Mill Motor Runs on Line Power 3P/240VAC But Not on VFD - Why??? - Page 2
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  1. #21
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    Quote Originally Posted by CalG View Post
    Is this a case where the VFD carrier frequency caused movement in the windings that resulted in random wire to wire shorts through the wire varnish insulation?

    That would be consistent with near normal amp draw with a decrease in motor torque.
    I've never seen a motor fail this way. Usually when the windings start to short, the current draw increases exponentially, leading to over heating, and finally with the magic smoke leaking out.

    This explanation also violates forward and back EMF. If the windings are shorting with reduced magnetic field strength, the reduced back EMF will cause an increase in current draw. This not what is occurring.

    Not saying it can't happen but in the context that the OP has not actually done anything to test the motor as suggested, I suspect an underlying problem that has not been solved yet. I would be looking at a high resistance wiring connection or bad wiring up to the motor.

    From what the OP has explained, the motor has reduced torque yet the current is remaining constant. This either means the input voltage is decreasing proportionately so that the torque output/ power input relationship is remaining proportional or else the motor has to overheat.

    It is a simple case of total power in must equal the total power out with heat losses included. This is not what the OP says he is experiencing.

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    Still no info on VFD and programming numbers...

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    All sorts of things can cause low torque with a VFD...

    First thing is a bad setting of max amps. It seems, but is not clear, that this is not the likely cause.

    However, many VFDs can be set to LIMIT amps without shutting down. That may even be the default. In that case, the VFD may weaken drive to one phase to compensate for shorts. The current will sit at the set point, and things will look pretty normal, but it would in fact be a bad motor.

    A true RMS meter can find that, because phase voltage will be low, even though current is fairly normal. (averaging meters may also find it, but they do not read correctly on VFD outputs).

    it is odd that regular 3 phase would be fine, but it is perfectly possible that the issue is aggravated by the spike voltages from the VFD. Powerco 3 phase would not have them.

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    If the windings are leaking from the harmonics of the VFD, they will also usually leak at some level with normal line voltage.

    Meggering the motor would have revealed this condition. Being that the motor is running at 240VAC, the voltage spike issues from the harmonics should not be as problematic as the 480VAC scenarios.

    I agree that it is likely that the VFD is limiting the max current flow due to parameter settings. This gets back to the VFD parameter issue that we don't know.

    This still points to the problem that the motor is not necessarily the issue.

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  8. #25
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    Quote Originally Posted by thermite View Post
    Was thinking a "similar" thing, more perhaps Corona damage, "pinholed" insulation, given the OEM motor was probably NOT "inverter duty".

    IIRC, Mark acquired a Rigol 'scope off the back of his Parker-SSD 10EE conversion project?

    I could be wrong.... But if so, it would be interesting to see what the output waveform and its bleeding edges show off the older VFD.

    Hopefully, the new VFD has cleaner output AND the new motor IS rated inverter duty?

    If not both, a salvaged dv/dt filter could be a sound investment.
    hey Bill, yes i do have a suspicion that the VFD did shorten the life if the motor. i do have the scope and could check out the motor, but not really sure what/how to test.

    here is the new motor. definitely inverter rated. actually it is the only way it can be run - not allowed to use line power.

    119987.00 LEESON 2HP 230VAC MOTOR C6T17FC264A

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    I haven’t read all the posts so sorry if this is a repeat.

    Inverters do shorten the life of motors, generally insulation breakdown creating turn to turn shorts in the stator windings which if done in an orderly manner give the result you experienced.

    The motor you linked is an interesting design. 2hp from a 56C frame motor at 1745 rpm with a 1.0 service factor . . . Generally 2hp motors rated at 1745 rpm are at least a 143 frame. I guess that means no overload to speak of. The peak speed of 4000 rpm indicates a relatively low breakdown torque rating which explains why it shouldn’t be operated across the line as it wouldn’t likely have enough torque to start its own inertia plus load.

    The motor shop should have megohm tested the motor as the first order of business, both phase to phase and phase to motor frame. For a 230 volt rated motor, this test should be done at ~750 volts (general rule of thumb is 2x VFD bus voltage x 1.1 factor of safety). No effort to rebuild a motor should be undertaken without first doing this test to determine stator winding health.

