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Old October 22nd, 2018, 10:54 PM   #1
Timeismoney08
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Soft Starter vs VFD

I've noticed a resurgence in Soft starters in custom Automation.

Besides them taking less space in a cabinet, are there any other pros and cons between the two?

Note: I know there are Ip67 VFD that can be mounted remotely


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Old October 22nd, 2018, 11:44 PM   #2
GaryS
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Soft starts generally require a bypass contactor to pull in after the motor is up to speed.
Some Soft Starts have the bypass contactor built in other require them to be supplied as an extra ( additional cost and room to mount it)
if don't need the change the motor speed them a soft start may be the way to go
but if you need variable speed them you need a VFD.

Although I do remember I think it was Louis- Allis made a drive for vibrating feed conveyors using a soft start and varying the phase angle of the scr's to hold the motor speed down. they used a special motor with class H insulation the motors all ran hot enough to cook on. That's the way they were designed.
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Old October 23rd, 2018, 01:28 AM   #3
OkiePC
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Soft starters can take less space when you get above a certain size. They can cost quite a bit less, and they will generate much less heat (when up to speed and bypass contacts are closed).

Soft starters do not ramp smoothly (or much at all) when lightly loaded. Soft starters often have a limited duty cycle (amount of time spent ramping/using SCRs).
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Old October 23rd, 2018, 05:09 AM   #4
PLC Pie Guy
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There would be no noise generated from the soft starter. Lessening the need to run expensive drive cables to motors.
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Old October 23rd, 2018, 06:09 AM   #5
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Power companies are getting a lot more strict on harmonics mitigation and rightly so. Harmonic currents cause voltage distortion for other customers and require oversizing of cables and transformers. If you have a large percentage of non linear loads on your site then mitigation measures are important.

If you dont need variable speed or slow ramps then use a DOL starter if it's small (<2kW) or soft start if its bigger.

Drives of course have their place; but they're not a panacea for **** poor mechanical design where an engineer doesn't know how to size a pump or fan or whatever correctly.
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Old October 23rd, 2018, 07:34 AM   #6
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Soft starters and VFDs have been around for approximately the same time.
They are different devices for different applications, although you can use a VFD as a high-end soft-starter and soft-stopper. But since the price difference used to be so high, this was extremely rare. I have only seen it once. What I am saying, is that in the past you would not see VFDs being used where softstarters could have been used instead. So the scenario you describe does not exist IMO.

As for any new development in this area I would say that both soft-starters and VFDs are becoming more prevalent due to their dropping prices. But I definitely do not see a trend that soft-starters are being used in place of VFDs. Since VFDs are dropping more than soft-starters (relatively speaking) I would guess that it would be the other way around. Especially since you can get STO with most VFDs nowadays, that is an easy and cost effective way to achive a high safety Performance Level, and you dont have this feature with any soft-starter.
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Old October 23rd, 2018, 08:52 AM   #7
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I use soft starts on long ramps and heavy load applications. The data that I have been looking at shows that soft starts can handle a greater amount of overcurrent for a longer time vs a VFD.
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Old October 23rd, 2018, 09:46 AM   #8
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All depends on how hard it is to start the machine.

I used Siemens Sirius starters with 75kW high pressure centrifugal fans that have a very difficult start, the start was smooth, with 30 seconds of ramp and without any belt slip.
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Old October 23rd, 2018, 10:18 AM   #9
Gene Bond
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Quote:
Originally Posted by JeffKiper View Post
I use soft starts on long ramps and heavy load applications. The data that I have been looking at shows that soft starts can handle a greater amount of overcurrent for a longer time vs a VFD.
But, you need to watch for motor heat. For a long ramp time, I've used VFD's to actually save energy and reduce motor heat. For instance, a centrifuge, which may require a 2-3 minute ramp. Some motors can't handle it. Since you can ramp the VFD at 100% motor current, there is no overload condition, unlike the Soft Starter, which runs the motor in it's non-linear area of the curve.

On the other hand, there are loads where you need the extra little bit of motor torque to start, and the VFD has to be oversized, where a properly sized, heavy duty Soft Starter can hit a little harder...
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Old October 23rd, 2018, 10:30 AM   #10
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When to use a Soft Starter or an AC Variable Frequency Drive
https://literature.rockwellautomatio...p007_-en-p.pdf

Choosing between a soft starter and a variable frequency drive to fit your application
http://www.eaton.com/ecm/groups/publ...ct_1110244.pdf

Last edited by tarik1978; October 23rd, 2018 at 10:33 AM.
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Old October 23rd, 2018, 10:19 PM   #11
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Quote:
Originally Posted by JeffKiper View Post
I use soft starts on long ramps and heavy load applications. The data that I have been looking at shows that soft starts can handle a greater amount of overcurrent for a longer time vs a VFD.
I always set my SS for current limit instead of the default SS setting. The default actually allows huge current spikes if I remember correctly. But haven't worked on large SS in awhile.
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Old October 23rd, 2018, 10:31 PM   #12
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I was starting large saw blades. 120" Diameter. The ramp time was crazy important. If you allowed the system to start in a linear ramp function the blade would hit a harmonic that would cause it to throw carbide inserts. When we let it start with a current limit and it got to through the harmonics much faster. I can't say for sure why the harmonic stayed so long in time based start but it did.
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Old October 24th, 2018, 10:27 AM   #13
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We had an application where a siemens soft start was being used with a 48" centrifugal fan driven by a 25hp motor.

This was my first experience with a soft start, but after the second failure. we ended up replacing it with a very basic siemens VFD. Cost wise the VFD we purchased was about 50% less than the soft start, which was shocking to me.

Luckily the panel had plenty of room so we were able to fit the VFD which was probably 4 times the size of the soft start. Still not real sure why the soft start would periodically go bad. As I recall the first one lasted about 3-4 years and then the second only 2.

When the first one failed we tripped the 250amp breaker that fed the machine. We started noticing strange occurrences around that time before it finally bit the dust. Second time, we started noticing some of the strange occurrences again, and decided to yank it in favor of the VFD after again checking the motor.

VFD has been in service about 9 months now with the same motor and fan without any issues, we'll see if we get better results in the long run.

Anyone ran into something like this?
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Old October 24th, 2018, 10:48 AM   #14
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dwoodlock,
what were the exact type numbers of both softstarter and VFD ?
what were the exact data from the motor name plate ?
what were the "strange occurences" you mention ?
was there a throttle element in front of the fan to ease the startup ?
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Old October 24th, 2018, 11:12 AM   #15
dwoodlock
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VFD: Sinamics V20 6SL3210-5BE31-8UV0
SS: Sirius 3RW3036-1BB04

Motor: ATB 160L/2F-11S+E3/0703
18kw
460v 60hz 27.4A 3545rpm 0,9 cos theta
IE3 classification

Basically the occurrences were the occasional dimming of lights around the machine, but it would come and go and happened inconsistently. This was 3-4 years ago, so I honestly cant remember everything. I do remember it happened over a few weeks before the soft start finally died out.

Blower is not throttled in any way, basically setup as regenerative with heating elements in its loop.
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