# Do I need a dv/dt output filters?



## rogee (May 2, 2013)

I was just about to order a VFD for my 3ph mill when I was told that because the motor in my mill was made prior to 1980 that I would need to also order  dv/dt output filters, at an additional cost of $360. I haven't read about the need for these filters previously, is this a fact or just the vender being overly cautious?


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## Ray C (May 2, 2013)

Is the person who suggested this also the seller of the $360 filter?

OK, I can't be too cynical w/o knowing all the facts... What's the story on the motor? -Type, specs, etc are needed.

Normally, filters are more a function of how much interference the VFD is causing to other items in the area. Don't use them unless they're needed is my general feeling. Also, for older motors, you should not go below about 30% of it's rated speed since it may require a certain amount of RPM for the fan to be effective. Never exceed it's rated RPM under any circumstance -and that goes with all motors -old or new.

Ray





rogee said:


> I was just about to order a VFD for my 3ph mill when I was told that because the motor in my mill was made prior to 1980 that I would need to also order dv/dt output filters, at an additional cost of $360. I haven't read about the need for these filters previously, is this a fact or just the vender being overly cautious?



- - - Updated - - -


And FWIW....  I have a 1950's 3phase motor in mint condition and have been using it with a VFD for the past 2.5 years.  It hasn't disintegrated yet and don't expect it to in the near future -venture to say, it will outlast me...


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## twstoerzinger (May 2, 2013)

I retrofitted a VFD to a 3 HP 3ph 240 VAC motor on a 1969 vintage lathe 3 months ago.
I was also concerned about the high freq switching generating high voltage spikes in the motor and damaging the internal insulation on a 40 year-old motor.
I put the question before the Yahoo Clausing group and got 6 or 8 responses saying that the users had run VFDs on older motors for years with no problems. No one responded with negative experience. That was good anecdotal evidence, but I also wanted an EE's opinion, so I talked with an EE where I used to work. He has a lot of experience applying VFDs to both new and existing motors. His advice was to skip the output filters as long as the following conditions are met:
* 1) Short leads between the VFD and the motor (like 5 or 10 feet)*. As you go past 100 feet, the rise time on voltage (dv/dt) begins to generate significant voltage spikes at the motor terminals.
*2) Run the VFD at the minimal switching frequency (usually around 3kHz or 5kHz)*. You have to listen to some motor whine, but the chance of generating voltage spikes is greatly reduced. (Newer VFDs have the option of running the switching freq at 10kHz or higher so you don't hear the whine).
*3) Don't use dynamic braking or regenerative braking - use the "coast to a stop" setting.* Braking can also induce spikes. Use the mechanical brake on your mill if it has one.
* 4) Only a single motor is being fed by the VFD *(this is only good practice anyways - the VFD expects to see the inductance a single motor)

There it is for what it's worth.
You have to make your own call.
I guess the $360 cost of the filters probably won't pay for a motor rewind, but appears to be a lot of older motors running without issues on VFDs.
Terry S.


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## rogee (May 2, 2013)

In answer to Ray's question,

The motor is a 2 speed 3/4hp / 3/8hp  motor that was manufactured in 1956. It is in use in a Hardinge TM mill, that I am trying to run on 240 single ph current.


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## twstoerzinger (May 2, 2013)

So I take it that this is a 3 ph motor and you need to VFD to come up with 3 ph with a single phase source?

I think the two speed feature will have to be abandoned. The usual rule is that no switching or disconnects of any type are allowed between the VFD and the motor. 
This may not be an issue since the VFD will give you the variable speed turn-down that the 2-speed motor otherwise provided. I would think that you would connect to the high speed terminals and leave the low speed terminals isolated.

Terry S.


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## Ray C (May 2, 2013)

Agreed.  Unless there are VFD's that support such a feature (and I highly doubt that) you can only use one of the motor's windings.  And as Terry suggested, the high-speed one is the way to go.


If this unit were mine, I would just get a modern motor and make whatever brackets are needed to mount it in place.  

Ray




twstoerzinger said:


> So I take it that this is a 3 ph motor and you need to VFD to come up with 3 ph with a single phase source?
> 
> I think the two speed feature will have to be abandoned. The usual rule is that no switching or disconnects of any type are allowed between the VFD and the motor.
> This may not be an issue since the VFD will give you the variable speed turn-down that the 2-speed motor otherwise provided. I would think that you would connect to the high speed terminals and leave the low speed terminals isolated.
> ...


