VFD Use With Non-Inverter Duty Motors

ptsmith · Aug 14, 2018

I recently acquired a 3HP Lagun FTV-2 Mill with variable speed pulleys for garage hobby use. I have it hooked up to a Teco L510 VFD. The variable speed pulleys (sounds like the one on the motor) have a problem. A loud knock that gets much worse as the speed is reduced to anything below top speed.

I haven't figured out how remove the motor yet, as I have a bad back, and probably couldn't do it with a good back, so I'm not sure what the fix entails yet.

But I'd just as soon use the VFD for speed control, regardless of what's wrong with the mill. Replace the variable pulleys with standard pulleys if that's feasible.

Which brings me to the reason for this post (finally, you're probably thinking). I read that non-inverter duty motors won't hold up to varying the frequency. *But*, I read here and see on youtube many people using VFDs with older mills and lathes with apparently no problems.

What do ya'll think?

billmac · Aug 14, 2018

The knock probably has nothing to do with the motor. You can use a VFD with non-inverter duty motors; they aren't designed for that type of drive but will be fine if you are sensible, and since it is hobby use its not likely to be an issue. You may well have a knock coming from your pulley area that has nothing directly to do with the VFD, but is perhaps caused by torque/speed variations as you change speed on the motor. This might cause the splines to rattle a bit?

FlatBeltBob · Aug 14, 2018

The main issues with using a VFD with an older motor relates to heat .
A motor designed to run at a constant speed has a cooling fan connected to the shaft .
CNC inverter drive motors will often add a small muffin fan so it gets some air flow even at slow motor speeds.
Knocking on a Bridgeport type mill with vari drive pulleys will often be a drive belt slapping , or the inner bushing on the moveable half of the pulley

grg12 · Aug 14, 2018

Motors connected to vfd should have higher isolation rating. google " motor insulation rating for vfd" . I also heard about problems with bearings damaged by stray currents but don't remember anything specific.

JST · Aug 14, 2018

grg12 said:
Motors connected to vfd should have higher isolation rating. google " motor insulation rating for vfd" . I also heard about problems with bearings damaged by stray currents but don't remember anything specific.

Both issues can be fixed to a large extent with an inductor installed between the motor and the VFD. That tends to block the high frequency pulse "edges" that cause the problems*. An actual "filter", including both an inductor and capacitors will essentially remove the problem if remotely correctly designed.

That does not fix the cooling issue, but if the range of change is restricted to perhaps 1/3 speed on the low side, problems should not occur.

When a motor is not easily replaced, and is an older one, that is very practical. And, older motors often have better insulation between coils, decreasing the "corona" problem.

The usual issues are:

1) voltages between wires in one coil, due to the inductance of the coils blocking the high frequency "edges" of the pulses. That can be fixed somewhat with better wire insulation, OR an external inductor.

2) Currents through the bearings due to the coils capacitance to the rotor passing high frequency "pulse edges". The inductor helps with that, a full inductor/capacitor filter will eliminate it.

3) Corona discharge from the coils which may cause "tracking" of the insulation between coils or between coil turns. That can lead to shorts and otehr problems. Older motors tend to have better insulation, but an external inductor cuts this problem down.

4) Extra heating of the motor due to high frequency pulses causing cirrents even though the motor core is laminated. External inductor will cut that down.

5) heating due to too low a speed decreasing fan capacity and increasing currents to maintain power. This is fixed with external fans, using larger than needed motors, or simply not changing speed that far, using a gearbox etc for low speed range.

*...There is an effect due to the VFD plus small coil-to-rotor capacitance being somewhat of a "current source" for the high frequency pulses, which makes a series impedance like an inductor less effective in blocking currents. That is why the full "filter" using inductor and capacitors does a better job in many cases.

rons · Aug 14, 2018

ptsmith said:
I recently acquired a 3HP Lagun FTV-2 Mill with variable speed pulleys for garage hobby use. I have it hooked up to a Teco L510 VFD. The variable speed pulleys (sounds like the one on the motor) have a problem. A loud knock that gets much worse as the speed is reduced to anything below top speed.

I haven't figured out how remove the motor yet, as I have a bad back, and probably couldn't do it with a good back, so I'm not sure what the fix entails yet.

But I'd just as soon use the VFD for speed control, regardless of what's wrong with the mill. Replace the variable pulleys with standard pulleys if that's feasible.

Which brings me to the reason for this post (finally, you're probably thinking). I read that non-inverter duty motors won't hold up to varying the frequency. *But*, I read here and see on youtube many people using VFDs with older mills and lathes with apparently no problems.

What do ya'll think?

