Are VFD's bad for motors? - Page 3
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  1. #41
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    Quote Originally Posted by Briney Eye View Post
    I know from experience designing small motor drivers that switching too fast can blow the H-bridges (I warned the engineer and he did it anyway - made a nice little light show), so I'm pretty much with you on this one. If you switch them too fast the transistors never have time to switch fully on or off, look like resistors, and cook. But I would assume that VFD carrier frequencies are limited to a "safe" value (mine are 15kHz) to keep that from happening. I have mine maxed out so I don't have to listen to them whistle, and haven't had any issues. All this new inverter tech is based on IGBT's, and they've gotten pretty good in the last ten years or so. Seems like they're typically rated for 30khz or higher switching frequencies. Earlier ones were slower.
    I run mine at 15kHz as well, because the whine annoys the hell out if me. I have poor hearing but somehow I hear even 15kHz, which should be impossible for me. Maybe I'm hearing a lower harmonic.

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    Quote Originally Posted by ptsmith View Post
    I run mine at 15kHz as well, because the whine annoys the hell out if me. I have poor hearing but somehow I hear even 15kHz, which should be impossible for me. Maybe I'm hearing a lower harmonic.
    VFD's haven't bothered my hearing in the least, near two years already.

    Far as I know, they are 20 feet down in a cut and cover landfill about 15 miles away from the shop.

    Annoying Phase-Perfect is going to have to go share a blanket in the back garden shed with the RPC, Delta => Wye transformer, and the Diesel though.

    Five each of 100' spools of #4 THNW Copper wire and some conduit and Square-D goods made that possible.

    Bog-standard electrical goods. Gotta LOVE the stuff!


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    Quote Originally Posted by DennisCA View Post
    What do you guys think?
    Here are some more guys.

    Mill Motor Runs on Line Power 3P/240VAC But Not on VFD - Why???

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    Quote Originally Posted by whidbey View Post
    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
    I have seen a motor that ran across the line for 15 plus years fail within 2 weeks of being on a VFD.

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    Quote Originally Posted by BMyers View Post
    I have seen a motor that ran across the line for 15 plus years fail within 2 weeks of being on a VFD.
    Of course you can do this, depends on how you setup the VFD and there are all sorts of things to tweak. It takes time and effort to get through all the options.
    Some engineers who commission such were obviously sleeping in class. I've seen that, this is not plug and play.
    You drag out the test equipment and check what is going on so that you know it will go another 15-30 years.
    Bob

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    Quote Originally Posted by Briney Eye View Post
    I know from experience designing small motor drivers that switching too fast can blow the H-bridges (I warned the engineer and he did it anyway - made a nice little light show), so I'm pretty much with you on this one. If you switch them too fast the transistors never have time to switch fully on or off, look like resistors, and cook. But I would assume that VFD carrier frequencies are limited to a "safe" value (mine are 15kHz) to keep that from happening. I have mine maxed out so I don't have to listen to them whistle, and haven't had any issues. All this new inverter tech is based on IGBT's, and they've gotten pretty good in the last ten years or so. Seems like they're typically rated for 30khz or higher switching frequencies. Earlier ones were slower.
    There is the direct "too fast" where the "on" time is too short as you describe.

    The "speed" of switching can mean the actual speed of the switching event, or it can mean the number of times per second that the device is switched. Carrier frequency is the latter. Each turn-on or turn-off event has a "loss" which appears as power dissipated in the device, since the device is essentially operating in a "linear" or "resistive" manner during the time of switching on or off. If switched too many times per second with insufficient heatsinking, the switching devices (usually IGBTs these days) simply overheat and fail, because the total energy dissipated per second is too high.

    IGBTs in particular have issues common to all "bipolar" transistors, that do not exist in a power MOSFET. These include a tendency to "stay on" after they are "turned off", due to what is called "tail current". MOSFETs , however, trade low losses during switching for higher losses when conducting current. IGBTs have lower losses when fully turned "on". Newer devices, both MOSFETs and IGBTs are getting much better, tending to mix the best features of IGBTs with the best features of MOSFETs in one device.

    The losses associated with turn-on and turn-off are also affected by the design of the gate drive. A weak gate drive leads to slow switching times, and more losses. A fast switching time has less loss, but takes more gate drive power (another form of "loss"), and also generates more EMI (radio interference). And the best devices tend to be the most expensive, which often dictates the use of a device type that is not the best possible, limiting the choices the designer has.

