• Spring 2024 meetup in Calgary - tentative date Saturday, April 20/2024. Other regions are also discussing meet ups. If you want one in your area get going on organizing it! discussion
  • We are having email/registration problems again. Diagnosis is underway. New users sorry if you are having trouble getting registered. We are exploring different options to get registered. Contact the forum via another member or on facebook if you're stuck. Update -> we think it is fixed. Let us know if not.
  • Spring meet up in Ontario, April 6/2024. NEW LOCATION See Post #31 Discussion NEW LOCATION

VFD questions

Susquatch

Ultra Member
Administrator
Moderator
Premium Member
I have often wondered how long a non VFD rated motor will last when used with a VFD In a hobby like environment?

I'm running a test, sample size one with my drill press, and an eBay "special" used motor that was used in a commercial dryer for who knows how long?

It's been about two years and so far it's working fine, who knows maybe it will blow up tomorrow?

I wonder that too. On a mill or a drill press in a hobby environment, I can't imagine a big concern. Loads are low cuz it isn't being pushed, duty cycle is low cuz we are not experienced enough to be rammy, cuts are shallow cuz we are scared of breaking a tool, and we have all day to make a $20 part.

That's prolly why we don't have $50,000 machines too.
 

BaitMaster

Super User
I have often wondered how long a non VFD rated motor will last when used with a VFD In a hobby like environment?

I'm running a test, sample size one with my drill press, and an eBay "special" used motor that was used in a commercial dryer for who knows how long?

It's been about two years and so far it's working fine, who knows maybe it will blow up tomorrow?
You will likely be fine for a very long time. Assuming, your running a motor rated for 230/460 3ph, intermittently, on 230v 3ph, and keeping the motor in a warm dry heated shop without moisture.

Problems with premature motor failure typically only pop up when the motor is run on 480v….
 

Bandit

Well-Known Member
Well, no indigestion so far. Room for thought about different applications. And then 3400 rpm motors mixed in. This ain't no rabbit hole, think it's an ant hill, which way , how far, How Much.
One shop I worked in had some old over head belt line equipment, truck transmissions and motors "updated" them. If the old timers could see this now.
 

Mcgyver

Ultra Member
The frequency is reduced in the same proportion that the voltage is reduced (to match the output voltage of the inverter) so as to maintain the design torque of the motor.

If a VFD is just connected with the default base frequency (which is typically 60 hz) the motor will provide less torque at whatever rpm you run it at compared to either providing the full voltage or setting the base frequency to the reduced frequency

same old same sold and I hate to bring it up again..,..but the fact is, the same torque at lower rpm means less power. Might be enough, and its sure a nifty way to power a 440V motor....but its not the same performance....its slower speed AND lower HP than what the motor is supposed to output. power = rpm x torque, there is no free lunch.
 

Susquatch

Ultra Member
Administrator
Moderator
Premium Member
same old same sold and I hate to bring it up again..,..but the fact is, the same torque at lower rpm means less power.

I was hoping you wouldn't bring it up again and I bet you were hoping I wouldn't provide my standard retort again. ;)

But I suppose others need to know we don't all agree on this.

Torque is what cuts. HP is what determines how fast you can cut in terms of material removal rate. At lower rpm you still need the torque to cut but you don't need the HP cuz you are not making as many chips.

As you say, Power = rpm x torque. There is no free lunch. But a free lunch isn't needed cuz the need for hp isn't there unless the operator is making massive chips.
 

TorontoBuilder

Ultra Member
A friendly reminder, the OP asked this:
Can a 440 volt be wired to run on 220 volt 3 phase?

They did not ask "what is the best practice for powering this particular lathe.
same old same sold and I hate to bring it up again..,..but the fact is, the same torque at lower rpm means less power. Might be enough, and its sure a nifty way to power a 440V motor....but its not the same performance....its slower speed AND lower HP than what the motor is supposed to output. power = rpm x torque, there is no free lunch.
Yes the above comment is true, there is a hit to performance in terms of speed and HP, but the torque curve is retained.

However, the solution does work to run the 440volt motor. It will allow an owner to operate their lathe as a temporary solution until they can implement a better or best practice option.

