes the ability to sense the line phase angle. Probable cost for a 30HP VFD like that, around $2000 US.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: VFD to use as a soft starter for 6 motor individualy

If I've read the OP correctly, he states that he needs FULL torque to start and get up to speed.  Then, he states that, when he does that, he gets mechanical failures (couplings, etc).

Under those conditions, I'm not so sure any solution is goin to really help.

For my money, I would use a reduced-voltage softstarter and set the starting torque just high enough to break into rotation and then set the accel time to keep the amps about equal to the starting amps until full speed is reached.

No "softer" start is possible since breakaway torque sets the low limit.  If you are still breaking parts under this start arrangement, you need either stronger components or some way of reducing breakaway torque.

In my view, a softstarter is going to be as good at this as an inverter since inrush current doesn't seem to be the primary concern.

RE: VFD to use as a soft starter for 6 motor individualy

LPS for you, jraef.

RE: VFD to use as a soft starter for 6 motor individualy

(OP)
DickDV

Sorry for my english. Maybe I did not explain myself clear.
I need full torque from 0 speed to nominal speed, but the speed must be increased softly (ramp) to avoid the mechanical shock from the DOL start that the machine has now.

lukin1977

RE: VFD to use as a soft starter for 6 motor individualy

lukin1977, thanks for clarifying but it seems to match what I said in my post.  To repeat, if you need full torque at zero speed (that would be breakaway torque) then that sets the level of torque that your system must be capable of handling.

You haven't mentioned it but maybe your problem is due to slack or looseness in your mechanical components that must be pulled out before the full torque for accelleration is applied.  In that case, the breakaway torque would be lower until the slack is all removed and then the full torque is required to accellerate.

If that is the case, I still think a reduced voltage starter on each motor would be the least expensive but I would program the accelleration with "S" curve rounding at start.  This gives a really slow, soft start for a couple of seconds to get the mechanicals all tightened up and then proceeds to the faster rate to get up to full speed.

If I still don't understand, please explain again what you need.

Good luck!

RE: VFD to use as a soft starter for 6 motor individualy

(OP)
The full torque from the start is needed due to the nature of the load. It is a wire drawing machine. The oposition (counterforce) of the wire is almost the same at any speed. In fact is a little bigger at the start because of the higher static friction between the wire and the die.

All the mechanical part of the machine are good

I understand that I can use 6 softstarters like you say but I think that if the money difference between 6 softstarter and 6 VFD is not that big, maybe I go for the VFD so I can have the machine working at lower speeds if needed

I wrote to some dealers and now I am waiting their offers

thanks again

RE: VFD to use as a soft starter for 6 motor individualy

To me, mechanical shock occurs due to the sudden take-up of the slack in the drive system causing mechanical impacts in the drive system. For example, the gears in a gear-box banging together when the motor is energized and just begins to rotate. Once all the slack is taken up and the system is accelerating then it usually doesn't undergo any more sudden shock loads.

You are contradicting yourself. You say you need to reduce the mechanical shock due to full-voltage motor torque yet you then also say you need the full-voltage torque to start the load. If a full-voltage start is too much torque then a soft-starter will help. We've put soft-starters on wire drawing machines before so they do work in this application and I think we could help your situation.

In your list of solutions.

#3 is a very poor option. You need a VFD that can do synchronous line transfer or you will still introduce a mechanical shock to the system on transfer. You will also have a system where you can't VFD start any of the motors when the VFD fails and you are waiting for a replacement.

#4 should not even be an option because it will cost as much as #2.

 

RE: VFD to use as a soft starter for 6 motor individualy

lukin1977, I now understand that you are not dealing with lots of slack in your power train but only a very hard start due to the nature of wire drawing.

Wired across the line, most common AC induction motors will develop about 150-160% of their rated nameplate torque at zero speed (locked rotor).  Are you saying that this is more torque than you need and mechanical damage is occurring?  Or, when you use the term "full torque" are you referring to the continuous nameplate torque rating?

If the 160% level is what is causing the damage, then a soft start will help you lower that value to the continuous nameplate level at start.  If you are referring to the continuous nameplate torque, then nothing is going to help because you need that level of torque to start the machine turning.  If that level of torque is damaging the mechanical components with a smooth start, then the problem is with the mechanical components or the machine loading, not the starting torque.

I am trying to help you see that, if you need all of the torque to just start your machine moving, you will end up spending a lot of money only to find that you still need full torque just to get started---which was what the magnetic starter was doing for you in the first place.

RE: VFD to use as a soft starter for 6 motor individualy

(OP)
When I said full torque I was refering to the nameplate torque. sorry
I know you all guys are trying to help me. Thanks!

one question: Is there a max starting time with the softstart? I mean, for example: can I start from 0 to full speed in 120 secs?

thanks,

lukin1977

RE: VFD to use as a soft starter for 6 motor individualy

OK, lukin1977, I've got it.

As to ramp-up-to-speed time limits, some softstarters are capable of many minutes of accel time.  The lighter duty (ie, smaller heatsink) softstarters will have shorter accel times.  You will need to check the spec's for whatever you buy.

Based on your desire to limit the motor to its continuous running torque rating and avoid any excursions into overload torque, you can choose a less-expensive softstarter or a more-expensive VFD.  Both will give you the desired torque limit.  The biggest difference, other than the ability to vary running speed, is that the softstarter will still require some inrush current where the VFD will be able to limit the torque without any inrush.  You didn't mention any desire to eliminate inrush currents so I'm not sure this difference is of any value to you.

