Using original motor controls with a VFD
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    Default Using original motor controls with a VFD

    Not having a great deal of experience with motor controls & none with a VFD I am trying to map out a plan for two machines I have coming on line. One 3 HP Bridgeport clone, with what I assume to be a drum switch( I don't have it here yet) and one older Sheldon lathe withe a 5 HP motor & a standard reversing electromagnetic starter . Both will be wired for 230V. Both are three phase one a single phase service. From what I see A VFD would be a no brainier for both machines. Only one motor to operate on each. Looks like I can buy two VFD's that will meet my requirements for less than a good RPC.

    I have searched through the forum but that only confused me some more. Basically I can wire up a standard motor starter without much problem. But the whole VFD thing is somewhat nebulous. I realize the information is probably at my fingertips, I just cannot seem to find it.

    It is my understanding that the VFD is used to start the motor, not the machine controls, but the machine controls can be used to signal the VFD.

    In the case of the lathe, do I simply use the buttons on the lathe control & bypass the Mag starter? and somehow wire the Fwd, rev & stop buttons into the VFD?

    How does the VFD interface with the Bridgeport reversing switch? Or do I simply bypass the switch & use the VFD control panel.

    I was thinking a Hitachi X200 for the Mill and Possibly A Teco 10HP 230 Volt AC Adjustable Speed Drive, Model MA7200-2010-N1. something like this for the Lathe

    Am I on the right path?

    Thanks,

    Warren

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    When you searched within the VFD forum, what search terms did you use?

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    Let me rephrase that, I have read about 20 or 25 posts in the forum that I thought to be relevant. They answered some of my questions, but in the process new questions have arisen. Therefore I asked.

    I did not do a search using the search feature, they never seem to work for me. I probably never use the correct terms or some such thing. Never the less I have better luck just asking the question on my mind.

    I am sure I am not the first person to ask this, as a matter of fact I know I am not. We all have to start someplace.

    If someone would be so kind to point me to a post that would answer my questions or simply answer them I would be very happy. In the mean time I will keep looking, including downloading & reading the manual of the unit I am interested in.

    Warren

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    Thumbs up Yes, you can use existing switches

    Hi Warren,

    Most VFDs give you several control-switch configuration options. You select the one best suited to your application.

    The control switches are simple contact closures. They operate at control voltage, typically 5 to 24 volts DC, and carry no significant current, so any small switch will work. (Check your VFD manual for specs.)

    The STOP switch uses a normally-closed contact pair, wired in a closed loop so that any open or broken wire will cause the VFD to stop. This is important. Some VFDs offer configuration options that use a normally-open STOP switch. I DO NOT recommend using any such configuration since it is not fail-safe.

    The other switches are normally-open, and can be momentary- or maintained-contact types on most VFDs. If they're maintained-contact types, the STOP circuit function overrides any run commands.

    If you use existing switches, clean the contacts. Dirty switch contacts can cause intermittent or unreliable operation. Due to the low current in the control circuits, switches in this application are not self-cleaning.

    Also, note that you do not use any other electrical components in the system when using the VFD, i.e. no starter, no contactor, no circuit breaker or thermal overload. These functions are all built into the VFD, and it's UL-approved as a motor starter and an overload device. Obviously you need a circuit breaker between the VFD and the power line.

    Do not connect any type of circuit interrupter (contactor, switch, etc) between the VFD and the motor. Opening those lines when the VFD is on can damage it.

    - Leigh
    Last edited by The real Leigh; 11-10-2010 at 09:42 AM. Reason: typo

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    Quote Originally Posted by The real Leigh View Post
    Also, note that you do not use any other electrical components in the system when using the VFD, i.e. no starter, no contactor, no circuit breaker or thermal overload. These functions are all built into the VFD, and it's UL-approved as a motor starter and an overload device. Obviously you need a circuit breaker between the VFD and the power line.


    Do not connect any type of circuit interrupter (contactor, switch, etc) between the VFD and the motor. Opening those lines when the VFD is on can damage it.

