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VFD in a lathe application
5

VFD in a lathe application

VFD in a lathe application

(OP)
Hi, does anyone have the rundown on using a VFD'd motor in a lathe?

We've put a 5hp VFD specific motor into a Monach lathe. The original motor was a DC motor run by tubes! This was later swapped out for SCRs in a DC controller in the 80's. That's crapped-out just like the company that made it and as a last resort we put in this VFD induction motor that even came with an encoder.

I'm running it in sensorless Vector mode since we're not trying to do positioning or holding.
Tuned it successfully with nothing attached to it. Hooked it up to a gear box whose output belt drives the spindle.

The lather came with 1hp, 3hp, and 5hp options.

Everything operates fine except at lower speeds you can grab the spindle and stop it with your hand. While I don't expect massive low end torque that's seeming... under-the-bottom.

On a second note I see large currents being sent into the motor even if it's stopped. !4A FLA and I'll see maybe 10A, .8A, 4A on the phases when the motor is stopped. Is that the drive trying to keep the rotor somewhere known? Can that be avoided?

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD in a lathe application

If it has an encoder why are you not using it? I wouldn't think that your application would need one but this will provide the best performance for all applications. The only reason not to run an encoder is cost.

RE: VFD in a lathe application

(OP)
That's not the only reason. You need to hunt down an additional expensive board and install it into the VFD.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD in a lathe application

Motor is actually taken down to zero speed and not stopped when you still see these currents, right? There is a difference. Stopped is VFD off.

RE: VFD in a lathe application

itsmoked: There are two modes involved with a VFD, usually. One is a region where the drive can effectively control power vs speed, and the other one - which is typically below about 5 Hz - is where it has difficulty. In the higher speed range, you'll pretty much see the torque you're expecting. At the bottom end, though - all bets are off, except that you're likely to see less torque than expected. One way to get around this is to acquire a MUCH better (and therefore more expensive by at least an order of magnitude) drive. Another is to just not operate "down in the mud" so to speak.

As to seeing current at nominal zero speed - that's precisely it. The drive is not "off", it's simply holding the shaft from spinning - kind of like riding the clutch on a hill. Remember, the control isn't all that accurate down there either, so the drive is using relatively coarse current adjustments as it "hunts" for the best position.

Converting energy to motion for more than half a century

RE: VFD in a lathe application

(OP)
Lionel; That's what I expected but nope! The external input set to STOP it still keeps the motor excited.. Still puzzled. The manual made mention of a timer one could set to stop, what I thought was this, when I wasn't even looking for it, but when I went back doing an extensive search I could never find it in an hour of hunting.


Hi Gr8blu; Thanks for that. I'm aware of the vulgarities of sensorless down below 5 hz or so. I set the minimum to 10Hz to avoid this region of mud. Didn't expect the gutless result. There is a torque-boost setting that ranges from 100% to 350% which I believe is being tried. I don't know the results yet.

I just wish the excitation would go away since one expects a lathe to spin freely when "STOPPED" and I'd like a stopped period to go towards cooling the motor where still cranking 10 amps thru it doesn't probably allow any net cooling.

I'm starting to wonder if I should go back to V/Hz mode and run in a PID mode, though, I'd think a PID mode would be for a fixed application like running a fan or a conveyor not an all-over-the-place application like a lathe.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD in a lathe application

PID for what, speed?

What make of drive?

RE: VFD in a lathe application

Have you considered that you may have an IGBT that has failed closed?

RE: VFD in a lathe application

Brand new? That puts you on the left side of the bathtub curve.

I'm thinking the stuck on gate only affects one phase. There are multiple gates in the inveter. Some time back I had a triac on a control board fail in short circuit. It controlled the contactor for a small air conditioning compressor. Twice we burned out compressors when they didn't shut down on high pressure cutout before we found the shorted triac.

RE: VFD in a lathe application

(OP)
Hi Tug; I see your points, they're pretty good. One point though is that those measurements I mentioned above were taken between several jogs and they changed around to different phases. For instance the 10 or so amp phase moved and sometimes the max was more like 6A and other phases got higher. They did seem to kind of algebraically always add up to some consistent value.

