A case study for running multiple motors off a single VFD

# Thread: A case study for running multiple motors off a single VFD

1. ## A case study for running multiple motors off a single VFD

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.
Last edited by Gundraw; 11-22-2019 at 03:29 PM. Reason: grammer

2. Aluminum
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You are not protecting either motor from overload, you cannot tune or adjust the VFD for the individual characteristics needed for each motor/controls. You are spending more than 3X the money at the end of the day to install a 20 Hp VFD which would require much higher installation costs when the job would be much more effectively be done by a 5 Hp and 1 Hp VFD. Put both in the same cabinet, have each one programmed for the specific motor parameters and controls. A single large VFD makes no sense in this application, not only on a pricing bases, but from a functional/safety stand point. I have installed multiple VFD drives on single mills, each sized and programmed for the specific motor it is driving. This way the mill/drives works the way they are suppose too. If you want a single VFD drive get a PhasePerfect. QED.

3. mksj -

I appreciate your response. However, I do not believe you read this (albeit wordy) write-up which confirms your claim. I simply shared this because I see a lot of VFD conversations getting shut down with only "you can't do it", or "it will burn up the VFD". I wanted to run the numbers to note two things:

1. You CAN run multiple motors with a single VFD, but only with certain design constraints considered(see the first link).
2. For a situation such as mine, there is no way to make a single VFD solution work in your favor, even looking at cost alone.

4. Aluminum
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I did read you post, and do not agree with your fundamental premise that this is an acceptable approach in this application. That you would be running multiple dissimilar motors off of the same VFD is not supported by the literature or the manufactures manuals from what I have read. Can it be done, sure it can, but doesn't make it correct. The typical multiple motor to single VFD scenario is using a VFD in a V/Hz mode running multiple motors that are the same and each with its own overload device. I have read the provided link in the past, it is a different application scneario. Running multiple dissimilar motors does not allow you to use sensorless vector, nor tune the VFD for the specific motor. You cannot operated the motors at different speeds. As I mentioned, you would be better off with a PhasePerfect or for that matter an RPC if you are looking for a plug and play option. Using a 20 Hp VFD as a form of overkill to overcome your design scenario, is not a good option in my opinion.

5. It sounds like you are well versed on this subject, but you seem to be refuting claims that I have never made, nor are made in the documents I linked. Perhaps this is just for the sake of argument?

I made it clear this was a replacement for my RPC in this scenario. Nowhere do I suggest or imply I would be running these motors at anything but their intended speeds. The automation.com paper makes this argument as well.

Your mention about VFD tuning and SVC is correct. It would have been prudent to mention that aside from cost, there are certainly performance enhancements one could take advantage of if using discrete VFD's over a ganged setup.

6. There is nothing wrong with the basic idea. Several folks on this forum say that they do this instead of an RPC. I believe it, because the "it will blow up your VFD" claims are way overblown. Just about all makers use the same type, and often essentially the same part number of IGBT as the utput devices, and commuting diodes, so the performance characteristics are the same. Why one mfgr says "never do that" and another allows it probably has much more to do with the speed f the commuting diodes, OR more likely, the degree of conservatism of the makers corporate policy.

Technically speaking, the VFD can stand a turn-off event if it can stand regular operation.... there are thousands of turn-off events per second, at every current level, in normal operation. One more is not much different if any. The Phase Perfect is just like a VFD, and it tolerates turn-on and turn-off at will. So do thousands of solar inverters. Those use the same type of output circuit, IGBTs and diodes.

The overload on startup when dropping a motor across the output is an issue, of course. The answer is given in the paper, oversizing for LRA.

The use of one large motor in a bunch of smaller ones will require a much bigger VFD than a mix of similar motors, such as 1 HP and 3 HP, where a bunch of 1 HP an one 15 HP would be different. That assumes random turn-on. If all the 1 HP were switched on at once, there would still be an issue. Reason being that the one large motor requires a large allowance for LRA, where a couple 1 HP at a time would have a much lower LRA.

It can be done, and is done, and can save money. It will also give a well-balanced system, where the RPC is always weak on the generated leg, unless specially made. It will not be cheaper than the RPC.

7. Aluminum
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It is not the purpose to make this as an argument, and that it can't be done. The objective is that that the discussion is a bit moot for the case study mill application vs. the link provided use which is a different scenario. So informative from the stand point that a VFD can drive multiple motors and the calculations you have done are informative, but I do not agree to the application/use you describe. Just a different perspective from my experience doing multiple system installs on these types of machines.

