VFD good or bad for old 3 phase motors

[Ron Bontz]

Long story short. I have been looking at a couple of power tools that have a 220v 3 phase motor. I was wanting to avoid having to install a rotary converter. But I have been told the VFDs that some folks use are bad for the motors and can ruin them. Can you tell I am not an electrical engineer? Any insight into this would be most helpful. Thank you in advance. Ron

[Loren Woirhaye]

Dunno, but you won't regret setting up a rotary converter. It's really not that big a deal. You'd have to learn to program the VFD anyway. The manual for the one I had was bewildering and I puzzled over it and my first 3 phase machine for many hours. The rotary once set up is plug and play.

[Dennis Ford]

Older motors will not last as long running on a VFD as they would running on a rotary converter BUT for hobby use, it is not a concern. If you were running the motor 24 hrs/day especially at low frequency; it would be a concern. My lathe gets used 10 - 20 hours per week; the motor was new 6 years ago but not inverter rated and is going strong. I expect it to last several more years (will probably outlast the inverter). A rotary converter would be a good choice for machines that don't need variable speed. VFDs are great for machines that benefit from variable speed.

[Dan Hintz]

Older motors will not last as long running on a VFD as they would running on a rotary converter BUT for hobby use, it is not a concern.

Dennis,

Why do you think that?

[Keith Hankins]

When i bought my first 3 phase tool had to do all that learning curve and in the end the consensus is if you are going to have a few old tools with 3ph then go converter. If one or two go the individual. I bought an American rotary phase converter 10hp that can do up to 7.5 hp and never looked back. It's quiet with a baldor motor and American made. That was several years ago. you will find 3 phase motors cheaper for sure!

[Dennis Ford]

http://www.sawmillcreek.org/images/misc/quote_icon.png Originally Posted by Dennis Ford http://www.sawmillcreek.org/images/buttons/viewpost-right.png (http://www.sawmillcreek.org/showthread.php?p=2218014#post2218014)
Older motors will not last as long running on a VFD as they would running on a rotary converter BUT for hobby use, it is not a concern.
Dennis,

Why do you think that?

I believe that 3 phase motors on hobby equipment are as likely to die of old age as they are to be damaged by VFD voltage spikes.

[David Kumm]

Old motors are pretty durable and even though the insulation on the windings is better now, the wires are tightly packed into small frame motors. The old ones dissapated heat quite well with their big frames. Wrecking a 220 volt motor with a vfd is not likely and I've seen and heard of more motor problems with a poorly balanced RPC than with a decent newer vfd. I 've run motors from the 1920s on vfds but hobby use isn't a fair test. Dave

[jack forsberg]

from a phase conversion stand point a VFD is the cleanest 3 phase power there is. it is the same as line power off the poll to the motor. A RPC on the other hand is very dirty power with a wild generated leg who's voltage can really only be balanced to a single hp. the cleanest power is Phase perfect and that's a VFD with out the veritable frequency. VFD are so easy to wire a monkey could it when it come to phase conversion . they don't waste power and developed full HP when use at 60 HZ for phase conversion. An older motor will last longer with a VFD when use with ramp up speed/soft start . One must remember that motors are dumb and you would be surprised what they will run on. Old ones do not have a problem in the 240 volt range. Anyway last i looked 3 phase motors were a dime a dozen just in case you were worried about frying your $10 motor.

I have 8 machines with VFDs and a RPC for the High voltage machines with a transformer. The VFD powered motor runs better and are cooler running too than the one on my RPC. Machine are 60 to 70 years old.

[Dan Hintz]

Older motors will not last as long running on a VFD as they would running on a rotary converter...


I believe that 3 phase motors on hobby equipment are as likely to die of old age as they are to be damaged by VFD voltage spikes.

I guess I should have shortened my quote from you a bit more. I wasn't asking about your statement from a hobby standpoint, I was asking why you believed an old motor was in any more danger from running on a VFD.