Slacken the belt and remove the motor and run it with just the VFD. This will probably indicate that the motor is fine and you have a problem with the pulley drive system.

You don't have to replace the variable pulleys, just strap the belt in the middle range and adjust VFD frequency up and down for speed changes. With regard to non-inverter duty motors and holding up to varying frequencies. The switching frequency of the base PWM waveform is going to heat up the windings in the motor, not the frequency of the waveform that determines the motor speed. The best you can do is keep the switching frequency low and make decent offerings in the collection plate in church.

You can drive over to Home Depot and hire a parking lot Mexican Bandito to remove the motor. Offer a twenty dollar bill up front and the promise of a couple Modelo Especials after the job is done.

Madis Reivik · Aug 14, 2018

JST said:
Both issues can be fixed to a large extent with an inductor installed between the motor and the VFD. That tends to block the high frequency pulse "edges" that cause the problems*. An actual "filter", including both an inductor and capacitors will essentially remove the problem if remotely correctly designed.

That does not fix the cooling issue, but if the range of change is restricted to perhaps 1/3 speed on the low side, problems should not occur.

When a motor is not easily replaced, and is an older one, that is very practical. And, older motors often have better insulation between coils, decreasing the "corona" problem.

The usual issues are:

1) voltages between wires in one coil, due to the inductance of the coils blocking the high frequency "edges" of the pulses. That can be fixed somewhat with better wire insulation, OR an external inductor.

2) Currents through the bearings due to the coils capacitance to the rotor passing high frequency "pulse edges". The inductor helps with that, a full inductor/capacitor filter will eliminate it.

3) Corona discharge from the coils which may cause "tracking" of the insulation between coils or between coil turns. That can lead to shorts and otehr problems. Older motors tend to have better insulation, but an external inductor cuts this problem down.

4) Extra heating of the motor due to high frequency pulses causing cirrents even though the motor core is laminated. External inductor will cut that down.

5) heating due to too low a speed decreasing fan capacity and increasing currents to maintain power. This is fixed with external fans, using larger than needed motors, or simply not changing speed that far, using a gearbox etc for low speed range.

*...There is an effect due to the VFD plus small coil-to-rotor capacitance being somewhat of a "current source" for the high frequency pulses, which makes a series impedance like an inductor less effective in blocking currents. That is why the full "filter" using inductor and capacitors does a better job in many cases.

Actually, RC filter is enough and much cheaper. It removes most of high freq components which cause heating and insulation problems.
GE inverters and probably others too have optional output RC filters listed in catalogues and any normal reseller can recommend a suitable filter. Not expensive and works.

ripperj · Aug 14, 2018

I ran a Rockwell Mill for 20 years on a VFD with no issues, typically around 60hz, but varied as needed. Same with a 13”SB lathe, both were original motors from the mid 60s. The lathe I varied a lot more.

Sent from my iPhone using Tapatalk Pro

Pathogen · Aug 14, 2018

I have run fans and pumps on VFD's at different speed rates on older motors (5-10 HP 3 phase) and the motors did better than on public utility power. 24 hours a day year round. I used the largest size copper wire that would fit it the VFD terminals, and an excellent grounding conductor.

If you are constantly varying the speed and load, that would be a concern.

JST · Aug 14, 2018

Madis Reivik said:
Actually, RC filter is enough and much cheaper. It removes most of high freq components which cause heating and insulation problems.
GE inverters and probably others too have optional output RC filters listed in catalogues and any normal reseller can recommend a suitable filter. Not expensive and works.

"RC" filters may mean one of two things.... Either an actual "RC filter", with a series resistor and a shunt capacitor, or, alternately, a "damper" unit, which consists of series resistor and capacitor units between lines, or line to ground.

An actual series R-C filter is not always practical, because the motor current heats the resistor, and for significant currents the resistor must be small in value or large in size, due to heating that is related to the current squared. Also, a low value resistor needs a larger value capacitor. Depending on the filtering needed, the capacitor may steal considerable current from the output, and can cause over-current trips of the VFD. With an inductor, the impedance is larger but not resistive, so the filtering can be more effective and not stress the VFD, and also does not waste power.

The "damper" can work, but can also overload the VFD and have heating issues if too large. Usually it is intended to reduce the effect of "reflections" that may cause voltage spikes, particularly with long lines from VFD to motor. The effects that are being discussed here can happen even when the "reflection" issues are not significant.

In either case, presumably if the filters are an R-C type, they are designed correctly and will function without issues, whether they solve the actual problem or not.. The only ones I have seen for fixing the "edge frequency" issues are L-C types, which may actually include a resistor that acts to reduce resonance effects..

ptsmith · Aug 14, 2018

Thanks for the responses!

