The general mantra is simply to never put a switched motor (or motors) on a VFD. I explored this option for myself, and figured I would share the results with the hope it helps someone else down the road.
My mill has a dedicated 5HP spindle motor and a separate 1HP table motor. My shop has 240V single phase wiring. I wondered if I could power all of these motors via a single VFD to avoid constantly running my large and(relatively) wasteful RPC. My plan: Have the 5HP motor spool up at startup with the VFD (the spindle has an electric clutch), and switch the 1HP table motor on and off as needed. I figured a VFD built for 7.5HP would easily handle the 1HP switched load plus the constant 5HP spindle motor. Here is what I came up with:
First, this document was an interesting find:
VFDs Can Control Multiple Motors
Going off the document, and assuming I get the overcurrent/short circuit protection installed, I got into the math.
The nameplate on my 5HP motor lists 11.8A @ 240V, and on my 1HP motor 4A. Since the 1HP will be switched, I need to determine the LRA (Locked Rotor Current) for that motor. Unfortunately, the motor nameplates don't have the code (perhaps my German is too rusty) to determine the LRA Amps. So I went a search to find how to determine a reasonable estimate. Three resources were useful:
1. If your have your "code" for your motor, use this calculator to find the LRA: Locked-Rotor Amps (LRA) Calculator and Chart
2. If you have no code like me, here is a seemingly realistic guess at LRA based on HP and voltage: 3 Phase Motor Full Load Amps. 3 Phase Motor Locked Rotor Amps. 3 Phase Motor Current. 3 phase FLA
3. Gives some estimations for NEMA codes based on HP (at the bottom): Electrical Motor - Locked Rotor Design Code Letters
Using these, I come up with 11.8A for the 5HP motor plus a staggering 34A LRA (estimated) for the 1HP motor, making a total VFD capacity of 46A. Realize that this is, of course, a worst case scenario: Starting the 1HP table motor while taking a heavy cut at full spindle load.
Using a single VFD:
Basic estimation says a 46A @240V would be around ~18.4HP. I will round up to 20 to fit the VFD options. As of this writing, I find one actual single phase input VFD for 20HP at the tune of $1700. Of course you can use the 2X multiplier to use a 3-phase VFD, and one of them was a bit cheaper at $1500. Total cost $1500-$1700.
Knowing we planned for a worst case scenario, lets just cover the switched 1HP load and assume negligible current in the main spindle motor. There are Ebay 10HP Single Phase input VFD's listed for $170 (Try your luck?) and amazon for $280. Aside from that, $750-$1000 seems to be the number for either a 3Phase 20HP, or a 10HP single input VFD. Total cost $170-$750.
Two seperate VFDs:
5HP single phase input VFD's go as low as $180 for imports, to name brand VFD's at around $500. 1Hp single phase VFD's range from $55 for an import to $200 for a name brand. When you add this up, running each motor with its own VFD will cost between $235-$680.
In short, switching loads of any reasonable size (1HP+) on a VFD is no trivial matter (by the numbers) due to the severe amount of inrush current the VFD must be able to provde. Either way you slice my scenario, it is cheaper (I'm not interested in the undersized Ebay 10HP option), and likely more responsible long term, to run two smaller dedicated VFD's instead of one big unit for both. Better said: by the time I add even fuse-blocks/fuses for overcurrent/short circuit protection, any savings will likely be nullified.
Single VFD Revisited:
The argument could be made for having both motors turn on simultaneously and avoid all the caveats of switching a motor on a VFD. This is true, although I intend to use the mill frequently with hand feeds, so again, there is a waste factor, which was the point of this exercise in the first place. Still yet, one could recommend having a hard switch to either allow only the spindle, or the spindle + table motor to come online at startup, depending on what the job required. Reasonable? Yes (know that VFD settings should be setup for each condition). The cheapest import 7.5HP VFD I could find on ebay was $270, while the more recognizable units were in the $700. Price $270-$700.
Conclusion: If you have multiple motors in a scenario similar to mine, the pricing of today's VFDs makes it quite difficult to justify a single VFD driver. If you aren't into Ebay no-name VFD's, this becomes even more apparent.