Hope this gives you the help you need.

RE: VFD to use as a soft starter for 6 motor individualy

It's very unlikely you can actually stretch out accelerating the motor to 120 seconds using a soft-starter. There are soft-starters that can handle a 120 second start but once you hit enough torque to accelerate the motor it usually accelerates quicker than that.

You want less torque than full-voltage gives? A soft-starter will do that. You want to control acceleration time to long ramps? A vfd will do that. You want to run at different speeds? A vfd will do that.

 

RE: VFD to use as a soft starter for 6 motor individualy

That is essentially true, although there is what is called "linear acceleration" using a tachometer feedback loop to the soft starter available still in a few brands (albeit a dieing feature). However, remember that a soft starter is providing INCREASED current to the motor in order to generate torque because the frequency is fixed. So it is unlikely that a standard motor can accelerate for 120 seconds without bypassing the overload protection scheme, which risks motor damage. If you want long acceleration like that, use a VFD.


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RE: VFD to use as a soft starter for 6 motor individualy

I'll also add that years and years ago, when VFDs were extremely expensive compared to soft starters, we used to use soft starters on winders and wire drawing machines, often with Tach Feedback as I mentioned above. But the results, although better than magnetic clutches, were far from spectacular. Once VFDs came along, almost everyone in that industry stopped using soft starters because all of the headaches in adjusting and tinkering with the soft starter settings over time (as the machines wore) were almost completely eliminated by using VFDs.

By the way, now that we know it is a wire drawing application, trust me, use the 6 separate VFDs; your life will be easier. And make sure they are at least open-loop vector type (although that is now relatively common).


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RE: VFD to use as a soft starter for 6 motor individualy

(OP)
Thanks!!
I received a very "nice" offer for 6 new VFDs

Brand: Delta
Type: VFD-220B

to be used for 6 AC motors - 30HP - 3x380V - 50Hz

Are they good?

thanks

lukin1977

RE: VFD to use as a soft starter for 6 motor individualy

I work for a competitor, so take this for what it's worth, but the general consensus in the VFD industry is that Delta is one of the cheapest drives on the market. If this were a simple conveyor, pump or fan application I wouldn't worry too much about it. But on a winder? If your vendor is very good and has good technical support available to you, then that is a more important criteria. But if they are offering it to you on-line or from a distance, I would be wary. Most of the low cost suppliers are able to do so by making the customers support themselves; the price is low in order to go after the volume OEMs who will have their own support staff.


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RE: VFD to use as a soft starter for 6 motor individualy

(OP)
thanks jraef
the invoice is very poor indeed (seller´s address is missing, just a PO box) we think this guy just want to steal money from us. We contacted him from his ebay store

Yesterday I wrote directly to DELTA and now they are preparing an invoice

The ebay price is 999 USD each

lukin1977

RE: VFD to use as a soft starter for 6 motor individualy

Buying drives off ebay?  Gee, want to buy a bridge?

RE: VFD to use as a soft starter for 6 motor individualy

Good grief! And a sensorless vector application too.

Please do your self a favor and buy a mainstream drive with sensorless vector capability and local programming and application support.

When its all said and done, you will be glad you did.

RE: VFD to use as a soft starter for 6 motor individualy

Huh, I'm confused as to how Delta is going to provide you with a receipt for something you bought off of a Flea-Bay vendor, nor do I understand what that peice of paper would be worth.

If you have not already purchased them, do yourself a favor. Call whatever toll free line is provided by Delta (if any) and ask them to help you step through the setup and programming tasks specific to your application. I'd be willing to bet they will either tell you a) you need to contact your vendor for that, or b) they have some liability reason why they can't help you. Option c) is the one I usually run into on cheap drives: the phone just rings off the hook and they never answer it.

Another thing I would do when buying ANYTHING off of Flea-Bay, and especially on VFDs:

Ask to see the Serial Numbers on the VFDs, then call the manufacturer to see how old the product is. A lot of these things that end up on Flea-Bay are old dead stock that distributors dumped when the manufacturer would not take them back. The problem peculiar to VFDs is that the capacitors have a shelf life. If they have been un-powered for more than a year, you must go through what is called a "reforming" procedure where you slowly increase the voltage applied to them over the course of a day, usually with a Variac (variable transformer). If they have been un-powered for over 5 years, they are likely boat anchors and will pop the caps as soon as you energize them. It usually costs more to replace the caps than they are worth.

Let us know how you fare.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: VFD to use as a soft starter for 6 motor individualy

jraef; I think that was a slip of the pen.  I think he meant a 'quote' not an 'invoice'.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD to use as a soft starter for 6 motor individualy

Oh, makes sense. My bad.

RE: VFD to use as a soft starter for 6 motor individualy

(OP)
Sorry,
yes, is a quote. an offer. a proforma invoice

We are not going to buy from the ebay.

If we buy something we have to do an "importation" from the US. So we probaly buy from DELTA

We are comparing prices. You would ask: Why dont you buy from your local vendor?... well, the answer is: they are extremly expensive and dont even have technical support.

Like I said, we are comparing numbers. This machine has been working with DOL start for years. We are not in a hurry to do anything
I will let you know how all this ends. If you are interested of course

thanks

lukin1977

RE: VFD to use as a soft starter for 6 motor individualy

We are ALWAYS interested.  Happy hunting.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD to use as a soft starter for 6 motor individualy

Of course we are interested - otherwise we might have to do actual work.  cheers

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