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    If a motor shop has been in business for any length of time then they are doing the right thing.

    You have not mentioned what the VFD carrier frequency is set to. The higher it is the more stress is put on the windings.

    What you have there is a old pancake motor that was manufactured before any modern VFD's were made. I have a motor similar to that. I have been reluctant to use a VFD with it.

    You should go back to the motor shop with the motor and your VFD. They will see that you are ready to prove your point. See what they have to say about all this. You might learn something. If you are not satisfied then ask for you money back or file a complaint with the BBB.

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    There's a saying in the motor business. "It's always the motor".

    That is to say...whenever there is any problem of any sort with anything, the motor gets blamed first and foremost. If the industrial world were a neighborhood, the motor would be the black teenager in the hoodie.

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    Another source of failure that we often see in VFD installations is motor lead wiring when pulled in through conduit by a careless electrician. Damaged wire insulation (again found with a megohm test) . . . just be sure to disconnect the motor leads from the VFD before lighting up the conductors.

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    I am a bit confused on why they rate the motor 30-60 hertz and only have a 1745 speed rating.

    The motor is only rated 3:1 constant torque speed range. That's not fantastic. Most motors bought today tend to be at least 4:1 and smaller ones like this one can easily be 10:1.

    This motor is designed to run high speed not low speed. I seriously doubt that you need the motor to run fast given that the old motor did not have that capability. You want it to be able to run at low speed and produce rated torque and not over speed.

    The reason some VFD motors are not rated to start across the line is because of rotor bar design. They optimize the bar design for VFD operation, and the rotor can overheat if started across the line with a load - will start fine with no load.

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    Quote Originally Posted by everettengr View Post
    hey Bill, yes i do have a suspicion that the VFD did shorten the life if the motor. i do have the scope and could check out the motor, but not really sure what/how to test.
    Under any legitimate load, "magnify" the leading-edges of the output pseudo-sine-wave on the output to the load.

    Look for spikes, the artifacts of solid-state switching - how far they rise or fall, and how wide ("the energy under the curve.." thing).

    Your Parker_SSD DC drive has those as well, simply all at a multiple of but the one line frequency. They vary power in the time or duty-cycle domain, percentage OFF or ON, each incoming excursion, the utility grid their "NON Variable Frequency" oscillator.

    Worse, DC drives lack the separate pre-rectifier and filter cap bank of a local "DC bus" that VFD have, so reflect that hash back up the local grid. Our use of full-isolation boost transformers blocked most of that nastiness, just as if they were filter chokes.

    VFD are pretty clean, upstream grid side, do the dirty onto the load side, mostly, and it differs, one make and model to another if filtered, and how well, etc.

    Just for reference, a Motor Generator 10EE is harder to regulate easily, but smoother than either of DC Drive or VFD. It operates in the Voltage and Current variability realm, directly, not by re-integrating as a switched thyristor drive must do.

    Technically, it "switches" one coil and commutator segment pair to the next, BUT.. they are laid near each other, the brushes span more than one segment as they transfer, so the result is a small ration of 'brush noise" - the sparking - yet a waveform almost as smooth as a "variable battery" or massive plain resistor, the variable Field at modest current through the Rheostat acting as a "lever" to alter the greater generator power.

    The roughly $300 each Hammond "ripple filter" chokes we used on the SSD output are to protect the old DC motors from similar effects of a solid-state DC Drive as a VFD burdens old AC motors with.

    The "Hollow state" OEM drive didn't need that because its power or "plate" transformer was "inside" the switching part of the circuit, serving as filter to BOTH incoming and load. Elegant system, actually.

    TANSTAAFL all about balancing various trade-offs one approach to the other.

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    Default Working Again

    FWIW, i got the new motor installed (after a little delay).
    it works awesome!!! i could not be happier and made the best decision.
    i cnc'ed a spacer plate and turned a longer shaft. it bolted right up...
    the motor shop made a mistake when they said the motor was good.
    and the VFD mfg (TECO) service center was wrong when they told how to test the VFD and said it was bad based on results.
    the original VFD is perfect and in use again. i didn't realize how bad the old motor was until i started using the new one.
    anyone want a non-working Tree mill motor, also have a brand new VFD in the box for sale :-)
    Mark
    img_0314-copy.jpg
    Last edited by everettengr; 11-09-2019 at 10:40 AM. Reason: typo

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    The motor runs on VFD only: line power is not permitted? How does that work? What's different?