    It's all a trade-off, and engineers are expected to find the best trade-off for a given application.

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    In an earlier post, the following comment was made:

    "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."

    Surely the voltage per turn is the same in both cases?. Same risk perhaps?

    I have a machine with a 4hp dual voltage motor, but it is not possible to find a 240v inverter of this rating.

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    Quote Originally Posted by Earl Sigurd View Post
    In an earlier post, the following comment was made:

    "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."

    Surely the voltage per turn is the same in both cases?. Same risk perhaps?

    I have a machine with a 4hp dual voltage motor, but it is not possible to find a 240v inverter of this rating.

    Failure mode may be different.

    There are straight-up electrical failures, insulation issues, in general. For these, yes, the volts per turn may be about the same. But the spike voltage tends to be only on the first few turns of the winding, and that voltage is higher at 480V. So any places of insulation weakness may end up as failure points.

    Then there are failures due to other causes. The 480V VFD has double the voltage change per switching "event". Failure from capacitive currents (induced in the rotor and going to to ground) gradually etching the ball bearings (like an EDM) is fairly rare at 240V, but considerably more common at 480V.

    Generally every form of stress is accentuated on the higher voltage motor, in comparison to the lower voltage, and so you would expect more failures.

    Your 4 HP motor would be perfectly happy with a 240V 5 HP inverter, which should not be hard to find. Just set the "motor rated current" etc, to match the motor.

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    Quote Originally Posted by rons View Post
    Describes what a square wave is, or a rectangular pulse train. The edges of the pulses have huge spikes. The inductor has to deal with them.
    A friend from the space biz went through my old Louis-Allis variable frequency drive when it exploded. This was one of the things Fred was muttering about, the spikes on the leading edge of the square wave when it flattened out. He put 'snubbers' in the circuit for that reason.

    Makes you wonder how many commercial products are not well-designed. The motor ran much better after he was through.

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    rons -

    Hate to argue with a fellow Californian, but seems that it would be hard to define an appropriate sample, given the variety of makes, models and, particularly, ages of 230 volt three phase motors that make up the universe of interest.
    Also pretty hard to see the relevance of the deaths, intentional or accidental by victim owned knives and forks.

    Monoblanco

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    Quote Originally Posted by Earl Sigurd View Post
    In an earlier post, the following comment was made:

    "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."

    Surely the voltage per turn is the same in both cases?. Same risk perhaps?
    most dual voltage motors are wound with two wires in parallel "in hand" so when you connect them for 480 volts every single wire everywhere in the motor has 240 volts between it and the next wire. the voltage spikes from a 480v vfd are by nature 700 volts. if you have a doubling of that voltage due to the square edges of the waveform interacting with the leakage inductance and capacitance of the motor and the transmission line between the vfd and the motor, then you've got 1400 volts available, and its going to find a pinhole in the insulation somewhere.

    a dv/dt filter which is just a resistor and a capactitor can substantially reduce the spikes at the leading and trailing edges of the waveform and they only have to eat up say, 15 watts of power for a 2-5hp drive (depends on the length of the transmission line how much energy must be burned up to get rid of the spike).

    but 30 watts worth of good quality resistors and three capacitors able to handle the pwm waveform 24/7 for 10 years costs.. i'm guessing 10$. which is why you don't find them on cheap vfds.

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    Actually, applied electric current is bad for electric motors.

    It makes them move, and movement causes wear of the bearings and deterioration of the insulation.

    Applied current causes heating, which is detrimental to bearing lubrication and winding insulation.

    Applied current causes shaft rotation which stresses the armature connection and the winding mounts.

    There are many other troubles associated with electricity when applied to electric motors. It is best NOT TO CONNECT MOTORS TO POWER LINES.
    if you want to preserve the electric motor.

    IF you want to get WORK done, ignore the above warnings. HAVE AT IT!

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    Quote Originally Posted by johansen View Post
    most dual voltage motors are wound with two wires in parallel "in hand" so when you connect them for 480 volts every single wire everywhere in the motor has 240 volts between it and the next wire. the voltage spikes from a 480v vfd are by nature 700 volts. if you have a doubling of that voltage due to the square edges of the waveform interacting with the leakage inductance and capacitance of the motor and the transmission line between the vfd and the motor, then you've got 1400 volts available, and its going to find a pinhole in the insulation somewhere.

    a dv/dt filter which is just a resistor and a capactitor can substantially reduce the spikes at the leading and trailing edges of the waveform and they only have to eat up say, 15 watts of power for a 2-5hp drive (depends on the length of the transmission line how much energy must be burned up to get rid of the spike).

    but 30 watts worth of good quality resistors and three capacitors able to handle the pwm waveform 24/7 for 10 years costs.. i'm guessing 10$. which is why you don't find them on cheap vfds.