So, I will stick with the RPC for now. Perhaps if wiring or the electronics becomes an issue on this old machine, I will revisit a potential dual VFD solution.
My mill has a dedicated 5HP spindle motor and a separate 1HP table motor. My shop has 240V single phase wiring. I wondered if I could power all of these motors via a single VFD to avoid constantly running my large and(relatively) wasteful RPC. My plan: Have the 5HP motor spool up at startup with the VFD (the spindle has an electric clutch), and switch the 1HP table motor on and off as needed. I figured a VFD built for 7.5HP would easily handle the 1HP switched load plus the constant 5HP spindle motor. Here is what I came up with:
First, this document was an interesting find:
VFDs Can Control Multiple Motors
Going off the document, and assuming I get the overcurrent/short circuit protection installed, I got into the math.
The nameplate on my 5HP motor lists 11.8A @ 240V, and on my 1HP motor 4A. Since the 1HP will be switched, I need to determine the LRA (Locked Rotor Current) for that motor. Unfortunately, the motor nameplates don't have the code (perhaps my German is too rusty) to determine the LRA Amps. So I went a search to find how to determine a reasonable estimate. Three resources were useful:
1. If your have your "code" for your motor, use this calculator to find the LRA: Locked-Rotor Amps (LRA) Calculator and Chart
2. If you have no code like me, here is a seemingly realistic guess at LRA based on HP and voltage: 3 Phase Motor Full Load Amps. 3 Phase Motor Locked Rotor Amps. 3 Phase Motor Current. 3 phase FLA
3. Gives some estimations for NEMA codes based on HP (at the bottom): Electrical Motor - Locked Rotor Design Code Letters
Using these, I come up with 11.8A for the 5HP motor plus a staggering 34A LRA (estimated) for the 1HP motor, making a total VFD capacity of 46A. Realize that this is, of course, a worst case scenario: Starting the 1HP table motor while taking a heavy cut at full spindle load.
Using a single VFD:
Basic estimation says a 46A @240V would be around ~18.4HP. I will round up to 20 to fit the VFD options. As of this writing, I find one actual single phase input VFD for 20HP at the tune of $1700. Of course you can use the 2X multiplier to use a 3-phase VFD, and one of them was a bit cheaper at $1500. Total cost $1500-$1700.
Knowing we planned for a worst case scenario, lets just cover the switched 1HP load and assume negligible current in the main spindle motor. There are Ebay 10HP Single Phase input VFD's listed for $170 (Try your luck?) and amazon for $280. Aside from that, $750-$1000 seems to be the number for either a 3Phase 20HP, or a 10HP single input VFD. Total cost $170-$750.
Two seperate VFDs:
5HP single phase input VFD's go as low as $180 for imports, to name brand VFD's at around $500. 1Hp single phase VFD's range from $55 for an import to $200 for a name brand. When you add this up, running each motor with its own VFD will cost between $235-$680.
In short, switching loads of any reasonable size (1HP+) on a VFD is no trivial matter (by the numbers) due to the severe amount of inrush current the VFD must be able to provde. Either way you slice my scenario, it is cheaper (I'm not interested in the undersized Ebay 10HP option), and likely more responsible long term, to run two smaller dedicated VFD's instead of one big unit for both. Better said: by the time I add even fuse-blocks/fuses for overcurrent/short circuit protection, any savings will likely be nullified.
Single VFD Revisited:
The argument could be made for having both motors turn on simultaneously and avoid all the caveats of switching a motor on a VFD. This is true, although I intend to use the mill frequently with hand feeds, so again, there is a waste factor, which was the point of this exercise in the first place. Still yet, one could recommend having a hard switch to either allow only the spindle, or the spindle + table motor to come online at startup, depending on what the job required. Reasonable? Yes (know that VFD settings should be setup for each condition). The cheapest import 7.5HP VFD I could find on ebay was $270, while the more recognizable units were in the $700. Price $270-$700.
Conclusion: If you have multiple motors in a scenario similar to mine, the pricing of today's VFDs makes it quite difficult to justify a single VFD driver. If you aren't into Ebay no-name VFD's, this becomes even more apparent.
So, I will stick with the RPC for now. Perhaps if wiring or the electronics becomes an issue on this old machine, I will revisit a potential dual VFD solution.
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