    And referring to rons post #30, just what does "rotor bar design" refer to? I am not a motor guy and have never heard that term before.



    Quote Originally Posted by everettengr View Post
    hey Bill, yes i do have a suspicion that the VFD did shorten the life if the motor. i do have the scope and could check out the motor, but not really sure what/how to test.

    here is the new motor. definitely inverter rated. actually it is the only way it can be run - not allowed to use line power.

    119987.00 LEESON 2HP 230VAC MOTOR C6T17FC264A

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    Quote Originally Posted by EPAIII View Post
    The motor runs on VFD only: line power is not permitted? How does that work? What's different?

    And referring to rons post #30, just what does "rotor bar design" refer to? I am not a motor guy and have never heard that term before.
    i'm not a motor guy either, but VFDs ramp up/down and may have harmonics that can damage insulation, cause eddy currents, etc. that can damage old style motor windings. i'm sure others can provide more detailed info or a interwebs search might yield additional info.

    this motor has a 10:1 turn down rating, meaning it can safely operate at a 10th of the base speed. not sure what 'rotor bar design' refers to?

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    Quote Originally Posted by EPAIII View Post
    The motor runs on VFD only: line power is not permitted? How does that work? What's different?

    And referring to rons post #30, just what does "rotor bar design" refer to? I am not a motor guy and have never heard that term before.
    That was markz528

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    Quote Originally Posted by everettengr View Post
    i'm not a motor guy either, but VFDs ramp up/down and may have harmonics that can damage insulation, cause eddy currents, etc. that can damage old style motor windings. i'm sure others can provide more detailed info or a interwebs search might yield additional info.

    this motor has a 10:1 turn down rating, meaning it can safely operate at a 10th of the base speed. not sure what 'rotor bar design' refers to?
    Probably more accurate to say it is stable and can be regulated under load over a 10:1 speed range.

    Sustained low RPM falls into the "blower duty" or "separately ventilated" category, wherein a separate, smaller, and independently speed controlled (temp sensors..) ELSE constant speed motor blows cooling air through the primary motor lest it overheat whilst "snailing". Not all of these are still DC motors, but many are:

    Reliance Electric 50 HP DC Motor C2514ATZ 1150 RPM 240V TR DP Shunt 50HP RPM III (NP0348-2) - River City Industrial

    Among the reasons why? From your Parker-SSD manual

    RPM range, integral current sensing. 20:1 @ 2% stability, typical.

    RPM range, tachogenerator feedback, 100:1 @ 0.1% stability, typical.

    Torque regulation, integral, 2%, typical.

    CAVEAT; The recommended Reliance tacho's are about $1,200, the mountings another $200-plus! Silver commutators? Probably. "Ordinary" Servo-tek tachos are under $400.

    VFD's look like a damned good deal, value for money, after all, yah?


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    Quote Originally Posted by EPAIII View Post
    The motor runs on VFD only: line power is not permitted? How does that work? What's different?

    And referring to rons post #30, just what does "rotor bar design" refer to? I am not a motor guy and have never heard that term before.

    Motor impedance, composed of inductance and resistance, limits turn-on surge. A motor with a low impedance will have a hellacious turn-on surge,potentially damaging to the motor. But, a low impedance motor is typically more efficient, it has less losses, and maintains torque well.

    A VFD mitigates these problems, because the VFD does a ramp-up instead of a slam across the line start. And, it can be set to maintain a given max current, and also has a V/Hz curve that is settable, so that it essentially stays within th allowable voltage.current envelope of the motor, while squeezing the most it can out of the motor..

    Essentially, the VFD can be set up to use all the good features of a low impedance motor, and also to mostly stay clear of the bad features. So the motor can then be made in such a way as to require what the VFD offers, to such an extent that it is not suitable for use across the line, where currents are not controlled as-supplied, and therefore the motor characteristics do not have to be relied upon to limit current, etc. The VFD can do that, which frees the motor designer to go all-out for more important specifications.