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## DMS (May 2, 2013)

I'm not sure what they mean by dv/dt filter, unless they are talking about a "line reactor" which is a filter (a big chunk of metal with wires wrapped around it) that helps to reduce stress on your motor (especially older and non "vfd rated" motors), and reduces "hum". I added one to mine, but it sure wasn't $360!. I think I payed around $80 for mine (it was a 1.5 HP model). Here is a 1HP model from the same supplier

http://www.automationdirect.com/adc...arts_-a-_Accessories/AC_Line_Reactors/LR-21P0

The unit I got was US made, and seemed of good quality. If this is what they are talking about... well, I wouldn't pay $360 for it, but $60 doesn't seem too bad.

Even if it is an older motor, if you are running it at full speed, and don't bog it down, it should be ok without. Others mentioned reducing switching frequency, and I think that would help too.

FWIW, I have a  Hitachi WJ200-015SF running a 1.5HP motor from the early 80s.


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## CluelessNewB (May 2, 2013)

My 1945 Logan lathe has it's original 3 phase motor and a Teco N3 VFD working fine with no filters.  
My 1976 Powermatic wood shaper has the original motor and a Teco FM50 VFD working fine with no filters.
My 1974 Delta wood jointer has the original motor a Teco 7300 working fine with no filters.
My 1968 Powermatic disk and belt sander has a mid 70's replacement motor working fine on a Ebay cheap VFD with no filters.  (This one I did need to adjust the carrier frequency higher because of motor noise)  

I'm not saying filters are never required but would sure try without.  I would shop around also.  I'm guessing the $360 line reactor is in a NEMA enclosure, if its just a chassis mount it's a total ripoff price.


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## Tony Wells (May 2, 2013)

I believe we have some EE's among us, and maybe they can give you a condensed explanation of the phenomenon you're protecting against, but if you want to wade through some technical reading, here's a couple of sites for you:

http://www.allaboutcircuits.com/vol_1/chpt_13/2.html

http://www.nhavfd.com/tech-stuff/dvdt-output-filters/


In particular, I will quote a paragraph from the second reference:

"It’s also important to note that 230 volt 3 phase motors are  adequately protected from failure due to voltage overshoot by present  insulation standards. Keep in mind the maximum voltage that is possible
 Example: 230 input volts x 1.1 = 253 x 1.414 = 357.7 x 2 = 715.4 possible volts.
 Most motors are rated for both 230 & 460 volts. So the maximum  output voltage from a VFD running 230 volts is well below the NEMA  standards MG-1 of 1600 volts used today. In other words if your system  voltage is 230, motor failure is very rare when using a VFD, but it can  still fail." 

End quote

Please note that they do refer to "present" NEMA standards, but if indeed your motor is a dual 230/460 voltage motor, and the leads are short (in this context, probably meaning <100 feet), you are unlikely to see spikes that are a real threat.


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## Ray C (May 2, 2013)

FYI:  dv/dt simply means "change in voltage with respect to time" or "delta voltage over (/ divided by) delta time".  In essence, voltage spikes.  Spikes can happen for all kinds of reasons and unless you have an environment prone to them, don't sweat it.  A simple motor on a branch circuit is no more prone to spike than any other appliance in your home (and I assume we're talking about a home-shop environment).  If that motor is sharing a line with big AC units or heater coils, welding units etc.  you might be prone so some issues and fixing the wiring would be the most appropriate course of action.

I looked at the 3Ph output of a VFD and it was pretty clean about 10 times cleaner than a typical generator output waveform.  Probably the most common reason for spikes are the old-fashioned blade switches and/or typical drum contactor switches.  The contacts bounce several times (3-5 is typical) in a matter of milliseconds before finally closing -so there you have your dv/dt.  Voltage goes from zero to 240 and back 3-5 times.  Now divide 230 by say 2x10[SUP]-3[/SUP] (2 milliseconds) and there you have your classic voltage spike.  Guess what, this happens every time you flip a mechanical switch and nobody sweats it.  Finally, during that few milliseconds while the voltage is stabilizing, there's something called a reflected wave whereby the electricity bounces back from from the motor connecting terminals.  It actually bounces back dozens of times and in so doing, by happenstance the criss-crossing waveforms can additively combine (or subtract) and this could lead to a much higher spike.  Believe it or not, this happens every darn time you flick on a light bulb.  The reason it's harmless is because it's all stabilizing in a matter of micro-seconds (x10[SUP]-6[/SUP]) and it's simply too short of a time for anything to heat up.  

