Direct on line after VFD startup
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    Default Direct on line after VFD startup

    Like many people, I hate the whine of VFDs. I have often thought of disconnecting the VFD after startup of an inertial load and connecting it to line voltage for the rest of the time it is running. I know VFD's don't like to contactors on their output, but that seems to be because they don't like loads being suddenly connected. I also am aware of line filters, but have heard varied results as far as quieting goes.

    The main reason I have never attempted this (other than not having a reason to) is because of concerns with phase matching during the switch. But, we just got a compressor and dust collector with star-delta starters that connect the motor in a star configuration, disconnect it, then connect it in delta. I can hear the delay between the two and I know the motors slow down to at least be a half cycle out of phase, and while there is a slight current peak it really doesn't matter.

    The reason I am revisiting this now is because I was considering making blower wheels for some dust collection/air purification/powder booth applications. Blower wheels like these have significant inertia, and would typically use a star delta starter. However, I also am considering two or more motor/blower pairs to run sets of filters so that one can blow off with the blower stopped while leaving the other filters uninterrupted. This would require less air and simpler to construct baffles.

    So my question is, what is stopping me from having, say, four 2hp motors with heavy blower wheels, each with a contactor to a single VFD and a contactor to mains power (preferably interlocked)? A motor would connect to the VFD, start, switch to mains, and repeat until all motors have started, then individual motors could stop and start the same way for a blowoff cycle on their filter stack.

    I know there would be some control circuitry but I see no reason a finite state machine or sequence of timers would have any issue controlling everything.

    And of course there is extra complexity here, but star-delta starters aren't much simpler or surge current/motor friendly, and I'm far more electrically capable than I am slip-clutch or similar capable.

    I will say I better implement a phase protection device, otherwise the first switchover from VFD to line with reversed phase would sound soul crushing at best.

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    Just 2 HP?

    Still might have a significant surge switching over. If the motor is still generating back EMF when reconnected, you could have a jerk and significant surge if the back EMF were to be nearly or actually opposed to the line volts.

    At that power level, there would not be any expected problem with disconnecting the motor. I'd still clear it with the VFD manufacturer..... some specifically say not to, and others say specifically it is fine, some are silent on the issue. (I do not know why some say not to specifically, presumably they know something, but I've never had a problem, and would not expect one. )

    One of my designs had the worst of all, switched off, AND "dropped onto" the VFD output to start the motor. No issues, but it was made to handle the start load. The disconnect was no issue.

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    just do it.

    buy the right sized 5% line/load reactor and install delta capacitors after the filter. put the vfd and the reactor inside a sound proof box and you wont hear the whine.

    2hp vfds are like $70


    btw, cheap VFDs measure the current on just two low side dc legs of the IGBT half bridge, not the current in the motor or high side switches. while it is theoretically possible for some kind of current surge or something blow the igbt, the reality is that motors are inductors, not capacitors, and they cannot produce more current than what was already flowing through them.

    if a vfd fails due to over current, its because the software wasn't fast enough to sense the current rising, and turn off the switch, given that motors are inductors, there is no excuse for this being a problem. secondly if you soften the motor up with an LC filter as i suggest, you have even more time for the vfd to fault.


    i have had no problem running 5% line load reactors and capacitors, and hot plugging motors, provided you don't trip the overcurrent sensors.

    typical 2hp drive might have an igbt bridge that can handle 20 amps indefinitly with 30 amp surge current, but the $2 heatsink and thermal goop limits it to 8 amps continuous.

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    Quote Originally Posted by JST View Post
    Just 2 HP?

    Still might have a significant surge switching over. If the motor is still generating back EMF when reconnected, you could have a jerk and significant surge if the back EMF were to be nearly or actually opposed to the line volts.

    At that power level, there would not be any expected problem with disconnecting the motor. I'd still clear it with the VFD manufacturer..... some specifically say not to, and others say specifically it is fine, some are silent on the issue. (I do not know why some say not to specifically, presumably they know something, but I've never had a problem, and would not expect one. )

    One of my designs had the worst of all, switched off, AND "dropped onto" the VFD output to start the motor. No issues, but it was made to handle the start load. The disconnect was no issue.
    I haven't really decided on the exact configuration because I haven't done any math on the blower wheel diameter/width/rpm to match to the filter cfm and static pressure needed for proper flow in the booth. It could be anywhere from 1-5 HP.

    I'm fairly certain the surge won't be worse than that from a star delta transition, but I'm curious if any form of snubber and a longer delay (possible overspeed a little and let it coast to synchronous speed) would reduce the inrush. I'm kind of curious about the possibility of an SSR that can only turn on at zero crossing, but I don't even know if that trick works with three phase.

