Are VFD's bad for motors?
Close
Login to Your Account
Page 1 of 3 123 LastLast
Results 1 to 20 of 46
  1. #1
    Join Date
    Oct 2017
    Country
    FINLAND
    Posts
    556
    Post Thanks / Like
    Likes (Given)
    89
    Likes (Received)
    170

    Default Are VFD's bad for motors?

    I have a friend who is adamantly against putting VFDs on old machines like lathes and mills and whatnot.

    It came up while I was showing him the progress of my FP2 renovation and while going over the electrical cabinet I mentioned a lot of people replace this with a VFD instead to get various features like being able to adjust speeds between gears even more finely, and to quickly stop and reverse the spindle for tapping procedures. I don't have need of that for my FP2 though I could see the use for one on my lathe to be able to stop the spindle quickly for threading to a shoulder perhaps.

    Well he said that's a really bad idea, that his father (retired electrical engineer) has told him that VFDs puts more strain on the motor and that the square wave they produce are bad for the motor and often lead to the motors windings risk burning out.

    That motors need to be designed for use with a VFD to begin with, and that especially older motors like in "old iron" machinery are susceptible to being destroyed if tried to run with a VFD. He was deeply critical of what he thought was the "modern trend of everyone and their dog mounting a VFDs on their machines".

    I should add we're not located in america but in Finland and 3-phase is standard residential power here so nobody needs a VFD to make an old industrial machine run in ones garage.

    I dunno, I have never heard of people burning out their motors on VFDs except when they try and run a motor too slow, then it tends to heat up and the fan also can no longer provide adequate cooling in those cases. I am somewhat skeptical of his claims but he does have someone who used to work a place that built induction motors.

    What do you guys think?

  2. #2
    Join Date
    Mar 2009
    Country
    UNITED STATES
    State/Province
    California
    Posts
    5,047
    Post Thanks / Like
    Likes (Given)
    91
    Likes (Received)
    922

    Default

    The father is right about that. He probably has seen it himself. The word "often" was used and that sounds pretty strong. Use a VFD and take your chances.

    You should use a quality VFD at a low carrier frequency with something like a Baldor Super-E motor.

  3. #3
    Join Date
    May 2017
    Country
    FINLAND
    Posts
    1,721
    Post Thanks / Like
    Likes (Given)
    484
    Likes (Received)
    793

    Default

    Risk is there but its like is if the winding insulation is going to fail after 50 years without VFD or 20 years with VFD.
    Not knowing the condition of the motor it could be also 10 hours without VFD or 1 hour with VFD.

    Not a problem for most of the old motors but if the insulation is already sketchy it might fail in short time.

  4. #4
    Join Date
    Sep 2007
    Location
    Cairns, Qld, Australia
    Posts
    2,795
    Post Thanks / Like
    Likes (Given)
    879
    Likes (Received)
    755

    Default

    Modern VFD's have an "autotune" feature that allows the VFD to set parameters like carrier frequency to suit the motor. This measures the motor's inductance among other things and prevents the carrier being high enough to cause excess heating losses and winding failure. That said, a proper inverter ready motor will have things like triple coated winding wires used in construction and the motor frame will be sized a little larger to lower the RPM at which magnetic saturation occurs which allows the motor to run at lower speeds without overheating.

    So the original motor can be used with a VFD if set up properly, which it hardly ever is.

  5. Likes MichaelP liked this post
  6. #5
    Join Date
    Oct 2007
    Country
    FRANCE
    Posts
    600
    Post Thanks / Like
    Likes (Given)
    92
    Likes (Received)
    112

    Default

    If you talk to motor companies, they will tell you not to use a vdf with old motors. I have also asked this question online before and have yet to find anyone actually experience a failure due to VFD use. So basically for low volume use (home workshop) it doesn't matter and for full time use you are going to need more modern equipment or at least more power than you can get out of a single 20amp outlet anyway so it won't be a problem.

    I have run the same machines on vfd and 3phase out of the wall and while I have no proof but they seem to run better with the real thing. If you can get 3ph it is the way to go and really I don't think you should be using a vfd with a fp2 as you have multiple motors and many switches in that machine. (The vfd needs to be wired directly to the motor with nothing in-between. This is also a problem with two speed motors for obvious reasons.

