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

 

[dpc]

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.

 

[lukin1977]

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

 

[dpc]

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

 

[lukin1977]

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

 

[lukin1977]

The motors are 30HP - 3x380V - 50HZ each

 

[Marke]

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

 

[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. 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.

 

[lukin1977]

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

 

[itsmoked]

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

 

[dpc]

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.

 

[lukin1977]

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

 

[jraef]

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


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[itsmoked]

lukin1977; Your list looks correct.

You are talking around $1,500/VFD @ 30hp.

http://www.factorymation.com/s.nl/sc.2/category.41/.f

If you are talking one regenerative/synchronizing then you're probably looking at $3.5k~$5k for the one. Probably less money but also less flexible.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

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

 

[DickDV]

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.

 

[edison123]

LPS for you, jraef.

 

[lukin1977]

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

 

[DickDV]

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!

 

[lukin1977]

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