    Seems odd. UL does not even count wire varnish as "insulation". It may be good for as little as 50 volts, and is expected to have "holidays" in it. It's OK for turn-to-turn voltages of from 1 to 20 volts, but not even close for 240V, let alone normal circuit spike voltages.

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    I was just looking in to specs for braking resistors and came across this info on thhn from VFD to motor failing due to corona discharge:

    Problems VFDs cause and cable types that help solve them

    Great summary on a lot of the VFD-specific failure modes and how to avoid them. Now I need to find where I can get shielded xlp or 20 mil PVC cables. Or is that overkill for a 10 ft cable length?

    On a related note, I'm running a 10 hp motor with a 10 hp VFD. If/when that motor fails, I have a spare 15 hp inverter rated motor that would fit. Is there a safe way to run the 15 hp motor on the 10 hp VFD by setting the VFD current limit to a value in the 10 hp range?

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    Quote Originally Posted by FCB84 View Post
    I was just looking in to specs for braking resistors and came across this info on thhn from VFD to motor failing due to corona discharge:

    Problems VFDs cause and cable types that help solve them

    Great summary on a lot of the VFD-specific failure modes and how to avoid them. Now I need to find where I can get shielded xlp or 20 mil PVC cables. Or is that overkill for a 10 ft cable length?

    On a related note, I'm running a 10 hp motor with a 10 hp VFD. If/when that motor fails, I have a spare 15 hp inverter rated motor that would fit. Is there a safe way to run the 15 hp motor on the 10 hp VFD by setting the VFD current limit to a value in the 10 hp range?
    for only 10 feet if you want to increase the life of the motor. (statistically, there is no guarantee.). buy a cheap dv/dt filter and install it at the motor. its a simple RC filter. it will do nothing for the common mode voltage which is what destroys the motor bearings. but it might double the life of the motor's insulation.


    regarding running a 15 hp motor on a 10 hp vfd. it will probably run fine, even up to 10hp load, because most vfds can dump more amps out than their "rating" would nominally account for. for example, a 10 hp vfd might have a rated load amps of 34. i have a 5 hp motor with full load amps of 12.3 (14-15 is normal) i assume 10hp motors with a 25 full load amps exist, but 28-30 is more typical and, i assume your 15hp motor will draw less than 34 amps supplying 10 hp to the shaft.

    There is a good chance the 15hp motor will operate with lower power factor and higher amp draw than the 10hp motor did, at 10hp shaft load. but it will be similar in efficiency.

    you may gain a few percent efficiency by programming the vfd for something like 190-210 volts/60hz (or 380-420 for a 480v motor) if the shaft hp is 10hp or less.

    the reason for this is that induction motors have an optimal volts per hz for a given load. the larger the motor the less this matters.

    the savings are on the order of 100$ a year for a 5hp motor, just dropping the voltage a little bit, dropping from say, 300-350 watts down to 200-250. if you've got an under loaded 15hp motor it might be 500 watts no load losses.. drop the voltage and it can still deliver say 7-10 hp at the shaft, no load losses might only be 250 watts. that's 250$ a year if its running 24/7.

    but it doesn't take much of a load to offset the 100 watts saved at no load.

    you really need a full account of the motors' load (both in magnitude and duration due to time required to overheat) to find the optimal volts per hz.

    but if you are trying to run, say, a 20hp motor from a 10hp vfd.. yes there is a volts per hz at which you can get 10hp out of the shaft.. and it is probably on the order of 2/3rds the nameplate volts per hz.


    your 15hp motor on a 10hp vfd is probably still within the range of what the vfd can do out of the box programmed for the nominal numbers.

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    I actually find the older motors to work better on VFD‘s.(50-70’s)They are a little bigger, they usually have a little more cooling and larger amateur and stater. I replaced the motor on my drill press with something from the 60s. The motor was out of an old city pump station(2hp). On the VFD I have set it to sensorless vector, which gives me more torque at lower speeds/hz. The motor is about three times the size of any 2 hp motor built today.

    I find some of the 80- 90s offshore brands motors to have trouble with a Vfd’s. I have an air compressor and a lathe that both have Baldor motors and have no problems.

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