    Luke

  7. #6
    Join Date
    Oct 2005
    Country
    UNITED STATES
    State/Province
    Pennsylvania
    Posts
    15,133
    Post Thanks / Like

    Default

    There is a new insulation scheme just for VFD driven motors, I'm not privy to all of it, but some details are where the turns comes out of the slots.

  8. #7
    Join Date
    Mar 2007
    Location
    England UK
    Posts
    1,832
    Post Thanks / Like
    Likes (Given)
    764
    Likes (Received)
    751

    Default

    I'd fit the VFD and keep an eye out for a spare. If there's insulation degradation it's bound to be a function of time and use so your lightly-used home shop mill motor will probably out-last the operator

  9. #8
    Join Date
    Mar 2019
    Country
    UNITED STATES
    State/Province
    Massachusetts
    Posts
    88
    Post Thanks / Like
    Likes (Given)
    30
    Likes (Received)
    51

    Default

    I am told you need an "inverter duty motor" for use with VFD's. putting them on old motors is not recommended.

  10. #9
    Join Date
    Jan 2005
    Location
    Houston
    Posts
    5,600
    Post Thanks / Like
    Likes (Given)
    501
    Likes (Received)
    2419

    Default

    There is no doubt that VFD's are bad for motors. The question is, 'how bad'? And, is the 'bad' worth the 'good' that comes with it? You have to decide that on a case by case basis.

    A VFD provides the motor with a 'choppy' waveform instead of the beautifully smooth sinusoidal waveform of a normal 3 phase power. The amount of choppy-ness differs with drive make and drive quality, but it's always there. So the insulation bears the brunt. Imagine driving your car 60MPH down a glass smooth road versus a car down a road that has a speed bump every 10 feet. Which car will last longer?

    You bring up a less-talked-about point, too. The whole 'stop the motor and instantly reverse it' stuff. Does anyone here really think that's gonna be good for ANY piece of mechanical or electrical equipment? A motor is half mechanical and half electrical, so the reversing bit hits you on both counts.

    Yes, there are definitely motors that are designed to handle that, and they do handle it, but you can be sure a 50 year old motor wasn't.

  11. #10
    Join Date
    Feb 2007
    Location
    St. George, Utah
    Posts
    875
    Post Thanks / Like
    Likes (Given)
    412
    Likes (Received)
    552

    Default

    I guess I've been lucky. I 'instant reverse' my lathe (1975 Sheldon) at least a hundred times on a busy day for over 15 years. No issues yet. I don't use a VFD though, just the drum switch. Actually there has been one issue, I do have to periodically tighten the set-screws on the jackshaft pulley.

  12. #11
    Join Date
    Oct 2005
    Country
    UNITED STATES
    State/Province
    Pennsylvania
    Posts
    15,133
    Post Thanks / Like

    Default

    Quote Originally Posted by Gordon Heaton View Post
    I guess I've been lucky. I 'instant reverse' my lathe (1975 Sheldon) at least a hundred times on a busy day for over 15 years. No issues yet. I don't use a VFD though, just the drum switch. Actually there has been one issue, I do have to periodically tighten the set-screws on the jackshaft pulley.
    "Plug reverse" is not the issue, spikes from the VFD is the issue.

    IIRC Forrest Addy did a good write up on making your own line reactors to help/fix.

  13. #12
    Join Date
    Apr 2004
    Location
    Modesto, CA USA
    Posts
    6,884
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    1150

    Default

    Time for Forrest to jump in and say a quality motor will have the windings dipped and baked at the factory. You can take the winding's to a shop and have them dip and bake them. The dip and bake glues the winding's in place so they do not vibrate against each other and wear through the insulation enamel. When I had to do a little work on my lathe motor I flooded the new work with floor varnish to encapsualte it and add support.
    Bil lD

  14. #13
    Join Date
    Oct 2005
    Country
    UNITED STATES
    State/Province
    Pennsylvania
    Posts
    15,133
    Post Thanks / Like

    Default

    Quote Originally Posted by Bill D View Post
    Time for Forrest to jump in and say a quality motor will have the windings dipped and baked at the factory. You can take the winding's to a shop and have them dip and bake them. The dip and bake glues the winding's in place so they do not vibrate against each other and wear through the insulation enamel. When I had to do a little work on my lathe motor I flooded the new work with floor varnish to encapsualte it and add support.
    Bil lD
    The windings aren't' being vibrated any different with a VFd, the voltage spikes are puncturing the insulation.