BTW:  A solid state switch (such as what's on a VFD) will not bounce -so, I venture to say, that part of the problem simply doesn't exist with a VFD.  What you will get from a VFD is ringing from the HSS (High Speed Switcher) because, the sine wave is produced by a series of square-wave pulses to approximate a sine wave.  Square waves are always prone to overshoot with a little ringing on the rising edge.  Modern IGBTs -especially at the low voltages of 240V have this problem pretty well licked.

Yes, these things are commonplace and natural.  I just don't understand why someone steered the OP in this direction of thinking and concern.  For practical considerations, the only time things like this have a noticeable effect is in high-speed switching circuitry in the multi-megahert range and in power company distribution lines (where the reflected waves have lot's of length to build-up).

The example over-voltage condition from that reference is pretty extreme and would pretty much indicate a dead short of all the leads.  With some of the really old motors, the wire was cloth-wrapped insulation and if it got totally soaked in oil, you could get some conduction there -but I doubt it would account for a full short at the low potential of 240 volts.  In all probability, you'd smell oil cooking off before it damaged the motor.  460V might be cause for concern but motors at that voltage are most likely newer with vinyl insulation.   

Just a few thoughts...

Ray




Tony Wells said:


> I believe we have some EE's among us, and maybe they can give you a condensed explanation of the phenomenon you're protecting against, but if you want to wade through some technical reading, here's a couple of sites for you:
> 
> http://www.allaboutcircuits.com/vol_1/chpt_13/2.html
> 
> ...


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## Tony Wells (May 2, 2013)

Well, the current (pardon the pun) concern is precisely because of the change to IGBT's where the switching is so fast, rather than the old Darlington pair. 

But I'd agree that this should not be a problem. I've done some power analysis on several hundred HP VFD's, and observed the reflected wave, but I can't see the additive effect amounting to enough to exceed the safety of any industrial motor insulation, and unless it was continuous, and perfectly phased wrong so as to present a short, there almost can't be an issue. I'm no EE, but I'm not ignorant of the subject either.


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## Ray C (May 2, 2013)

The Darlington Pair... Weren't they from the Beverly Hillbillies or was it Andy of Mayberry?

... Nah, it's not a problem and if something does rear-up, there are other factors at-play.  About the only filter someone might consider is a line choke on the supply lines to the VFD.  These are just the plastic encased pieces of ferrous ring that you often see on laptop power supplies (aka. wall warts).  VFD can produce some conducted emission back into the house wiring and screw-up your TV reception etc...  If that happens, try tweaking the VFD frequency and if that still doesn't work, make a 1" diameter ring of soft iron and loop all three supply conductors once through the ring.  Most of the stray signal will be induced into the ring and dissipated as a tiny amount of heat only measurable by theoretical calculation and not detectable with any common form of thermometer.  Oh, before you make any kind of inductor ring, check the house grounding first.  That's the most probably root cause.

Ray





Tony Wells said:


> Well, the current (pardon the pun) concern is precisely because of the change to IGBT's where the switching is so fast, rather than the old Darlington pair.
> 
> But I'd agree that this should not be a problem. I've done some power analysis on several hundred HP VFD's, and observed the reflected wave, but I can't see the additive effect amounting to enough to exceed the safety of any industrial motor insulation, and unless it was continuous, and perfectly phased wrong so as to present a short, there almost can't be an issue. I'm no EE, but I'm not ignorant of the subject either.


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## Tony Wells (May 2, 2013)

Ahh, Briscoe Darling. It was the Andy Griffith Show. Actually, that "band" was The Dillards, and they still perform. I think only one of them has passed away. Briscoe, played by actor Denver Pyle was not part of the real band. Neither was Charlene, if I remember right. She was played by Maggie Peterson.


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## rogee (May 2, 2013)

This thread has been most useful to me, answering all of my questions. Your answers have been easy to understand, directly to the point. Thank you all, I greatly  appreciate your effort in answering this post.


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## Tony Wells (May 2, 2013)

That's what we're here for, rogee. Glad you found some answers.


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## Ray C (May 2, 2013)

Those guys were great.  I used to love those episodes!  

Have you ever listened to the soundtrack from the movie "Oh Brother, Where Art Thou"?  Lot's of great music there too.  I have bluegrass on one of my Pandora music stations.  Great stuff!


-Far enough off-topic.

Nite y'all.


Ray



Tony Wells said:


> Ahh, Briscoe Darling. It was the Andy Griffith Show. Actually, that "band" was The Dillards, and they still perform. I think only one of them has passed away. Briscoe, played by actor Denver Pyle was not part of the real band. Neither was Charlene, if I remember right. She was played by Maggie Peterson.


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