I certainly never touted this as the best practice and again if you look at my other comments on VFDs I've been very clear as to the best practices I would recommend.

I thought the idea was answer the questions asked. I did. I gave plenty of warnings and caveats as well.
 

Mcgyver

Ultra Member
I was hoping you wouldn't bring it up again and I bet you were hoping I wouldn't provide my standard retort again. ;)

But I suppose others need to know we don't all agree on this.

Torque is what cuts. HP is what determines how fast you can cut in terms of material removal rate. At lower rpm you still need the torque to cut but you don't need the HP cuz you are not making as many chips.

We need to get you on perpetual motion! :)

However, the solution does work

I think its a neat idea, wasn't meaning sound critical, just noting that there is a performance drop other than speed
 

TorontoBuilder

Ultra Member
We need to get you on perpetual motion! :)



I think its a neat idea, wasn't meaning sound critical, just noting that there is a performance drop other than speed
please do be critical... I am of the idea too. I am doing it on my lathe just until I get a new motor.
 

Susquatch

Ultra Member
Administrator
Moderator
Premium Member
We need to get you on perpetual motion! :)

Not sure if it's me you want in perpetual motion or a debate on the subject.

Either one would be most unpleasant for everyone - including me and you!

All of the outside inventions for my company used to get routed across my desk for review. If I had a dollar for every one who claimed to have invented a power source that could run on free energy (water, perpetual motion, or more power out than in), I'd be a millionaire!

I also have some hilarious stories on the subject best told another time.
 

Darren

Ultra Member
Premium Member
On both of my lathes I can take a pretty heavy cut at some low HZ numbers and the lathe doesn't slow down. So am I losing torque? Or HP? If it maintains the same speed and doesn't bog down, then it doesn't matter.
 

Susquatch

Ultra Member
Administrator
Moderator
Premium Member
On both of my lathes I can take a pretty heavy cut at some low HZ numbers and the lathe doesn't slow down. So am I losing torque? Or HP? If it maintains the same speed and doesn't bog down, then it doesn't matter.

You are losing HP but not torque. And you are right - it doesn't matter. ;)
 
There are constant torque VFD's and they maintain the torque throughout the range. How Variable Frequency Variable Voltage, current is what kills motors.

Now most regular VFD's don't suffer too much at the lower side of the scale, however as you exceed the range you notice a power drop. General rule of thumb is 60hz motor, +/-20 is ok.
 

BaitMaster

Super User
On both of my lathes I can take a pretty heavy cut at some low HZ numbers and the lathe doesn't slow down. So am I losing torque? Or HP? If it maintains the same speed and doesn't bog down, then it doesn't matter.
I mean for intermittent duty it won’t matter. It DOES slow down though, as slip is what causes torque in an AC motor…. And current draw. But an imperceptibly small amount of slowing down. Like 30-60 rpm in a 4 pole (1800rpm synchronous speed) motor at 60hz at rated FLA... Or likewise 150 ish rpm in a 2 pole (3600 rpm synchronous) motor at FLA at 60 hz. If a motor is forced to work harder, it slips more, and draws more current, and goes into an overload state…. At 60 or any other hz.

A major problem with drives is that typically they control average voltage and frequency, and monitor current for overload. And the current is simply a function of the amount of torque needed by the motor to maintain as close to synchronous speed as possible, and not a directly controlled variable…. So if a motor is ran on a drive at 30hz and is a 4 pole motor, now synchronous speed is 900rpm…. And the motor at its normal 2 ish percent slip, runs at 870 or so rpm at FLA, creating rated torque (but half the horsepower @Susquatch ;))…. But creates the same amount of heat as at 60hz because it’s drawing the same amperage…. But has less then 1/2 of the cooling capacity because of the centrifugal fan…. And doesn’t trip on overload because it’s drawing FLA, and then smells like burnt pickles and drips molten enamel out of the housing…

(Unless it’s inverter duty motor designed for constant torque at the frequency needed and provided adequate cooling)

You won’t see this in intermittent duty applications where the motor isn’t at or near FLA…. And has time to cool down. But it will happen if you work it at slip / current rating for long periods of time, especially at reduced speeds.