  15. #14
    Join Date
    Feb 2007
    Location
    St. George, Utah
    Posts
    875
    Post Thanks / Like
    Likes (Given)
    412
    Likes (Received)
    552

    Default

    Quote Originally Posted by digger doug View Post
    "Plug reverse" is not the issue, spikes from the VFD is the issue.
    I agree that it is something about the VFD. I posted in response to GregSY's comments in post 9, inferring that quick reverse in and of itself is detrimental.

  16. #15
    Join Date
    Mar 2009
    Country
    UNITED STATES
    State/Province
    California
    Posts
    5,047
    Post Thanks / Like
    Likes (Given)
    91
    Likes (Received)
    922

    Default

    Quote Originally Posted by DennisCA View Post

    I dunno, I have never heard of people burning out their motors on VFDs except when they try and run a motor too slow, then it tends to heat up and the fan also can no longer provide adequate cooling in those cases.
    I think the category of uses will make a difference.

    The VFD is running a motor on a air compressor which has intermittent use.
    The VFD drives a machine tool that is run more often and at many varying speeds.

  17. #16
    Join Date
    Mar 2017
    Country
    LATVIA
    Posts
    341
    Post Thanks / Like
    Likes (Given)
    80
    Likes (Received)
    176

    Default

    Quote Originally Posted by digger doug View Post
    The windings aren't' being vibrated any different with a VFd, the voltage spikes are puncturing the insulation.
    the sine wave is being generated by short impulses which in theory can make windings vibrate at other frequencies which wouldn't be there if it was driven from the constant 50/60Hz source, and the ones I had used had just a regular 3ph rectifier in them, resulting DC was then used to create the sine in the output - meaning - you can't get voltage spikes because the source for the new sine is the same as it would have been without the VFD - so you simply can't get any voltage spikes

    I have no proof to the following, just personal speculation regarding the problems people see with VFD driver motors and IMHO they are from oversizing the VFD for the given motor and then not limiting the current via programming of said VFD, using fast acceleration/deceleration times just because they can without thinking about the extra power required to achieve this which results in overheating the windings and ruining insulation, motors being already old and may have had some wear and tear on the winding already

  18. #17
    Join Date
    Mar 2006
    Location
    Auburn CA
    Posts
    430
    Post Thanks / Like
    Likes (Given)
    4
    Likes (Received)
    39

    Default

    My understanding of the primary failure mode is (as Digger Doug stated) insulation puncture from the fast rise time of the pulses that VFDs generate. Another failure mode is in the bearings. The pulses tend to generate voltage in the armature shaft that is conducted to ground through the bearings. Fortunately bearings are generally inexpensive and easy to replace.

    A well built motor intended for VFD use will have better insulation, an external cooling fan, and a discharge brush for the armature.

    Another feature of some VFD rated motors is rating for overspeed. On my mill I have a base speed 1800 RPM motor that is rated for operation to 6000 RPM.

    In the US many three phase motors are dual rated for 240/480 VAC operation.
    A motor run at 480 with a VFD is much more likely to have a failure than running the motor at the lower voltage.

  19. Likes Conrad Hoffman liked this post
  20. #18
    Join Date
    Sep 2009
    Location
    barcelona, spain
    Posts
    2,388
    Post Thanks / Like
    Likes (Given)
    530
    Likes (Received)
    1432

    Default

    VFDs are not bad for motors. As such.

    But bad VFD tuning, can wear out or break even a good motor.

    Some possible examples.
    Excessive acceleration (peak currents very high),
    high ripple in acc/decc,
    excessive e-stop speeds,
    excessive tracking via pid resulting in "hunting" for the right speed, etc.

    My 1940s-era 3-phase Bridgeport motor runs perfectly well, with a VFD (Hitachi 2.2 kW 220 series iirc).

    Imo, ime, a VFD will run any 3-phase industrial motor as well or better than straight current will.
    With suitable, conservative settings.