    - Leigh
    Thanks Leigh,
    This is for some reason what was not clear to me. I think I have it now.

    Also thanks for the tip about cleaning the switch contacts.

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    Warren,

    The reversing mags on the lathe can be utilized as mere switches to signal the drive to start/stop/reverse. This allows you to use the factory buttons in their current location.

    The caveat here is that you must have the specific voltage available to match the coil ratings...220 volts is what you mention in your post. Your drive will probably be powered at this same voltage so this doesn't seem to be an issue.

    The output of the drive goes directly to the motor, no interruptions, while the control for the drive is handled through the separate logic buss and the simple switch closures offered by the many contacts on the existing mags. These contacts are no longer powered, merely opened or closed by the mag which itself is powered and controlled by the factory mounted buttons and a separate source of 220v...or 120v if you replace the coils!

    I don't know if this confuses you more, I tend to do that with my posts.

    Stuart

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    Quote Originally Posted by atomarc View Post
    Warren,

    The reversing mags on the lathe can be utilized as mere switches to signal the drive to start/stop/reverse. This allows you to use the factory buttons in their current location.

    The caveat here is that you must have the specific voltage available to match the coil ratings...220 volts is what you mention in your post. Your drive will probably be powered at this same voltage so this doesn't seem to be an issue.

    The output of the drive goes directly to the motor, no interruptions, while the control for the drive is handled through the separate logic buss and the simple switch closures offered by the many contacts on the existing mags. These contacts are no longer powered, merely opened or closed by the mag which itself is powered and controlled by the factory mounted buttons and a separate source of 220v...or 120v if you replace the coils!

    I don't know if this confuses you more, I tend to do that with my posts.

    Stuart
    Stuart,

    I think what you are saying is, use control coils in the electrical box to operate the VFD contacts.

    My control circuit is 120V to work the mag coils BTW

    Something like this then?

    Route the input & motor wires to the VFD.

    Use the forward & reverse contactors in the electrical box as wired. (I assume you remove the overload heaters from the mix) to signal the VFD control terminals. Using a separate circuit to control the controls or taking a leg from the input side to run the mag switches would probably work OK too I would think as that is there now.
    That would certainly be a lot neater to wire up

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    Warren,

    Yes, that's what I attempted to say.. The overload heaters can stay or go, they are of no consequence. If you supply 120 volts to the control circuit, remove all the motor wires from the mags you end up with contactors that respond to the factory buttons but switch nothing.

    Now you run the FWD/OFF/REV control wires through these naked contactors and they become the maintained switches that command the drive to operate the lathe through the logic buss.

    Stuart

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    People keep saying that switching the VFD output can damage it with very little theoretical backing for the statement. I've been running this way for several years now with one 3 motor machine with no issues whatsoever. I'll increase the system to three machines and 8 motors in time . If the VFD manual for that model doesn't specifically warn against it, then just do it. If you're still worried fit a braking resistor and output reactor to the VFD.

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    Default Vfd

    The solution is in the unit's manual. External contacts may be used but do not inject any power into the switch circuit.. There is an old line that fits in many cases---RTDM (read the dxx manual)

    JRW

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    Quote Originally Posted by J.R. Williams View Post
    but do not inject any power into the switch circuit.
    JRW
    Modern VFD's with braking resistor facilities neatly cope with that issue. The DC rail in the VFD gets clamped to the designed voltage by shunting excess current through the braking resistor.

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    Exclamation

    We're getting some rather bizarre answers here.

    DO NOT, under any circumstances*, connect any external voltages to the control inputs of the VFD. You'll blow it in a heartbeat.

    The VFD provides its own control voltage through a terminal. Wire the control circuits as described in the manual, using that terminal as the voltage source.

    BTW, J.R.
    I always thought that was RTFM (Read The FINE Manual)

    - Leigh

    *Note: There is one exception, which I hesitate to mention because you won't be using it. A 4-to-20 milliamp current-loop interface may use an external voltage supply, although it can be driven from the VFD's own control supply if the VFD is designed to support that configuration.

