Soft starter question

I have a milling machine that's starting to show its age. It has a 5 Hp, 208V, 3 Ph spindle motor that's currently on a full voltage Fwd/Rev starter.
Motor really slams when turned on and has damaged the variable speed sheaves and keyways.
Was thinking of adding a soft start system in series with the original starter. Considered a VFD but have heard that using one with a motor not designed for such makes problems. Even adding a line reactor to remove harmonic content may be required. All told, a soft starter appears to be the best choice.
Does anybody have a favorite unit to recommend? Ones I've seen so far have a SCR in series with two of the three legs and control starting voltage and ramp-up time. If you keep them powered, they can also control ramp-down time. A contactor bypasses the reduced voltage system after the ramp time. All fine and well but only some of the vendors seem to rate # of starts per hour. I would assume that this is due to heat generation when the SCRs are carrying the load and the need to dissipate it.
Thanks in advance for any answers Oppie
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You might ask if there is anyone that has had problems using a vfd on an old motor. I know it is possible to have a problem, but I have not had a problem. However I have not used that many VFD's with old motors. So I would ask others with more experience.
Dan
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By its very nature a vfd can be configured to "soft start" as one of many selectable parameters. Most old motors weren't designed for vid operation but many of them are ok so long as their insulation has not become too degraded over time.
Bob Swinney
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You might ask if there is anyone that has had problems using a vfd on an old motor. I know it is possible to have a problem, but I have not had a problem. However I have not used that many VFD's with old motors. So I would ask others with more experience.
Dan
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Howdy, Bob.. haven't seen you around for a couple of blue moons.
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I think what saves us using old motors on VFDs is that most HSMs use 220 volt single phase feeds, and the motors were built for dual-voltage, typically 220/440, and so have stout insulation.
Joe Gwinn
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I have two mills that I've installed VFDs on. The WELLS INDEX 645, has a 3HP C-Face motor not inverter duty ~1968. I've used this now for 11 years without problems. Hitachi SJ300. Also a Rockford horizontal mill, that someone a long time ago put a overhead conversion to drive the flat belt of this antique beast. This has a 5HP motor from some time in the 50s, this has a TECO 7200 drive. Also put a 10HP drive on a 7.5 HP lathe motor (not inverter duty). But I generally never push the HP envelope while machining, as I dry cut, so slow feed and SFPM on the mills, and ceramic tooling light feeds on the lathes. I've installed 3ph motors on my 1976 Jet 12x24 lathe, drill press and vertical bandsaw, but these were 1HP motor with drive kits from dealerselectric.com and were the older TECO FM100 that would not run the motors smoothly down below 20-30Hz (the motors were inverter duty). I recommend the sensorless vector drives, as I've had very good luck with controlled low RPM operation. All my wiring is short, all 240VAC single phase in. So as of yet I have not punched through any insulation of the motors and caused arching. Someone here will probably beat me up over this. ignator
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Found this link <http://www1.eere.energy.gov/manufacturing/tech_deployment/pdfs / motor_tip_sheet14.pdf> This references standing waves with long cables. Maybe that's why I've been lucky. I'm just using standard SO cord to connect the mains to the VFD and VFD to the motor. ignator
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wrote:

Informative, Thank you!
I had read in the past that putting a "non-inverter" qualified motor on a VFD caused the motor to overheat (due to iron hysteresis and eddy current losses) from the high frequency harmonics and bearings to fail from circulating currents that caused current flow through the bearings. The exception was if a "line reactor" (basically a 3 ph inductor) was added between the VFD and motor to smooth out the high frequency content of the drive waveform. You seem to have disproved this though. VFD technology is rapidly changing and hard to keep up with.
I had originally posted this inquiry to news:sci.electronics.design under thread "Soft starter or VFD" and added a bunch of links for soft starters I had found.
Cheers - Oppie
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really only an issue if you are trying to squeeze every little bit of power out of the motor. AS I understand them, "inverter duty " motors largely have better insulation.

This can happen to motors run from any source, but is worse in motors run from a vfd, but not really a big deal IMO, unless again, you are running the motor to the ragged edge.

