10 hp VFD for 7.5 hp motor

Next week I bring home a new to me lathe that has a 7.5 hp 3 phase motor. My garage shop power is limited to a 240 volt / 40 amp circuit (unused dryer outlet) so I plan to run a VFD to supply power to the machine. I have two questions:

(1) Can I run a 10 hp VFD to power the 7.5 hp motor?
In reading other posts, it appears that the ideal setup would be to run a VFD that’s 2x the rated motor hp. My concern is that if I did that I’d pull more amps than the 40 amp circuit has available. Is it viable to run this motor on a 10 hp VFD without tripping my breaker repeatedly? I don’t plan on machining very heavy loads that would strain the motor. I’ve considered replacing the motor with a 5 hp alternative but I’d rather not buy a new motor if I can avoid it.

(2) Should I setup VFD just as power source or tie in control panel buttons?
Can I use the VFD just as a power source or do I need to rewire the control panel relays / switches for power on/off, jog, forward/backward & e-stop? I don’t plan on using the VFD for variable motor speed control so it seems easier to use it only as a power source and rely on the lathes existing wiring to handle all the usual features.

This will be my first VFD installation so I’m new to the whole configuration options.

Unlike RPCs (rotary phase converters) or uber expensive electronic phase converters, VFDs cannot be used as "a simple power source": you must rewire your lathe controls so that there are no switches between the VFD and the lathe motor. All switching has to be done by the VFD: either by its own buttons or through switches remotely connected to it. That's one of the main reasons why I try to avoid VFDs unless they're really needed for variable speed control, soft start, quick stop, etc.

If that's not enough, you have to realize that every motor needs to be fed by a separate VFD unless you're particularly masochistic and like to jump over hoops.

If this is the only 3-phase machine/motor you'll ever have (which I doubt), a VFD can be considered as an option. But a single RPC would feed all your machinery and require no rewiring. Think about it.

EDIT: I need to correct my mental fart in the original reply. Yes, if VFDs not designed for it are fed with a single phase power, you'll have to derate your VFD up to x2. But since you mention you're not going to load your lathe even close to the motor's capacity, I see no problem with 10hp VFD feeding your lathe.

The risk you run will be based on the VFD internal protection capabilities. A 10HP drive fed with single phase for a 7.5HP 3 phase motor is not large enough to provide the proper DC bus capacitance to handle the extra ripple caused by the single phase input, and the diodes of the rectifier will not be rated for the added current. So what will happen at BEST is that when your load torque requirement gets to the equivalent of around 5HP, the drive will hopefully detect the added ripple on the DC bus and shut down to protect itself. Worst case, it isn’t sophisticated enough to do that and you damage the capacitors and/or transistors in the drive, making it a door stop. There is also the possibility of shorting out the diodes on the front end of the drive, same end result, irreparable damage.

The DC bus ripple and the current through the diodes is based on the current drawn by the motor, so if the drive has the ability to do what’s called “current limiting”, then what you can do is to program the current limit for what the drive can handle, which you must find out from the drive mfr. So for instance if they say the drive must be de-rated by 50% for single phase input, and your 10HP drive is rated for 28A, you must set up the current limit to be 14A. Your 7-1/2HP 230V motor is likely rated for around 22A FLC, so that means you can only load that motor to about 64% of its full capability. If the drive mfr says the de-rate is 65% as some do, then you must set up the current limit to 9.8A or just 45% of the motor’s capacity.

What will happen with current limiting however is that as you start to load it to more than that value, the drive will back off to limit the output current (and thus torque) regardless of what you are asking it to do. In machining then, that might end up affecting the speed or quality of your work, tool life, etc.

TXManka said:

Next week I bring home a new to me lathe that has a 7.5 hp 3 phase motor. My garage shop power is limited to a 240 volt / 40 amp circuit (unused dryer outlet)...

Click to expand...

Just FIX that, first, then, and you'll have better options. My 10 HP RPC or 10 HP Phase-Perfect EACH want about 55 A, single-phase side per the books - for the several 5 HP, 7 HP, to 7.5 HP loads, here, driven one at a time. Mostly, anywy. One mill has four motors, but uses only three at a go, max. One lathe has four as well, uses minimum three at a go.

OUTPUT side of the P-P is limited to just under 30A, but then again, there are now two more phases, so "that works".



And that 55 A is BEFORE I drop a supplemental 5 HP or 7.5 HP idler onto the RPC "if/as/when need be" for hard-starting or hard running. Phase-Perfect wasn't cheap. I try to "baby it" a tad and hammer the near-as-dammit "bulletproof" RPC instead.

Even if in rented space, your service entrance is probably 100A if not 200 A. A 60A load center, wired per code to a code-compliant breaker, main load center, and NOW you have a "shop".

Have to do it in "stealth" mode? Have the dryer branch circuit upgraded to 50 A.

Or the 60 A I use for the custom electric range & oven.

Any experienced "lectrician has more than one way to deliver the needfuls safely, and well within code.