If both/multiple motors ran a fixed speeds in say a conveyor/fixed automation system then the post would have more meaning. RPC's have been used for years in shop machinery application with good enough performance characteristics, but do have some drawbacks. The better RPC shop installations I have seen, they have been quiet and drove a wide range of machinery flawlessly. In many cases I have added VFDs to specific machinery supplied by the RPC to better harness the individual motor control and the VFD features.

8. Originally Posted by JST
If all the 1 HP were switched on at once, there would still be an issue. Reason being that the one large motor requires a large allowance for LRA, where a couple 1 HP at a time would have a much lower LRA.
This is a good point. A few documents I have read note the necessity to add some room to the VFD spec (although I do not recall them mentioning the LRA specifically) when running multiple motors simultaneously. Others seem to imply simply adding the HP together, which isn't really appropriate. It is apparent from typical LRA calculations that two 1HP motors will have an LRA greater than one 2HP motor.

9. Of course, if all have to turn on at once, you can use the VFD to spin them up.

Worst case is then really the case of one stays on, and the rest turn on and off, maybe sometimes all at once. Then, staggering the turn-on would take care of the issue.

10. Stainless
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you can likely get by with an ebay 10hp drive. to ramp up the 5hp spindle, and provided the 5hp motor is at low to medium load, you may have enough current left to start the 1hp drive, depending on whether or not a 34amp 10hp drive can briefly handle 40 amps output.

also you can use say, 3 60 uf caps connected delta after a line/load reactor and this will offset 9 amps reactive current, continuously all the time if my math is right.

i'm assuming locked rotor current is 50% lagging power factor, so your 34 amps LRA will drop to about 28 at the vfd. this leaves 6 amps left for the vfd to be running the 5hp motor.

assuming 50% lagging power factor at locked rotor condition, the vfd only has to supply 24 real amps, the other 24 inductive amps is recirculated in the vfd. adding 60uf delta caps after the line reactor can offset that 24 amps down to 15, which makes for 28 amps LRA

11. My band saw has been running 3 motors from 1 vfd since late '10 with no issues to date...except when the blade scuffs my hand because I can't use the hydros without the blade.

Sent from my SM-G973U using Tapatalk

12. Stainless
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So if I have a 30hp 480v vfd, could I use it to run multiple machines, none over 5hp, maybe 3 machines running at same time, in place of the rotary converter? Could I run 220v thru a transfomer to boost to 440v, feed that into vfd, run output thru a breaker panel that then feeds machines? I have the vfd, can get a transformer for pennies. I have 3ph on the pole, but power co wants 5k deposit, plus independent sparky costs for bringing it into building (guesstimate 10k total for deposit and install). I have a rotary converter, hate the noise, and the wasted energy it consumes.

13. You need to have the capability to run the motors you want to run, and to start any of them at the same time as running any motors that may need to be on. Figure the current surge from the kVA class marked in the motor, rated FLA for any motors that will already be running when you start that motor, and add a bit for margin.

If the VFD will supply that, you should be good to go. To avoid really oversizing, you can check the 2 minute, and short term overload capability of the VFD. It is fairly common to have ratings around 150% for 1 to 2 minutes, and sometimes 200% for several seconds.

Those numbers can help you with the turn-on surges, but do not figure them closely.... you need a margin for expansion, and also because motors do not always do exactly what the rating plate says. Generally they will be within 10 or 20 percent on the high side. The low side you do not need to worry about except on the VFD. There is also the consideration that the allowable overload time may vary with VFD heatsink temperature. If the VFD is already hot, those overload times may be considerably shorter.

14. Just idle curiosity since I have three phase in my shop, would there be any benefit to having a large idler motor on the VFD? The motor could be larger than the VFD is rated for because it would never be loaded. The VFD could be set up to ramp up and the idler would be running continuously. The idler would act as a flywheel, helping smooth out surges. Admittedly this sounds like the worst of both worlds, but it might be a solution to the multiple motor problem. Running the idler that way eliminates the phase shift with varying load problem of RPCs.

Bill

15. Stainless
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If the motor is a synchronous motor it might help but only according to the resistance of the vfd and the impedance of the motor.

If you can program the vfd to drop the voltage and allow the idler to push current into the load, it would work. But you also have to drop the frequency as the idler slows down to keep the current flowing.

The impedance of a vfd with a 3% reactor on its output is still far less than a large induction motor, so its not going to do much.

Anyhow you need a pid control loop for both frequency and voltage to get this to work right.