Using original motor controls with a VFD - Page 3
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  1. #41
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    Being new to this forum, I did not know there was a second page button at the bottom of the replies list! I thought all posts were shown by scrolling down the page so did not see page 2 replies until I stumbled on this page button just now.

    I wanted to just give a small rebut reply to the inductive kick is no big deal reply and also say good point in another good reason for not putting contact on output side of the drive. I have been sent to many places all over the US to solve weird VFD problems that were not being solved otherwise. On more than 1 machine I found a loose wire in the motor leads between the drive and motor. The failures that were resulting from these were two fold: 1) drive output IGBT failures, & 2) motor insulation failures. Scoping the output voltage showed clearly that normal IGBT switching of 480v input drives with 650vdc nominal bus had typically 12-1500vdc 'inductive kick' at each IGBT switch off. But what opening the motor lead presented was 2500-3500v kicks. The result of these much higher voltage spikes in the motor was it caused corona between motor individual winding wires anywhere there as a gap or air space left by incomplete varnish potting. This corona generated ozone. Ozone eats varnish. I learned from this experience hi voltage VFD rated motor wire is good for around 2kv max spikes and exceeding this level slowly damages it in a cumulative way. So until this loose connection was found the result was motor failures every 2-24 hours! The customer went through about 10 motors before I got there. They also blew 1 drive in the process. A lot of drives use 1200v rated IGBTS (repetative peak reverse voltage) today. I suggest (but am not 100% sure) the reason disconnecting the motor while current is flowing produces a higher voltage spike is that when the contact opens, there is no longer the backwards IGBT diode hooked up to allow the current to flow and dissipate, so the voltage does not get clipped by it. So it continues rising towards the level it needs to go to. But as it is rising, the 'contact' is an airgap - and when the voltage gets high enough, it ionizes the air between the open contacts, causing it to become a conductor, and so we get the big spark. That spark is already a very high voltage so the reverse diode in the IGBT pack cannot stop it before it starts, so it clamps it but too late. The result is the higher voltage spike.

    I believe it is this much higher voltage when you disconnect a motor from drive that makes it different than the smaller more controlled spike each PWM off cycle.

  2. #42
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    Depends how it happens... and what sort of power we are talking about

    The voltage is related to the di/dt, and if the break happens in a way that is relatively "clean", it may develop a large voltage.

    A contactor normally has some "rubbing" where the contacts are opening but still have a small gap only. The arc-over voltage on a small gap may be rather low. 12V circuits arc even without discrete inductors. So quite a bit of energy is dissipated in "contact bounce" at lower voltages.

    it is always possible to set up a worst-case situation that manages to provide maximum stress to the system. No protection circuit is perfect.

    The VFD generally uses the smallest components that will do the job, because they are cheaper. AND the larger the VFD, the slower the components are. Slower diodes etc, may actually allow larger voltages in situations of wire breaks etc. The stored energy in the windings of a big motor is much larger than in a small one in the 1 to 10 HP area. Problems of a similar type may be catastrophic in a 200 HP system that are merely an annoyance in a 2 HP system

    That said, hundreds of thousands of off-grid inverters power motors every day, 'fridges, shop tools, air compressors, lawn tools, you name it. There has never been a "generic" problem with these things and motor switching.

    But you STILL don't want to use contactors, because of turn-on overloads. Only if the inverter/VFD is a lot larger than the load motor HP is it practical to do that, and that situation is usually not economically practical, although there are generator sets that operate with inverter output.

    The VFD normally will provide better control than a contactor, with braking capability etc that is not available with a simple disconnection. So aside from the simplicity suggested by "plugging into a VFD", there isn't much advantage to trying to keep the contactor operational in-circuit. Wire around it, and set the start-stop to operate the VFD controls.

    usually there is a way to emulate the usual start-stop system with the VFD control inputs, as I think has been described above.

  3. #43
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    [QUOTE=atomarc;1456001]
    Quote Originally Posted by Hopefuldave View Post

    Dave,

    .......Maybe I am wrong on this assumption, I would love to find out that it can be configured for momentary inputs that latch internally.

    Stuart
    Stuart, I believe there is ONE low cosgt hitachi model that will not accept momentary for run/stop, but the rest do. Also, apps support guys can sometimes be confused with all the questions they get and possibly to0old you wrong? Please tell what your Hitachi part no is and I will be happy to look it up also.

  4. #44
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    Default Adding three-wire control to older Hitachi VFDs

    In the Hitachi documentation, at least the X200 and SJ200 documents, the phrase to find is "three wire control".

    Industrial Inverters | AC Variable Speed Drives | Industrial Systems : Hitachi America, Ltd. is a good place to start looking for documentation.


    If the product is L100 and SJ100, yes, that feature wasn't built-in.

    Hitachi has an application note to address creating that feature; unfortunately, it is visible only after giving some personal information to the Hitachi website (or at least filling in the fields with gibberish ).