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    Quote Originally Posted by The real Leigh View Post
    We're getting some rather bizarre answers here.

    DO NOT, under any circumstances*, connect any external voltages to the control inputs of the VFD. You'll blow it in a heartbeat.

    The VFD provides its own control voltage through a terminal. Wire the control circuits as described in the manual, using that terminal as the voltage source.
    Sorry, I took it to mean the "switched circuit" i.e output, not the "switch circuit" i.e control input

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    I sure hope I have been making myself perfectly clear on this issue! I am merely advocating using contacts on existing contactors as switches for the logic circuit! The pull in mag is powered but this does not in anyway 'power' the contacts, only closes them.

    Stuart

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    Quote Originally Posted by Mark Rand View Post
    People keep saying that switching the VFD output can damage it with very little theoretical backing for the statement. ............................
    Good post Mark! Actually as a drives designer, I can give the 100% valid theoretical reason for these comments: opening any contact between the drive output and motor can very easily blow the output transistors!

    This is from a thing we learned in electrical engineer school: flowing current does not want to stop thru an inductor. If you insist, and force the circuit to open, the current in the inductor will continue to flow by reversing its voltage. the result is a voltage spike who
    s size is determinted by di/dt - a thing that says the more inductance, the larger the voltage spike.

    a motor is a BIG inductor. So if you are put contact in series with it and its current source - the drive - and open it while current is flowing, you WILL generate a HUGE voltage. You have seen this by opening a contactor on a running ac motor - the contacts SPARk big time! the reason is the current is trying like heck to keep flowing at any cost! but you opened the circuit, so the voltage reverses and continue to try to jump the gap. being thousands of volts, it easily arcs thru the airgap between the opening contacts. the result will be potentially thousands of volts! guess what ur output IGBT transistors are rated in that inverter drive? about 1200v peak reverse voltage (PIR). yep, the transistors tend to blow up when hit with this 2-4kv instantaeious spike! Often this blow up sounds just like a 12 gauge shotgun going off!

    all that said, if someone MUST have a contactor in the output legs of the inverter, there is a potential way to get away with it by making sure you disable the drive BEFORE the contact opens - if the mfgr aqrees this is acceptable. But there is no valid or safety reason I see today to put contactor in output of a drive.

    Warren, also as an aside, did u know that Hitachi's UL rating for their VFDs typically say they are acceptable as a circuit breaker? Ie., no CB is required in front of them. Little known fact. I would still put a CB in front of them.

    Also Warren, why would you consider Two DIFFERENT brands of inverters?? Make them the same so u dont have to learn 2 drives! for commonality of parts (consider making both 5hp models). for more leverage with the supplier (if we sell u BOTH Hitachi drives instead of just one, we will be more likely to give better price since u r now an even bigger customer of ours! we would be happy to quote these to u also

    It may be more work, but getting rid of the present 120v coils and such and just using the switches direct into the Hitachis sounds like better solution to me too - I like the KISS principle. Less stuff means less stuff to break later. also it is not uncommon for folks to mess up a wire and actually apply 120v into the 24v input circuit on the drive and blow it up. no 120v, no chance of this.

    Warren consider scanning the machines wiring diagrams to computer and sending to folks u ask to quote ur VFD drives and ask them if they will mark them up showing how to wire up the new drives using the switches already there - we do this for customers for a small fee - can get u a lot further along out of the gate!

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    Quote Originally Posted by Mark Rand View Post
    If the VFD manual for that model doesn't specifically warn against it, then just do it. If you're still worried fit a braking resistor and output reactor to the VFD.
    Not meaning to start an argument, Mark, but most manuals don't tell you to avoid putting your finger in a light socket either.

    The braking resistors are only in the circuit when the VFD is in the STOP condition. That would not be the case if the output circuit to the motor is interrupted while the VFD is in operation.

    - Leigh

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    Get a suitable VFD

    Take off the magnetic starter and its box, either put on a shelf, or sell it.