Some drives have functions to compensate for loading / heat buildup at reduced frequencies, but typically only ($$$) higher end models.

Ya it’s “fine” for a couple cuts, but definitely be aware of heat buildup if your working the thing like that for 2 hours.
There are constant torque VFD's and they maintain the torque throughout the range. How Variable Frequency Variable Voltage, current is what kills motors.

Now most regular VFD's don't suffer too much at the lower side of the scale, however as you exceed the range you notice a power drop. General rule of thumb is 60hz motor, +/-20 is ok.
Yes current can / does kill motors in VFD or contactor applications via overloading. BUT In VFD applications voltage (inductive spike / cable impedance mismatch, dV/dt) and frequency (resonance, harmonics, carrier frequency) kill motors (insulation, bearings) as well….
 
Last edited:

Darren

Ultra Member
Premium Member
@BaitMaster you also have a good grasp of what is going on here, obviously. My post was more or less saying 'who cares, it works'.

I've monitored temps on both my lathes, and compared to my air compressor motor, I have nothing to worry about. The lathe motors might hit 120-130*F after a few hours use. I don't run them at low hz for long, as i'd suspect would be true for most anyone here. My air compressor motor hits 150-160 after a cycle. I will be looking at better cooling for it. It works hard in my shop.

For hobbyist use, keep it above 20-30 hz and the stock fan should be fine. If you need to run it slower , add a 120mm computer case fan off the vfd aux circuit. Super easy to do, might save a motor.

My Kent mill, which was a cnc knee mill originally, came with a 6000 rpm EVS head. Its a VFD controlled factory setup. It has a big auxiliary fan on the motor that runs no matter what speed you run the motor at. As soon as the spindle starts, it comes on. The motor never even gets warm even drilling multiple 1" holes at 120ish rpm (1:1 drive). As for the TQ vs HP argument, at 50 rpm a 1.25" annular cutter drilling plate is a bit much. You need to speed it up a bit. I wish it had backgear for that stuff.
 
I mean for intermittent duty it won’t matter. It DOES slow down though, as slip is what causes torque in an AC motor…. And current draw. But an imperceptibly small amount of slowing down. Like 30-60 rpm in a 4 pole (1800rpm synchronous speed) motor at 60hz at rated FLA... Or likewise 150 ish rpm in a 2 pole (3600 rpm synchronous) motor at FLA at 60 hz. If a motor is forced to work harder, it slips more, and draws more current, and goes into an overload state…. At 60 or any other hz.

A major problem with drives is that typically they control average voltage and frequency, and monitor current for overload. And the current is simply a function of the amount of torque needed by the motor to maintain as close to synchronous speed as possible, and not a directly controlled variable…. So if a motor is ran on a drive at 30hz and is a 4 pole motor, now synchronous speed is 900rpm…. And the motor at its normal 2 ish percent slip, runs at 870 or so rpm at FLA, creating rated torque (but half the horsepower @Susquatch ;))…. But creates the same amount of heat as at 60hz because it’s drawing the same amperage…. But has less then 1/2 of the cooling capacity because of the centrifugal fan…. And doesn’t trip on overload because it’s drawing FLA, and then smells like burnt pickles and drips molten enamel out of the housing…

(Unless it’s inverter duty motor designed for constant torque at the frequency needed and provided adequate cooling)

You won’t see this in intermittent duty applications where the motor isn’t at or near FLA…. And has time to cool down. But it will happen if you work it at slip / current rating for long periods of time, especially at reduced speeds.

Some drives have functions to compensate for loading / heat buildup at reduced frequencies, but typically only ($$$) higher end models.

Ya it’s “fine” for a couple cuts, but definitely be aware of heat buildup if your working the thing like that for 2 hours.

Yes current can / does kill motors in VFD or contactor applications via overloading. BUT In VFD applications voltage (inductive spike / cable impedance mismatch, dV/dt) and frequency (resonance, harmonics, carrier frequency) kill motors (insulation, bearings ) as well….
Slip remains the same and why this works is very simple, the motor thinks it doing full speed, whatever its 60hz speed is.