[Ed Aumiller]

Mr Jack Forsberg has it correct... a VFD is much cleaner than an RPC... have had both... got rid of the RPC and went exclusively to VFD's...
There is nothing wrong with RPC's if properly sized, but are not as efficient as VFD's....
Also VFD's take less space and provide speed control IF DESIRED......
Motors do not know the difference between VFD's and RPC's unless the RPC is not properly done... many RPC's are not true RPC's but simply spare 3-phase motors wired up to emulate an RPC, this is not the same as a commercial (& real) RPC....

[CPeter James]

Jack id dead on. I have two VFD powered machines and would like to convert my 17" drill press to 3 phase just so I could use a VFD. The only issue is running the older motors at extremely slow speeds for extended periods of time IF, and I say IF they are fan cooled. if the are not fan cooled as the motor on my Wadkin lathe is, even that should not be a problem. VFDs are they way to go. And Jack is correct, connecting is simple and programming is not hard. I use Automation Direct and the programming sequence is printed on a simple to read card. And with a VFD you get variable speed and reverse. A real asset on a drill press.

CPeter

[David Kumm]

There is such a thing as inverter duty rated motors. The nature of the current conversion can cause motors that run continuously to run hotter with a vfd. My understanding is that those motors use insulation that you can barely burn off with a torch. I still don't worry about my old motors with a vfd. Dave

[Ed Aumiller]

Quote "The nature of the current conversion can cause motors that run continuously to run hotter with a vfd."

When you have near perfect 3 phase voltage/current applied to a motor, it does not matter if it comes from the power company (which, incidentally, is not 'perfect') or from some other source (RPC's or VFD's) how does a motor know to run 'hotter' ?????

If you use some RPC's or "Static converters" which DO NOT properly provide the third phase, but only enough to make a motor run, then yes you may experience some heating as the motor is basically only providing power on slightly over 2 phases....

VFD's provide full power on each phase and are actually better or as good as commercial power...

[Mike Henderson]

The only problem I've ever heard of from running "older" motors off a VFD is that the motor is often cooled by an internal fan. When you run the motor slower you also slow down the fan which can lead to overheating of the motor. But the amount of heat generated depends on how heavily loaded the motor is. You can mitigate the problem by putting an external fan blowing on the motor.

If you run an "older" motor on a VFD at rated Hertz, the motor should run the same as if you ran it on mains power. The output waveform of some VFDs will contain harmonics of the primary frequency because of the shape of the "sine" wave but the motor is a very good low pass filter and will generally filter out those higher frequencies.

VFDs and induction motors are used extensively today in place of DC motors where speed control is required because an induction motor requires less maintenance than a DC motor (no brushes).

Mike

[David Kumm]

Way above my pay grade here. The little I know about inverter duty motors is that they use different insulation because a vfd MAY cause a motor to run hotter. Either from running slowly and not cooling itself, or because the conversion of DC back to a " chopped " sine wave can create voltage spikes. Again, I run old standard motors all the time on vfds and am not afraid to do so. Doesn't change the fact that " inverter duty motors " have been created to run on vfds in certain applications. Might be all marketing but I'm not clever enough to know. Dave

[Jim Neeley]

Motors, especially those which are fan cooled, can overheat when run at lower speeds. Technically overspeed can be an issue too but I haven't seen that (what appears to be an overspeed caused failure) manifest itself very often.

At least in the industrial world you can specifically buy VFD rated motors. These motors are typically heavier, likely to have fins on the exterior and are otherwise designed for superior cooling.

Heat is not your friend in electrical devices. :-)

Just my $0.02.. YMMV.

Jim

[Loren Woirhaye]

many RPC's are not true RPC's but simply spare 3-phase motors wired up to emulate an RPC, this is not the same as a commercial (& real) RPC....

Would you be able to back that up with a link or technical screed?

