Problem starting 1.5 hp lathe on VFD....

Fellas (and bnelson),

OK, here's a situation. I've got a 70's import lathe with a 1.5 hp motor on it. Been sitting for a few years too. I've been cleaning it up this week - now it looks good but it won't run!

I'm using an ABB VFD rated to power 3 hp max. The lathe has a reversing switch on the front panel, routed to the backside into a box holding some kind of something I don't know about. The wiring from the motor and the reversing switch both go into this thing.

I turn on the VFD, get it up to 60 hz, hit the run button, and then hit the 'power on' switch on the lathe. What happens at this point is the thing on the back kinda hums, spattering too at first. (Sounds like a contact not getting enough juice.) Now here is when the trouble starts - when I take the reversing switch out of the middle neutral position to get the motor going, everything just stops cold. No more funny noises from the thing on the backside of the lathe, and the VFD does a mild 'depowering' sound, like it is being sucked dry and coming back up to speed. Only way to get the lathe to 'power on' after this is to shut down the VFD and start all over, to no avail once more.

The VFD is about eight years old and hasn't been used in about three years. Do these things have capacitor blocks or something that go bad and perhaps keep them from starting a motor it should be able to start? Or is it more likely my motor is to blame? I can turn it by hand at the chuck, although with effort.

Any general ideas out there about why it won't start, or things to look for? Thank all for your time.

Regards,
J. Elliott

Joel,

I know almost nothing about VFDs. Never even had my hands on one. However I've learned some things on this forum, for example, that you are better off letting the VFD ENTIRELY control your machine, and not use any of your machine's own controls.

So have you tried (with power off) setting your lathe to be in FORWARD and RUN, then starting it with the VFD? That would be my first step.

If that doesn't work . . .

Do you have another three phase machine(s) in your shop you could get going, and using that as a crude phase converter, try to power on your import lathe with its three feed wires being in parallel to the feed wires of your running machine?

If your running machines are rated at least 3 hp, I'd say your import lathe should start up.

Regarding your question about a capacitor block in the VFD circuit, there is one, called a DC linking capacitor. If that went bad I would think the VFD would be nonfunctional.

Bnelson is probably correct, use the VFD to control the motor, not switches. If I switch the motor on or off while the vfd is set to run, the vfd will shut down to protect itself. I have a Hitachi 2hp vfd on a 2hp motor. (this particular Hitachi model was advertised as not needing to be de-rated for single phase to 3 phase use)

The good news is, if the vfd is bad, a new one is not terribly expensive. www.dealerselectric.com has good deals.

Good luck.

Eric

You need to get the VFD connected directly to the motor leads. Your lathe is probably using a relay to turn the motor forward/reverse/off and this relay is between the VFD and motor, not good. You want to get it out of the loop.

Try wiring the VFD directly to the motor and use the VFD controls to run it. It should work ok. Then dig into the manual and see if you can use your current forward/reverse/off switch to remotely control the VFD. It sounds like your switch is not a drum switch, but simple contacts to energize a relay and should be pretty easy to wire up to the VFD.

Guys,

Great, I'll try to hook up the VFD right to the motor. I'll let y'all know what happens.

bnelson - I just re-read one of your other posts. I might have a problem with grounding this lathe/power setup. I supply the VFD with a fused disconnect using 30 amp time-delay fuses. This new switch box has a provision for grounding, so I've grounded it back to the main bus grounding bar (and bonded the ground bar to the case). I then ground the VFD through this switch box. The problem comes next.

I feed the VFD output into a three-pole fused disconnect using 30 amp time-delay fuses. BUT - this is an older switch box and it has no innards for a ground contact. AND - the lathe motor has no line or lug for grounding it either.

I've bought a grounding bar and plan to screw it into the three-pole switch box. But - do I ground it daisy-chain style to the VFD, or somewhere else? And should I tap the motor housing and route a ground wire up to the three-pole switch (the last switch before the load)?

This raises a question - can grounds be properly routed daisy-chained? Or is each supposed to be a straight shot back to the main bus ground?

Joel,

It warms my heart that you are concerned about proper grounding. I don't think it can be stated here too often that it's really quite easy to kill yourself with the lathes, phase converters, transformers, VFDs, etc. we deal with on this forum if proper grounding is left out.

It is acceptable to 'daisy-chain' ground wires (NEC calls them 'equipment grounding conductors') so long as certain rules are followed. The term which applies is 'feed-through' ground.

Basically the rules are: NO home-made devices, NO use of a bonding screw for any other purpose than grounding a frame (it cannot also serve a mechanical purpose).

What you are doing is entirely appropriate using grounding bars. I like them because they automatically ground the switch housing. After you get everything grounded, start at the most distant point (machine), and follow the green wire back through the switches and other devices (transformers, phase converters, etc) all the way to the main panel ground bus. There must be a direct connection leading all the way back, though it does not have to be an unbroken home-run.

I would think the frame of your VFD is grounded if it's metallic. But if only three wires come out of the VFD, it's likely that none of them are grounded. However, it is MOST IMPORTANT that the frame of your motor be grounded. This is a matter of life and death, because an internal short from the motor windings to frame could kill the lathe operator.

So if your VFD's case is metal, mount a lug on it and ground it to one of your grounding bars. If your VFD has a green ground wire on the output side, bond that to your grounding bar. But by all means, mount a big lug on your motor frame and run at least a #10 wire (I always use at least #8 myself) back to your grounding bar.

Then, to give yourself a sense of safety, take a multimeter and test resistance between a cover screw on your house main panel and your lathe frame; it should measure zero ohms.

Guys -

Hey, y'all were right! I went straight to the windings and the thing came to life.

Sloooowwwly though. Seems the VFD has a 'soft-start' ramping act going on. The initial voltage was only 181, causing, I suppose, the relay to quit. Don't know how, but this is the way it seemed.

Nice pointer, many thanks.

Good to hear you got it going! You can usually set parameters on the VFD to change the accelerate and decelerate times. I've got a Teco, so I can't tell you how exactly. On mine, I set the accelerate time to 2 seconds with no decelerate, it coasts to a stop. I've got a threaded lathe spindle and don't want to unwind it, something to watch out for.