The increased amperage that usually occures during start up doesn't because the running slippage doesn't change if the VFD is set up correctly creating less load on the circuits used by the VFD vs motor alone.
 

Susquatch

Ultra Member
Administrator
Moderator
Premium Member
I mean for intermittent duty it won’t matter. It DOES slow down though, as slip is what causes torque in an AC motor…. And current draw. But an imperceptibly small amount of slowing down. Like 30-60 rpm in a 4 pole (1800rpm synchronous speed) motor at 60hz at rated FLA... Or likewise 150 ish rpm in a 2 pole (3600 rpm synchronous) motor at FLA at 60 hz. If a motor is forced to work harder, it slips more, and draws more current, and goes into an overload state…. At 60 or any other hz.

Here we go...... It's dark in that rabbit hole!

I "mostly" agree with what you say. I say mostly, because many modern VFD's can measure speed, calculate slip, and compensate to bring RPM back to what is required. They can do this in one of two ways. Using a speed sensor on the motor shaft or using sensor less vector mode. In sensor less vector mode, there is no sensor but the VFD learns the motor characteristics and figures out what the motor is doing by measuring current and back emf. It then compensates by changing the output frequency and voltage to change the motor speed back to where it should be. I tested this on my Mill motor. In sensor less vector mode, there is zero change in motor speed between loaded and unloaded. I have verified this with my setup.

Of course, there is still slip. It's just that the VFD output frequency goes up to tell the motor to go faster. So the slip is from a higher speed reference. The motor does what motors do. But the VFD changes the motors target rpm so slip can go up without an actual speed change. Lots of current maintains constant torque and everyone is happy in the rabbit hole.

The result is a totally AMAZING improvement is smoothness, control, and motor noise - it has to be seen to be believed. No amount of reading such things will convince you. You just have to try it! Seeing really is believing. Put a tach on it while you test too. The rpm stays rock solid.

I also put a scope on the VFD output. As you increase the load, you can watch the target frequency go up. It's really cool!

A major problem with drives is that typically they control average voltage and frequency, and monitor current for overload. And the current is simply a function of the amount of torque needed by the motor to maintain as close to synchronous speed as possible, and not a directly controlled variable…. So if a motor is ran on a drive at 30hz and is a 4 pole motor, now synchronous speed is 900rpm….

As above, a modern VFD with SLV (sensor less vector) control mode can control voltage and frequency and can also control current indirectly by fine tuning voltage and frequency to increase rpm and current flow.

I do not know at what price point sensor less vector control is offered. I do know that most TECO brand VFD's have SLV Control.

And the motor at its normal 2 ish percent slip, runs at 870 or so rpm at FLA, creating rated torque (but half the horsepower @Susquatch ;))….

Let's be CLEAR here. I DID NOT SAY that horsepower is maintained! Horsepower MUST go down as rpm goes down. HP, Torque, and rpm are linearly related. HP = Torque x rpm. To maintain torque, hp MUST go down with RPM.

What I DID say, is that the HP does not NEED to be maintained. As RPM goes down, the workload also goes down (there are fewer chips). So constant horsepower is not needed. In other words, losing HP as RPM goes down is no big deal. The machine will keep making chips, just fewer of them. Which is totally what you would expect.

But creates the same amount of heat as at 60hz because it’s drawing the same amperage…. But has less then 1/2 of the cooling capacity because of the centrifugal fan…. And doesn’t trip on overload because it’s drawing FLA, and then smells like burnt pickles and drips molten enamel out of the housing…

(Unless it’s inverter duty motor designed for constant torque at the frequency needed and provided adequate cooling)

You won’t see this in intermittent duty applications where the motor isn’t at or near FLA…. And has time to cool down. But it will happen if you work it at slip / current rating for long periods of time, especially at reduced speeds.

Some drives have functions to compensate for loading / heat buildup at reduced frequencies, but typically only ($$$) higher end models.

Ya it’s “fine” for a couple cuts, but definitely be aware of heat buildup if your working the thing like that for 2 hours.

Yes current can / does kill motors in VFD or contactor applications via overloading. BUT In VFD applications voltage (inductive spike / cable impedance mismatch, dV/dt) and frequency (resonance, harmonics, carrier frequency) kill motors (insulation, bearings) as well….

