Where to wire the VFD
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    Default Where to wire the VFD

    Hello there,

    I've managed to acquire a SAG12 graziano three phase lathe. Its 380/220v three phase, 3hp motor; I want to run it on my standard 220v supply. I purchased a VFD (5hp) that outputs 220 three phase. I'm wondering where i need to wire in the VFD? do i need to directly connect it to the motor wiring (after they've been reqired to delta for 220v) or can i simply put it upstream of the whole machine (simply wire it into what was unplugged from the wall in the shop where it was with three phase electricity supply) The lathe does have electromagnetic DC clutches that from reading forums needed a steady supply. I don't want to use the VFD to control motor speed so I would just be using it produce the three phase 220V.

    I think i realize from reading other posts that it's generally better to a VFD directly to the motor, but in this case doing that would mean getting a second VFD and figuring out how run the DC clutches on something else (which would involved starting from scratch and figuring out basic electronics that i have no experience with -- is there any reason that the VFD can't just be wired upstream and sned in the threephase as it would have been with a standard three phase supply?

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    VFDs can't supply enough starting current to start a stalled motor at full frequency.

    You need to replace the motor contactor(s) with the drive.

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    Quote Originally Posted by SomeoneSomewhere View Post
    VFDs can't supply enough starting current to start a stalled motor at full frequency.

    You need to replace the motor contactor(s) with the drive.
    Not strictly true. A sufficiently oversized VFD will have the short burst current overload capability to start a motor when running at line frequency. Three times oversize on an old style V/F (voltage / frequency drive) one should do it. Dunno about the modern self tuning Vector Drive types. Of course this also pushes up all the overload sensing set up parameters so, unlike a right sized VFD, its not going to protect the motor.

    How do I know. My shop has been running just fine off a 10 hp Teco VFD modified to give 440 volt output wired in the same manner as an RPC using the machine controls to switch them on and off. Largest motor is 3 hp and its quite happy supplying two machines at a time.

    That said I'd not try anything like that without input from someone who knows what's going on. Guy who supplied mine does it as a part of his business.

    In your case there is no problem just using the VFD to run the motor. The DC clutches will take their supply from a transformer and rectifier unit connected to one phase of the incoming power. Just a matter of connecting the transformer to the single phase input. Maybe change a tapping too.

    Clive
    Last edited by Clive603; 03-11-2021 at 03:41 AM.

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    I was in the exact same situation as you, just a different Euro brand lathe. I did what Someonesomewhere suggested, which I believe is the correct solution, and re-wired the entire lathe to replace the AC contactors (one forward/one reverse) with the VFD and maintaining nearly all existing controls. This required figuring out the existing control logic with the help of a wiring diagram, creating my own incorporating the VFD, and then ripping out a bunch of wires lol. This was not that bad but if you’ve never done and or minimal wiring you’ll be blown away when you open up a 3 phase lathe electrical cabinet. The VFD actually simplifies the setup, and the hardest part is the clutch/brake logic and control. Mine was significant enough that I made a new wiring diagram so I or the next guy have a good shot at troubleshooting in the future.

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    Like Clive, I run my entire home workshop off one VFD outputting a fixed 50Hz 415V. Electronics have moved on over the last four decades.

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    Fair, it does work, but you do need a large drive for it.

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    I run my entire home workshop off of a 240v single phase panel, each machine's spindle motor has a VFD getting fed single, and pushing out three phase. As a result, each VFD is tuned specifically to the spindle motor's drive circumstances to yield best performance, and each drive is set up with dynamic braking on E-stop, and sensible accelleration curves commensurate to the actual machine.

    The most important benefit of the VFD, is that one can intimately control the tool's spindle... accelleration, run speed, slow down, dynamic braking, coasting, DC injection braking... and best of all... deadman controls that stop it prompt if you bump the emergency bar.

    Using a VFD for a simple phase converter is possible, but doing so throws away 905 of the VFD's greatest benefits.

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    Wait.. I'm very confused now. I can just run everything off a vfd (the same way you would use a rotary phase converter? Am i right in that ye are just wiring your VFD upstream of the entire lathe (essentially where it would plug into the wall if you had three phase to your shop) and it's working? I have a 5hp vfd and a 3hp motor on the lathe, is that enough of larger vfd to motor size? If this was true it would literally save me over a thousand quid, the vfd was €150, the rotary phase converter is €1550 plus vat. This would make my day, tell me its true!!!

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    Sort-of.

    The issue is that when you just "switch-on" a motor it draws a lot of current to get moving. Up to 7x or more the regular draw even at full power.

    So the essence of the matter is that you need a VFD that has the current capacity to supply that. A 3 HP motor draws 10 or so amps to run at full load. So you can figure that the current needed to start it is going to be from 50 to 80 amps for a short time.

    Most VFDs can carry 150% of max for a short enough time to start a motor (some can do 200%). So if your VFD can put out 60A continuous, it likely can start any 3HP that is just "switched on" as a load. That would be (conservatively) about a 20 or 25 HP VFD. A 15 HP might do it.

    Some VFD may do better, some motors pull more current (Weg seem to be notorious for that).

    A 5 HP motor draws around 15A, a 15 HP VFD may have a current rating of 16 or 17A, meaning about 26A short term. Probably not going to do the job reliably.

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    As a datum, the one I'm using is a 7.5hp Teco Speecon 7200. This is rated at 10.3kVA and 12A with 150% overload for one minute. The largest single motor that I start at any one time is 2hp but the highest load I put on it is 5hp worth of motors.

    The VFD needs enough output capability to cope with the motor starting conditions and good enough current sensing to either trip or load limit (either frequency or voltage dropping) on overload to protect itself from abuse.

    I did have a much larger 23kVA Danfoss unit (cost me £10 ), but one of its output current sensing modules went psychotic, so it kept tripping.

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    Quote Originally Posted by Dinky O'Sullivan View Post
    Wait.. I'm very confused now. I can just run everything off a vfd (the same way you would use a rotary phase converter? Am i right in that ye are just wiring your VFD upstream of the entire lathe (essentially where it would plug into the wall if you had three phase to your shop) and it's working?
    In theory, you could... and some do. In practice, it wastes most of the VFD's greatest features, and of course, is never really 'matching' the motor.

    The proper way to get the most value and performance, is to dedicate the VFD to the motor it'll be driving. That's the way they were designed to work- as a motor control SYSTEM... not just providing 3 phase power, but variable speed, accelleration and braking control, overcurrent protection, soft starting.

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    to finish up this thread. I've bought a 7.5hp digital phase converter to try to power the lathe. I was told by the company that sold it that it would work to wire it upstream of the entire machine like a rotary phase converter. Seemed like a dream come true but it arrived today and reading the manual i saw this...


    "If switching devices like contactor are installed between the output end of the inverter and the motor, it should ensure that the on/off operation is conducted when the inverter has no output. Otherwise the modules in the inverter may be damaged.""

    now i only know what i've read from googling but I think my lathe would have contactors to start the motor, so they've be inbetween the inverter and motor, and I don't get how you could turn a machine on without power coming from digital phase converter. Am i missing something, or was I just sold a bill of nonsense about this thing being designed to operate upstream of the lathe.

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    Default Where to wire the VFD

    VFD definitely do not like to have the load disconnected under power.
    You could test the lathe by just putting it upstream, but I would use the VFD buttons to turn the lathe off, vice just using the lathe lever or switch.
    I just converted my lathe over to a VFD , and it’s a bit of work wiring up all the controls so the brake pedal and fwd/rev lever work properly. I used a 24v control power transformer and 24v relays for the forward and reverse logic, along with the stop circuit.

    Edit-I was really referring to a VFD sized for the load.I don’t think the 7.5 is big enough to reliably be used in the manner you are considering, I guess you could buy a big one (15-20hp??) and use it like a rotary


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    From the terminology, the device is from Drives Direct. Just use it. If, for some reason, it failed, then claim under the guarantee, Making use of the Sale of Goods Act and the Consumer Rights Act in case of difficulty. I think you will be very happy with the setup.

    Edit to add:- This assumes that it was from Drives Direct and you weren't just calling it a digital phase converter when you meant off-brand VFD.

    PS:- VFDs don't give a shit about load disconnections under power...
    Last edited by Mark Rand; 03-17-2021 at 12:19 PM.

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    Quote Originally Posted by Mark Rand View Post
    From the terminology, the device is from Drives Direct. Just use it. If, for some reason, it failed, then claim under the guarantee, Making use of the Sale of Goods Act and the Consumer Rights Act in case of difficulty. I think you will be very happy with the setup.

    PS:- VFDs don't give a shit about load disconnections under power...
    I’m not claiming to be an expert, but I have read in many places that if you are going to open a disconnect or contactor on the load side of the drive, there should be a timed contact that trips the drive before the load is disconnected


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    Most of the lore about inverters is just that:- lore. Unfortunately, almost all of it is based on devices that used fairly early and fragile thyristors with separate flywheel diodes in their outputs instead of IGBTs with integral flywheel diodes. There is no comparison between the reliability of those mid '80s designs and modern ones.

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    Aright I'm just going to go for it. I bought it off jfkelectricalni.com. I'm gonna ring them and just double check their warranty and then hook it up. Hopefully this is the end of my three month foray into three phase electricty conversion, it's mad how unclear it is what the best option is. I could easily have spent 1700 on a rotary phase converter, or 150 on a vfd and spent the time to learn how to rewire the whole lathe, but ended up with an oversized digital phase converter.. thanks for the replies and i'll let ye know when the thing blows up.

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    As ye might realize from the next thread I created.. I bought the digital phase converter and it doesnt work to power the lathe, so either something got jiggled in the move or a connection went bad while it sat for two months. -- I'm starting to figure out how to troubleshoot, i can get the motor to run when i wire it directly to the phase converter, but am having trouble troubleshooting where the issue is. The transformer is putting out voltage, and power seems to be getting through all the fuses, but the start buttons do nothing. I think i need to find a multimeter that can measure 380v. as all of mine stop at 300.

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    Quote Originally Posted by Dinky O'Sullivan View Post
    I think i need to find a multimeter that can measure 380v. as all of mine stop at 300.
    Dinky, you are using 220 volts supply, so won't be finding anything greater than that on your lathe (it seems to me anyway )

    Edit: oops, I thought you were using a VFD. I have not heard of a Digital Phase Convertor, maybe it steps up the voltage as well as supplying three phases?

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    Only measuring up to 300V is generally a sign of a poor-quality or electronics-duty meter that shouldn't be seeing mains voltage use. Pretty much anything decent is going to be 600V Cat III or better.

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