I can shift the drive to V/Hz and that would kill any and all static currents. If that happens then a 'stuck' pass-element wouldn't be.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD in a lathe application

I'm not familiar enough with that drive to know what it might be doing, but stopped should be off. There can be an odd mode occur where you set the drive to force the motor to zero speed before it stops and the motor regenerating can't be taken to zero speed causing the drive to get stuck trying to stop but never getting to the stopped condition where it would shut down. It might be that you've got this happening even though the load is not regenerating to cause that.

I do agree that something is definitely wrong if you can just grab the chuck and stop the machine with the motor running at 10Hz or more.

There must be someone at WEG with enough of a clue about that VFD to suggest some programming changes to make it work correctly.

A VFD has 2 IGBT in series across the DC bus for each output phase. If one IGBT is failed on, the drive will immediately blow up as soon as the other IGBT turned on. Since the IGBTs switch on and off at the carrier frequency it'd blow within milliseconds of being given a start command. Well, it might not blow up but rather trip on a hardware short, it's kind of a crap-shoot on which would happen. If it did fault and you reset and tried it again you'd greatly increase your odds of getting a nice explosion. You've got to know how a VFD works before suggesting how it might have failed.

RE: VFD in a lathe application

We had always set a minimum speed.
There comes a point at lower freq where the waveform of the VFD and the winding style of the motor just don't like each other.
I would never have thought of trying to run down to 10Hz, or anywhere near that.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: VFD in a lathe application

Our winches use cam operated limit switches on the speed potentiometer to force the stop command when the lever is placed in the center position. However, I believe in itsmoked's case the drive is indicating that it is in fact stopped but still has output?

Can you hear the switching or carrier frequency in the motor when it is supposed to be stopped?

RE: VFD in a lathe application

(OP)
I did not hear the carrier though the motor is a NonVentedTotallyEnclosed model that does wonders for bottling up VFD excitation noise.

I did try hand turning the spindle while "stopped" and I could turn it but you could tell it was excited in some form as it would take effort to turn then advanced herky-jerky.

As Hutz mentions though, if one channel was stuck ON it would be crow-bared almost instantly.

I'm going to give another try searching for that "Note" in the manual I subsequently couldn't find again. I saw it while speed reading the manual the first time just looking for things that would matter for this job not actually realizing that note WOULD matter to me.

LionelHutz; Thanks for confirming the hand-stopping. I haven't messed with a motor this size at low speed in vector to have much judgement about hand stopping it. (Especially since it's being geared down some thru a gearbox!)

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD in a lathe application

A motor might never stop if you set the VFD to current limit and the motor is being driven hard enough to overcome the decelerating torque the motor is producing with the VFD current limit. The VFD timer may be to protect against this where it will time out and eventually trip off if told to decelerate but it can't get the motor to decelerate. It'd be odd for the VFD to be thinking a stopped motor is still rotating, but maybe it is.

RE: VFD in a lathe application

I had one application where the motor would spin at 25 rpm when clutched out but once clutched in the gearbox load was enough to stall it. Nobody ever noticed anything but I could hear the carrier frequency faintly through the motor casing. I hooked up Connected Components and saw the motor speed. I never did get to troubleshoot that one as we transferred the vessel out of our fleet.

RE: VFD in a lathe application

Some vector drives will keep the "magnetization current" active even when there is no request for orthogonal "torque current". This way, when there is a request for torque current, there is not the delay in getting the rotor to magnetize.

One of the trickiest things about "sensorless" (actually, shaft-sensorless) vector control is calculating the rotor angle at very low speeds. The back EMF on the phases is so low that you have a horrible signal-to-noise ratio. This can mean that what is intended to be torque current is not oriented correctly.

I don't know if these factors are what you are seeing, but they should be considered.

Curt Wilson
Omron Delta Tau

RE: VFD in a lathe application

You might try this for the STOP mode.
I am assuming that Coast is what we call free rolling. ponder

P0229 Stop Mode Selection
0 = Ramp to Stop
1 = Coast to Stop
2 = Quick Stop

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: VFD in a lathe application

If it's using DC injection braking then it may cause the situation described but if that were the case the drive status should be "running" and not stopped.