I agree with all of this although I do think it sounds scarier than it needs to sound. In any event, this is why I love inverter duty rated motors coupled with a VFD that has sensorless vector control mode. They are not perfect, but for my usage they work just fine.

From everything I have read in the technical info provided by most manufacturers, (NOT YouTube) +/- 20% input frequency range is a reasonably safe intermittent use operating range for most non-vfd rated motors. 20% is 40 to 80 Hz which is a 2:1 motor speed adjustment range. Not bad at all.

Of course, a VFD really shines when coupled with a VFD Rated motor. These come in different grades. Mine has a 1000:1 turn down ratio. It is zero Hz rated. It will provide rated torque at 0 rpm. Of course the control frequency does go up and a crap load of current flows, but the motor just holds the torque. Nobody I know recommends this though. I have set my VFD to a minimum frequency of 6Hz, and a maximum or 120hz. That's a 20:1 speed ratio. Who needs gears! Yes, I kept most of the belt changes, but I hardly ever use them.
 
Last edited:

Mcgyver

Ultra Member
On both of my lathes I can take a pretty heavy cut at some low HZ numbers and the lathe doesn't slow down. So am I losing torque? Or HP? If it maintains the same speed and doesn't bog down, then it doesn't matter.

You're losing power. Reducing speed on a machine tool is usually because the diameter has increased meaning you need greater torque. Imagine a motor 1:1 to spindle, no transmission and mount a 1/4 end mill. It should work, right? Now hook up a 4" fly cutter. Doesn't cut despite having the same torque. You need much higher torque to drive 4"

When you quarter the speed via a mechanical transmission you quadruple the torque. i.e. A mechanical transmission increases torque as the speed goes down, maintaining power.

When you did that low Hz cut, I bet you are in low gear mechanically? That has multiplied the torque.

then it doesn't matter.

100%, that's why I put in "Might be enough".

At some point it matters else the industry would put smaller motors on machines. It also matters how much you reducing the speed by and for a correct understanding of the merits of different approachs; there is a performance degradation with electronic speed reduction.

Which is the only point I've ever made, is that there IS a performance degradation. You lose power. Whether it matters is case by case. (machine, operator, operation, how much you are trying to reduce it, etc) I don't know how that can be argued, but it seems contentious. :)
 
Last edited:

Susquatch

Ultra Member
Administrator
Moderator
Premium Member
You lose power. Whether it matters is case by case. (machine, operator, operation, how much you are trying to reduce it, etc) I don't know how that can be argued, but it seems contentious. :)

I am beginning to think it's contentious because we seem to have different baselines. We agree on almost everything except the baseline requirements. But your reply to @Darren suggests to me a few opportunities to at least understand our different perspectives and perhaps even get on the same page. If not, then we can always agree to disagree as we have in the past.

In any event, I am hopeful that we can get there from your reply to Darren. Wouldn't that be nice!

Please bear with me one more time.

You're losing power. Reducing speed on a machine tool is usually because the diameter has increased meaning you need greater torque. Imagine a motor 1:1 to spindle, no transmission and mount a 1/4 end mill. It should work, right? Now hook up a 4" fly cutter. Doesn't cut despite having the same torque. You need much higher torque to drive 4"

Your example assumes no speed change (1:1 spindle, no gear box). I totally agree that if rpm is constant, and diameter increases, the power requirements go up. Not just a little, a lot! In fact, exponentially! So does the torque requirement to an extent. It requires more torque to plow faster as surface speed increases. But this cutting force is not linear. But I am quibbling on that point. Fundamentally I agree with your example for a constant speed motor.

Do I hear applause?

When you quarter the speed via a mechanical transmission you quadruple the torque. i.e. A mechanical transmission increases torque as the speed goes down, maintaining power.

Yup. Totally agree again. There are other factors at play here too, but again they are quibbles.

When you did that low Hz cut, I bet you are in low gear mechanically? That has multiplied the torque.