I'm good for over 150 A if i need it for RPC, Phase-Perfect, ++, but the 200A service entrance happened to be right IN the garage/shop/annex already!



In the long run, the upgrade can be cheaper and waaay more convenient than trying to squeeze by on too light a feed year after year.

Just a data point for you. I have a 10 hp lathe on a 7.5 VFD and a 30 amp circuit for 10 years now.

Did have to wire the lathe forward and reverse to the VFD inputs for forward and reverse. the coolant pump was changed out to 110.

never had a problem.

Karl_T said:

Just a data point for you. I have a 10 hp lathe on a 7.5 VFD and a 30 amp circuit for 10 years now.

Did have to wire the lathe forward and reverse to the VFD inputs for forward and reverse. the coolant pump was changed out to 110.

never had a problem.

Click to expand...

"Moderation in all things." Nothing wrong with being frugal.

I've never tried to run my XJ8-L to its track-tested 172 MPH whilst going 'round the block for groceries, either! No need.



That said, having plenty of basic power AVAILABLE makes for long years of reduced nuisance and aggravation, first machine.. fifth one... can ultimately save-back its modest cost several times over.

I might go at this one with a 7.5 HP idlered RPC, 5 HP supplemental idler dropped-on once up to speed, drop EITHER of them off the line for average running once the load-motor is up and active.

ELSE a VFD specifically BUILT FOR hauling this size load FROM single-phase. One already tested and rated appropriately, no further de-rating maths required.

And a sine filter to protect the old motor. Cheap enuf insurance, those are, used-but-good.

Lots of those single-phase-in rated VFD out there now that didn't even exist ten years ago. Why try to re-invent that wheel with one NOT so rated by its own maker?

Thank you for your reply. I haven’t worked with VFDs in the past so please help me understand something.

My machine’s existing 3-phase power input ties into three separate power loads: two magnetic switches and a transformer.

- the first magnetic switch / relay powers the lathe motor,
- the second magnetic switch/ relay powers the coolant motor, and
- a transformer that provides 110v power to the panel switches that control the magnetic switches / relays, a work light and a pilot light.

Can’t the VFD be set to provide constant 60hz 3-phase power to the machine and keep all the control infrastructure and 110v power supply intact? I have no need for speed control so bypassing those featureS, if possible, isn’t a problem for me.

VFDs don't like that.

Get a VFD for both 3 phase loads. You can take the 110 off the 220 line to the machine.

You'll need a low voltage (power from VFD) on/off switch for the coolant.

A forward/off/reverse switch for the spindle. A speed pot is almost mandatory to control lathe speed. Even if your lathe has a speed adjustment, the VFD speed change is so easy you'll end up using it.

A braking resistor is also more than nice. Otherwise you will have to set a VERY LONG slow down time when stopping.

TXManka said:

Thank you for your reply. I haven’t worked with VFDs in the past so please help me understand something.

My machine’s existing 3-phase power input ties into three separate power loads: two magnetic switches and a transformer.

- the first magnetic switch / relay powers the lathe motor,
- the second magnetic switch/ relay powers the coolant motor, and
- a transformer that provides 110v power to the panel switches that control the magnetic switches / relays, a work light and a pilot light.

Can’t the VFD be set to provide constant 60hz 3-phase power to the machine and keep all the control infrastructure and 110v power supply intact? I have no need for speed control so bypassing those featureS, if possible, isn’t a problem for me.

Click to expand...

"Sort of". But a VFD was BUILT to be "tuned" or matched to a single load, and a dynamic one. It's equivalant to a brain or personality and the sensors that inform it adhere to that goal. So they can get confused and ornery when lighter, heavier, "just different" loads come and go... with switching spikes, yet.

By contrast, a Phase-Perfect uses a different conversion approach than a VFD does, and is optimized the OTHER direction. To hold stable as varying loads DO "come and go".

So it does pretty much exactly what you seek.

It does better-yet. It can handle heavy starting loads without need of "sneaking up" on them. Even when among multiple mixed load types as they are being activated or idled through the day.

Needess to say a Phase-Perfect is waaay bigger and heavier than a VFD of the same nominal HP. Got lots of big old-fashioned Iron and Copper lumped-inductances in it as well as more than one type of large capacitor.

TANSTAAFL, though.

My 10 HP one also cost me about $4,000, new, with warranty, for ten HP capability.

And then I chose to add a TCI 3-Phase 'Sine Guard" Filter to the output and a pair of line-side chokes to keep it's HF hash out of my OLD motors, load side, and the WiFi, LAN(s), audio gear, and household appliances, LINE side.

Call it $4,500 USD with NEMA enclosures or equivalant for the filter goodies?

My 10 HP RPC was cheaper, even when done from all-new parts. RPC generate some harmonic distortion back toward the grid, but are otherwise pure sine-wave critters - no hash filters needed, line side nor load, either one.