I've been reading this thread and Googling, and reading and Googling some more. I'm not finding anything that says "get this" for VFD use with older motors. So it's great news that I can use my VFD for speed control, but I still have no idea what to get. I keep finding mention of "reactors". Will this do what I need to do? http://www.wolfautomation.com/media/pdf/linereactors/mte/mte-rlreactor-brochure.pdf

I ran the mill a little more today with VFD set at 25Hz. I'm afraid to run it too much as it doesn't sound good. It's a loud metal on metal banging sound. It always bangs, but it gets louder as I dial the speed slower. Much louder. It really sounds like it's coming from the rear pulley. Motor speed changes doesn't change the sound much, nor does back gear changes. Pulley speed changes do, so it really doesn't seem like it's the motor.

I can get a couple of younger guys to lift the motor off, but I don't trust them not to drop it and hurt themselves and/or the motor. I can get an engine hoist that's tall enough to get it off, but it's heavy too. It's hell getting old! :-\

Madis Reivik · Aug 15, 2018

I meant "damping" unit with small RC values, for limiting dU/dT and of course this means less noise (even acoustical) and less heating.

Eurybates · Aug 15, 2018

Hmmm

I would take a look at rotor to see if it is poling. You will probably see marks on one end (most likely DE) and also marks on inside stator laminations. Then it's a matter of bearing housing/journal slogged out, or bearing itself. Had this with a few motors over the years on VFD and variable voltage when magnetic pull (induction) of rotor is less when up to speed knocking goes away.

Would be good of you to post back and tell us all what the issue was for future reference!.
Good luck

GregSY · Aug 15, 2018

The bigger issue you will face is the motor on the mill will not perform well at lower speeds. For one thing, it most likely has a cooling fan which will not pump enough air and while you might get good cooling at full RPM, you will overheat at low RPM. You probably also will not produce enough torque to make low RPM, heavy cuts.

Fix the mechanical issue you have (the pulleys), run the motor at 60hz (or close to it) and you will be a happier dude.

adama · Aug 15, 2018

Go look at my thread in the bridgeport section, cant recommend getting rid of all the vari speed crap, couple of nice pulleys and new belt, picking your favourite flavour and just use it. Duty cycle on a manual mill is never going to be enough to cause issues with the motor over heating at lower speeds. Your just never going to run it slow enough for long enough to get hot. I have still not managed to even get my 1.5kw motor more than warm to the touch and it was chewing through a fair bit of 45HRC hardox last week!

dgfoster · Aug 15, 2018

rons said:
<Snip>
You can drive over to Home Depot and hire a parking lot Mexican Bandito to remove the motor. Offer a twenty dollar bill up front and the promise of a couple Modelo Especials after the job is done.

Yes, the world needs a little more racism. That was so clever. Do your part to promote racism in whatever form. It will make you feel better and look smarter.

Denis

GregSY · Aug 15, 2018

Mexican is not race, it's an ethnicity.

dgfoster · Aug 15, 2018

GregSY said:
Mexican is not race, it's an ethnicity.

Oh sure!

Denis

kenton · Aug 15, 2018

GregSY said:
Mexican is not race, it's an ethnicity.

I thought nationality?

markz528 · Aug 15, 2018

I promised to do a full right up on VFDs and bearing currents on another thread but just have not had time.

First - what insulation system does the motor have - will be on the nameplate. If its A, then you got no chance. Preference would be a Class F insulation, but B would be tolerable. H is the highest standard class.

VFD motors run hot due to harmonics - roughly 20% higher losses on a vfd - also run around 20% hotter. As stated earlier, the slow speed of the fan at slower speeds does heat the motor but for what you are doing I would not worry about it.

Reflected wave causes high voltages on the motors leads - keep the motor leads to the drive as short as possible and should be fine.

In a belted application, only circulating currents and stator to rotor capacitive currents apply for bearing currents - in this hp range I would not be concerned. Stator to frame capacitive currents are most damaging but they go away in a belted application.

Output reactors are a good idea. They slow the dv/dt of the wave form (how fast the pulse rises) - the vfd vendor should be able to recommend an appropriate reactor. I don't know what a reactor costs, but at 3 hp would not surprise me if the reactor cost more than the drive. I just paid under $300 for a quality single phase input 3 hp drive.

VFD Use With Non-Inverter Duty Motors

Cast Iron

Stainless

Stainless

Plastic

Diamond

Diamond

Hot Rolled

Stainless

Hot Rolled

Diamond

Cast Iron

Hot Rolled

Plastic

Diamond

Diamond

Diamond

Diamond

Diamond

Stainless

Hot Rolled

Similar threads