Not necessarily the way I would make your point. Even if you don't change gears mechanically, you do reduce rpm through the VFD in order to maintain surface cutting speed. Here I won't quibble. It takes a certain force to push the cutter forward in the metal at a certain speed. That tangential cutting force is translated via the torque applied to the cutter from the motor via a moment arm which is the radius of the part or tool (lathe/mill). Thus the torque required to cut the metal at that constant surface speed has to go up to maintain the same size chip with the same cutting force.

I believe we agree on this point.

Where we disagree is within the parameters of what you have called performance loss. Not so much in terms of actual performance but rather what I would call available performance. I think this will get clearer when I debate your small motor argument next.

At some point it matters else they industry would put smaller motors on machines. It also matters for a correct understanding of the merits of different approachs; there is a performance degradation with electronic speed reduction.

I don't think that's how it happens. The industry doesn't specify motors based on minimum requirements. I believe they are specified in order to meet maximum requirements. The maximum dictates the motor size and everything else happens below the maximum.

This where our debate converges, and also where your performance criteria enters the picture. I think it's also a good place to understand the difference between hobby use and professional use. Those who make their living on machining are always looking to maximize performance. ROI is what drives profit and pays the bills. Hobbiests on the other hand usually don't care that much about maximizing performance. They just want to make parts. Payback and cost justification take a back seat to getting the job done.

So yes, if the AVAILABLE performance is greater than the necessary performance everyone is happy (pros and hobbiests).

In most cases, the pros look to maximize performance pushing cutting depth, chip size, insert design, etc to the limits. That's what pays the bills.

On the other hand, the hobbiest looks to produce a part. As long as the "available" performance is greater than the "required" performance, all is well. Furthermore, if the required performance isn't less than the available performance, the hobbiest is totally prepared to make adjustments (shallower cuts, slower feed rates, lower load inserts,) etc to get the job done.

Which is the only point I've ever made, is that there IS a performance degradation. You lose power.

I think this is where the circle closes.

I would insert the word "potential" into your point which if you accept it, I believe allows us to agree.

"there is a POTENTIAL performance degradation".

You only lose power if the requirements exceed what is available. But if what is available exceeds what is required, then everything works just fine.

Whether it matters is case by case. (machine, operator, operation, how much you are trying to reduce it, etc) I don't know how that can be argued, but it seems contentious. :)

There is NOTHING contentious about that statement at all. At least not for me. I agree 100%.

I would only add that most hobbiest machines are capable of much greater performance than we Hobbiests require. So, we can easily be happy with less horsepower (using your words) because we simply don't need it. We don't push our machines to their limits and there is usually LOTS of room between available and required. When there isn't, we simply reduce our requirements.

What do you say? Can we agree to agree now? Or is there still more ground to close between us?

FWIW, I shall change my statements about VFD's going forward to be more clear about the difference between available and required.

I think this clarification also perfectly addresses @Darren's observations and mine too cuz our requirements simply do not exceed what is available.

I'll only add one statement that you may or may not agree with. That is that "the low speed constant torque capabilities of a VFD dramatically improve the margin between what is available and what is required."
 

Darren

Ultra Member
Premium Member
@Mcgyver all good points of course. I was speaking strictly about my uses as a hobbyist.

The other day I was doing some work in high gear on the 3hp Emco at 20hz (lowest its allowed) and was surprised by the power. Had it been an issue, I would have selected a lower gear for sure.

On some of the 10ee Ac motor swaps they are running no back gear box and the factory pulley ratio's -1.5ish to 1. They upsize the motor to compensate.
 

Chipper5783

Well-Known Member
I’ve had a VFD on a 1970s drill press (3/4 hp, 6 pole motor is not VFD rated) for ~10 years. The VFD is an Allen Bradley, vintage the late ‘90s. Per comments above it is a low duty cycle. I had one job of tapping hundreds of holes and set the drive up for rapid reversal on a foot controlled switch. The torque for actual job (3mm spiral point tap on a through hole) was minor - the endless fwd - rev - fwd - rev - fwd …….. (you get the point) at about 25Hz (on my lowest belt speed) should have been a workout for the drive/motor. All good, no problem.

Advise I was given was to keep the motor leads as short as practical (the longer the motor leads, the greater the potential for harmonic issues?).
 
Top