    If you want to reuse the pushbuttons, then do it. Those 'simple' motor control stations can get a bit pricy, for "just two buttons in a box"

    Let the VFD be hard-wired to the motor, with nothing in-between. That's what the makers expect and (usually) specify.

    If you have a drum switch, let it 'talk' to the VFD, at the low logic voltages specified, on the terminals specified. If you have the drum switch hooked to "that could kill you" voltage, you are doing it wrong.

    If you have an emergency stop button, it can "talk" to the VFD, at the low logic voltages specified, on the terminals specified. If you have the emergency stop button hooked to "that could kill you" voltage, you are doing it wrong.

    If you have two (Run, Stop) button, or three button (Forward, Stop, Reverse)control station, it can "talk" to the VFD, at the low logic voltages specified, on the terminals specified. If you have the control station hooked to "that could kill you" voltage, you are doing it wrong.

    Any or all of this will require reading the manual. Any or all specific information from the board members will require knowing the VFD model being used.

    If you buy some bare board OEM oddball VFD off of ebay, don't expect many of us to know anything about it, just like those bare board computer motherboards that get tossed at work, sold at auction, found on a shelf, etc. If you don't get the manual, I suggest you don't buy it.

    I have a favorite (duckling syndrome) brand, but there are a handfull of brands that are mentioned on this board. I can't recall them, I just know mine.


    It's too late for you to edit the title of this thread (Go Advanced is how to do it, for the first 24 hours), but it really should be titled something that would pop up in a search, something like:
    Using Start/Stop or Forward/Stop/Reverse Buttons or Drum Switch with VFD

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    Exclamation still need a breaker

    Quote Originally Posted by mike_kilroy View Post
    Warren, also as an aside, did u know that Hitachi's UL rating for their VFDs typically say they are acceptable as a circuit breaker? Ie., no CB is required in front of them. Little known fact. I would still put a CB in front of them.
    A service drop breaker protects the wiring and connectors (if any) between the service panel and the VFD in the event of an overload.

    It's required by code regardless of the ratings of the VFD, unless the VFD is mounted IN the service panel.

    - Leigh

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    Quote Originally Posted by atomarc View Post
    I sure hope I have been making myself perfectly clear on this issue! I am merely advocating using contacts on existing contactors as switches for the logic circuit! The pull in mag is powered but this does not in anyway 'power' the contacts, only closes them.

    Stuart
    Yes Stuart That was abundantly clear from the get go, at least to me anyway. I have read some of the X200 manual now and even figured out where the wires go. And kinda sorta how to set the parameters. we will see once I get everything hooked up how simple it really is

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    Quote Originally Posted by mike_kilroy View Post

    Warren, also as an aside, did u know that Hitachi's UL rating for their VFDs typically say they are acceptable as a circuit breaker? Ie., no CB is required in front of them. Little known fact. I would still put a CB in front of them.
    I figured that much, and planned on a CB for each machine

    Also Warren, why would you consider Two DIFFERENT brands of inverters?? Make them the same so u dont have to learn 2 drives! for commonality of parts (consider making both 5hp models). for more leverage with the supplier (if we sell u BOTH Hitachi drives instead of just one, we will be more likely to give better price since u r now an even bigger customer of ours! we would be happy to quote these to u also
    I was using the Teco as an example, first one I came to

    It may be more work, but getting rid of the present 120v coils and such and just using the switches direct into the Hitachis sounds like better solution to me too - I like the KISS principle. Less stuff means less stuff to break later. also it is not uncommon for folks to mess up a wire and actually apply 120v into the 24v input circuit on the drive and blow it up. no 120v, no chance of this.
    That could easily be done with the way the control panel is set up on my lathe

    Warren consider scanning the machines wiring diagrams to computer and sending to folks u ask to quote ur VFD drives and ask them if they will mark them up showing how to wire up the new drives using the switches already there - we do this for customers for a small fee - can get u a lot further along out of the gate!
    If I go that route, I may indeed do that, I see what Stuart is saying about that, & understand how to do it. But new switches may be the better route, especially since they need to be clean to operate with low voltage


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