  21. #19
    Join Date
    Aug 2004
    Country
    UNITED STATES
    State/Province
    California
    Posts
    2,703
    Post Thanks / Like
    Likes (Given)
    25
    Likes (Received)
    623

    Default

    ... Well he said that's a really bad idea, that his father (retired electrical engineer) has told him that VFDs puts more strain on the motor and that the square wave they produce are bad for the motor and often lead to the motors windings risk burning out.
    Nobody has been using square waves on VFDs now for over 20 years, maybe longer.

    VFDs work by "tricking" a motor into thinking that it is getting an AC sine wave, by taking advantage of the fact that the motor is itself an inductor, and there is a phenomenon called the "inductive time constant" that means that current cannot change instantly in an inductor, it has a specific rate of change that cannot be different. So what a VFD does is to send a "PWM" (Pulse Width Modulation) output of little short pulses of DC into the motor, then before the inductive circuit lets the current rise all the way, it turns it off again, then on, then off, then on then off, at a rate of thousands of times per second. The timing of the length of the pulses and the length of the gaps between them is what controls the actual RMS voltage (Root Mean Squared, a form of averaging) seen by the motor and then how often the pulse strings change from positive to negative dictates the frequency.

    In this process, there can be a capacitive coupling effect in the wires to the motor that can cause what's called a "standing wave" of higher than normal voltage superimposed on top of the normal voltage. That standing wave then can reflect back and forth between the motor terminals and and the VFD terminals, building up like waves on a pond until some of the waves become spikes of voltage. These spikes can in theory reach 2.5x the peak line voltage (peak vs RMS, so peak is RMS x 1.41), maybe more under the right circumstances. So on a 480V system where the peak voltage is actually 677V, the pikes can routinely get to over 1350V. The problem gets worse as the distance from VFD to motor gets longer because there is more wire to have the capacitive coupling effect. In general though if the total circuit length (VFD to motor and back again) is under 25ft, the spike issue is insignificant. So if you are within 10ft of the motor, I would not worry too much about this issue.

    On older motors, it was common to use 1000V or 1200V insulation in the magnet wire used for the windings. So if you have an older 480V motor with 1000V insulation and the spikes can reach 1350V, those spikes will eventually punch their way through the insulation and you get what is called a "turn-to-turn short" within a single winding, effectively reducing it's capacity. That then can lead to motor failure. So NEW motors are now using higher voltage ratings on the windings; the NEMA standard now is to use 1488V s a minimum, but some mfrs are using 1600-2200V insulation.

    BUT... if you are using 240V, the peak is 334V, so 2.5X is 846V as the typical spike. So even with 1000V insulation it is going to be rare that a 230V rated motor is going to suffer this kind of damage at all, and if combined with the VFD being within 10ft of the motor, again, I would not worry about it.

    If you have a very oddball motor that would be difficult to replace, you can use what's called a "sine wave filter" on the output of the VFD to remove the capacitive coupling effect altogether. But these filters often cost as much as the VFD and more than the motor. So I only recommend them under those special circumstances.

    The bearing issue is similar, but is the result of a capacitive coupling effect between the stator and rotor causing a voltage differential to build up between them. Voltage always wants to flow to it's source, via ground in most cases, so because the motor is grounded, that rotor voltage tries to flow across the bearings to ground to get back to the VFD. In that process, it etches the bearing surface and the raceway, which just gets worse and worse until the etching looks like frosting, then begins to look like a washboard until the bearings fail. The solution is to give that voltage an EASIER path to ground than the bearings by putting a shaft grounding ring on the motor with little carbon brushes making contact with the rotor shaft. There are other ways to protect it too, that's just the simplest. But this phenomenon too is not absolute, lots and lots of people never experience it, because there are lots of factors that lead to it and a shaft grounding ring for a 1HP motor is likely going to cost more than a new motor, so most people in the small machine tool world just take their chances.

  22. Likes Yan Wo, mksj, Jim Kennedy, Pathogen, JST and 4 others liked this post
  23. #20
    Join Date
    Aug 2005
    Location
    Bay Area, CA
    Posts
    93
    Post Thanks / Like
    Likes (Given)
    203
    Likes (Received)
    13

    Default

    Thanks, Jraef -
    Good to get a response from someone who has the facts and numbers. I have VFDs on five different motors in my shop, various ages, all at 230 volts, never any problems, nor do I know of any 230 volt VFD user among my associates who has had a problem. I almost consider the subject to be an urban myth.
    Regards,
    Monoblanco


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •