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IndustryArena Forum > CNC Electronics > Spindles / VFD > Limit Spindle Current / Derating spindle?
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  1. #1
    Join Date
    Jan 2020
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    3

    Limit Spindle Current / Derating spindle?

    Through a combination of miscommunications and also bad timing, I have a 7.5kw 220v 3ph spindle, a single phase to 3ph 220v VFD, and only 230v 15a outlets on a 40a breaker with some other gear. I'm no expert but pretty sure that doesn't really add up.

    I do have a 415v 20a 3ph supply, and the obvious fix seems to be a 3ph VFD, however soon(-ish (hopefully)) I'll be relocating and should have a 415v 32a 3ph supply, and want to put off any more equipment purchases until that's all settled and confirmed.

    On the other hand I'm a bit impatient, so I wanted to get this thing spinning, make sure its not a dud, and ideally get it attached to a machine.

    To sanity check, I've wired it up on the bench and got it spinning, seems to be in order with no load at nameplate hz/v/rpm, VFD is saying its ~7a

    My question is, realistically what can I expect if I were to set "Motor rated current" in the VFD to lower than the nameplate rated current, and tried to actually use this thing? My very basic understanding is voltage relates to speed and current relates to torque/hp, if I simply limit current, will I effectively have a lower powered motor, and use it as a 3hp motor until the power situation is sorted? Would the relation to torque/hp/current be somewhat linear, or curved in a way that makes running it at a lower current basically useless?

    Is there any other risks/problems I'm totally missing?

    I unfortunately have very little info on the motor aside from the nameplate, the model number does not appear on google, and the manufacturers specs for equivalent motors don't totally line up to the nameplate, and even if they did, they don't really provide any useful info on performance anyway.

    Also, If I wanted to limit the VFD input current to something specific, I'm guessing the max motor current setting would need to be something like (Max input current * VFD Efficiency * VFD PF) / 1.73, is that roughly the right idea? I'm assuming motor efficiency/PD is only relevant to working out the real power, not the draw.

    Any guidance here is appreciated, cheers.

  2. #2
    Join Date
    Apr 2018
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    131

    Re: Limit Spindle Current / Derating spindle?

    Quote Originally Posted by butchery View Post
    ... and only 230v 15a outlets on a 40a breaker with some other gear.
    Assuming you are in the USA, it seems unlikely to me that you really have a 40a breaker on that circuit.

    What seems more reasonable is that you have a 20a breaker that has two 'handles' (each marked as 20amp) mechanically linked together. A 240v breaker [internally] has separate mechanisms for each of the two 120v legs, which results in the 240v available at the screw terminals of the breaker. This would result in a 240v 20amp circuit.

    If you have "some other gear" on this same circuit, subtract their loads from the 20a circuit to see what you really have available.

  3. #3
    Join Date
    Jan 2020
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    3

    Re: Limit Spindle Current / Derating spindle?

    I'm not in the US, sorry, mains from street is single phase 230v (by regulation I guess, but where I am specifically it's really 240-250ish) and 415v 3ph, there's 3 different panels around the property but all the machinery runs from one thats fed from a line with a single pole 40a breaker.

    Putting that aside, I'm not too worried about trying to maximize the draw, I can pull ~25-30a at 230/240v from a few outlets with no problems while running other gear that needs to stay on, which I do already with other equipment. The CNC the spindle is going on would be run mutually exclusive to those, so I can do ~25-30a between the VFD and the other electronics for the CNC, while running the other gear needed in the shop, as I already do that with other equipment.

    That said I only really want to draw 15-20a for the VFD itself, which is still not half of what I believe I'd need to run it properly, thus the questions

  4. #4
    Join Date
    Jan 2005
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    15362

    Re: Limit Spindle Current / Derating spindle?

    Quote Originally Posted by butchery View Post
    I'm not in the US, sorry, mains from street is single phase 230v (by regulation I guess, but where I am specifically it's really 240-250ish) and 415v 3ph, there's 3 different panels around the property but all the machinery runs from one thats fed from a line with a single pole 40a breaker.

    Putting that aside, I'm not too worried about trying to maximize the draw, I can pull ~25-30a at 230/240v from a few outlets with no problems while running other gear that needs to stay on, which I do already with other equipment. The CNC the spindle is going on would be run mutually exclusive to those, so I can do ~25-30a between the VFD and the other electronics for the CNC, while running the other gear needed in the shop, as I already do that with other equipment.

    That said I only really want to draw 15-20a for the VFD itself, which is still not half of what I believe I'd need to run it properly, thus the questions
    That is not possible 7.5Kw on 15A or a 20A single phase supply is going to trip the breaker as soon as you put any type of load on it

    The 7A you see on the VFD Drive KeyPad is 7A per phase, so you are at 21A just running no load, you can't derate the motor, you would need a 60A supply to run that motor on single phase and the input for the VFD Drive would have to have a 50A rating or it will fail

    So, what kind of motor do you have, a regular AC 3Ph motor or a high-speed spindle motor 18,000 RPM / 24,000RPM

    Dumbing down the VFD Drive the spindle will stall as it will have no torque that you could uses for cutting anything
    Mactec54

  5. #5
    Join Date
    Jan 2020
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    3

    Re: Limit Spindle Current / Derating spindle?

    Quote Originally Posted by mactec54 View Post
    That is not possible 7.5Kw on 15A or a 20A single phase supply is going to trip the breaker as soon as you put any type of load on it

    The 7A you see on the VFD Drive KeyPad is 7A per phase, so you are at 21A just running no load, you can't derate the motor, you would need a 60A supply to run that motor on single phase and the input for the VFD Drive would have to have a 50A rating or it will fail

    So, what kind of motor do you have, a regular AC 3Ph motor or a high-speed spindle motor 18,000 RPM / 24,000RPM

    Dumbing down the VFD Drive the spindle will stall as it will have no torque that you could uses for cutting anything
    Yeah going off of the numbers I have, I arrived at ~46a, and I'm taking their numbers with a grain of salt so I assumed 50a+. Like I said, the real plan is to run it from a 3ph VFD instead, which will be happening soon, just not sure if 'soon' is '1 month' or '16 months'.

    It's a 18,000rpm high speed type. Originally ordered a lot of 220v/240v gear (few dozen different things) and also a 380v 3Ph spindle/VFD, the supplier was having stock issues and wanted to replace some things with equivalent parts from different manufacturers, at one point they asked about sending 110v stuff for some of the other gear and I said we need things in the 220-240 range, and I guess they applied that to the spindle as well, and sent a 220v spindle/VFD instead of the 380v. Nobody took a good look at it when we took delivery, and it sat in the shop for 12 months while we worked through the other stuff in the order.

    It's been long enough, and shipping is bad enough right now, that it's probably not worth trying to go back and argue so it is what it is. Been looking at local VFD options and the real annoyance is the 220v spindle has a nameplate current of 26a, most equivalents here are ~400v and 15a, so most brand name VFDs here in that power range max at 19a, so might need to oversize the replacement VFD? Talking to local suppliers, it's turning into a can of worms all on its own as even the drives at 11kw normal duty are <26a, and the next step up from the local supplier overshoots what I expect our 3ph supply will be.

    Was sort of hoping to just push the problem down the line until the shop is relocated, even if we could run it at low loads at 20% or 30% of its advertised power or torque, it'd be usable for something else, until the power is properly sorted. Sounds like thats not an option, which is a bit of a bummer but I sort of expected it haha.

  6. #6
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    Jan 2005
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    15362

    Re: Limit Spindle Current / Derating spindle?

    Quote Originally Posted by butchery View Post
    Yeah going off of the numbers I have, I arrived at ~46a, and I'm taking their numbers with a grain of salt so I assumed 50a+. Like I said, the real plan is to run it from a 3ph VFD instead, which will be happening soon, just not sure if 'soon' is '1 month' or '16 months'.

    It's a 18,000rpm high speed type. Originally ordered a lot of 220v/240v gear (few dozen different things) and also a 380v 3Ph spindle/VFD, the supplier was having stock issues and wanted to replace some things with equivalent parts from different manufacturers, at one point they asked about sending 110v stuff for some of the other gear and I said we need things in the 220-240 range, and I guess they applied that to the spindle as well, and sent a 220v spindle/VFD instead of the 380v. Nobody took a good look at it when we took delivery, and it sat in the shop for 12 months while we worked through the other stuff in the order.

    It's been long enough, and shipping is bad enough right now, that it's probably not worth trying to go back and argue so it is what it is. Been looking at local VFD options and the real annoyance is the 220v spindle has a nameplate current of 26a, most equivalents here are ~400v and 15a, so most brand name VFDs here in that power range max at 19a, so might need to oversize the replacement VFD? Talking to local suppliers, it's turning into a can of worms all on its own as even the drives at 11kw normal duty are <26a, and the next step up from the local supplier overshoots what I expect our 3ph supply will be.

    Was sort of hoping to just push the problem down the line until the shop is relocated, even if we could run it at low loads at 20% or 30% of its advertised power or torque, it'd be usable for something else, until the power is properly sorted. Sounds like thats not an option, which is a bit of a bummer but I sort of expected it haha.
    The 7.5Kw spindle would be 29A with a power factor of.85 which is normal for a VFD Drive powered motor so 29A x1.732=50A at the input of the VFD Drive for single phase supply plus required by code the Breaker is sized from this which would be a minimum of 60A

    Name plate current is close at 26A that would mean using 26Ax1.732=45A at the VFD Drive plus the circuit Breaker requirement would be 56A so back to a 60A circuit to run just that spindle motor using single phase supply, if using 3ph supply using the 26A you would want around a 32A circuit @ 220/240v

    You could use a 3Ph Buck Transformer to go from 400v to the 220/240v and run that spindle also

    The name plate current they are using is using a low power factor around .76 to get the 26A, VFD Drives the power factor in most quality VFD Drive is never below .85 with some having almost no loss as they correct for input power factor losses, so this puts up the Amp/current of the motor with is good for performance

    For Single Phase use the VFD Drive should always have a larger capacity than for 3Phase supply

    You could try it using your 40A circuit it may have enough torque to do light cutting
    Mactec54

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