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    Quote Originally Posted by JST View Post
    Motor impedance, composed of inductance and resistance, limits turn-on surge. A motor with a low impedance will have a hellacious turn-on surge,potentially damaging to the motor. But, a low impedance motor is typically more efficient, it has less losses, and maintains torque well.

    A VFD mitigates these problems, because the VFD does a ramp-up instead of a slam across the line start. And, it can be set to maintain a given max current, and also has a V/Hz curve that is settable, so that it essentially stays within th allowable voltage.current envelope of the motor, while squeezing the most it can out of the motor..

    Essentially, the VFD can be set up to use all the good features of a low impedance motor, and also to mostly stay clear of the bad features. So the motor can then be made in such a way as to require what the VFD offers, to such an extent that it is not suitable for use across the line, where currents are not controlled as-supplied, and therefore the motor characteristics do not have to be relied upon to limit current, etc. The VFD can do that, which frees the motor designer to go all-out for more important specifications.
    Write this on "a" wall somewhere, because it has gone beyond even into what we take for granted in the way of cordless power hand tools, not even fully aware of just how well optimized they have become.

    Ablilty to design motor, controller, and for some time now, "smart" battery, as a cooperatively optimized "system" has changed the whole landscape. Other flavours, fit for THEIR purpose, run servo'ed & VFD'ed CNC spindles.

    Our beloved "BFBI" yet turbine-smooth 10EE motors, built off the back of massive rations of Iron and Copper truly are "Dinosaur Current" in their simplistic ancient glory.


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    Quote Originally Posted by everettengr View Post
    this motor has a 10:1 turn down rating, meaning it can safely operate at a 10th of the base speed. not sure what 'rotor bar design' refers to?
    The rotor is compromised of rotor bars. The rotor bars cut the magnetic lines developed by the stator current. This causes current to flow in the rotor bars which creates a rotor magnetic field. The rotor magnetic field counteracts the magnetic forces developed by the stator and that causes mechanical rotation.

    When a motor tries to start across the line, the rotor sees a 60 hertz slip. The rotor bars need enough resistance to generate the proper rotor bar current/magnetic field to generate the proper motor starting torque. The rotor bars are designed for this and are always a compromise.

    In a VFD application, the rotor never sees the 60 hertz slip - technical reason is that on a VFD the motor is always working on the right side of the speed/torque curve and across the line when starting it is on the left side of the curve. Since the rotor bars in a VFD application don't need to operate above approximately a couple of hertz, the shape can be optimized for VFD performance. The current is induced deeper into the bars in a VFD application during starting than across the line.

    Unless you are using this motor different than the original design, then this motor is really not the design you want - there were much better choices. 600 to 4000 rpm is the wrong choice and gives you a 3:1 speed range (based on the original 1800 rpm motor) not the 10:1 that you may think. This is my interpretation of the data sheet. Is the 1800 rpm at 30 or 60 hertz?

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    Quote Originally Posted by markz528 View Post
    The rotor is compromised of rotor bars. The rotor bars cut the magnetic lines developed by the stator current. This causes current to flow in the rotor bars which creates a rotor magnetic field. The rotor magnetic field counteracts the magnetic forces developed by the stator and that causes mechanical rotation.

    When a motor tries to start across the line, the rotor sees a 60 hertz slip. The rotor bars need enough resistance to generate the proper rotor bar current/magnetic field to generate the proper motor starting torque. The rotor bars are designed for this and are always a compromise.

    In a VFD application, the rotor never sees the 60 hertz slip - technical reason is that on a VFD the motor is always working on the right side of the speed/torque curve and across the line when starting it is on the left side of the curve. Since the rotor bars in a VFD application don't need to operate above approximately a couple of hertz, the shape can be optimized for VFD performance. The current is induced deeper into the bars in a VFD application during starting than across the line.

    Unless you are using this motor different than the original design, then this motor is really not the design you want - there were much better choices. 600 to 4000 rpm is the wrong choice and gives you a 3:1 speed range (based on the original 1800 rpm motor) not the 10:1 that you may think. This is my interpretation of the data sheet. Is the 1800 rpm at 30 or 60 hertz?
    you know a lot more than me regarding motor design and application. i am relying on the info provided by the mfg and the distributor and this motor appears to be exactly what i want. here is the placard. let me know what you think? BTW, it works great so far...
    Mark

    leeson-placard.jpg


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