    This is the application note link:
    And this is what it says:

    Application Note:
    Three-Wire Control
    (STA, STP, F/R) with L100 and SJ100 Series Inverters
    Please refer also to the L100 or SJ100 Inverter Instruction Manual
    AN021003-1 Rev. 1 11-14-03
    Hitachi America, Ltd.
    © 2003 Hitachi America, Ltd.
    Application Note for “Three-Wire Control” with
    Hitachi L100 & SJ100 Series Inverters
    Sometimes it may be desirable to use a remote START/STOP pushbutton arrangement together with an inverter. This often occurs when a machine is being retrofitted or upgraded with the addition of a Hitachi inverter as a replacement for a motor starter. The pushbuttons that interfaced with the original motor starter would typically be a single-pole normally-open momentary pushbutton for the START button, and a single-pole normally-closed momentary pushbutton for the STOP button. Sometimes a switch may also be present for selecting the forward or reverse direction of rotation.
    In the case of the L300P or SJ300 Series of inverters, a function is built-in to directly interface to a START/STOP pushbutton arrangement of this type without any additional hardware. Additionally, a FORWARD/REVERSE signal can be accommodated, hence the term “three-wire” control. The three signals are wired directly to three intelligent input terminals.
    In the case of the L100 and SJ100, such a function is not built in. However the START/STOP functionality (without direction control, i.e. FWD only) can be accomplished simply with the addition of a single external control relay, using a circuit similar to that in the diagram below. This is a simple seal or latch circuit.[the diagrams do not copy]


    The above circuit does not include FWD/REV direction change functionality. If this is a requirement, a more elaborate arrangement is needed. Two relays would be necessary, and the STOP pushbutton would have to be DPST NC type, as shown in the example that follows on the next page.

    [the diagrams do not copy]

    In this circuit, pushing the STA FWD button would start the inverter in the forward direction, while pushing the STA REV button would start reverse operation. If it is desired to change rotation direction while running, the STOP button would have to be pushed first, then the desired run direction button. That is because the inverter will ignore a REV run command when a FWD run command is already present, and vice versa. Pushing the buttons in this sequence will cause the inverter will ramp the motor down to a stop, then ramp up in the opposite direction, according to its configuration settings.
    The 'hidden' application notes are found at Inverter Technical Support Library : Inverters : Hitachi America, Ltd.


    This document addresses derating for single phase operation, and defeating single phase detection: http://www.hitachi-america.us/suppor...ngle-Phase.pdf
    Last edited by S_W_Bausch; 11-18-2010 at 10:18 AM.

  5. #45
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    Default I'll be dammed, it actually works!

    Please welcome me into the 21 first century! I managed to get the milling machine wired, power up tested & controls hooked up without having to call tech support! We are now making chips!

    One down & one to go. The lathe is the next target. And I do not have any doubts about getting it going. Thanks for all the help folks, your posts gave me a better understanding of why things work. The Hitachi manual is not hard to follow. Lots of information in there though.

    As far as the controls go for the milling machine. It is a standard Bridgeport style drum switch. I just sent the motor leads to the VFD and wired the drum switch into the control bus for Fwd/stop and Rev/stop. Works just like normal. I used some control wire I had handy.

    About the only parameter I changed besides the required data for the motor type and the input parameters was the acceleration & deceleration time. I went from the default 10 seconds to 5 seconds. The motor now ramps up & slows down nicely and does not seem to put undue stress on anything. Since I am not needing a quick stop, I did not specify braking resistors.

    I recycled a Square D motor starter enclosure for the VFD. Plenty of air space for cooling and a few extra holes for air

    Warren

  6. #46
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    Warren,

    Using the drum switch is a piece of cake. Tweaking the ACCEL/DECEL parameters was good...but if you fiddle with the DC brake injection you can get the spindle to come to a stop in a respectable amount of time and this requires NO resistors or any other monkey motion. The drive has the ability to brake the load, up to a point, with nothing other than programming.

    You don't need to get nuts, just find the programming parameter and start dialing it up until the spindle cruises to a stop a tad faster than a coast...or more if you want. This will allow you to tap with the machine as you transition from FWD to REV in a very fluid motion.

    Stuart

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    Stuart,

    I never gave much thought to tapping with it. I'll have a look at that. We may be talking some more. I would assume I need to make the acceleration a bit faster?

    I tap with my Milwaukee mag drill. The electronic control in it stops the motor & starts it again nice & soft like. Just by cycling the fwd / rev buttons. i'll see if I can figure this out. seem to remember seeing something on braking in the manual other than using external resistors. I'm on it!

  8. #48
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    I am designing a simple two station remote control for a Vacon X4 drive. Is it possible to use two remote key pads with a single VFD to allow speed, stop and emergency stop from two stations on opposite sides of the machinery. The emergency stops may require resetting at the station that initiated the E-stop but the speed and stop functions should be from either station. Any advice?
    Last edited by denno; 10-05-2019 at 01:11 PM. Reason: spell check


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