[Ron Bontz]

Hey guys. Thanks for the insight. I have been looking/ wanting a decent metal lathe and mill for a long time. But every time I find a reasonably priced old one, it is 3 phase. I have plenty of single phase outlets as I just rewired my little shop. But no 3 phase. Always something. :)

[Carroll Courtney]

I have a 1960 PM90 that I have a VFD on had no issues.But I don't use it several hrs a day maybe couple hrs at a time,I also have a home made 5hp rotary phase converter that I have use several hrs at a time,also no issues from motors that date form the 50's on up.Just afew yrs ago elec motor companys started to put on the name plate VFD rated,just to help make a sale.At work have both types of motors and one does not last any longer than the other.What really counts is your power provider,black out,brown outs,floating power outage,etc.To get a VFD to help with these problems will cost you more money.

[Dennis Ford]

I guess I should have shortened my quote from you a bit more. I wasn't asking about your statement from a hobby standpoint, I was asking why you believed an old motor was in any more danger from running on a VFD.


As I understand it; the output from a VFD is not a true sine wave; it has some harmonic distortion that creates voltage spikes. Over time, these spikes can damage insulation that would not be harmed by a "clean" power source. My statement that a motor would last longer on a RPC was an error. (based on invalid assumptions)

[John McClanahan]

I have read about the voltage spike thing, too. My gut feeling is that the old motors get undercooled when running at below rated speed, resulting in the winding insulation baking and failing.

John

[John McClanahan]

Would you be able to back that up with a link or technical screed?

Loren, I don't have proof to present, but I agree with Ed. A true phase converter would be a motor/alternator setup. The reason is the phases of 3 phase are 120 Deg. apart. Single phase is 180 Deg. apart. The 3 phase needs to be generated, not converted, to be real 3 phase. A lot of the lower priced RPCs are a 3 phase motor with a static converter to get it started and some capacitors to try to balance the wild leg (3rd phase).

John

[Mike Henderson]

As I understand it; the output from a VFD is not a true sine wave; it has some harmonic distortion that creates voltage spikes. Over time, these spikes can damage insulation that would not be harmed by a "clean" power source. My statement that a motor would last longer on a RPC was an error. (based on invalid assumptions)
The output of a VFD may have a waveform that's more like a bunch of square waves which form the general shape of a sine wave at the primary frequency. Because of that, the waveform will contain harmonics of the primary frequency. But that should not cause voltage spikes. The motor acts like a low pass filter and blocks most of the higher frequency components of the input waveform, leaving a waveform of the primary frequency in the shape of a pretty good sine wave.

Mike

[Mike Henderson]

My gut feeling is that the old motors get undercooled when running at below rated speed, resulting in the winding insulation baking and failing.

John
Exactly right. Increasing the operating temperature by 10 degrees C above the rated temperature halves the life of the motor. This is for continuous operation but even short operation above rated temperature decreases the life of the motor.

Mike

[Dan Hintz]

The output of a VFD may have a waveform that's more like a bunch of square waves which form the general shape of a sine wave at the primary frequency. Because of that, the waveform will contain harmonics of the primary frequency. But that should not cause voltage spikes. The motor acts like a low pass filter and blocks most of the higher frequency components of the input waveform, leaving a waveform of the primary frequency in the shape of a pretty good sine wave.

This was essentially going to be my answer. The output of a VFD is a PWM signal (square waves), but it's run at such a high rate compared to the sine wave you're duplicating there is no more wiggle than if you were looking at a typical signal from the power plant (assuming a halfway decent VFD). The slower cooling motor is about the only thing I can think of that would kill an "older" motor, but that would apply to new fan-cooled motors, too.

[Dan Schrecongost]

https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/motor_tip_sheet14.pdf

For process and HVAC fans and pumps that will be controlled by variable frequency drives, inverter-rated motors are usually part of the VFD upgrade. These loads usually have long hours of operation and reliability of the motor/drive system is important.

Power tools, different story...

[Clay Fails]

My understanding is that to get long life with VFD a motor should rated inverter duty with insulation class H. Anything less will result in shorter life. I'm not an electrician but that's what I've been told.

[jack forsberg]

what i think so funny is you get a 3 phase machine and buy a VFD to run it. Then your told the motor won't last so you replace it with a inverter duty one. No more worry about burning that motor out now is there:rolleyes:

Why not try to burn it out first with the VFD. I have been trying for years and will let you know when it happens