That is not really why they are there. The motor itself acts as sufficient inductance to smooth out the current. It is the voltage that is a problem that the line reactors (which are just 3 inductors) are meant to solve.
The sharp rise time of the voltage wave forms are more stressful to the insulation. The frequency is also an issue (switching frequency that is) because with LONG cable lengths you can reach a point where the Cable capacitance, and the system inductance resonate, causing MUCH worse voltage stress at the motor. Unless you are putting the motor 100s of feet away from the VFD, I wouldn't worry. It is an issue for something like a down hole pump.

He doesn't really disprove it, just shows how much of a non issue it really is for his sort of usage.
If he wants to run his motors at max power or 24/7 then the issue could be more critical.
Simple basic motors, in basic applications are run from VFDs all the time in the real world, without a problem.

As A PS, Soft starters are not intended to be SOFT for the Load, but instead are SOFT (ish) for the power system that feeds them. They may not do a whole lot for your issue. jk
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The two basic cautions for VFD use are that the output of VFDs consists of a fast risetime voltage waves, similar in appearance to squarewaves (as opposed to the gradual rise/fall of sinewaves normally applied to a motor without a VFD).
Ideally, a VFD should be used with motors rated for "inverter duty" which means they have a higher voltage rated insulation.. but many dual-voltage motors operate without problems when the output of the VFD is set to the lower dual-voltage spec (at 240V rather than the 480V rating), and the motor's power leads are arranged/wired for the lower voltage, of course.
The second issue is heat related, not so much because the output of VFDs cause more internal motor heat, but instead, less cooling efficiency of motors running at reduced speeds.. since most heavy duty induction motors utilize a cooling fan (even totally enclosed cases), and when the fan spins slower at reduced motor RPMs, the fans move less air. Many HSMs install a separate, constant speed fan or small blower to maintain adequate airflow over the motor at all times.
Soft-start doesn't necessarily imply that the mortor starts softly, the term used for increasing the duration of time for the motor to reach it's full operating speed is a feature of many VFDs, and is typically a programmable preference for the speed "Ramp Up" or increase of time during starting.
As JK commented, the actual "soft start" feature is intended to put a minimal strain on the machine's supply voltage, such as in installations where the high inrush startup currents of a large HP motor would cause problems with other equipment connected to the same supply (lights dimming excessively, unecessary/frequent circuit breaker trips/interruptions etc).
I don't have much experience with VFDs, but this is the first time I've heard of a recommended limited number of starts-per-hour for 3-phase motors with VFDs.
Please let us know what solution you find which best fits your application.
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WB
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    With a bit of a complication. Those square waves repeat quite a few times per cycle, starting out on for a small percentage of the time and off for the rest, then a bit more on and less off until it reaches the time when you should have the peak voltage of the sine wave, at which point is is on the full time before it starts decreasing again. This is to approximate the shape of the sine wave, and is more gentle to the motor than a square wave of the full width of the half cycle would be. This is called PWM (Pulse Width Modulation).
    I've already spent too much time with ASCII graphics elsewhere in this thread, and I won't bother doing it again here. :-)
    Enjoy,         DoN.
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Given a VFD gives you lots of pluses, you may reevaluate this. They are inexpensive. You'll get soft-start, variable speed, and protection, all in one package.
Yes, you want a load reactor. BFD, it's off the shelf. You can skip it, but especially on older motors, that's carrying a KICK ME sign. Here is why....
The PoCo gives you nice smooth sine waves, gently undulating up and down. {It says so right here in their ad...}. The VFD gives you square waves.
Now a clever guy name Fourier proved that you can mathematically create a waveform from a series of sinewaves....but for square waves, some of those are HIGH voltage sine waves.
What this means is: square waves have a HV component that is hard on motor insulation. A load reactor smoothes them down, and gives you less-square, more sine-ish, waveforms; and thus is less stressful on the motor insulation.
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The harmonics are of lesser amplitude than the fundamental frequency.
http://mathworld.wolfram.com/FourierSeriesSquareWave.html
Dan
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That's not my memory, but it's been 25 years since I did that homework, so I could easily be wrong.
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Take a look at the url. It shows that a square wave is composed of the fundamental and the odd harmonics. The amplitude of each harmonic is 1/n where n is the harmonic number. That is the 3rd harmonic is 1/3 of the fundamental, the 5th harmonic is 1/5 of the fundamental, ect.
Dan
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