RE: VFD in a lathe application

I don't understand what do you mean by "drive status should be "running" and not stopped"?

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: VFD in a lathe application

On the HMI for the drive, the ones I have seen usually have some sort of indicator of what the drive is doing. If the drive is controlling the motor it will say "running". If the drive is not controlling the motor it will say "stopped". Braking would mean the motor is under control of the drive and it will indicate running. Coast to stop means the drive is not controlling the motor and will indicate stopped.

This is an example from my phone.

RE: VFD in a lathe application

(OP)
I can't use coast-to-stop as this is a lathe and the users always want it to stop INSTANTLY even though that's idiotic and I have to ask them if they also want the work piece ejected into their faces and if they want to replace all the motion hardware every two years. That usually gets them to reduce their expectations out to something like 3 seconds... Coast would take 60 to 300 seconds.

I'm not using DC braking as there's a BIG braking resistor and this is an oversized drive so 3 seconds should work. DC braking dumps a ton of heat into the motor.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD in a lathe application

Quote (Itsmoked)

I just wish the excitation would go away since one expects a lathe to spin freely when "STOPPED" and I'd like a stopped period to go towards cooling the motor where still cranking 10 amps thru it doesn't probably allow any net cooling.

Quote (Itsmoked)

I can't use coast-to-stop as this is a lathe and the users always want it to stop INSTANTLY

Okay I misunderstood, I thought you wanted it to "spin freely"/coast. smile

You meant after it stopped you wanted it to be powerless?

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: VFD in a lathe application

3
Hmm maybe it is this one then. ponder

P0181 = 1

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: VFD in a lathe application

(OP)
Gadzooks!!!

You found it Red! That was what I perused past and hadn't been able to find again.

Thanks so much. I'll report back.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD in a lathe application

You are welcome. smile

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

RE: VFD in a lathe application

I agree with Curt, this could be the magnetization current, also sometimes called "flux-up" current, meaning the drive pre-establishes the motor flux to reduce the time it takes for the torque to get to the shaft. It might be that your "Stop" signal is not set as a true Stop, as in it is a "Zero Speed" command, which is not the same in a VFD, especially one under any kind of vector control.

Looking at the manual, I'm wondering what you have P0229 set for, the "Stop Mode" selection. Do you have it set for "Ramp to Stop" by chance? If so, and you have the Decel Ramp TIME set out long, even if the LAOD causes the motor to come to a stop, the DRIVE will continue that decel ramp of the output for the full amount of Ramp time. Then if you have P0229 set for "Quick stop", it uses the setting of P0106 as the Decel ramp time, and the factory default is 5 seconds, but someone could have set that longer too. Also look at P0300, which is the DC Braking Time at Stop. With DC Injection Braking, DC current is sent into 2 poles of the output. If that is turned on, then look at the next couple of parameters, which are the frequency at which is begins to brake and the voltage of that DC injection. Any of those might be causing what you are experiencing.

D'oh! Didn't scroll down all the way before responding...
Never mind!


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: VFD in a lathe application

Quote (cfw500 programming manual 11.2 I/f MODE (SENSORLESS))

The minimum speed recommended for the operation of the Sensorless Vector Mode is 18 rpm for 60 Hz IV pole motors, and 15 rpm for 50 Hz IV pole motors. If P0182 ≤ 3 rpm the inverter will always operate in Sensorless Vector mode, i.e., the I/f function will be disabled.
If your users need torque at speeds slower than 18 r/m Vector with encoder will get you there, but at zero speed unless intentionally set to freewheel, the motor will lock in place with current proportional to the force the operator applies to rotate the spindle. At zero speed no torque voltage will be very low, and current will be close to magnetizing current. If you need freewheel, do that command after a short time delay.
I did exactly this in a hoisting application. IE motor to a stop with test torque applied, set the shoe brakes, then set drive to freewheel after time delay. and also the reverse.
But with a hoisting duty drive (now obsolete) so I am not going to suggest programming. Enough good hints above.wink

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