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

[lukin1977]

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

 

[LionelHutz]

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.

 

[DickDV]

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.

 

[lukin1977]

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

 

[DickDV]

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.

 

[LionelHutz]

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.

 

[jraef]

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.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[jraef]

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).


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[lukin1977]

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

 

[jraef]

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.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[lukin1977]

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

 

[davidbeach]

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

 

[DickDV]

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.

 

[jraef]

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
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

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

 

[jraef]

Oh, makes sense. My bad.

 

[lukin1977]

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

 

[itsmoked]

We are ALWAYS interested. Happy hunting.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

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