Efficiency is lower for the RPC, but not terribly so. And .. it doesn't need new capacitors every three years as the P-P does, nor every 7, 9, or 12 years ("RTFM") as VFD are MEANT to have.

The RPC was less than $700, no used parts involved in the core 10 HP. Seldom-needed add-on or "supplementary" idlers, used is good enough. Mostly they just sit and wait anyway. All of it "mostly sits", really. Other things to do in life.

$700 "sunk" cost for a 10 HP RPC vs $4500 "sunk" cost for a 10 HP Phase-Perfect?
And/or/else one can build an RPC out of salvaged, surplus, NNB, or NOS parts?
So an RPC can be near-as-dammit "free".

Annnnd an RPC won't mess-up old motors from switching artifacts nor corona's fluting bearing races, confuse yer microwave, tell you the check is in the mail, nor leave funky fluids in yer bodily orifices, neither!

Splinters, maybe. Ask Jim Rozen.

Better yet. DON'T ask Jim. We've seen his fugitive from a wooden garden-gate RPC so often it's beginning to put the hogs off their feed!

But it works. So, too, one John Oder has that is "minimalist", low-tech. Rope start was it?

Thank you for this

Karl_T said:

Just a data point for you. I have a 10 hp lathe on a 7.5 VFD and a 30 amp circuit for 10 years now.

Did have to wire the lathe forward and reverse to the VFD inputs for forward and reverse. the coolant pump was changed out to 110.

never had a problem.

Click to expand...

Thank you I was uncertain about this but was thinking so long long as we stayed below the threshold of the vfd it shouldn’t be a problem seeing as it’s producing the proper voltage just doesn’t have enough amperage to run at full capacity but who is gonna be trying to run a large machine at full capacity in a home shop I’ll be purchasing my 10hp vfd today for my 7.5hp Koping lathe

TXManka said:

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Can’t the VFD be set to provide constant 60hz 3-phase power to the machine and keep all the control infrastructure and 110v power supply intact? I have no need for speed control so bypassing those featureS, if possible, isn’t a problem for me.

Click to expand...

The problem with this is mainly in starting the machine. Motors typically draw 5 to 8 times their rated current as they start. The VFD cannot supply that, due to its current limits.

What generally happens is that the VFD just cuts off, and shows an overcurrent fault. That is the default response, to just "stop trying". Obviously that is not helpful.

You CAN do that, and many do, but they have VFDs which are rated for at least around 5x the current of the biggest motor. Since there is usually a 150% to 200% short term current capability, that allows starting the motor. Bigger is better for any VFD used in that way.

First of all I may be wrong but I suspect you will never load the motor on the lathe to 7.5hp or near that. Also, the extreme inrush for motor startup is for across line startups. With a VFD you will have a ramp up time so the inrush shouldn't be a problem. Considering that I suspect a 10hp vfd will work fine. I wouldn't be afraid to go this way. I had a 7.5hp vfd on my 5hp lathe for years and it worked great.

Second, at one time I lusted for a phase perfect phase converter but, based on some of the issues that some have had with these and considering how proud phase perfect is of them and considering some of the reports I have read of their service they couldn't give me one. My old American Rotary 30hp phase converter was a fraction of the price and just runs day in and day out and never complains.

Third, with a lathe or a mill a VFD can be sort-of a nice value added addition. I have one on my southbend lathe. I put the control and some other associated stuff in a hoffman box mounted on the back and made up a nice little control station. Using Ebay sourced allen bradley switches I have a forward/reverse switch, and a 3 position selector switch that selects 20,40, or 60hz output for quick speed changes. You get some really nice modernized controls but it does take some effort.

It not about running a motor at a full 7.5HP continuously, and sure you can dial it down anyway you want, but it does make a difference which VFD you buy, what your driving and the specific VFD derating factors/output amps vs. motor amps. Some VFD's the single phase derating can be as much as 4, in particularly the many newer lighter duty models. Often the cost difference between a 10 Hp VFD vs. 15 Hp VFD may not be significantly more, so something like a WJ200-075LF (7.5kw) is $737 vs. the WJ200-110LF ((11kW) is $757. Why scrimp out for $20 difference. A DC choke will help lessen the current draw peaks, and decrease the fusing requirements. Size of the breaker needed and wiring to the VFD, well it is one way to burn your shop/house down if you do not do right. There are a multitude of factors to consider, and different ways to implement them, but the general statement it worked for me doesn't mean it is going to work for you.

Pete Deal said:

First of all I may be wrong but I suspect you will never load the motor on the lathe to 7.5hp or near that. Also, the extreme inrush for motor startup is for across line startups. With a VFD you will have a ramp up time so the inrush shouldn't be a problem. .................

Click to expand...

If you use the VFD controls, sure.

Most folks doing the "constant 60 Hz" deal use the VFD the way a Phase Perfect works..... just supplies 60 Hz, and you turn on loads and turn off loads as needed.

If you do that, you do need a larger one.

It's not clear what the OP is asking..... could be either scenario.