    Luckily I'm in a building with enough power to do everything I need to, so I don't need to try damping surges with resistor networks and whatnot anymore. My bigger concern would be sustained power draw from starting multiple inertial loads in sequence, and also motor longevity from the relatively frequent starts and stops, though my brain may still be in single phase mode where that is a bigger issue.

    The star delta starter on our compressor still draws 500 amps for a few seconds and a bit more than that on contact due to magnetization. You can hear and see the transition all over the shop.

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    Quote Originally Posted by johansen View Post
    just do it.

    buy the right sized 5% line/load reactor and install delta capacitors after the filter. put the vfd and the reactor inside a sound proof box and you wont hear the whine.

    2hp vfds are like $70


    btw, cheap VFDs measure the current on just two low side dc legs of the IGBT half bridge, not the current in the motor or high side switches. while it is theoretically possible for some kind of current surge or something blow the igbt, the reality is that motors are inductors, not capacitors, and they cannot produce more current than what was already flowing through them.

    if a vfd fails due to over current, its because the software wasn't fast enough to sense the current rising, and turn off the switch, given that motors are inductors, there is no excuse for this being a problem. secondly if you soften the motor up with an LC filter as i suggest, you have even more time for the vfd to fault.


    i have had no problem running 5% line load reactors and capacitors, and hot plugging motors, provided you don't trip the overcurrent sensors.

    typical 2hp drive might have an igbt bridge that can handle 20 amps indefinitly with 30 amp surge current, but the $2 heatsink and thermal goop limits it to 8 amps continuous.
    2-3 times that for one I have any confidence in, but yeah, they aren't that expensive. Motors are inductors with significant cores that hold significant magnetization energy, (essentially a transformer coupling) which can result in huge transients. Starting loads of motors are 3-5 times their FLA (typically) but on that first cycle (and on any transitions after) they can pull up to 10 times FLA. Transformers have the same issue. Also, since the rotor of an induction motor has a relatively static magnetic field, it can remain magnetized enough to keep the motor generating, which can then be out of phase of the power being switched to it.

    But really since the only sudden connections occur to line power in my setup, the VFD wouldn't ever see it to care about it. My concern with cheap VFDs is primarily roasted capacitors and shoddy construction.

    You say you have experience running load reactors and capacitors. How well do those mitigate the VFD whine? That and maybe a little efficiency boost would be the reason I don't want to have one VFD per motor.

    My dad is actually completely tone deaf to I think 5khz and 7khz. He can hear 6khz fine, but he's spent years near big motors modulated at those frequencies and now he can't even tell if the VFD is on or not.

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    2 HP can generally be set to 16 kHz, where you will not hear them.

    The motor is a lossy inductor (losses are out the shaft, mostly). With a higher voltage across the inductor, the rate of increase of current is higher in proportion.

    So the initial current will not be very different from what is already flowing, but it will be increasing faster than normal, so the current surge can potentially be more. It is similar to a "plug reverse" situation.

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    Quote Originally Posted by JST View Post
    2 HP can generally be set to 16 kHz, where you will not hear them.

    The motor is a lossy inductor (losses are out the shaft, mostly). With a higher voltage across the inductor, the rate of increase of current is higher in proportion.

    So the initial current will not be very different from what is already flowing, but it will be increasing faster than normal, so the current surge can potentially be more. It is similar to a "plug reverse" situation.
    Unfortunately I'm young enough that I can hear 16khz just fine.

    Honestly I don't think the surge will be a big deal.

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    Quote Originally Posted by Strostkovy View Post
    Unfortunately I'm young enough that I can hear 16khz just fine.

    Honestly I don't think the surge will be a big deal.

    Well, I'm older and so can I. But the harmonics are not audible to either of us. So there is always the 32 kHz setting, although it produces considerably more heating of the VFD.

    The surge may not be, it just produces a torque and some motor heating, and may cause some issues of protection rating.

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    Quote Originally Posted by Strostkovy View Post
    My dad is actually completely tone deaf to I think 5khz and 7khz. He can hear 6khz fine, but he's spent years near big motors modulated at those frequencies and now he can't even tell if the VFD is on or not.
    While my hearing isn't what it used to be, I can still hear to 25khz fine... and I hear my VFDs' primary and overtones pretty well, so I can sympathize with anyone who finds it uncomfortable. In contrast, My dad said that playing in a 15-piece rock band would make me deaf... I spent years singing in a competing barbershop quartet, which was certainly much worse... When you get intonation 'just right' for a room, the overtones take off, and hit incredible SPL at close range. At least being onstage with my band means I'm 14 feet from those crash and ride cymbals...

    The thing about the kHz range audio, is that it is occurring as the motor's natural reaction of the motor armature to the PWM switching. One can cover up the motor and get SOME relief, but the motor is still coupled through to a load, which becomes an excellent path for the mechanical action, and anything thus connected to it, becomes a 'speaker surface' of sorts...

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    This is actually not too uncommon practice when using staged systems for capacity control. It's still not very common though.

    Generally you want to use a pair of mechanically interlocked contactors to switch between DOL and the drive, and have the drive control which is engaged.

    See for example figure 10 on page 32 of this Vacon application manual.

    A soft starter is potentially a cheaper option if you're intending to run it only at full speed. They're not as nice for the motor as a VFD but aren't as rough as a star-delta.

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    Anything over the carrier frequency the drive was designed for increases the heating of the VFD, most are designed around 4kHz. Of those responsible enough to include a de-rating chart, 16kHz requires de-rating of the VFD capacity by 50%. Those that don't provide this information are just ignoring the issue, but that doesn't make it go away. High carrier frequencies also increase the possibility of reflected wave damage to the motor windings.

    Opening a contactor while the motor is running on the output of a more modern drive (made in the last 5 or 6 years) is not the death knell it once was, because the manufacturers of the power devices pretty much all build in some additional protection into the transistor package now (cheap Chinese junk drives not included). But if you use an older used drive, it's a concern.

    There will be a current surge when reconnecting the motor A-T-L, but the risk of damaging anything on a motor this small is very low. I have had a500HP motor shear the shaft on transition though... The bigger risk is in reconnecting to the VFD while running, because although the VFD can control the current and catch the motor on the fly (most can anyway), if there is still residual magnetism in the motor when you reconnect to the drive, there can be a VOLTAGE spike that can destroy the transistors. So the way to avoid that is to use a timer set to about 1 second on the contactor that applies the VFD output so that you endure that the motor fields are gone before connecting the VFD.

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    Its the current spike that would destroy the drive, voltage is clamped by diodes to the dc rail of the vfd.

    I measured peak current for 1/2 line cycle at double the locked rotor current on a small 1/3 hp single phase bench grinder. Took a lot of attemps to switch it on and off to get the scope triggered and capture the spike. Torque produced was enough to loosten an additional lapping plate i had on, hand tight. I was somewhat supprised.

    Takes about 2 seconds for voltage to decay to 10% of line voltage at turn off.

    Anyhow a vfd should be able to handle a short circuit in the motor,(limited inductance to slow the rise) amd shut the vfd off. The current climbing at about double the rate locked rotor current builds up is a much slower rise time, like 100 times slower, no excuse not to be able to sense it and turn off the switch.


    Thought occurs to me that older drives had a lot of inductance between the transistors and the diodes because they were not integrated on the die yet, amd they had snubbers designed to capture the energy.

    Opening the output contactor was not properly snubbered, because the drive wasnt designed for that event.


    Many vfds do not have a snubber, but the igbt located less than 1cm away from the diodes solves the problem

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    It's nice to see sense talked about downstream switching with inverters/VFDs instead of the more common hysterical nonsense.


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    Snubbers of the R-C type don't do well on modern sine wave VFDs, as the cap passes too much current. The diodes are right there on many IGBTs. Not usually on the die (that's MosFets that have an intrinsic diode) but next door to it in the package. Good enough.

    Old drives may have done six step, or other LF switching, and could use an R-C snubber.

    "Hysteria" is close.... I have no idea why any modern unit should have a problem at normal line lengths etc. They handle thousands of turn-off pulses per second already, a few more are not an issue.

    There may be pathological cases where there is a lot of stored energy. Or there may be cases where the diodes are rated lower, or are not as "fast" as they might be. But in general with 3 HP and below there is no issue. At higher power, there can be a lot of energy, especially with long lines, inductors, etc, I'd not do it.

    As for the heating.... That depends on the choice of devices. Not every device will need derating of 50% at 16 kHz, although the choices available in the one-piece packages may not be so good.

    We did a 200 kW 5 phase inverter that ran around 18 kHz maximum pulse rate, and there was no unusual heating because the devices were somewhat newer. Each "IGBT" was an array in a package that was bigger than two paperback books. That was a custom part from England, IIRC. It was to "motor" a 5 phase 200 kW generator at 60,000 RPM for testing. (You could pick up and carry that generator head by hand).

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    If you reconnect the motor while the field is collapsing and its out of phase, then you get a nasty violent current spike in the motor. You need to let the magnetic field decay in order to avoid nasty spikes. The good news is that the time constant to decay the field in these small motors is quite short, but you really want to delay the restart until the field collapses.

    I would shut the drive off (coast stop) then swap over. That should give you enough delay to collapse the field and protect the drive. Motor will decel some during the swap - it will either way you do it.

    On big motors (hundreds to thousands of hp) we prevent restart by several seconds to prevent damage. I got a trip to Russia over 4th of July holiday one time because we didn't have restart protection and took out a 1000 hp or so motor on a compressor.


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